Mid-Infrared Nanophotonics for Biochemical Sensing: a Review

Romanian Reports in Physics 72, 408 (2020)

MID-INFRARED NANOPHOTONICS FOR BIOCHEMICAL SENSING: A REVIEW

O. TAKAYAMA DTU Fotonik – Department of Photonics Engineering, Technical University of Denmark, Ørsteds Plads 343, DK-2800 Kgs. Lyngby, Denmark Email: [email protected] Received April 26, 2020

Abstract. The mid-infrared (IR) wavelength region is widely utilized for absorption spectroscopy to detect molecules in gas, liquid, and solid phases by taking the advantage of characteristic infrared absorption peaks unique to molecular bonding. Mid- IR absorption spectroscopy offers a unique label-free detection method with a wide variety of applications from environmental monitoring to medical diagnosis. How- ever, the challenge lies in the huge mismatch of molecular sizes, typically on the order of nanometer as opposed to microns of mid-IR wavelengths, making detection of low concentration of them extremely challenging. Various nanophotonic phenomena enable us to localize mid-IR light at nanoscale to enhance light absorption by molecules and thus sensitivity. We present an overview of nanophotonic phenomena, such as waveguide modes, both propagating and localized plasmon modes, together with micro- and nanostructured platforms that host these phenomena, including dielectric planar waveguides, photonic crystals, plasmonic, and phononic nanostructures. Key words: mid-infrared absorption spectroscopy, label-free detection, surface plasmon polariton, metamaterial, metasurface, waveguide, photonic crystal, antenna, surface-enhanced infrared spectroscopy, SEIRA.

1. INTRODUCTION Mid-infrared (IR) light whose wavelength spans between 2.5 µm - 20 µm 1 1 (4,000 cm - 500 cm ) is extensively exploited for molecular detection known as mid-IR absorption spectroscopy (vibrational modes spectroscopy) [1–3]. Mid- IR absorption spectroscopy is a powerful tool to identify speciﬁc chemical species and bonding thanks to their particular absorption bands. It offers numerous applications, such as industrial process monitoring for pharmaceutical production [4], gas sensing for environmental monitoring [5], label-free detection of proteins [6–11] to study their folding, unfolding and membrane characteristics. Determination of each molecule leads to multiple applications. For instance, the sensing of carbon dioxide (CO2) that exhibits strong absorption peaks at 4.3 µm and 15.0 µm in wavelength ﬁnds applications in environmental monitoring, air conditioner, clinical diagnosis, such as anesthesia analysis, blood gas analysis. Moreover, mid-IR absorption spectroscopy has been applied for the clinical and biomedical analysis of human breath (c) 2020 RRP 72(0) 408 - v.2.0*2020.7.28 —ATG Article no. 408 O. Takayama 2

[12], bio-ﬂuids [13], such as urine, serum, blood to diagnose diabetes [14], can- cers [15], and different viruses [16]. Clinical studies have been conducted for the detection of biomarkers of acute myocardial infarction, such as cardiac troponin T [17]. On-chip biochemical sensors based on mid-IR vibrational spectroscopy may become reality, thanks to the advancement of recent nanofabrication technologies and such mid-IR light sources as LED, thermal light source, and quantum cascade lasers (QCLs), which can cover the broad mid-IR wavelength range [3, 18].

Fig. 1 – Three major mechanisms for mid-IR absorption enhancement. (a) Long interaction length. (b) Field enhancement by localized optical modes. (c) Large active surface area with strong electric ﬁelds.

However, due to the huge spatial difference between the wavelength on the order of microns and target molecule size of nanometers, three orders of magnitude, light-matter interaction is weak. Therefore it is challenging to detect a trace amount of molecules or small change of their concentration. In order to address this issue, there have been considerable efforts to improve the infrared light absorption by analyte molecules. As depicted in Fig. 1, the typical strategies adopted for the enhancement of IR light absorption are to

1. prolong interaction length of light with molecules,

2. enhance ﬁelds by highly localized optical modes, such as, surface waves, bulk waves, guided modes, cavity modes, localized plasmon modes, and

3. increase active surface area where strong ﬁelds are present, overlap, and interact with molecules.

These concepts can be expressed in terms of Beer-Lambert law for infrared absorption spectroscopy, in which the transmitted intensity I is given by: I()=I ()exp[ ↵()()L], (1) 0 (c) 2020 RRP 72(0) 408 - v.2.0*2020.7.28 —ATG 3 Mid-infrared nanophotonics for biochemical sensing Article no. 408 where I0() is the incident intensity, ↵() is the absorption coefficient of analyte molecules that depends on analyte molecule and its concentration, () is the me- dium-specific absorption factor determined by enhanced light-matter interaction, L is interaction length, and exp[ ↵()()L] corresponds to the absorption by analyte in transmission. Therefore, the strategy is to increase () or L by nanostructures and nano-optic phenomena. Figure 1(a) represents making the interaction length, L, longer. Figures 1(b) and 1(c) correspond to increasing of () since the vibrational absorption signal is proportional to E/E 2, where E and E are the local field and | o| o the field of incident light, respectively. In this review, we clarify the underling optical phenomena for the enhancement of mid-IR light absorption by molecules and related structures. We focus on three main stream micro- and nanostructure platforms:

1. Guided modes in planar dielectric waveguides.

2. Surface waves, cavity modes, slow light on photonic crystal structures.

3. Propagating and localized surface plasmon polaritons in nanoantennas, metasurfaces, 2D materials, techniques termed as surface enhanced infrared absorption (SEIRA) spectroscopy.

Based on these categories, this review article is organized as follows. First, we start with guided modes in planar dielectric waveguide structures in Sec. 2. In Sec. 3, we discuss several dielectric photonic crystal structures that support surface waves, cavity modes, and slow light. Section 4 deals with various plasmonic and phononic structures, plasmonic nanoantennae, metasurfaces, 2D materials, and high aspect ratio nanostructures for surface enhanced infrared absorption where tight localization of infrared light via plasmonic and phononic effects are exploited. Finally, in Sec. 5 we conclude this paper.

2. GUIDED MODES IN PLANAR DIELECTRIC WAVEGUIDE

A dielectric waveguide structure is formed by a core layer with high refractive index sandwitched by low refractive index cladding layers (superstrate and substrate) as illustrated in Fig. 2(a). In this manner, light is trapped in the core layer and propagates without both radiation and leakage into cladding layers while the tail of evanescent ﬁeld stretches into the cladding layers. In the superstrate, the evanescent tail interacts with analyte molecules that absorb infrared light at certain characteristic wavelengths. By measuring the transmission spectrum through such waveguide structures, one can detect molecules and their concentration. One of the major criteria for the core materials is wide band transparency for mid-IR wavelengths, which can (c) 2020 RRP 72(0) 408 - v.2.0*2020.7.28 —ATG Article no. 408 O. Takayama 4 covers more characteristic wavelengths of different molecules. Various materials for the waveguide structures have been studied in terms of the core refractive index and their transparency window in the mid-IR wavelength region so that the interaction length of waveguide, L in Eq. (1), can be longer for better sensitivity.

Fig. 2 – (a) Conceptual illustration of mid-IR absorption sensing by planar dielectric waveguide structures. The sensing system consists of a light source and coupling element, lens or grating, the waveguide structure as sensing element, and a detector. The presence of analyte is detected by lower transmission at certain waevlengths corresponding to the characteristic light absorption of molecular bonding. (b) A ridge wavegudie, (c) pedestal waveguide [20], (d) free-standing (membrane) [24, 25], and (e) Si-liquid-Si slot [26] waveguide structures.

Silicon (Si)-based materials have been extensively investigated for mid-IR wa- velenths thanks to the advent of Si photonics. Apart from optical telecommunication at near-IR region around 1.5 µm, there have been movements to extend the operating wavelength range to mid-IR wavelengths beyond 2 µm for sensing [19]. Si photonics is compatible with conventional CMOS microelectronics process and suitable for integrated on-chip sensing platform for mid-IR wavelengths. Si has a refractive index of 3.4 - 3.5 with a transparency window 1.1 µm - 8.0 µm, covering the absorption peaks of many molecules. Si core on various cladding materials have been studied, such as silicon-on-insulator (SOI) with silica, SiO2 (transparency window: 1.1 µm - 3.7 µm), silicon-on-sapphire (SoS, 1.1 µm - 5.5 µm), and free-standing silicon membranes (1.1 µm - 8.0 µm). Note that in these structures the limiting factor of upper transparency window is the cladding material’s absorbance. In order to increase the surface area where analyte can be hosted and to reduce the absorp- tive SiO2 cladding (SOI configuration), Si waveguide structures with pedestal shape (c) 2020 RRP 72(0) 408 - v.2.0*2020.7.28 —ATG 5 Mid-infrared nanophotonics for biochemical sensing Article no. 408 with air-silica cladding underneath were fabricated [20] as shown in Fig. 2(c). In this manner, the bandwidth of sensing system is extended up to 6.5 µm as opposed to 3.7 µm of SOI ridge waveguide. This system was used to measure the specific absorption band of C-H stretch of hexane (C6H14) dissolved in water at 3.55 µmin wavelength in order to distinguish hexane from other chemicals in water, such as acetone ((CH3)2CO), methanol (CH3OH), and toluene (C7H8). Instead of SiO2 cladding that is opaque above 3.7 µm, other cladding materials have been studied in order to extend operating the wavelength of Si core-based waveguides. A sapphire substrate can be used as a cladding material (SoS configuration) that can extend the transmission window of the Si core waveguide structures up to 5.5 µm [21, 22]. Amorphous Si ridge waveguide that consists of epitaxial barium titanate (BTO) thin films as a cladding layer deposited on lanthanum aluminate (LAO) substrate by the pulsed laser deposition technique has been demonstrated [23]. BTO film is transparent from 2.5 µm to 7.0 µm. Furthermore, since BTO film is a ferroelectric, BTO cladding layer may enable tunability for mid-IR microphotonics. As illustrated in Fig. 2(d), Si membrane (free-standing Si) waveguides provide the largest bandwidth up to 8.0 µm since the core is not in contact with the absorbing cladding layer. Besides, free-standing Si waveguides provide extra surface area where the analyte can interact with the guided modes though the long free- standing waveguide structures can be a fabrication challenge [24]. The free-standing Si waveguides were demonstrated for CO2 sensing at 4.3 µm [25]. Moreover, Si- liquid-Si slot waveguide structures were developed for liquid analyte sensing [26] as shown in Fig. 2(e). The liquid core with 100 nm spacing between Si cladding with 1.5 µm width guides mid-IR light confined within the nanofluidic liquid core where analyte resides. The field enhancement in the liquid core is achieved by the large refractive index contrast (n = 3.5 - 1.5 = 2) between Si cladding and liquid analyte. This sensing system was able to distinguish several liquid analyte, such as toluene, isopropanol, and to determine the ratio of chemical species, for instance, between acetonitrile and ethanol at low concentration (< 5 µL/mL) in the wavelength region from 3.1 to 3.6 µm. Apart from Si core, other materials as core layer have been studied and developed: silicon nitride (Si3N4, refractive index, n = 2.0, the transparency window up to 8.5 µm) [27–31], silicon germanium (SiGe, n = 3.5 - 4.1, 8.5 µm) [32, 33], ⇠ ⇠ chalcogenide glass (Ge Sb Se , n = 2.1 - 2.2, 12 µm) [34–40], aluminum nitride 23 7 70 ⇠ (AlN, n = 2.0 - 1.3, 10 µm) [41], and germanium (Ge, n = 4.0, 14 µm) [42–44]. ⇠ ⇠ A more detailed review on materials for planar waveguides [3] and guided structures for sensing [45] can be found elsewhere.

3. PHOTONIC CRYSTAL

A photonic crystal (PC) is a periodic arrangement of dielectric or metallic elements forming a unit cell and exhibits a photonic bandgap or a stop band where light is prohibited for certain wavelength range. PCs have been studied extensively in the last three decades thanks to their ability to control the ﬂow of light [46, 47]. The sensitivity enhancement is realized by ﬁeld enhancement () of surface waves, cavity modes, and slow light, as well as the interaction length, L. As in the case of dielectric planar waveguides in the previous Section, the detection of analyte is in general performed through transmission spectra via PC structures.

Fig. 3 – (a) One-dimensional photonic crystal (1DPC) structures for mid-IR sensing composed of alternating layers of ZnSe as high refractive index and YbF3 as low refractive index layers on a CaF2 substrate, which supports Bloch surface waves [49]. (b) 1DPC cavity with high quality factor [51]. The cavity is made on Ge33As12Se55 chalcogenide glass ridge waveguide with periodic air holes. The width and height of the ridge waveguide is w = 2.4 µm and h = 0.9 µm, respectively. (c) Two-dimensional (2D) PC Si slotted waveguide with the lattice constant a = 845 nm on sapphire that supports strong electric ﬁelds in the slot (gap) [56].

3.1. BLOCH SURFACE WAVE

A one-dimensional (1D) photonic crystal is a periodic multilayer structure composed of alternating low and high refractive index layers, as illustrated in Fig. 3(a). When the terminating layer is truncated with different thickness from other periodic layers, a type of surface electromagnetic waves, named Bloch surface wave (BSW), is supported [48, 49]. The field maxima of BSW is located at the vicinity of the terminating layer and its field tail stretches into the air. The use of BSWs for mid-IR absorption sensing was proposed since the evanescent field tail can provide field enhancement [49]. Their 1DPC structure consists of three periods of alternating layers of ZnSe as high refractive index and YbF3 as low refractive index layers on a CaF2 substrate. These layers have thicknesses between 1.30 µm and 1.50 µm and the terminating layer is a 0.23 µm thick ZnSe film. BSWs were observed in the wave- 1 1 lengths between 6.6 µm (1515 cm ) and 10.6 µm (943 cm ). BSWs on 1DPC can (c) 2020 RRP 72(0) 408 - v.2.0*2020.7.28 —ATG 7 Mid-infrared nanophotonics for biochemical sensing Article no. 408 function similarly to planar dielectric waveguides since BSWs can propagate with longer interaction length with analyte, because of the transparent dielectric layers and the field enhancement by localized surface waves.

3.2. CAVITY MODE

An optical cavity is a structure that is composed of a pair of reflective elements within and traps light at certain resonance wavelength by bouncing light between the reflective elements. Cavities can be made within photonic crystals by introducing a defect in the structure that breaks the periodicity of PC structure and generates a transmission peak inside the forbidden band where transmission is low. When this transmission peak overlaps the absorption band of the analyte, accumulated photons in the cavity interact with analyte molecules and its concentration can be determined by measuring the transmission peak of the cavity modes. 1DPC cavity made of a ridge waveguide with periodic air holes of different diameters as illustrated in Fig. 3(b) can trap light and therefore host strong electric field within [50, 51]. The cavity is made of Ge33As12Se55 chalcogenide glass (n = 2.5) ridge waveguide on CaF2 substrate (n = 1.4) fabricated by the combination of UV lithography to fabricate the ridge waveguide and focused ion beam (FIB) milling techniques to make air holes. The devices exhibit 15 % transmission and a quality factor of 104 at the mid-IR ⇠ wavelength of 5.26 µm. In theory, the PC cavity device could detect nitric oxide (NO) gas up to the sensitivity of 10 ppb (part per billion). The device has a small foot print of 150 µm2 with the interaction length of several dozens of µm and can potentially serve as a sensing element for on-chip gas sensing applications. Another type of cavity in 3D macroporous Si PCs was also proposed for spectroscopic gas sensing [52].

3.3. SLOW LIGHT

The group velocity of guided modes in 2DPC waveguide can be significantly smaller than that of conventional dielectric waveguides [53–55], the so-called slow light. Slow light in 2DPC slotted waveguide structures has been used to achieve high sensitivity for infrared absorption spectroscopy on chip [56]. The PC waveguide is made of hexagonal lattice orientation of air hole array on Si core with radius r = 0.25 a, where a is the lattice constant (period) of hexagonal lattice PC and has the value a = 845 nm, as illustrated in Fig. 3(c). The slot (air gap) is 130 nm betweeen 600 nm wide rail, and the thickness of the waveguide is h = 585 nm on SoS (n = 1.7) substrate. The slot waveguide embraces strong field of the guided mode in the air slot between the high refractive index Si waveguides and 13 times enhancement is achieved compared with PC waveguide without the slot. The group (c) 2020 RRP 72(0) 408 - v.2.0*2020.7.28 —ATG Article no. 408 O. Takayama 8 velocity of the slow light mode is 20, in other words, 20 times slower than the speed of light in vacuum. Triethylphosphate (TEP, (C2H3)3PO4), a chemical warfare agent simulant, was detected up to a concentration of 10 ppm (parts per million) via the transmission at 3.43 µm through 800 µm long waveguide. The strong field in the slot that overlaps gas analyte and the slow light are combined to enhance the sensitivity of gas molecules in the device.

4. SURFACE ENHANCED INFRARED ABSORPTION (SEIRA) SPECTROSCOPY

Over last three decades, a technique called surface enhanced infrared absorption (SEIRA) spectroscopy has been developed by using plasmonic nanostructures. Initially SEIRA adopted percolated silver films that support localized surface plasmon polaritons between metal islands, the so-called ”hot-spot”, which confine mid- infrared light and enhance local electric fields, boosting the photon-matter interaction and the sensitivity toward molecules [57]. In this Section, we discuss a wide variety of plasmonic nanostructures, as well as phononic nanostructures, such as nanoantennas, metasurfaces, high aspect ratio nanostructures, and materials used for SEIRA. In the case of SEIRA, the interaction length is small, typically a few hundreds nm or less. However, the field enhancement related to in Eq. (1) can be 104 - 107, which enhances the absorption signal and could make the foot print of sensing device small.

4.1. LATERAL NANOANTENNA

A metallic bar antenna can efficiently confine electromagnetic radiation at certain resonance wavelength when electric fields oscillate parallel to the metal bar so as to excite the localized plasmon modes in the antenna. As illustrated in Fig. 4(a), the resonance wavelegnth, R, for a dipole antenna is related to the antenna length, La, and roughly follows [58, 59]

R =(2/m)neff La + C, (2) where m is the mode number, neff is the effective refractive index between superstrate and substrate, and C is a constant. Note that the effective refractive index of 2 2 1/2 background is expressed as neff =(nsubstrate + nsuperstrate) [60]. For instance, neff = 2.55 for air superstrate and Si substrate of nsuperstrate = 1 and nsubstrate = 3.45. Gold-based lateral antennas with the length of La = 1.3 - 2.5 µm have been extensively studied and realized, which gives resonance wavelength for 3 - 7 µm [2, 59–68]. In order to increase the active surface area, periodic arrays of antennas were usually fabricated. Various specimens, such as lipid membranes [65], silk protein layers [61], proteins [60, 63, 64, 67], fullerene [68], have been detected in these nanoantenna array systems. Typically, the measurement is conducted by taking re- (c) 2020 RRP 72(0) 408 - v.2.0*2020.7.28 —ATG 9 Mid-infrared nanophotonics for biochemical sensing Article no. 408

Fig. 4 – Illustration of plasmonic nanoantennas for mid-infrared absorption sensing. (a) Lateral antennas of metallic bars, such as gold, doped semiconductors, transparent conductive oxides [2, 59–68]. (b) Asymmetric Au dipole antennas with Lx = Ly [69]. (c) InAs particle or disc antennas 6 [72]. (d) Dipole antennas made of Si-doped InAsSb [77]. (e) Vertical plasmonic antennas made of ITO [82], Au-coated PMMA [83], and SiC nanopillars [84, 85].

flection from the structures with and without analyte whose difference corresponds to the absorption signal of analyte. The resonance wavelength of antennas depends on antenna length as in Eq. (2), and the antennas work for certain wavelength range and one polarization. The asymmetric cross design, as illustrated in Fig. 4(b), enables the simultaneous detection of multiple infrared vibrational resonances over a broad mid-IR spectrum by creating two different resonances [69]. For one axis, antenna length, Lx, gives resonance at certain wavelength and the other orthogonal axis with length, Ly, exhibits resonance at another wavelength. Searic acid (C18H36O2) molecules functionalized to the asymmetric Al antennas were detected based on their carboxylic (–COOH) functional groups absorption peaks around 3.5 µm and 6.6 µm. Apart from noble metals, various doped semiconductors have been studied for mid-IR wavelengths above 4 µm since plasmonic properties (negative permittivities) and their wavelength ranges can be tuned by the doping level of semiconductors [70], including IV-IV semiconductors, such as Si, Ge [71], III-V semiconductors, such as InAs [72], InP [73], a transparent conductive oxide (TCO) as indium tin oxide (ITO) [74]. InAs semiconductor antennas in the form of plasmonic disc array, as shown in Fig. 4(c), were fabricated to detect polymethyle metacrylate (PMMA) as a model analyte [72]. Note that PMMA is a polymer used for photo resist, which (c) 2020 RRP 72(0) 408 - v.2.0*2020.7.28 —ATG Article no. 408 O. Takayama 10 can be spin-coated on wafer with well-controlled thickness, gives many absorption peaks, and therefore can serve as a model analyte. Si-doped InAsSb nanoantennas on GaSb substrate [75] for vanillin (C8H8O3) detection was performed in the wavelength range of 13 - 17 µm [76]. N-doped germanium (Ge)-based dipole antennas, a pair of metallic bars as illustrated in Fig. 4(d), were fabricated to detect chloroethyl 1 methyl sulfide (CEMS) around the wavelength 11 µm (900 cm ) [77]. CEMS is an agent simulant of common explosive materials and is a transparent liquid, which is hard to distinguished from harmless liquids except by spectroscopy. Graphene, a monolayer carbon sheet and a 2D material, is a plasmonic material whose permittivity becomes negative in mid-IR wavelength region and can be used for SEIRA [78]. Graphene nanoantenna array offers two advantages as opposed to conventional Au nanoantennas: One is that the localized fields on graphene antennas are more tightly confined. Typically, the electric fields of Au antennas are localized within several hundreds nm from antenna surface while those of graphene antennas are within 15 nm. Furthermore, the resonance wavelengths of the graphene antennas can be tuned by voltage to shift the resonance towards absorption peaks of analyte molecules to optimize the sensitivity. The resonance wavelength can be 1 1 tuned from 5 µm (2000 cm ) to 11 µm (900 cm ), covering the absorption signals of many molecules [79]. The main vibrational fingerprints of proteins are amide I, 1 C=O stretch vibration at 6.0 µm (1660 cm ) and amide II bands, N-H bending and 1 C-N modes at 6.5 µm (1550 cm ). The graphene nanoantennas detected the absorption signals of amide I and II of proteins with voltage tuning. Apart from proteins, graphene nanostructures were demonstrated for gas detection, such as NO, N2O, NO2, SO2 [79], CO2 [80]. To enhance sensitivity further, a layer that absorbs and immobilizes gas molecules can be adopted to keep the gas molecules spatially close to the hot-spots of graphene plasmons and achieve the enhanced light absorption by gas molecules. A 10 nm thick polyethylenimine(PEI) that captures CO2 molecules is deposited on graphene antennas, detecting CO2 in the concentration range of 390 ppm (atmospheric concentration) to 2000 ppm [80]. Moreover, hexagonal boron nitride (hBN), which exhibits negative anisotropic permittivities for 6.2 µm (1610 1 1 cm ) – 7.3 µm (1360 cm ) was demonstrated as nanoantenna material for SEIRA [81].

4.2. VERTICAL NANOANTENNA

As illustrated in Fig. 4(e), an array of vertical plasmonic or phononic pillars whose permittivity becomes negative also functions as nanoantennas for mid-IR wavelengths. The advantage of vertical antennas is that they offer more active sensing surface area for analyte, and consequently more overlap between localized fields and analyte than lateral antennas on substrate that blocks the access of analyte molecules (c) 2020 RRP 72(0) 408 - v.2.0*2020.7.28 —ATG 11 Mid-infrared nanophotonics for biochemical sensing Article no. 408 to the antennas. Several studies have been reported for the vertical antenna systems. Nanorod array of indium tin oxide (ITO) with height La = 1.8 µm, diameter w = 100 nm, and period ⇤ = 600 nm were realized and functionalized with biotin and 1 streptavidin [82]. Vibration signals of the amide I at 6.0 µm (1660 cm ) and amide 1 II at 6.5 µm (1537 cm ) from the bonding between streptavidin and biotin were detected. Moreover, the antenna resonance wavelength was tuned by the incident angle since the polarization components parallel to antennas change with the incident angle. At near-normal incidence, only amide I’s absorption signal was observed. As the incident angle increased, the intensity of amide I absorption became smaller while the amide II absorption became more prominent and eventually exhibited a stronger intensity than the amide I signal. Moreover, an array of vertical porous Au antennas with PMMA core was studied [83]. The core of the vertical antennas were made of PMMA pillars covered by sputtered Au. A 3 nm thick SiO2 layer as a model analyte was conformally deposited around antennas by atomic layer deposition (ALD) and ALD deposited SiO2 can be a model analyte that gives well-defined thickness and absorption feature. SiO2 exhibits absorption around the wavelengths of 8 µm for its phonon resonance and detected by reflection spectra. Apart from the plasmonic (metallic) antennas, a polar material has been studied for vertical nanoantennas. Silicon carbide (SiC) is a polar material that exhibits negative permittivity in the specific wavelength region sometimes referred to as Rest- 1 1 strahlen band, 10.3 µm (973 cm ) - 12.5 µm (797 cm ), due to its optical phonons [84, 85]. The imaginary part of SiC’s permittivity, which corresponds to the loss of the material, is lower than those of noble metals and doped semiconductors by one order of magnitude or more. Therefore, low-loss phonon polariton modes supported by polar materials can be a potential alternative to conventional plasmonic material- based nanostructures. Nanopillar arrays of 6H-SiC[84] and 4H-SiC [85] were shown to support highly localized modes, suggesting that for those arrays, analyte with characteristic absorption peaks in the wavelength range could be detected.

4.3. METASURFACE - NANOSTRUCTURED METAL FILM

Compact and cost-effective monolithic mid-IR sensing devices that integrate all optical components in a single chip including a light source, sensing element, and detector are highly desirable for numerous future sensing applications. A monolithically integrated sensor with a quantum cascaded laser (QCL) as the light source and detector, and dielectric-loaded plasmon waveguide as the sensing element was demonstrated [86], as illustrated in Fig. 5(a). The dielectric-loaded plasmon waveguide consists of SiN dielectric layer on Au film with the interaction length of L = 50 µm, which enhances the light localization and modifies the mode profile for efficient end-fire coupling from and to the QCL and detector. The monolithic device was (c) 2020 RRP 72(0) 408 - v.2.0*2020.7.28 —ATG Article no. 408 O. Takayama 12

Fig. 5 – Illustration of various plasmonic nanostructures for SEIRA. (a) Monolithic mid-infrared on-chip sensor. The device comprises of a laser, a dielectric-loaded plasmon waveguide, and a detector [86]. (b) Au nanorings on Si3N4 pedestals [87]. The nanoring antennas are arranged in a square lattice array with ⇤ =3µm period. (c) Coaxial structures with gap plasmon in circular slit with 5 - 10 nm gap in Au film [89]. (d) Au metasurface of square Au patches on SiN membrane and 2.7 µm thick MOF layer. The thickness of Au film is 40 nm, patch period 2.08 µm, and gap 250 nm on 200 nm thick SiN membrane [90]. submerged in a mixture of ethanol (C2H5OH) as analyte and water (H2O) solution in order to determine the concentration of ethanol in water in the range of 0 - 60 % concentrations via transmission through guided plasmon modes. Plasmonic nanoring antennas on Si3N4 nanopedestal and membrane as illustrated in Fig. 5(b) were fabricated [87]. The circular geometry of ring antennas is polarization insensitive as opposed to bar antennas so that unpolarized incident light can be used. The pedestal bottom of ring antennas gives larger surface areas for analyte and more accessible localized field profile. The field volume is 2.6 times larger than for ring nanoantennas directly on substrate. The absorption signals by 8 nm thick protein layer were also found to be around 2.6 times higher, showing that the enhancement in the absorption signal is proportional to the overlap between the optical fields and the biomolecules in the vicinity of the ring antennas. A nanometer scale gap up to 10 nm between metals can support extremely localized modes, called gap plasmons, and can be exploited for enhancing light absorption by analyte molecules [88]. For gap plasmons in general, the smaller the gap, the stronger the electric fields. Al-air-Al structures were theoretically and ex- perimentally studied and it turned out that the localized field enhancement factor E/E 2 can be on the order of 104 for 2 nm gap while 103 for 10 nm gap. The | o| (c) 2020 RRP 72(0) 408 - v.2.0*2020.7.28 —ATG 13 Mid-infrared nanophotonics for biochemical sensing Article no. 408 resonance wavelength also depends on the gap and 2 nm - 10 nm gap leads to the resonance wavelengths of 4 µm-7µm. Furthermore, in order to densely pack the nanogap with strong fields and enhancement factor of 104 - 105, coaxial nanogaps in hexagonal lattice arrangement were fabricated in Au film on Si substrate [89], as illustrated in Fig. 5(c). The circular gap has the diameter of 440 nm - 770 nm with 7 nm - 10 nm gap, giving a resonance in the range of 4.7 µm - 6.8 µm. A 5 nm thick monolayer silk protein was detected via amide I absorption signal at 6.0 µm in wavelength. The reflection difference caused by amide I is around 20 %, which is significantly higher than Au nanoantennas of typically several % [61]. This is because even though the nanoantennas possess large field enhancement of 104 - 107, it occurs at the tip of antennas and the active sensing surface area is not as large as that of circular gaps geometry. SEIRA for on-chip gas sensing has a challenge to accumulate sufficient amount of gas molecules at the vicinity of localized plasmon modes since the absorption coefficient of gas, ↵, and the interaction length, L, are small even though the field enhancement related factor, in Eq. (1) is large. Compact on-chip CO2 sensors based on the planar periodic plasmonic metasurface structures, as illustrated in Fig. 5(d), achieved a detection limit of 52 ppm for 2.7 µm in wavelength [90]. This was realized by the combination of the plasmonic nanopatch antennas that localize light, and 2.7 µm thick metal-organic flamework (MOF) layer that captures and immobilizes carbon dioxide (CO2) gas molecules within its nanoporous structure. The plasmonic metasurface structure was based on periodically structured Au films that support highly localized plasmon modes on a 200 nm thick free-standing silicon nitride (Si3N4) membrane. The use of a material that absorbs and immobilizes gas molecules close to the localize plasmon modes was also used for graphene nanoantennas [80] as a sensitivity enhancement mechanism.

4.4. HIGH ASPECT RATIO PLASMONIC AND PHONONIC GRATING/TRENCH STRUCTURE

Apart from planar plasmonic nanostructures, high aspect ratio plasmonic structures can be used for mid-IR sensing. Deep subwavelength plasmonic trench (grating) structures as shown in Fig. 6(a) have been demonstrated to enhance light absorption by solid analyte. The structures were composed of aluminum-doped zinc oxide (AZO) plasmonic trenches with deep subwavelength period of 400 nm as opposed to the wavelength around 8 µm [91]. AZO is a transparent conductive oxide and becomes plasmonic for mid-infrared wavelengths [92]. Such high aspect ratio nanostructures can be fabricated by the combination of deep UV lithography (DUVL), deep reactive ion etching (DRIE), and atomic layer deposition (ALD) techniques with different materials [92–97]. The trench structures support surface waves, sometimes referred to as Dyakonov plasmons, and bulk plasmon polaritons, which propagate (c) 2020 RRP 72(0) 408 - v.2.0*2020.7.28 —ATG Article no. 408 O. Takayama 14 on the top surface and inside of trenches, respectively [92]. Such high aspect ratio trench structures also function as a hyperbolic metamaterial (HMM) [98, 99], a type of nanostructures that exhibits different signs of effective permittivity ten- sor components, generating hyperbolic isofrequency contours in the wavevector (k) space. HMMs made of Au nanowires [100] and alternating layers of aluminum oxide (Al2O3) and Au [101] have been demonstrated for refractive index and biosensing in visible and near-IR wavelength regions, yielding extremely high bulk refractive index sensitivity up to 30000 nm / RIU (refractive index unit). As a model analyte, 5 nm thick SiO2 layer that absorbs light around 8 µm in wavelength was conformally coated around the AZO plasmonic trenches by the atomic layer deposition (ALD) technique. The enhanced absorption is achieved by the combination of bulk plasmon modes that propagate in the trenches and large surface area of the nanotrench structure. Over 9 % reﬂection difference that corresponds to IR light absorption by SiO2 layer was achieved as opposed to 6 % by Si trenches with the same dimension, showing the potential of high aspect ratio plasmonic trench HMMs as a platform for biochemical sensing platform. The additional 3 % absorption is due to the ﬁeld enhancement of bulk plasmon polaritons in plasmonic AZO trenches.

Fig. 6 – Illustration of high aspect ratio (a) plasmonic AZO-based trench HMM structures [91], and (b) phononic 4H-SiC grating structures with liquid analyte in gaps [103]. The inset in (a) shows 5 nm thick SiO2 analyte layer coated around AZO trench structures.

An epsilon-near-zero (ENZ) material is an optical material whose relative permittivity is close to zero for a certain wavelengths range [102]. Moreover, a periodic nanostructure made of either dielectric or plasmonic material can behave as effective ENZ material or zero-index material (ZIM) as a whole for certain wavelengths. Such ENZ materials or ZIM support a highly localized optical mode, called ENZ mode, within the material or nanostructures. Recently, it has been reported that highly local- (c) 2020 RRP 72(0) 408 - v.2.0*2020.7.28 —ATG 15 Mid-infrared nanophotonics for biochemical sensing Article no. 408 ized ENZ modes could be another sensitivity enhancement mechanism [103]. Silicon carbide exhibits negative permittivity for = 10.3 µm - 12.5 µm and it becomes an ENZ material (" 0) around = 10.3 µm. High aspect ratio gratings of 4H-SiC have ' been fabricated with period ⇤ = 5 - 10 µm and height h = 0.8, 11.5, and 24.8 µm as illustrated in Fig. 6(b). The grating structures support gap plasmon modes in SiC-air- SiC geometry and propagate down towards the substrate. The grating forms a cavity for these modes that are reﬂected by top and bottom interfaces and are trapped in the grating region. One of these cavity modes is ENZ mode and is used for liquid analyte sensing. The localized ﬁelds of the ENZ mode in the gap enhance the vibrational coupling of liquid analyte, cyclohexane (C6H12). Strong coupling between the ENZ mode and the vibration of analyte was observed in terms of the splitting of both resonances in wavelength for different analyte concentration where the splitting of two resonances is the indication of strong coupling as in the case of hBN antennas [81]. This work also showed that the grating period can be tuned in order to control the cavity modes wavelength and overlap the mode with the vibrational mode of analyte.

5. SUMMARY AND CONCLUSION In this review, we have discussed nano-optic phenomena to conﬁne infrared light at nanoscale and to enhance infrared absorption by analyte molecules, provid- ing a broad overview of sensitivity enhancement mechanisms and material platforms for biochemical sensing. In general, the difﬁculty of fabrication of micro- and nanostructures for mid-IR regime is relatively eased thanks to the longer wavelength and re- quired minimum features for nanostructures compared with visible to near-IR wavelengths. This also enables novel nanostructures where their optical properties are tailored to support the desired localized modes at the wavelengths of interest. More- over, 2D materials, such as graphene and hBN, are natural plasmonic and phononic materials for mid-IR wavelengths and support highly localized modes as novel sensitivity enhancement mechanism. These nanomaterials and nanostructures may lead to new platforms for mid-IR absorption spectroscopy and mid-IR nanophotonics.

Acknowledgements. The author would like to acknowledge Independent Research Fund Den- mark, DFF Research Project 2 ”PhotoHub” (8022-00387B) for ﬁnancial support.

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