Advances in Photonic Devices Based on Optical Phase-Change Materials

molecules

Review Advances in Photonic Devices Based on Optical Phase-Change Materials

Xiaoxiao Wang 1, Huixin Qi 1, Xiaoyong Hu 1,2,3,*, Zixuan Yu 1, Shaoqi Ding 1, Zhuochen Du 1 and Qihuang Gong 1,2,3

1 Collaborative Innovation Center of Quantum Matter & Frontiers Science Center for Nano-Optoelectronics, State Key Laboratory for Mesoscopic Physics & Department of Physics, Beijing Academy of Quantum Information Sciences, Peking University, Beijing 100871, China; [email protected] (X.W.); [email protected] (H.Q.); [email protected] (Z.Y.); [email protected] (S.D.); [email protected] (Z.D.); [email protected] (Q.G.) 2 Peking University Yangtze Delta Institute of Optoelectronics, Nantong 226010, China 3 Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China * Correspondence: [email protected]

Abstract: Phase-change materials (PCMs) are important photonic materials that have the advantages of a rapid and reversible phase change, a great difference in the optical properties between the crystalline and amorphous states, scalability, and nonvolatility. With the constant development in the PCM platform and integration of multiple material platforms, more and more reconﬁgurable photonic devices and their dynamic regulation have been theoretically proposed and experimentally demonstrated, showing the great potential of PCMs in integrated photonic chips. Here, we review the recent developments in PCMs and discuss their potential for photonic devices. A universal overview of the mechanism of the phase transition and models of PCMs is presented. PCMs have injected new life into on-chip photonic integrated circuits, which generally contain an optical switch, Citation: Wang, X.; Qi, H.; Hu, X.; an optical logical gate, and an optical modulator. Photonic neural networks based on PCMs are Yu, Z.; Ding, S.; Du, Z.; Gong, Q. another interesting application of PCMs. Finally, the future development prospects and problems Advances in Photonic Devices Based that need to be solved are discussed. PCMs are likely to have wide applications in future intelligent on Optical Phase-Change Materials. Molecules 2021, 26, 2813. https:// photonic systems. doi.org/10.3390/molecules26092813 Keywords: phase change materials; photonic devices; reconﬁgurable; modulator; optical switch; Academic Editors: Miguel Levy, optical logic devices; vanadium dioxide; chalcogenide; photonic neural networks Dolendra Karki and Long Y. Chiang

Received: 23 March 2021 Accepted: 7 May 2021 1. Introduction Published: 10 May 2021 Photonic devices and applications have attracted much attention over the last two decades owing to their unique advantages. Photonic devices can achieve the same function Publisher’s Note: MDPI stays neutral as electronic devices using photons rather than electrons. The advantages of photonic with regard to jurisdictional claims in devices are their ultrafast information-processing speed owing to the propagation speed of published maps and institutional affil- light, very large information capacity because of light’s abundant degrees of freedom, and iations. ability to achieve parallel computing and device interconnection owing to no ohmic loss and Coulomb’s law. Tremendous progress has recently been made in photonic integrated circuits (PICs), which have the striking advantages of small footprints, low power consumption, ultrahigh Copyright: © 2021 by the authors. information-processing speed, and wide frequency bandwidth. PICs provide a scalable Licensee MDPI, Basel, Switzerland. hardware platform to solve the contradiction between the rate of energy efficiency im- This article is an open access article provement and the rapidly increasing computational load [1]. There are multiple material distributed under the terms and platforms to achieve PICs. The silicon-on-insulator platform fabrication infrastructure is conditions of the Creative Commons compatible with complementary metal–oxide–semiconductor (CMOS) technology, and it Attribution (CC BY) license (https:// can significantly reduce production costs. However, electrically pumped efficient sources creativecommons.org/licenses/by/ of silicon are still a challenge because silicon is an indirect bandgap material. To solve this 4.0/).

Molecules 2021, 26, 2813. https://doi.org/10.3390/molecules26092813 https://www.mdpi.com/journal/molecules Molecules 2021, 26, 2813 2 of 20

problem, other materials that have a variety of functions, such as InAs, GaAs, LiNbO3, and InP, are directly placed on silicon to obtain a heterogeneous silicon PIC platform. The heterogeneous silicon PIC platform provides not only low power and low cost but also high capacity and high volume. Furthermore, some of these platforms can provide modulators with high performance, nonlinearities, and magnetic properties [2]. The functional materials that can achieve proactive control described above completely depend on the external drive. The properties of the materials will be completely or partly restored after removing the external drive, so the response of the optical device will return to the initial state. That is, optical devices based on the materials mentioned above do not have the ability to remember. Electronic integrated circuits have the ability to be redesigned by field-programmable gate arrays, and they provide “programmable” capability based on electronic “rewiring” for various applications. Phase-change materials (PCMs) that have traditionally been used in electrical memory have recently emerged as an ideal material platform to realize optical programmability [3,4]. PCMs offer a way of tuning the properties of the nanostructures that strongly interact with light. Optical property modulation in PCM systems originates from a change in the bonding configuration [5]. There are some phase-change models that correctly describe the mechanism of phase transitions. Density functional theory (DFT) is usually used to predict the phase and electronic structures of PCMs [6]. The local density approximation plus dynamical mean-field theory has recently been proven to be very successful in modeling the symmetry conserving metal– insulator transition of some PCMs, and it is generally used for calculating the electronic spectrum, energy gap, and local magnetic moment [7,8]. The Stefan problem can describe the phenomenon of the phase change by transforming the problem into determining the temperature distribution in the one-dimensional liquid phase at a certain time. It improves the accuracy and stability of the existing solutions of the one-dimensional Stefan problem with time-dependent Dirichlet boundary conditions [9]. PCMs have also attracted much attention for use in reconfigurable photonic devices because of their outstanding properties, such as fast and reversible switching of the states, good thermal stability, drastic nonvolatile changes in the optical properties between the crystalline and amorphous states accompanying considerable modulation in the electrical resistivity, and optical constants with a wide spectral region [10,11]. These excellent properties greatly improve the performance of optical switches, as well as optical modulators in some aspects. Furthermore, phase-change memory offers multilevel data storage and can be applied in both neuro-inspired and all-photonic in-memory computing [12]. The progress in photonic applications focusing on realizing reconfigurable and reprogrammable functions has recently caught up with electronic devices [13]. However, many PCMs, such as Ge–Sb–Te (GST) alloys, simultaneously exhibit large contrast of both the refractive index and optical loss. The concurrent index and loss change fundamentally limit the scope of PCMs in many optical applications [6]. In this review, we provide a general overview of PCMs classified into two types (chalcogenide compounds and metal oxides) and compare the properties of the two types of PCMs. The physical mechanism of the phase transition and the models of PCMs are presented. The role of PCMs in different photonic devices is introduced, ranging from multilevel optical switches and ultracompact and low-loss optical modulators to large- scale photonic deep neural networks. The review emphasizes the important role of PCMs in emerging phase-change photonic devices, such as large-scale photonic deep neural networks and integrated photonic on-chip devices. The main issues facing PCMs and the possible solutions are discussed. The outlook for successfully implementing PCMs in photonic computing is also discussed.

2. Phase-Transition Mechanism and Properties of Prototypical PCMs

The most attractive PCMs are chalcogenide compounds (e.g., Ge2Sb2Te5, GeTe, and Ge–Sb–Se–Te) and transition-metal oxides (e.g., VO2 and NbO2). There are distinct differ- ences between PCMs, such as the volatility, refractive index contrast before and after the phase change, and phase-transition speed [14]. Ge2Sb2Te5 (GST) is a commonly used PCM with fast crystallization speed, large optical and electrical contrast, and good reversibility between the amorphous and crystalline states. The amorphous GST film exhibits semiconductor characteristics, while crystalline GST films exhibit semimetallic and metallic Molecules 2021, 26, 2813 properties in different crystalline states at specific temperatures. The characteristic mate-3 of 20 rial of another type of PCM is vanadium dioxide (VO2). By increasing the temperature, VO2 undergoes an insulator-to-metal transition around its phase-transition temperature, so VO2 has substantial optical contrast [15]. Upon decreasing the temperature of VO2 be- lowphase the change, phase-transition and phase-transition temperature, speed its state [14]. reverts Ge2Sb2 backTe5 (GST) to the is initial a commonly structure used [16]. PCM As awith result, fast VO crystallization2 is unsuitable speed, for large nonvolatile optical and and electrical reconfigurable contrast, applications. and good reversibility Therefore, PCMbetweens can the help amorphous to realize novel and crystalline photonic devices states. Theand amorphouscatch up with GST the film requirements exhibits semi- for aconductor higher level characteristics, of integration, while adjustability crystalline, and GST performance films exhibit [4]. semimetallic and metallic propertiesBefore in understanding different crystalline how PCMs states can at specific be applied temperatures. to optical Thedevices, characteristic more needs material to be knownof another about type the of phase PCM-transition is vanadium procedure dioxide itself, (VO2 both). By theoretically increasing the and temperature, experimentally. VO2 Inundergoes this section an, insulator-to-metal we introduce the transitionmechanism around of the its phase phase-transition transition, as temperature, well as the models so VO2 andhas substantialproperties of optical PCMs. contrast [15]. Upon decreasing the temperature of VO2 below the phase-transition temperature, its state reverts back to the initial structure [16]. As a result, 2.1.VO 2GSTis unsuitable and Other C forhalcogen nonvolatile-Based andOptical reconfigurable PCMs applications. Therefore, PCMs can help to realize novel photonic devices and catch up with the requirements for a higher 2.1.1. Models and Physical Mechanism of the Phase Transitions of GST level of integration, adjustability, and performance [4]. KolobovBefore understanding et al. [17] developed how PCMs a model can beto appliedexplain the to optical mechanism devices, of the more phase needs-transi- to be tionknown process about of the the phase-transition GST alloy that fits procedure the experimental itself, both data theoretically well. The distinction and experimentally. between theIn thistwo section, states was we introducemeasured the by mechanismextended X of-ray the absorption phase transition, fine-structure as well asspectroscopy the models andand Raman properties scattering of PCMs. spectroscopy to construct the model and explain the mechanism. The obtained results (Figure 1a,b) were in good agreement with previous X-ray diffraction measurements.2.1. GST and Other Chalcogen-Based Optical PCMs 2.1.1.They Models determined and Physical that the Mechanism true structure of the of Phase the crystalline Transitions phase of GST is not a typical type of rockKolobov-salt structure et al. [17, ]although developed it is a modelsimilar to. The explain Ge and the mechanismSb atoms may of the swap phase-transition places, pro- ducingprocess randomness of the GST alloy, and that each fits additional the experimental building data block well. may The rotate distinction by 90° between in an arb theitrary two direction.states was In measured addition by, the extended ideal model X-ray exhibits absorption a stacking fine-structure sequence spectroscopy of –Sb–Te–Ge and–Te Raman–Te– Gescattering–Te–Sb– spectroscopyTe–, which is toidentical construct to the the stable model hexagonal and explain structure the mechanism. of GST, thus The predicting obtained theresults phase (Figure change1a,b) to were some in extent. good agreementThe structure with is previous shown in X-ray Figure diffraction 1c. measurements.

FigureFigure 1. 1. TheThe m modelsodels and and physical physical mechanism mechanism of of Ge Ge2Sb2Sb2Te2Te5 (GST5 (GST)) phase phase transitions. transitions. (a) Fourier (a) Fourier-- transformedtransformed raw raw extended extended X X-ray-ray absorption absorption fine ﬁne-structure-structure spectroscopy spectroscopy (EXAFS (EXAFS)) spectra spectra for for crystallized and laser-amorphized samples. Spectra measured at the K-edges of Ge, Sb, and Te. On amorphization, the bonds become shorter (as shown by shifts in the peak positions) and stronger, that is, more locally ordered (as shown by increases in the peak amplitudes and concurrent decreases in the peak widths). (b) Raman scattering spectra for crystallized and re-amorphized GST layers. (c) The crystal structure of laser-amorphized GST. A schematic two-dimensional image of the lattice distortion of the rocksalt structure due to charge redistribution between the constituent elements. (d) The ideal structure of GST, constructed from the above building blocks and fragments of the three-dimensional GST structure constructed from rigid building blocks.

They determined that the true structure of the crystalline phase is not a typical type of rock-salt structure, although it is similar. The Ge and Sb atoms may swap places, producing randomness, and each additional building block may rotate by 90◦ in an arbitrary direction. Molecules 2021, 26, x FOR PEER REVIEW 4 of 21

crystallized and laser-amorphized samples. Spectra measured at the K-edges of Ge, Sb, and Te. On amorphization, the bonds become shorter (as shown by shifts in the peak positions) and stronger, that is, more locally ordered (as shown by increases in the peak amplitudes and concurrent de- Molecules 2021, 26, 2813 creases in the peak widths). (b) Raman scattering spectra for crystallized and re-amorphized GST4 of 20 layers. (c) The crystal structure of laser-amorphized GST. A schematic two-dimensional image of the lattice distortion of the rocksalt structure due to charge redistribution between the constituent elements. (d) The ideal structure of GST, constructed from the above building blocks and frag- mentIn addition,s of the three the- idealdimensional model GST exhibits structure a stacking constructed sequence from ofrigid –Sb–Te–Ge–Te–Te–Ge–Te–Sb– building blocks. Te–, which is identical to the stable hexagonal structure of GST, thus predicting the phase changeLaser to- someinduced extent. amorphization The structure results is shown in a drastic in Figure shortening1c. of covalent bonds of GST (Te–GeLaser-induced and Te–Sb bonds) amorphization and a decrease results in in a the drastic mean shortening-square relative of covalent displacement, bonds of demonstratingGST (Te–Ge and a substantial Te–Sb bonds) increase and a in decrease the degree in the of short mean-square-range order relative. To obtain displacement, further insightdemonstrating into the amorphous a substantial structure, increase Kolobov in the degree et al. [17] of short-range performed order.X-ray Toabsorption obtain further near- edgeinsight structure into the simulations. amorphous The structure, mechanism Kolobov of the et phase al. [17 transition] performed can X-ray be explained absorption as followsnear-edge. The structure Ge atoms simulations. play a key Therole mechanism in the amorphization of the phase procedure transition. They can be occupy explained the octahedralas follows. and The tetrahedral Ge atoms play symmetry a key role positions in the amorphization in the crystalline procedure. and amorphous They occupy states, the respectively.octahedral and The tetrahedral difference symmetryis that several positions weaker in covalent the crystalline bonds andconnect amorphous the Ge atoms states, withrespectively. other atoms. The Upon difference application is that of several an intense weaker laser, covalent the weaker bonds bonds connect will thebe ruptured, Ge atoms andwith the other Ge atoms.atom will Upon flip application into the tetrahedral of an intense position laser,. the By weakerchecking bonds the Ge will–Te be distance, ruptured, thisand explanation the Ge atom fits will the ﬂip experimental into the tetrahedral data well. position. They suggested By checking that electronic the Ge–Te excitation distance, this explanation ﬁts the experimental data well. They suggested that electronic excitation creating nonequilibrium charge carriers is crucial for weakening and subsequent rupture creating nonequilibrium charge carriers is crucial for weakening and subsequent rupture of of the subsystem of weaker Ge–Te bonds. Photogenerated nonequilibrium carriers popu- the subsystem of weaker Ge–Te bonds. Photogenerated nonequilibrium carriers populate late these states, making them more susceptible to thermal vibration-induced dissociation. these states, making them more susceptible to thermal vibration-induced dissociation. The The local arrangement around the Sb atoms remains unchanged, enhancing the overall local arrangement around the Sb atoms remains unchanged, enhancing the overall stability. stability. Both of these factors contribute to amorphization. Both of these factors contribute to amorphization.

2.1.2.2.1.2. Properties Properties of of GST GST and and O Otherther C Chalcogen-Basedhalcogen-Based O Opticalptical PCMs PCMs GSTGST is is widely widely used used and and commercialized commercialized in in materials materials that that are are used used in in optical optical data data storagestorage devices devices,, such such as as digital digital videodisks videodisks ( (DVDs).DVDs). Researchers Researchers are are attempting attempting to to discover discover newnew properties properties while while searching searching for for new new applications applications of ofthe the currently currently known known properties properties of GSTof GST.. By Byslight slightlyly changing changing GST GST and and exploring exploring its new its new properties, properties, research research has hasbeen been ex- tendextendeded from from GST GST to some to some general general chalcogen chalcogen-based-based optical optical PCMs PCMs (O- (O-PCMs).PCMs). ChalcogenChalcogen-based-based PCMs PCMs consist consist of of some some particular particular elements elements that that are are widely widely used used for for memory.memory. The The commonly commonly used used elements elements are are shown shown in Figure in Figure 2a 2[18]a [ .18 Other]. Other than than these these elements,elements, researchers researchers have have also also discovered discovered other other components components that that have have potential potential phase phase-- changechange properties properties;; for for example, example, AuTe AuTe2, which2, which is also is also included included in the in chalcogen the chalcogen-based-based ma- terialsmaterials family. family.

FigureFigure 2. 2. (a(a) )Typical Typical elements elements in in the the chalcogenide chalcogenide family family used used for for optical optical storage storage.. (b () bT)he The tertiary tertiary GeGe-Sb-Te-Sb-Te phase phase diagram diagram with with some some popular popular chalcogenide chalcogenide alloys alloys highlighted.

TheThe discovery discovery of of a a new new class class of of materials materials along along the the GeTe GeTe–Sb–Sb2TeTe3 pseudopseudo-binary-binary line line broughtbrought about about the the initial initial use use of of GST GST alloy alloys.s. The The basic basic properties properties were were known known long long before, before, andand it it was was not not until until the the emergence emergence of of the the pseudo pseudo-binary-binary line line that that GST GST alloy alloyss attracted attracted muchmuch attention attention because because of of their their properties. Alloys Alloys along along this this line line with with compositions (GeTe)(GeTe)mm(Sb(Sb2Te2Te3)3 )have have the the ability ability to to rapidly rapidly switch switch between between the the phases phases and and have have strong strong electrical/optical contrast [19]. After doping the material with elements (mostly those in Figure2a, crystallization will change, which could result in an even faster switching speed or a steadier amorphous phase [20,21]. The GeTe–Sb Te pseudo-binary line and the 2 3 previously mentioned alloys are shown in Figure2b [18]. GST alloys and other chalcogen-based PCMs have similar optical properties that are beneﬁcial for potential applications. They work by forward crystallization and backward Molecules 2021, 26, x FOR PEER REVIEW 5 of 21

electrical/optical contrast [19]. After doping the material with elements (mostly those in Figure 2a, crystallization will change, which could result in an even faster switching speed or a steadier amorphous phase [20,21]. The GeTe–Sb2Te3 pseudo-binary line and the pre- Molecules 2021, 26, 2813 5 of 20 viously mentioned alloys are shown in Figure 2b [18]. GST alloys and other chalcogen-based PCMs have similar optical properties that are beneficial for potential applications. They work by forward crystallization and backward amorphization,amorphization, which which is is a a way way of phase transformationtransformation thatthat isis reversible.reversible. The The procedure procedure is isshown shown in in Figure Figure3[ 313 [13].]. The The two two states states of of these these materials materials show show tremendous tremendous contrast contrast of theof theoptical optical constants. constants. We We mostly mostly pay pay attention attention to theto the complex complex index index of refraction, of refraction, optical optical loss, lossand, and electrical electrical resistivity resistivity [22]. [22] These. These properties properties are fundamentalare fundamental to optical to optical storage, storage, light lightmodulators, modulators and, and other other types types of devices. of devices.

FigureFigure 3. 3. TheThe structural structural phase phase transition transition of of the the GST GST chalcogenide chalcogenide PCM PCM between between its its (a ()a amorphous) amorphous andand (b (b) )crystalline crystalline state states.s.

TheThe phase phase-change-change process process is is bas baseded on on the the thermal thermal effect. effect. By By applying applying a a specific specificallyally designed laser pulse, the material will be heated above its crystallization temperature designed laser pulse, the material will be heated above its crystallization temperature but but below its melting point. After slow cooling, in contrast to amorphization, the atoms below its melting point. After slow cooling, in contrast to amorphization, the atoms will will show long-range order, so the material is in the crystalline state. If the material is show long-range order, so the material is in the crystalline state. If the material is heated heated above the melting point and then rapidly quenched to room temperature, it will above the melting point and then rapidly quenched to room temperature, it will reach the reach the amorphous state. The two states have different electronic densities and transition amorphous state. The two states have different electronic densities and transition matrix matrix elements, that is, the oscillator strength of the optical transitions due to structural elements, that is, the oscillator strength of the optical transitions due to structural rear- rearrangement. Another advantage of GST alloys and other chalcogen-based PCMs is that rangement. Another advantage of GST alloys and other chalcogen-based PCMs is that the the phase-change processes of these materials can be very fast, and they are faster than phase-change processes of these materials can be very fast, and they are faster than those those of almost all known materials. of almost all known materials. Other properties are also important for this type of material to be used in devices. Other properties are also important for this type of material to be used in devices. Requirements such as stability at room temperature, large scalability of the capacity, a Requirementsnonvolatile nature, such as and stability extended at read/writeroom temperature, endurance large are allscalability fulfilled of by the GST capacity, alloys and a nonvolatileother chalcogen-based nature, and extended PCMs, and read/write they are necessaryendurance for are applications. all fulfilled by GST alloys and other chalcogenIn addition-based to the PCMs, optical and properties, they are the necessary thermal for properties applications. should also be taken into In addition to the optical properties, the thermal properties should also be taken into consideration. The thermal conductivities of the different phases of Ge2Sb2Te5 range from consideration.0.14 to 1.76 W/(m The thermal·K) [23]. conductivities The low thermal of the conductivity different phases can significantly of Ge2Sb2Te reduce5 range energy from 0.14consumption to 1.76 W/(m·K) in the crystallization[23]. The low thermal and amorphization conductivity procedure, can significantly which isreduce beneficial energy for consumptionthe optical properties. in the crystallization and amorphization procedure, which is beneficial for the optical properties. 2.2. VO2 and Other Transition-Metal Oxide O-PCMs 2.2.2.2.1. VO Models2 and Other and T Physicalransition- MechanismMetal Oxide ofO- thePCMs Phase Transitions of VO2

2.2.1. ModelsVO2 is one and of P thehysical strongly Mechanism correlated of the electron Phase materials Transitions that of are VO typically2 characterized by aVO variety2 is one of phaseof the transitionsstrongly correlated that occur electron as a result materials of competing that are interactionstypically character- between izedthe charge,by a variety spin, orbital,of phase and transitions lattice degrees that occur of freedom as a result [24]. of First-order competing phase interactions transitions be- in tweensolids, the such charge, as VO spin,2, are orbital, notoriously and lattice challenging degrees to studyof freedom [25]. The[24] transition-metal. First-order phase dioxides tran- sitionsat the beginningin solids, such of the as d VO series2, are have notoriously been studied challenging by DFT-based to study electronic [25]. The structure transition calcu-- metallations, dioxides which at are the capable beginning of correctly of the d describing series have the been electronic studied and by magneticDFT-based properties electronic of structurethe metal, calculations as well as, both which the are ﬁrst capable insulating of correctly monoclinic describing (M1) and the theelectronic second and insulating mag- neticmonoclinic properties (M2) of phases.the metal By, as using well recentlyas both the developed first insulating hybrid monoclinic functionals, (M1 considerable) and the secondprogress insulating has been mademonoclinic in understanding (M2) phases. the By physics using of recently VO2, including developed the electron– hybrid electron interaction, and thereby improving the weaknesses of the semilocal exchange functionals provided by the local density and generalized gradient approximations [26]. Although there are limitations in the local density approximation, the theory establishes a reference that can be directly compared with experimental data [7]. The results of calculations based on DFT using hybrid functionals are shown in Figure4b,c , which are in good agreement with the experiment. The group of bands strad- Molecules 2021, 26, x FOR PEER REVIEW 6 of 21

functionals, considerable progress has been made in understanding the physics of VO2, including the electron–electron interaction, and thereby improving the weaknesses of the semilocal exchange functionals provided by the local density and generalized gradient approximations [26]. Although there are limitations in the local density approximation, the theory establishes a reference that can be directly compared with experimental data [7]. Molecules 2021, 26, 2813 6 of 20 The results of calculations based on DFT using hybrid functionals are shown in Fig- ure 4b,c, which are in good agreement with the experiment. The group of bands straddling the Fermi energy is dominated by the V3d states. The V3d and O2p contributions in the en-

ergydling range, the Fermi where energy the respective is dominated other by orbitals the V 3ddominate,states. The result V3d fromand Ohybridization2p contributions be- tweenin the energy these states. range, Therefore, where the respective the phase other transition orbitals is dominate, accompanied result by from strong hybridiza- orbital switching,tion between which these has states. beenTherefore, confirmed the by phase polarization transition-dependent is accompanied X-ray-absorption by strong orbital spec- troscopyswitching, measurements, which has been as shown confirmed in Figure by polarization-dependent 4d. In addition, a constant X-ray-absorption critical free-elec- spec- trontroscopy concentration measurements, is needed as shown on the in insulating Figure4d. side In addition, to trigger a constant the insulator critical-to free-electron-metal tran- sitionconcentration [24]. is needed on the insulating side to trigger the insulator-to-metal transition [24].

FigureFigure 4. 4. ((aa)) Schematic Schematic band band diagrams diagrams for for VO VO22 in in the the metallic metallic (rutile) (rutile) phase phase and and the the semiconduct- semiconduct- ing(monoclinic)ing(monoclinic) phase. ( b) Partial DOSDOS ofof rutilerutile VO2 VO2 as as calculated calculated using using the the HSE HSE functional. functional. (c )( Partialc) PartialDOS of DOS M1-VO2 of M1- asVO calculated2 as calculated using using the HSEthe HSE (bottom) (bottom) functional. functional. (d) ( Experimentald) Experimental V L2;3V L2;3 XAS XASspectra spectra of VO2 of VO in the2 in insulatingthe insulating M1 M1 phase phase (top (top panel, panel, T = T 30 = ◦30C) °C) and and the the metallic metallic R phaseR phase (bottom (bottom panel, T = 100 °C) taken with the light polarization→ 퐸⃗ //c (solid lines) and→ 퐸⃗ ⊥ c (dashed panel, T = 100 ◦C) taken with the light polarization E//c (solid lines) and E⊥c (dashed lines). lines). 2.2.2. Properties of VO2 and Other Transition-Metal Oxide O-PCMs 2.2.2. Properties of VO2 and Other Transition-Metal Oxide O-PCMs VO2 is a promising reconﬁgurable and reprogrammable active optical PCM because of itsVO distinctive2 is a promising insulator-to-metal reconfigurable phase and reprogrammable transition. It provides active optical orders ofPCM magnitude because ofchange its distinctive in the resistivity insulator and-to large-metal changes phase in transition. absorption It and provides the refractive orders index,of magnitude which is changeaccessible in the in theresistivity range from and near-large tochanges far-infrared in absorption wavelength and [the27]. refractive index, which ◦ is accessibleAt a critical in the temperature range from near of T-C to= far≈67-infraredC (340 wavelength K), VO2 undergoes [27]. a signiﬁcant and reversibleAt a critical insulator-to-metal temperature of phase TC = transition67 °C (340 in K), response VO2 undergoes to an increase a significant in the tempera- and re- versibleture [28 ].insulator In the process-to-metal of phase the phase transition transition, in response the structural to an increase transition in the occurs, temperature with a [28]change. In the from process a low-temperature of the phase transition monoclinic, the phase structural to a high-temperature transition occurs rutile, with phase. a change The fromfundamental a low-temperature atomic structures monoclinic of VOphase2 are to showna high- intemperature Figure5[ 7rutile]. The phase. insulator The funda- with a mentalmonoclinic atomic structure structures is shown of VO in2 are Figure shown5a, andin Figure its bandgap 5 [7]. The is about insulator 0.7 eV. with The a monoclinic metal with Molecules 2021, 26, x FOR PEER REVIEW 7 of 21 structurethe rutile is structure shown in is Figure shown 5a in, and Figure its 5bandgapb. The large is about and 0.7 small eV. spheres The metal denote with thethe metalrutile structureand ligand is shown atoms, respectively.in Figure 5b. The large and small spheres denote the metal and ligand atoms, respectively. The phase transition of VO2 can also be induced by an applied terahertz electric field [29], hot-electron injection [30], strain [31], and all-optical pumping [32]. For all-optical pumping, the time of the phase transition is ~75 fs [33].

Figure 5. Crystal structures of (a) semiconducting(monoclinic) and (b) metallic (rutile) phases of VO2. Figure 5. Crystal structures of (a)semiconducting(monoclinic) and (b)metallic (rutile) phases of VO2. The phase transition of VO2 can also be induced by an applied terahertz electric ﬁeld [29], hot-electron injection [30], strain [31], and all-optical pumping [32]. For all- opticalIn the pumping, telecommunication the time of the wavelength phase transition bands isnear ~75 1310 fs [33 and]. 1550 nm, the changes of the realIn theand telecommunication imaginary parts of wavelengththe refractive bands index near of VO 13102 are and obvious 1550 nm,. At the 1310 changes nm, the of complexthe real and refractive imaginary index parts of VO of2 the changes refractive from index about of 3.2 VO +2 0.5iare in obvious. the insulating At 1310 phase nm, the to about 1.5 + 2.5i in the metallic phase. At 1550 nm, the complex refractive index of VO2 changes from about 2.9 + 0.4i in the insulating phase to about 2.0 + 3i in the metallic phase [34]. The wavelength dispersions of the refractive index (n) and extinction coefficient (k) of sputtered VO2 thin films in both states (semiconductor at T < TC and metal at T > TC) are shown in Figure 6 [35]. There are very large changes in the optical, dielectric, and electrical properties of materials when the transition occurs with thermal excitation [36]. The transition from the dielectric to the metallic state can be as fast as femtoseconds (“on”), while the transition from the metallic state back to the dielectric state is slower, with transition times ranging from ~1–10 ps (“off”).

Figure 6. Wavelength dispersions of VO2 thin films. (a) Refractive index, n and (b) extinction coefficient, k. Thin films at temperatures below and above the phase transition temperature of 67 °C determined by UVISEL spectroscopic ellipsometry.

VO2 has been incorporated into silicon photonics, plasmonics, and other hybrid nanocomposites to achieve improved performance, and it also provides platforms for reconfigurable photonics and enables the construction of ultrafast optical switches, modulators, and memory elements [27]. However, VO2 is unsuitable for nonvolatile and reconfigurable applications.

3. Photonic Devices and Applications Based on PCMs PICs provide a scalable hardware platform to solve the contradiction between the rate of energy efficiency improvement and the rapidly increasing computational load. PCMs have injected new life into on-chip PICs because of their programmability and wide use in reconfigurable photonic devices. In this section, we report the PCMs used in PICs, which contain an optical switch, an optical logical gate, and an optical modulator. Pho- tonic neural networks based on PCMs are another interesting application.

Molecules 2021, 26, x FOR PEER REVIEW 7 of 21

Figure 5. Crystal structures of (a)semiconducting(monoclinic) and (b)metallic (rutile) phases of VO2.

In the telecommunication wavelength bands near 1310 and 1550 nm, the changes of the real and imaginary parts of the refractive index of VO2 are obvious. At 1310 nm, the complex refractive index of VO2 changes from about 3.2 + 0.5i in the insulating phase to about 1.5 + 2.5i in the metallic phase. At 1550 nm, the complex refractive index of VO2 Molecules 2021, 26, 2813 changes from about 2.9 + 0.4i in the insulating phase to about 2.0 + 3i in the metallic 7phase of 20 [34]. The wavelength dispersions of the refractive index (n) and extinction coefficient (k) of sputtered VO2 thin films in both states (semiconductor at T < TC and metal at T > TC) are

complexshown in refractive Figure 6 [35] index. of VO2 changes from about 3.2 + 0.5i in the insulating phase to aboutThere 1.5 are + 2.5ivery in large the change metallics in phase. the optical At 1550, dielectric nm, the, and complex electrical refractive properties index of ma- of VOterials2 changes when fromthe transition about 2.9 occurs + 0.4i in with the insulatingthermal excitation phase to [36] about. The 2.0 +transition 3i in the from metallic the phasedielectric [34]. to The the wavelength metallic state dispersions can be as of fast the as refractive femtosecond indexs ( n(“on”)) and extinction, while the coefﬁcient transition (fromk) of sputteredthe metallic VO state2 thin back ﬁlms to in the both dielectric states (semiconductor state is slower, at withT < transitionTC and metal times at Tranging> TC) arefrom shown ~1–10 in ps Figure (“off”)6[.35 ].

FigureFigure 6.6. WavelengthWavelength dispersions dispersions of of VO VO2 thin2 thin film films.s. (a) Refractive (a) Refractive index, index, n and n ( andb) extinction (b) extinction coef- coefficient,ficient, k. T k.hin Thin films films at temperatures at temperatures below below and and above above the the phase phase transition transition temperature temperature of of67 67 °C◦ C determineddetermined byby UVISELUVISEL spectroscopicspectroscopic ellipsometry. ellipsometry.

ThereVO2 has are been very largeincorporated changes into in the silicon optical, photonics, dielectric, plasmonic and electricals, and properties other hybrid of materialsnanocomposites when the to transitionachieve improved occurs with performance thermal excitation, and it also [36 provide]. The transitions platforms from for the re- dielectricconfigurabl toe the photonics metallic and state enable can bes the as fast construction as femtoseconds of ultrafast (“on”), optical while switches, the transition modu- fromlators the, and metallic memory state elements back to [27] the. dielectric However, state VO2 is is slower, unsuitable with for transition nonvolatile times and ranging recon- fromfigurable ~1–10 applications. ps (“off”). VO2 has been incorporated into silicon photonics, plasmonics, and other hybrid nanocomposites3. Photonic Devices to achieve and Applications improved performance,Based on PCMs and it also provides platforms for reconfigurablePICs provide photonics a scalable and enableshardware the platform construction to solve of ultrafast the contradiction optical switches, between mod- the ulators,rate of energy and memory efficiency elements improvement [27]. However, and the VOrapidly2 is unsuitable increasing forcomputation nonvolatileal load. and reconfigurablePCMs have injected applications. new life into on-chip PICs because of their programmability and wide use in reconfigurable photonic devices. In this section, we report the PCMs used in PICs, 3. Photonic Devices and Applications Based on PCMs which contain an optical switch, an optical logical gate, and an optical modulator. Pho- tonicPICs neural provide networks a scalable based hardware on PCMs platform are another to solve interesting the contradiction application. between the rate of energy efficiency improvement and the rapidly increasing computational load. PCMs

have injected new life into on-chip PICs because of their programmability and wide use in reconfigurable photonic devices. In this section, we report the PCMs used in PICs, which contain an optical switch, an optical logical gate, and an optical modulator. Photonic neural networks based on PCMs are another interesting application. 3.1. Basic Concepts and Realization Principles First, we introduce some basic concepts and nanostructures that are usually used in photonic devices. Fiber optics process parallel streams of data at high rates. The application of plastic optical fibers in communication has been limited to short data links and local area networks because the comparatively high attenuation and modest bandwidth of these fibers limit the transmission rate. However, these limits can be overcome by using a W-type fiber in which the waveguide dispersion is smaller than in the single-cladding fiber. The W-type fiber is also easier to splice. Moreover, the intermediate layer decreases the fiber’s effective numerical aperture, and therefore the number of guided modes, and confines the guided modes tighter to the core. As a consequence, it has a wide transmission bandwidth and lower bending losses compared with the corresponding single-cladding fiber [37]. The wide bandwidth is important for the computing speed. Photonic crystals are artificial microstructures with a spatially periodic dielectric distribution. Because of the distinctive modulation actions of the periodic refractive-index distribution in space on the incident electromagnetic waves, a photonic bandgap (also called Molecules 2021, 26, 2813 8 of 20

the stop band) is generated, which leads to the specific ability of a photonic crystal [38]. The refractive index is changed by a microstructured pattern of very small holes running along the length of the photonic crystal fibers. Generally, a photonic crystal fiber has a solid core and a holy cladding. The pattern of holes lowers the effective refractive index of the cladding, enabling the fiber to guide light. There are additional adjustable geometric parameters that provide greater design flexibility, so the bandwidth can be improved by tuning many parameters [39]. Microring resonators are compact bent dielectric waveguides in a small loop configuration. Controlling the signal light’s output can be realized by changing the coupling state between the waveguide and the microring through the control light. PCMs can change the index and other optical properties by controlling the phase transition, so optical routing and optical switching can be well realized. Metamaterials refer to materials that are artificially structured to exhibit remarkable electromagnetic properties that cannot be found in nature. Metamaterials are structured on a sub-wavelength scale and can be described by an effective medium approach [4]. The use of metamaterials is the creation of arbitrary light field distributions on a sub-wavelength scale, and they can change the propagation direction, spatial amplitude, phase pattern, and polarization state of the incident light in some way. However, all of these concepts are limited to volatile applications. The reversibly switchable optical properties of PCMs enable active nonvolatile nanophotonics.

3.2. Photonic Devices and Applications 3.2.1. Integrated Photonic On-Chip Devices The explosive growth of global data has put forward an increasing demand on information processing technology for ultralarge data transmission capacity and ultrahigh information processing speed. PICs provide a scalable hardware platform to realize ultrahigh response speed, low photon power threshold, high conversion efficiency, and relatively low device cost. With the constant development in the PCM platform and integration of multiple materials platforms, more and more reconfigurable photonic devices and their dynamic regulation have been theoretically proposed and experimentally demonstrated, showing the great potential of PCMs in integrated photonic chips. In this part, we introduce the recent developments in PCMs and discuss their potential for integrated photonic on-chip devices. All-optical switches with the properties of high on-chip performance, ultracompact structure, high speed, and low power are highly desirable as the essential building blocks of integrated photonics. An all-optical switch can be defined as a structure with a pump light controlling the ON/OFF transition of the signal light. In 2018, Zhang et al. [40] designed an O-PCM-based photonic switch that is broadband nonvolatile. They used a new O-PCM, Ge2Sb2Se4Te1 (GSST), to reduce the optical attenuation. The structure they designed is undisturbed, resulting in low-loss device operation beyond the classical figure-of-merit limit. The basic principle of this switching is to change the propagation states by phase transition. The electric-field intensities when GSST is in the amorphous (a-GSST) and crystalline (c-GSST) states are shown in Figure7a,b, respectively. The even and odd supermodes in a two-waveguide system are shown in Figure7c (i) and (ii), one of which is covered by a-GSST. The case of c-GSST is shown in Figure7d. They designed a 1 × 2 switch (Figure7e). When GSST is amorphous, the two waveguides are well coupled, so the light transfers into the other waveguide. When GSST is crystalline, the light will propagate without transfer. They also designed a 2 × 2 switch (Figure7f), which works the same way. Molecules 2021, 26, x FOR PEER REVIEW 9 of 21

In 2018, Zhang et al. [40] designed an O-PCM-based photonic switch that is broadband nonvolatile. They used a new O-PCM, Ge2Sb2Se4Te1 (GSST), to reduce the optical attenuation. The structure they designed is undisturbed, resulting in low-loss device operation beyond the classical figure-of-merit limit. The basic principle of this switching is to change the propagation states by phase transition. The electric-field intensities when GSST is in the amorphous (a-GSST) and crystalline (c-GSST) states are shown in Figure 7a,b, respectively. The even and odd supermodes in a two-waveguide system are shown in Figure 7c (i) and (ii), one of which is covered by a-GSST. The case of c-GSST is shown in Figure 7d. They designed a 1 × 2 switch (Figure 7e). When GSST is amorphous, the two Molecules 2021, 26, 2813 waveguides are well coupled, so the light transfers into the other waveguide. When GSST9 of 20 is crystalline, the light will propagate without transfer. They also designed a 2 × 2 switch (Figure 7f), which works the same way.

FigureFigure 7. 7. (a(,ab,)b Modal) Modal intensity intensity profiles profiles of ofa SiN a SiN waveguide waveguide loaded loaded with with a GSST a GSST strip stripin the in (a the) (a) amorphousamorphous and and (b (b) )crystalline crystalline states. states. The The inset inset in ( ina) (illustratesa) illustrates the thewaveguide waveguide cross cross section; section; (c) (c) intensity profiles of (i) even and (ii) odd supermodes in a two-waveguide system, one of which is intensity profiles of (i) even and (ii) odd supermodes in a two-waveguide system, one of which is topped with GSST, while the other is not. The GSST layer is in (c) the amorphous state and (d) the topped with GSST, while the other is not. The GSST layer is in (c) the amorphous state and (d) the crystalline state. All modes are TE-polarized. (e) 1 × 2 switch: cross-sectional structure of the 1 × 2 × × switch.crystalline Here state., Wc All= W modesg = 500 are nm, TE-polarized. Ws = 720 nm, ( eW) 1p = 4002 switch: nm, h cross-sectional= 450 nm, and hp structure = 60 nm; of the(f) sche-1 2 maticswitch. illustration Here, Wc of = the Wg 1 =× 2 500 switch, nm, Wsthe =rectangle 720 nm, marks Wp = the 400 cross nm, section h = 450 depicted nm, and in hp (e =). 60(g) nm;2 × 2 ( f) switch:schematic cross illustration section and of ( theh) perspective 1 × 2 switch, view the of rectangle the switch. marks Here the, W crossc = 512nm, section W depicteds = 730nm, in (We).p (=g ) 400nm,2 × 2 switch: Wg = 562 cross nm, section h = 450 and nm, (h )and perspective hp = 60 nm view. Wc: of thethe switch.width of Here, the GSST Wc =- 512loaded nm, silicon Ws = 730ni- nm, trideWp= (SiN) 400 nm,waveguide. Wg = 562 Wg: nm, the h =width 450 nm, of the and gap hp between = 60 nm. two Wc: waveguides. the width of theWs: GSST-loadedthe width of siliconthe SiNnitride waveguide (SiN) waveguide. without GSST. Wg: theWp: width the width of the of gap the betweenPCM. h: twothe height waveguides. of the the Ws: SiN the waveguide width of the withoutSiN waveguide GSST. hp: without the height GSST. of Wp:the PCM. the width of the PCM. h: the height of the the SiN waveguide without GSST. hp: the height of the PCM. In 2019, Xu et al. [41] reported a low-loss and broadband nonvolatile GST-based phaseIn-change 2019, Xudirectional et al. [41 coupler] reported (DC) a switch. low-loss The and basic broadband principle nonvolatile is the same GST-based as that of Zhangphase-change et al. [40] directional. They demonstrated coupler (DC) both switch. a 1 × 2 DC The switch basic principle(numerical, is Figure the same 8a; asexper- that iment,of Zhang Figure et al. 8b [)40 and]. Theya 2 × 2 demonstrated DC switch (numerical, both a 1 Figure× 2 DC 8d switch; experiment, (numerical, Figure Figure 8e). The8a; experimentalexperiment, Figure results8b) are and in agood 2 × 2agreement DC switch with (numerical, the numerical Figure8 results.d; experiment, Figure8e) . The experimental results are in good agreement with the numerical results. In 2019, Wu et al. [42] reported a PCM-based low-loss integrated photonic switch using GST as the PCM. Their system contains two waveguides and a microring. The microring is partly covered by GST. Schematic diagrams of the system are shown in Figure9a,b . The through-port and the drop-port transmission are shown in Figure9c–f. In 2020, Zhang et al. [43] reported a PCM-based reconfigurable DC switch using a photonic crystal ring resonator. A schematic of the structure is shown in Figure 10. When the index of refraction is different, the port of the output light changes. They also realized double-wavelength switching, and the principle is the same. The 2 × 2 switch has five cross states, two dropping directions, works at two wavelengths, and only requires two materials. Molecules 2021, 26, x FOR PEER REVIEW 10 of 21

MoleculesMolecules 20212021,, 2626,, x 2813 FOR PEER REVIEW 1010 of of 21 20

Figure 8. (a) Schematic of the design of the 1 × 2 DC switch. (b) Experimental results of the 1 × 2 DC switch, optical microscope image of the fabricated switch. (c) Measured transmission at the cross and bar ports with the GST in the amorphous and crystalline states. (d) Schematic of the design of the 2 × 2 DC switch. (e) Optical microscope image of the fabricated switch. (f) The measured transmission at the cross and bar ports with the GST in the amorphous and crystalline states. FigureFigure 8. 8. ((aa)) Schematic Schematic of of the the d designesign of of the the 1 1× ×2 DC2 DC switch. switch. (b) ( bExperimental) Experimental results results of the of the 1 × 12 × 2 In 2019, Wu et al. [42] reported a PCM-based low-loss integrated photonic switch DCDC switch switch,, o opticalptical microscope image of thethe fabricatedfabricated switch.switch. ((cc)) MeasuredMeasured transmission transmission at at the the cross using GST as the PCM. Their system contains two waveguides and a microring. The mi- crossand barand ports bar ports with with the GSTthe GST in the in amorphousthe amorphous and and crystalline crystalline states. states. (d )(d Schematic) Schematic of of the the design de- croringsign of the is partly2 × 2 DC covered switch. by(e) OpticalGST. S chematicmicroscope diagram image ofs theof thefabricated system switch. are shown (f) The in m Figureeas- of the 2 × 2 DC switch. (e) Optical microscope image of the fabricated switch. (f) The measured 9ureda,b. transmissionThe through at-port the crossand theand dropbar ports-port with transmission the GST in theare amorphousshown in Figure and crystalline 9c–f. states. transmission at the cross and bar ports with the GST in the amorphous and crystalline states. In 2019, Wu et al. [42] reported a PCM-based low-loss integrated photonic switch using GST as the PCM. Their system contains two waveguides and a microring. The microring is partly covered by GST. Schematic diagrams of the system are shown in Figure 9a,b. The through-port and the drop-port transmission are shown in Figure 9c–f.

FigureFigure 9. 9. (a()a The) The 1 × 1 2× optical2 optical switch switch operates operates by triggering by triggering the phase the phase transition transition of the of GST the GSTfilm film embeddedembedded in in a a microring microring resonator. resonator. When When the the GST GST is is in in the the amorphous phase, the signal resonantlyreso- nantlycouples couples into the into microring the microring and outputs and outputs from from the drop the drop port withport with a very a very low loss.low loss. When When the GSTthe is GST is in the crystalline phase, the signal is decoupled from the microring and outputs directly in the crystalline phase, the signal is decoupled from the microring and outputs directly from the from the through port to avoid any optical loss. (b) The measurement scheme uses pulses from a through port to avoid any optical loss. (b) The measurement scheme uses pulses from a pump laser pump laser to optically control the phase transition of the GST, thereby switches the output of the Figure 9. (a) The 1 × 2 optical switch operates by triggering the phase transition of the GST film probeto optically laser between control the through phase transition and drop of ports. the GST, electro thereby-optic switches modulator the (EOM) output, erbium of the probe-doped laser embedded in a microring resonator. When the GST is in the amorphous phase, the signal reso- fiberbetween amplifier the through (EDFA) and, variable drop ports.optical electro-optic attenuator (VOA) modulator, photon (EOM), detector erbium-doped (PD). (c) Through fiber amplifier port nantly couples into the microring and outputs from the drop port with a very low loss. When the and(EDFA), (d) drop variable port opticaltransmission attenuator spectra (VOA), near photona resonance detector of the (PD). microring (c) Through when port the andGST ( dis) dropin a port GST is in the crystalline phase, the signal is decoupled from the microring and outputs directly transitiontransmission from spectra the crystalli nearne a resonancephase (blue) of to the the microring amorphous when phase the (red). GST is(e in,f) aLog transition scale spectra from the from the through port to avoid any optical loss. (b) The measurement scheme uses pulses from a highlighting the low insertion loss of 0.46 dB (0.75 dB) and crosstalk −14.1 dB (−22 dB) at the pumpcrystalline laser phaseto optically (blue) control to the amorphousthe phase transition phase (red). of the (e ,GST,f) Log thereby scale spectra switches highlighting the output theof the low through (drop) port. probeinsertion laser loss between of 0.46 the dB through (0.75 dB) and and drop crosstalk ports.− electro14.1 dB-o (ptic−22 modulator dB) at the through(EOM), erbium (drop) port.-doped fiber amplifier (EDFA), variable optical attenuator (VOA), photon detector (PD). (c) Through port and (d) drop port transmission spectra near a resonance of the microring when the GST is in a transition from the crystalline phase (blue) to the amorphous phase (red). (e,f) Log scale spectra highlighting the low insertion loss of 0.46 dB (0.75 dB) and crosstalk −14.1 dB (−22 dB) at the through (drop) port.

Molecules 2021, 26, x FOR PEER REVIEW 11 of 21

In 2020, Zhang et al. [43] reported a PCM-based reconfigurable DC switch using a photonic crystal ring resonator. A schematic of the structure is shown in Figure 10. When the index of refraction is different, the port of the output light changes. They also realized Molecules 2021, 26, 2813 double-wavelength switching, and the principle is the same. The 2 × 2 switch has five11 cross of 20 states, two dropping directions, works at two wavelengths, and only requires two materials.

FigureFigure 10. 10. SchematicSchematic structure ofof thethe proposedproposed PCM-based PCM-based 2 ×2 ×2 2 PCDCS PCDCS (a ).(a). The The electric electric field field distribution distribution in the in the nested nested ring ringresonant resonant cavity cavity at 1530 at 1530 nm ( bnm–d )(b and–d) itsand corresponding its corresponding transmission transmission spectra spectra at ports at B,ports C, and B, C D, and (e–g D) for(e– theg) for PCM-based the PCM- 2 based 2 × 2 PCDCS with different refractive indexes of dielectric rods. (b,e) nall = 3.6, (c,f) ninner = 4.1 and nother = 3.6, (d,g) × 2 PCDCS with different refractive indexes of dielectric rods. (b,e) nall = 3.6, (c,f) ninner = 4.1 and nother = 3.6, (d,g) nall = nall = 3.75. Electric field distribution at 1530 nm (h,j) and 1550 nm (i,k) when nall = 3.6 (h, i) and ninner = 3.95 and nother = 3.6 3.75. Electric field distribution at 1530 nm (h,j) and 1550 nm (i,k) when n = 3.6 (h,i) and ninner = 3.95 and n = 3.6 (j,k). (j,k). all other Mimicking and implementing basic computing elements on photonic devices is one of the firstMimicking and most and essential implementing steps toward basic all-opticalcomputing computers. elements on Programmable photonic devices optical is logicone ofdevices, the first such and as most logic essential “OR” and step “NAND”s toward gates, all-optical are the computers. basic computing Programmable elements optical [44]. logic devices, such as logic “OR” and “NAND” gates, are the basic computing elements In 2019, Zhang et al. [45] reported a Si–Ge2Sb2Te5 hybrid photonic device induced [44]by. optical pulses for logic operation. The device structure is shown in Figure 11a, which is composedIn 2019, Zhang of a silicon et al. multimode[45] reported interferometer a Si–Ge2Sb2Te (MMI)5 hybrid crossing photonic covered device withinduced a small by opticalcircular pulses piece offor a logic 30 nm-thick operation GST. The layer device and a structure 30 nm-thick is shown ITO layer. in Figure A heavily 11a P++, which doped is composedsilicon strip of orthogonallya silicon multimode crosses interferometer at the center of (MMI) the MMI crossing with covered a Ti/Au with layer a depositedsmall cir- cularon top piece at the of a end 30 ofnm the-thick doping GST regionlayer and as ana 30 electrode, nm-thick working ITO layer. as aA resistive heavily heater.P++ doped The silicontime delay strip betweenorthogonally the electrical crosses at pulse the center and optical of the pulse MMI affects with a the Ti/Au GST layer phase deposited state and onthus top the at responsethe end of of the the doping probe light, region as shownas an electrode, in Figure working11b. Such as a a device resistive can heater. perform The the timeBoolean delay logic between operation. the electrical When the pulse relative and transmission optical pulse change affects exceeds the GST a certainphase state threshold and thuslevel, the the response logic output of the can probe be regarded light, as asshown “1,” otherwisein Figure it11 isb. “0.” Such With a device low-energy can perform pulses, the inputs “00,” “10,” and “01” give no or small transmission change, while the input “11” generates a very large change. If the threshold level is set as a 25% transmission change, it then accomplishes the “AND” logic operation. If the energy of the pulses is increased,

only one pulse can signiﬁcantly change the transition, exceeding the threshold. It then accomplishes the “OR” logic operation. Molecules 2021, 26, x FOR PEER REVIEW 12 of 21

the Boolean logic operation. When the relative transmission change exceeds a certain threshold level, the logic output can be regarded as “1,” otherwise it is “0.” With low- energy pulses, the inputs “00,” “10,” and “01” give no or small transmission change, while the input “11” generates a very large change. If the threshold level is set as a 25% transmission change, it then accomplishes the “AND” logic operation. If the energy of the Molecules 2021, 26, 2813 12 of 20 pulses is increased, only one pulse can significantly change the transition, exceeding the threshold. It then accomplishes the “OR” logic operation.

FigureFigure 11.11. ((aa)) Schematic Schematic of of the the GST GST-gated-gated silicon silicon multimode multimode interferometer interferometer (MMI (MMI)) cro crossing.ssing. (b) (b) Co-interactionCo-interaction of optical pulses (green) and electrical pulses (blue) and the resulting power profile proﬁle (orange).(orange). (c,,d) Transmission change ∆T∆T varies varies with with the the time time difference difference between between the the two two pulses pulses ∆t∆ t inin ((cc)) amorphizationamorphization and ( d) crystallization processes. processes. ( (ee)) T Thehe truth truth table table of of the the “AND” “AND” and and “OR” “OR” logiclogic gates.gates.

AnAn optical modulator modulator is is one one of ofthe the most most important important integrated integrated optical optical devices devices in high in- high-speedspeed and short and- short-distancedistance optical optical communication. communication. Optical Optical modulation modulation converts converts an electri- an electricalcal signal signal into an into optical an optical signal, signal, and the and modulated the modulated light lightwave wave is sent is sentto the to receiving the receiving end endthrough through the optical the optical channel, channel, where where the optical the optical receiver receiver identifies identifies its changes its changes and then and re- thenstates restates the original the originalinformation. information. According According to the modulation to the modulation principle, optical principle, modulator opticals modulatorscan be divided can be into divided electro into-optic, electro-optic, thermo-optical, thermo-optical, acousto- acousto-optic,optic, and all- andoptical. all-optical. When WhenPCMs PCMstransform transform from the from amorphous the amorphous state to the state crystalline to the crystalline state, their state, optical their properties optical propertiesdrastically drasticallychange. Short change. optical Short or electrical optical pulses or electrical can be pulses used to can switch be used between to switch these betweenstates, making these states, PCMs making fit in with PCMs the fit production in with the of production optical modulator of opticals. modulators. PCMs show PCMs enor- showmous enormouspotential with potential respect with to the respect device to footprint, the device optical footprint, bandwidth, optical and bandwidth, extinction and ra- extinctiontio because ratio of their because high of contrast their high and contrast broadband and broadbandoptical properties. optical properties.Next, we report Next, sev- we reporteral optical several modulators optical modulators based on PCM baseds. on PCMs. InIn 2014,2014, AppavooAppavoo andand coauthorcoauthor [[46]46] proposedproposed aa typetype ofof modulator.modulator. TheThe principleprinciple ofof thethe phase-changephase-change modulator modulator based based on on Au Au and and VO VO2 nanostructures2 nanostructures is shown is shown in Figure in Figure 12a. 12Thea. VO The2 VOnanostructure2 nanostructure can adjust can adjust the resonance the resonance of the ofgold the plasma gold plasma structure structure in the device, in the device,thereby thereby adjusting adjusting the characteristics the characteristics of the ofhybrid the hybrid modulator modulator.. Almost Almost at the atsame the sametime, time,using using the demonstrated the demonstrated ultrafast ultrafast dynamics dynamics of VO of2 VO under2 under optical optical excitation, excitation, a Si/VO a Si/VO2 hy-2 hybridbrid ring ring resonat resonatoror that that can can realize realize high high-speed-speed optical optical modulation modulation with with out out-of-plane-of-plane ul- ultrafasttrafast optical optical excitation excitation was was reported reported [47] [47].. The The proposed proposed device geometry is shownshown inin FigureFigure 12 12b,b, which which can can achieve achieve an an extinction extinction ratio ratio greater greater than than 3 dB 3 withdB with ~1 dB ~1 insertion dB insertion loss µ overloss the over entire the C-band entire C (1.53–1.565-band (1.53–m).1.565 In 2015, µm). MarkovIn 2015, et Markov al. [48] reported et al. [48] a Si/VO reported2/Au a electro-optic modulator based on the near-field plasmonic coupling that is expected to Si/VO2/Au electro-optic modulator based on the near-field plasmonic coupling that is ex- µ demonstratepected to demonstrate ~9 dB/ m ~9 extinction dB/µm extinction ratio/length ratio/length (Figure ( Figure12c). Additionally, 12c). Additionally, the use the of use an Au/VO2 hybrid pattern on a silicon waveguide has been proposed for in-waveguide of an Au/VO2 hybrid pattern on a silicon waveguide has been proposed for in-waveguide all-optical modulation [49]. A schematic of the modulator is shown in Figure 12d. all-optical modulation [49]. A schematic of the modulator is shown in Figure 12d. An ultracompact and ultralow energy consumption electro-optical modulator based on PCMs was reported by Yu and co-workers in 2018 [50]. By electrically triggering the phase of GST on the silicon waveguide, the absorption coefficient of the transverse electric mode switches. The structure of the GST-based compact electro-optical modulator is shown in Figure 13a, which is composed of a silicon bottom layer, a GST active middle layer, and a copper (Cu) top layer. In 2019, Ghosh et al. [51] reported a graphene-based electro-optic modulator in a slotted micro-ring resonator. Partially overlapping graphene layers, via graphene–Al2O3–graphene stack, were used above and below the slotted waveguide. A low-energy consumption electro-optic modulator based on the photonic bandgap shifting phenomenon in a PhC slab waveguide developed in a GeSe PCM layer has been reported [52], whose phase change from crystalline to amorphous, and vice versa, is by Molecules 2021, 26, x FOR PEER REVIEW 13 of 21

Molecules 2021, 26, 2813 13 of 20

triggering electrical or optical pulses, thereby resulting in changes in the optical proper- Figure 12. (a) Schematic of a phase-change nanomodulator based on Au and VO2 nanostructures. ties of the materials. The advantages of this modulator are its high extinction ratio. By (b) Schematic view of the hybrid plasmonic modulator based on vanadium dioxide insulator- metalexploiting phase thetransition. reversible The andinset sub-nanosecondshows the cross-sectional fast switching view of the of antimony optical device. trisulﬁde (c) Schematic (Sb2S3) illustratingfrom amorphous regions of to VO crystalline,2 metallization Chamoli when eta voltage al. [53] is reported applied across a reﬂection the gold modulator nanodisk chain. based Molecules 2021, 26, x FOR PEER REVIEW 13 of 21 (ond) 3D the waveguide metal–dielectric–metal structure, illustrating structure pump in 2021.and signal The advantageslight injection, of p thisoynting design vector are magni- its small tudefootprint, when VO2 ease is of dielectric fabrication, and metallic, low cost, for and a 2D high slab frequency waveguide of modulator. modulation.

An ultracompact and ultralow energy consumption electro-optical modulator based on PCMs was reported by Yu and co-workers in 2018 [50]. By electrically triggering the phase of GST on the silicon waveguide, the absorption coefficient of the transverse electric mode switches. The structure of the GST-based compact electro-optical modulator is shown in Figure 13a, which is composed of a silicon bottom layer, a GST active middle layer, and a copper (Cu) top layer. In 2019, Ghosh et al. [51] reported a graphene-based electro-optic modulator in a slotted micro-ring resonator. Partially overlapping graphene layers, via graphene–Al2O3–graphene stack, were used above and below the slotted waveguide. A low-energy consumption electro-optic modulator based on the photonic bandgap shifting phenomenon in a PhC slab waveguide developed in a GeSe PCM layer has been reported [52], whose phase change from crystalline to amorphous, and vice versa, is by triggering electrical or optical pulses, thereby resulting in changes in the optical properties of the materials. The advantages of this modulator are its high extinction FigureFigure 12. 12. ((aa)) Schematic Schematic of a phase-changephase-change nanomodulatornanomodulator based based on on Au Au and and VO VO2nanostructures. nanostructures. (b ) ratio. By exploiting the reversible and sub-nanosecond fast switching of2 antimony trisul- (Schematicb) Schematic view view of the of hybridthe hybrid plasmonic plasmonic modulator modulator based based on vanadium on vanadium dioxide dioxide insulator-metal insulator- phase fide (Sb2S3) from amorphous to crystalline, Chamoli et al. [53] reported a reflection mod- metaltransition. phase The transition. inset shows The inset the cross-sectional shows the cross view-sectional of the opticalview of device.the optical (c) Schematic device. (c) illustrating Schematic ulatorillustrating based regions on the of VOmetal2 metallization–dielectric –whenmetal a voltagestructure is appliedin 2021 across. The theadvantage gold nanodisks of this chain. de- regions of VO2 metallization when a voltage is applied across the gold nanodisk chain. (d) 3D sign(d) 3D are waveguide its small structure, footprint, illustrating ease of fabrication pump and ,signal low costlight, injection,and high p oyntingfrequency vector of modula-magni- waveguide structure, illustrating pump and signal light injection, poynting vector magnitude when tion.tude when VO2 is dielectric and metallic, for a 2D slab waveguide modulator. VO2 is dielectric and metallic, for a 2D slab waveguide modulator. An ultracompact and ultralow energy consumption electro-optical modulator based on PCMs was reported by Yu and co-workers in 2018 [50]. By electrically triggering the phase of GST on the silicon waveguide, the absorption coefficient of the transverse electric mode switches. The structure of the GST-based compact electro-optical modulator is shown in Figure 13a, which is composed of a silicon bottom layer, a GST active middle layer, and a copper (Cu) top layer. In 2019, Ghosh et al. [51] reported a graphene-based electro-optic modulator in a slotted micro-ring resonator. Partially overlapping graphene layers, via graphene–Al2O3–graphene stack, were used above and below the slotted waveguide. A low-energy consumption electro-optic modulator based on the photonic bandgap shifting phenomenon in a PhC slab waveguide developed in a GeSe PCM layer has been reported [52], whose phase change from crystalline to amorphous, and vice versa, is by triggering electrical or optical pulses, thereby resulting in changes in the opti- FigurecalFigure properties 13. 13. (a()a Schematic) Schematicof the materials. of ofthe the proposed proposed The advantageshybrid hybrid Si-GST Si-GST-Cu of-Cu this waveguide modulator waveguide modulator. are modulator. its high The Theinsetextinction insetillus- il- tratedratiolustrated. theBy cross theexploiting cross-section-section the view reversible view of the of hybrid the and hybrid Sisub-GST- Si-GST-Cunanosecond-Cu waveguide. waveguide. fast (switchingb) Three (b) Three-dimensional-dimensional of antimony (3D) trisul- and (3D) twofideand- dimensional two-dimensional(Sb2S3) from (2D) amorphous schematic (2D) schematic toof crystalline,the of proposed the proposed Chamoli electro electro-optic-optic et al. modulator. [53] modulator. reported (c) Schematic (ac )reflection Schematic of the mod- of the ulatorproposed based electro-optic on the metal modulator.–dielectric Insets–metal illustrate structure the cross-sectional in 2021. The views advantage in differents of planes. this de- (d) signSchematic are its of small proposed footprint, optical reﬂectionease of fabrication modulator., low cost, and high frequency of modulation. The excellent properties of PCMs, containing fast and reversible switching of the

states, considerable modulation in the electrical resistivity, and optical constants with a wide spectral region, greatly improve the performance of optical switches, as well as optical modulators in some aspects. The progress in photonic applications focusing on realizing reconﬁgurable and reprogrammable functions will give rise to new designs for an optical switch, optical modulator, and optical logic gate. The future possibilities of switchable dielectric and metallic nanostructures with PCMs are countless. Incorporating nonvolatile PCMs in integrated photonic devices enables indispensable reprogramming, reconﬁgurability, higher level of integration, and adjustability for integrated photonic on-chip devices.

Figure 13. (a) Schematic of the proposed hybrid Si-GST-Cu waveguide modulator. The inset illus- trated the cross-section view of the hybrid Si-GST-Cu waveguide. (b) Three-dimensional (3D) and two-dimensional (2D) schematic of the proposed electro-optic modulator. (c) Schematic of the

Molecules 2021, 26, x FOR PEER REVIEW 14 of 21

proposed electro-optic modulator. Insets illustrate the cross-sectional views in different planes. (d) Schematic of proposed optical reflection modulator.

The excellent properties of PCMs, containing fast and reversible switching of the states, considerable modulation in the electrical resistivity, and optical constants with a wide spectral region, greatly improve the performance of optical switches, as well as optical modulators in some aspects. The progress in photonic applications focusing on realizing reconfigurable and reprogrammable functions will give rise to new designs for an optical switch, optical modulator, and optical logic gate. The future possibilities of switchable dielectric and metallic nanostructures with PCMs are countless. Incorporating nonvolatile PCMs in integrated photonic devices enables indispensable reprogramming, re- Molecules 2021, 26, 2813 configurability, higher level of integration, and adjustability for integrated photonic14 ofon 20- chip devices.

3.2.2. An Emerging Research Hotspot: Optical Convolutional Neural Networks 3.2.2. An Emerging Research Hotspot: Optical Convolutional Neural Networks ArtificialArtificial neural networks are alreadyalready widelywidely usedused inin performingperforming taskstasks suchsuch asas faceface and speechspeech recognitionrecognition [54 [54]]. In. In more more complex complex applications, applications, such such as medical as medical diagnostics diagnostics [55] and[55] autonomousand autonomous driving driving [56], [56] fast, datafast data processing processing is more is more important. important. Conventional digitaldigital computers computers usually usually adopt adopt a vona von Neumann Neumann structure structure of operation,of opera- wheretion, where computing computing architectures architectures physically physically separate separate the core the computing core computing functions functions of memory of andmemory processing. and processing. The data areThe stored data are inthe stored memory in the and memory executed and in executed the central in processingthe central unit.proces Assing a result, unit. fast,As efficient, a result, andfast, low-energy efficient, and computing low-energy is difficult computing to achieve. is difficult Computer to architecturesachieve. Computer that can architectures somehow fuse that the can two somehow basic parts fuse ofthe processing two basic andparts memory of processing break throughand memory the bottleneck break through in terms the bottleneck of the overall in terms speed of of the operation overall speed and amount of operation of energy and consumption.amount of energy The consumption neurosynaptic. The system neurosynaptic consisting system of neurons consist anding synapses of neurons mimics and syn- the biologicalapses mimics brain, the and biological it is one brain, of the and computer it is one architectures of the computer that can architectures break through that thecan limitationbreak through mentioned the limitation above. mentioned above. TheThe computationallycomputationally specificspecific integrated integrated photonic photonic hardware hardware accelerator accelerator (tensor (tensor core) core) is capableis capable of of operating operating at at speeds speeds of of trillions trillions of of multiply-accumulate multiply-accumulate operationsoperations perper second,second, whichwhich cancan meetmeet the the requirement requirement of of highly highly parallel, parallel, low-energy low-energy consumption, consumption, fast, fast and, scal-and ablescalable hardware hardware [57]. [57] The. tensorThe tensor core iscore defined is defined as the as optical the optical analogue analogue of an integrated of an integrated circuit thatcircuit has that a specific has a specific application, application and it is, and always it is usedalways in aused neurosynaptic in a neurosynaptic system or system network. or Photonicnetwork. neurosynapticPhotonic neurosynaptic networks promisenetworks access promise to the access high to speed the high and highspeed bandwidth and high inherentbandwidth to opticalinherent systems, to optical thus systems, enabling thus direct enabling processing direct ofprocessing optical telecommunication of optical telecom- andmunication visual data and [ 58visual]. One data way [58] to. implementOne way to quantum implement machine quantum learning machine parallels learning classical par- photonicallels classical deep neural photonic network deep accelerators, neural network as shown accelerators, in Figure as 14 shown. The stages in Figure of the 14 linear. The waveguidestages of the meshes linearare waveguide connected meshes by activation are connected layers, but by theseactivation activation layers, layers but mustthese haveacti- strongvation coherentlayers must (reversible) have strong nonlinearities. coherent (reversible) It is called nonlinearities. a quantum optical It is called neural a quantum network, andoptical its taskneural is trainingnetwork the, and phases its task in is the training waveguide the phases mesh throughin the waveguide supervised mesh learning through on thesupervised input and learning output on quantum the input states and [output59]. Incorporating quantum states nonvolatile [59]. Incorporating PCMs in integrated nonvola- photonictile PCMs devices in integrated enables photonic indispensable devices reprogramming, enables indispensable reconfiguration, reprogramming and in-memory, reconfig- computinguration, and capabilities in-memory for computing on-chip optical capabilities computing. for on-chip optical computing.

Figure 14. Quantum optical optical neural neural network network based based on on programmable programmable photonics. photonics. Such Such a network, a network, implementingimplementing the the matrix matrix operations operations W1, W1, W2 W2,, and and W3, W3, is fed is fedby single by single photons photons and nonlinear and nonlinear activation (for example, nonlinear materials or atomic nonlinearities). The ﬁnal state may be measured to complete a quantum computation or passed into a quantum network.

activation (for example, nonlinear materials or atomic nonlinearities). The final state may be measured to complete a quantum computation or passed into a quantum network.

For example, in 2017, Feldmann and co-workers [60] embedded nanoscale PCMs with a nanophotonic waveguide crossing array (Figure 15a), and selective addressing and manipulation with two overlapping pulses provides sufficient power to switch the de- sired PCM cell. The integrated central element for an all-optical calculator, a photonic abacus, was experimentally demonstrated, as shown in Figure 15b. This circuit operates at telecom C-band wavelengths (1530–1565 nm) with picosecond optical pulses. The single processing unit consists of a straight waveguide with a 7 μm-long PCM on top (Figure 15c). Transmission of the waveguide sensitively depends on the phase state of the PCM Molecules 2021, 26, 2813 15 of 20 cell in a full switching cycle. Starting from the amorphized state, the PCM cell will be crystallized stepwise by identical picosecond pulses with pulse energies of 12 pJ. Every crystallization step needs five consecutive pulses, and the device is re-amorphized with tenWith 19 picosecond pJ pulses. opticalWith picosecond pulses, this optical system performspulses, this the system fundamental performs arithmetic the fundamental operations arithmeticof addition, operations subtraction, of multiplication,addition, subtraction, and division. multiplication, For instance, and they division. demonstrated For instance base-, the10 multiplicationy demonstrate ofd “4base×-3,”10 multiplication which can be implemented of “4 × 3,” which by successive can be implemented addition (Figure by 15suc-d). cessiveAs expected, addition the (Figure ﬁnal states 15d) of. As the expected, PCM cells the after final four states successive of the PCM pulse cells sequences after four and suc- one cessivecarryover pulse were sequences “2” and and “1,” one thus carryover representing were “ the2” and correct “1,” result thus representing of “12.” Each the PCM correct cell resultrepresents of “12 a.” single Each PCM place cell value represents (ones, tens, a single hundreds, place value and so (ones, forth) tens, corresponding hundreds, and to the so forthdifferent) corresponding rods of an abacus. to the Thedifferent rectangular rods of crossed an abacus. waveguide The rectangular array is scalable, crossed andwave- its guidesize is array limited is byscalable, the absorption and its size of theis limited PCM cells. by the This absorption research providesof the PCM the cells. ﬁrst stepsThis resetowardsarch provides light-based the non-von first steps Neumann towards arithmetic.light-based non-von Neumann arithmetic.

FigureFigure 15. 15. ((aa)) Sketch of aa waveguidewaveguide crossingcrossing array array illustrating illustrating the the two-pulse two-pulse addressing addressing of individualof indi- vidualphase-change phase-change cells. (b cells.) Optical (b) Optical micrograph micrograph of a studied of a studied crossed-waveguide crossed-waveguid photonice photonic array (Scale array bar (Scaleis 100 barµm). is (100c) Level μm). definition:(c) Level definition: the pulse the energies pulse are energies set in suchare set a wayin such that a clearlyway that distinguishable clearly distinguishable and repeatedly accessible levels are obtained. In this example, each step downwards and repeatedly accessible levels are obtained. In this example, each step downwards consists of consists of a group of five picosecond pulses, and the reset pulse of a group consists of ten ps a group of five picosecond pulses, and the reset pulse of a group consists of ten ps pulses. (d) pulses. (d) Multiplication: “4 × 3 = 12,” computed applying sequential addition. Multiplication: “4 × 3 = 12,” computed applying sequential addition. In 2021, Feldmann et al. [57] reported a tensor core that achieves parallelized pho- In 2021, Feldmann et al. [57] reported a tensor core that achieves parallelized photonic in-memory computing using PCM memory arrays and photonic chip-based optical tonic in-memory computing using PCM memory arrays and photonic chip-based optical frequency combs. The optical tensor core is scalable, has an ultralow loss, and is CMOS frequency combs. The optical tensor core is scalable, has an ultralow loss, and is CMOS compatible.compatible. AA conceptualconceptual illustration illustration of theof the fully fully integrated integrated photonic photonic architecture architecture is shown is shown in Figure 16a, where an on-chip laser pumps an integrated Si3N4 microresonator to in Figure 16a, where an on-chip laser pumps an integrated Si3N4 microresonator to gen- generateerate a broadband a broadband soliton soliton frequency frequency comb. comb. The The key key is is encoding encodingof of thethe imageimage data onto thethe individual individual comb comb teeth teeth of of an an on on-chip-chip frequency frequency comb comb and and subsequent subsequent encoding encoding of the of fixedthe fixed convolutional convolutional kernels kernels in the in amorphous the amorphous or crystalline or crystalline phase phase of the of integrated the integrated PCM cellsPCM that cells evanescently that evanescently couple coupledd to a matrix to a matrix of interconnected of interconnected photonic photonic waveguides waveguides.. The researchersThe researchers experimentally experimentally demonstrate demonstratedd convolution convolution using using sequential sequential matrix matrix-vector-vector multiplicationmultiplication operati operationsons (Figure (Figure 1166bb–g).–g). Similarly, in 2021, Wu and co-workers [1] proposed a multimode photonic computing core composed of an array of programmable mode converters based on on-waveguide metasurfaces made of the GST PCM. They then demonstrated a prototypical optical convo-

lutional neural network that can perform image processing and recognition tasks with high accuracy, broad operation bandwidth, and a compact device footprint. A three-dimensional schematic diagram of the conﬁguration is shown in Figure 17a, consisting of a linear array of GST nanoantennae integrated on a silicon nitride waveguide. Each GST nanoantenna scatters the waveguide mode and causes a phase shift Φ, which depends on its width and the refractive index of its material. The GST’s distinctive refractive index between the amorphous and crystalline phases can modify the scattered phase of each GST nanoantenna and control the conversion of the waveguide’s two spatial modes (modes TE0 and TE1), as shown in Figure 17b. The measurement and control scheme and a scanning electron microscope image of the complete phase-change metasurface mode converter device are shown in Figure 17c. With these advances and after overcoming the scaling challenge, the Molecules 2021, 26, x FOR PEER REVIEW 16 of 21

Molecules 2021, 26, 2813 16 of 20

Molecules 2021, 26, x FOR PEER REVIEW 16 of 21 photonic neural network accelerator will be very promising for artiﬁcial intelligence in data centers where massive optical interconnects have already been deployed. Figure 16. (a) Conceptual illustration of a fully integrated photonic architecture to compute convolutional operations. (b–g) Experimental result of convolving an image of 128 × 128 pixels showing a handwritten digit (b) with four image kernels of size 3 × 3 (corresponding to a 9 × 4 filter matrix). The kernels are chosen to highlight different edges of the input image. g, Combined image from c– f showing edge highlighting.

Similarly, in 2021, Wu and co-workers [1] proposed a multimode photonic computing core composed of an array of programmable mode converters based on on-waveguide metasurfaces made of the GST PCM. They then demonstrated a prototypical optical convolutional neural network that can perform image processing and recognition tasks with high accuracy, broad operation bandwidth, and a compact device footprint. A three- dimensional schematic diagram of the configuration is shown in Figure 17a, consisting of a linear array of GST nanoantennae integrated on a silicon nitride waveguide. Each GST nanoantenna scatters the waveguide mode and causes a phase shift Φ, which depends on its width and the refractive index of its material. The GST’s distinctive refractive index between the amorphous and crystalline phases can modify the scattered phase of each GST nanoantenna and control the conversion of the waveguide’s two spatial modes (modes TE0 and TE1), as shown in Figure 17b. The measurement and control scheme and aFigure scanning 16. (( aaelectron)) Conceptual Conceptual microscope illustration image of aa fullyfully of the integratedintegrated complete photonicphotonic phase architecturearchitecture-change metasurface to to compute compute convolu- convo-mode converterlutionaltional operations. operations device are (.b (–bg –shown)g Experimental) Experimental in Figure result result 17c of. ofWith convolving convolving these advances an an image image of andof 128 128 after× ×128 128 overcoming pixelspixels showingshowing the a scalingahandwritten handwritten challenge, digit digit (b the(b) with) withphotonic four four image image neural kernels kernels network of of size size accelerator 3 3× × 33 (corresponding(corresponding will be very to apromising 9 × 44 filter filter matrix).for matrix). ar- tificialThe kernels intelligence are chosen in data to highlight centers different where massive edges of thetheop tical inputinput interconnects image.image. g,g, CombinedCombined have alreadyimageimage fromfrom been c–fc– f showing edge highlighting. deployed.showing edge highlighting. Similarly, in 2021, Wu and co-workers [1] proposed a multimode photonic computing core composed of an array of programmable mode converters based on on-waveguide metasurfaces made of the GST PCM. They then demonstrated a prototypical optical convolutional neural network that can perform image processing and recognition tasks with high accuracy, broad operation bandwidth, and a compact device footprint. A three- dimensional schematic diagram of the configuration is shown in Figure 17a, consisting of a linear array of GST nanoantennae integrated on a silicon nitride waveguide. Each GST nanoantenna scatters the waveguide mode and causes a phase shift Φ, which depends on its width and the refractive index of its material. The GST’s distinctive refractive index between the amorphous and crystalline phases can modify the scattered phase of each GST nanoantenna and control the conversion of the waveguide’s two spatial modes Figure 17. (a) 3D illustration of the devices (b) Finite Difference Time Domain (FDTD) simulation of Figure(modes 17. TE0 (a) 3D and illustration TE1), as shownof the devices in Figure (b) Finite 17b. TheDifference measurement Time Domain and control(FDTD) simulationscheme and the scattered electric field by one nano-antenna when the GST is in amorphous phase (left panel) and ofa thescanning scattered electron electric microscopefield by one nano image-antenna of the when complete the GST pha is sein -amorphouschange metasurface phase (left mode crystalline phase (right panel), respectively, showing the distinctive difference. (c) Scanning electron panel)converter and crystalline device are phase shown (right in panel),Figure respectively, 17c. With these showing advances the distinctive and after difference overcoming. (c) the microscope (SEM) image of the complete device and the measurement and control schematics. The scaling challenge, the photonic neural network accelerator will be very promising for ar- complete phase-change metasurface mode converter device consists of an encapsulated GST phase tificial intelligence in data centers where massive optical interconnects have already been gradient metasurface (red box) and a mode selector (yellow box). the transverse electric fundamental deployed. mode (TE0), the first electric fundamental mode (TE1). 4. Conclusions Optical PCMs are promising materials that can be used in reconfigurable and reprogrammable applications. They can also be incorporated into silicon photonics, plasmonics, and other hybrid nanocomposites to improve optical device performance. Two types of PCMs are widely used: chalcogenide compounds and metal oxides. One of the metal oxides is VO2, although it is unsuitable for nonvolatile and reconfigurable applications. Chalcogenide compounds, such as GST, have attracted more attention than other PCMs because of the high contrast in both the electrical and optical properties between the amorphous and crystalline states. The optical properties and the physical mechanism of the phase transition of these optical PCMs have been introduced in this review. In addition, the Figure 17. (a) 3D illustration of the devices (b) Finite Difference Time Domain (FDTD) simulation of the scattered electric field by one nano-antenna when the GST is in amorphous phase (left panel) and crystalline phase (right panel), respectively, showing the distinctive difference. (c)

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models and experimental data have been reported. Optical PCMs can be used in different micro/nano-optical structures, such as waveguides, microring resonators, surface plasmon polaritons, photonic crystals, and metamaterials, to realize various optical devices. The basic concepts have been introduced. On-chip photonic devices, such as optical switches, optical modulators, and optical logic gates, are necessary to achieve PICs. The materials used in integrated optics are not suitable for active tunability. Optical PCMs have the potential to solve this problem, and they play an important role in improving the performance in terms of faster data processing speed, shorter response time, higher contrast, and lower energy consumption. The application of PCMs in reconﬁgurable and reprogrammable photonic devices has attracted extensive attention because of their excellent performance, and optical convolutional neural networks are also attracting great interest. Novel all-optical storage can realize high-density storage with multilevel storage [61], and it is useful to construct photonic neural networks. Integrated photonic hardware accelerators based on PCMs can break through the bottleneck in terms of the overall speed of operation and amount of energy consumption, as well as accelerate the development of future photonic circuits. In conclusion, incorporating nonvolatile PCMs in integrated photonic devices enables indispensable reprogramming, reconﬁgurability, higher level of integration, adjustability, and in-memory computing capability for on-chip optical computing.

5. Discussion and Outlook PCMs offer a versatile material platform for reconﬁgurable photonic devices because of their outstanding properties, such as fast and reversible switching of the states and drastic nonvolatile changes in the optical properties between the crystalline and amorphous states, accompanied by considerable modulation in the electrical resistivity and optical constants with a wide spectral region, across ultraviolet, visible, and infrared frequencies. During the last decade, PCM platforms have been developed for application in telecommunication, imaging, data storage and display technologies. Limitations in the optical applications based on PCMs remain, as well as unsolved problems and technical problems that need to be solved. The limitations will be described in detail below, and we will propose some possible solutions. First, in visible spectra, chalcogenide compounds simultaneously exhibit large extinction of both the refractive index and optical loss. The concurrent index and loss change fundamentally limit the scope of many optical applications. One possible solution is to discover new materials as the foundation of the research on optical PCMs. For example, a new class of optical PCMs based on GSST has been introduced because it can break the traditional coupling of both the large contrast of the refractive index and the large contrast of the optical loss [6]. Moreover, the researchers pointed out that isoelectronic substitution with light elements is a generic route in the search for new O-PCMs optimized for low-loss photonic applications. Second, the mechanisms of the phase transitions in some PCMs are challenging to study, such as for VO2. The problem of metal–insulator transitions in transition-metal compounds has been a matter of ongoing controversy for a long time. The center of the problem is the relative role of the electron–lattice interactions and corresponding structural distortions versus the electron correlations [62]. The mechanisms of the phase transitions and models describing the metal–insulator transitions should be further investigated. A more appropriate method should be applied in the calculations, and then the theory will be in better agreement with the experimental data, such as X-ray absorption spectroscopy data. The sub-wavelength structures of photonic devices based on PCMs are mostly fabricated by the following methods: ultraviolet (UV) photolithography, electron beam lithog- raphy (EBL), and gallium focused ion beam (FIB) milling [63]. The utilization of UV photolithography for nanoscale patterning is limited by the resolution, design and fabrication of the photomask, and diffraction effects. EBL can provide higher resolution and precision, but it might be incompatible with other nanofabrication processes, such as Molecules 2021, 26, 2813 18 of 20

high-temperature chemical vapour deposition (CVD). Moreover, a tradeoff should be made between the unexpected phase transition caused by baking the photoresist at a high temperature and the resolution and accuracy of the pattern by baking at a lower temperature. FIB milling is a direct nanostructuring technique that is unconstrained by the limitations of the etchant and substrate compatibility, and it is incredibly versatile and compatible with a large variety of materials. However, proper calibration of the ion beam parameters is important to avoid redeposition of the milled material and gallium contamination. Differ- ent methods have their own advantages and disadvantages. Considering that there is no perfect fabrication method, the fabrication method should be carefully chosen. We look forward to the development of a better micro/nanofabrication method in the future. The area of the devices using GST is usually large because they operate in the near- and mid-infrared frequency range, leading to poor integration. To reduce the size of optical devices based on PCMs, they should operate at a shorter wavelength. However, for photonic devices operating in the visible to near-infrared visible wavelengths, GST is not ideal owing to the relatively small change in its refractive index but large extinction coefficients, resulting in high losses [13]. A solution is to search for another PCM that is transparent at a shorter wavelength and possesses high optical contrast for the refractive index. The most promising candidates are GSST and Sb2S3. Sb2S3 has larger bandgaps for both the amorphous (Eg = 2.05 eV) and crystalline (Eg = 1.7 eV) phases than GST (Eg = 0.7 and 0.5 eV). Finally, fully reconfigurable photonic devices are expected to have a large cycle number. Commercial PCRAM devices, which are based on GST, can be switched more than 108 times, and the switching time can be as short as <500 ps. However, most of the photonics devices based on PCMs can only switch from amorphous to crystalline. As a result, the realization of such characteristics as those of commercial phase-change random access memory (PCRAM) devices within optical on-chip devices is still difficult [64]. This is because a fully reversible phase transition in PCMs via electrical and optical pulses needs to be performed with electrodes and protective layers, which influences the design and function of the optical devices. Although some problems need to be solved, we believe that workable plans will be applied to break through the limitations of the applications of PCMs, and rapid progress will be made in photonic integrated circuits with ultrafast, ultrahigh bandwidth, and low-energy signaling. The reconfigurability will also give rise to new designs for smart glasses, displays, and reprogrammable devices. The future possibilities of switchable dielectric and metallic nanostructures with PCMs are countless. Many structures with different functionalities, such as filters, lenses, absorbers, and sensors, can be imagined. With the rapid development in technological advances, a nascent technology platform and proliferation of phase-change-based nanophotonic devices into commercial device platforms will occur in the next few decades.

Author Contributions: Investigation and outline, X.W.; revised the manuscript and refined it to the final form, X.W.; wrote part of the article, X.W., H.Q., Z.Y., S.D., and Z.D.; review and editing, X.H. and Q.G. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Key Research and Development Program of China under Grant Nos. 2018YFB2200403 and 2018YFA0704404 and the National Natural Science Foundation of China under Grant Nos. 61775003, 11734001, 91950204, 11527901 and the Beijing Municipal Science & Technology Commission No. Z191100007219001. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Conflicts of Interest: The authors declare no conflict of interest. Molecules 2021, 26, 2813 19 of 20

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