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Sharelatex Example Reconfigurable Flat Optics with Programmable Reflection Amplitude Using Lithography-Free Phase-Change Materials Ultra Thin Films S´ebastienCueff1, Arnaud Taute1;2, Antoine Bourgade3, Julien Lumeau3, St´ephane Monfray2, Qinghua Song4, Patrice Genevet4, Xavier Letartre1, and Lotfi Berguiga5∗ 1 Universit´ede Lyon, Institut des Nanotechnologies de Lyon - INL, CNRS UMR 5270, Ecole Centrale de Lyon - CNRS, 69134 Ecully, France. 2STMicroelectronics, 850 Rue Jean Monnet, Crolles, 38920, France 3Aix Marseille Univ, CNRS, Centrale Marseille, Institut Fresnel, F-13013 Marseille, France 4Universit´eC^otedAzur, CNRS, CRHEA, rue B. Gregory, 06560, Valbonne, France and 5Universit´ede Lyon, Institut des Nanotechnologies de Lyon - INL, CNRS UMR 5270, INSA Lyon - CNRS, 69621 Villeurbanne, France (Dated: September 8, 2020) We experimentally demonstrate a very large dynamic optical reflection modulation from a simple unpatterned layered stack of phase-change materials ultrathin films. Specifically, we theoretically and experimentally demonstrate that properly designed deeply subwavelength GeSbTe (GST) films on a metallic mirror produce a dynamic modulation of light in the near-infrared from very strong reflection (R > 80%) to perfect absorption (A > 99; 97%) by simply switching the crystalline state of the phase-change material. While the amplitude of modulation can lead to an optical contrast up to 106, we can also actively "write" intermediate levels of reflection in between extreme values, corresponding to partial crystallization of the GST layer. We further explore several layered system designs and provide guidelines to tailor the wavelength efficiency range, the angle of operation and the degree of crystallization leading to perfect absorption. I. INTRODUCTION modulation of PCMs has been used in specifically de- signed nanostructures to enable active beam-steerers [5], Adjusting the absorption, reflection and transmission dynamic modulation of light emission [6], light absorp- properties of systems is the basis of most photonic de- tion [7] or light transmission [8]. vices engineering, from mirrors to dispersion gratings Most of these studies leverage optical frequencies meta- as well as photodetectors and solar cells. The recent atoms which require in-plane nanostructuration of mate- progress in metamaterials and metasurfaces has provided rials that make their large-scale fabrication and industrial new methods to precisely control these features to an un- development difficult. Alternatively, other less technolog- precedented degree. For example, through careful spa- ically constraining methods exist to engineer the optical tial arrangement of dielectric meta-atoms with multipo- properties of devices using lithography-free planar thin- lar resonances we can design metasurfaces tailored for films. The most well-known example is the anti-reflection specific optical functionalities. This concept has been ex- coating, for which the thickness of a transparent thin- ploited to demonstrate flat optics such as lenses, polar- film is set at λ/4n to minimize the reflection through izers, retroreflectors, holograms, perfect absorbers, etc. destructive interferences while simultaneously maximiz- [1{4] that hold promise to surpass the performances of ing transmission. By introducing PCMs in similar en- conventional diffractive optics components. gineered thin-films, recent works demonstrated tunable However, both the nanoscale of meta-atoms and the structural coloration [9] or tunable near-perfect absorp- standard materials used for metasurfaces can represent a tion [10{16]. So far, these experimental works demon- roadblock for modulation and reconfiguration purposes. strated actively switchable optics that are either binary Indeed, nanopatterning thin-film materials through etch- and/or volatile. However, the complex features of chalco- ing processes definitively set their geometries and limit genide PCMs, and most notably the possibility to ac- their functionalities to a designated purpose. To take tively set them into a state of controlled partial crystal- a simple example, a TiO2-based metasurface hologram, lization may be used to a much deeper extent. Indeed arXiv:2009.02532v1 [physics.optics] 5 Sep 2020 once fabricated will only display one holographic image, their complex refractive index can be encoded into ar- what severely restricts the potential of this technology. bitrary intermediate values between those of amorphous In that context, recent works demonstrated the potential and those of fully crystalline. This multilevel crystal- of phase-change materials (PCM) for tunable nanopho- lization produces stable, non-volatile states that can be tonics. Indeed, this class of materials enables a very large driven optically or electrically via pulsed inputs [17, 18]. optical modulation at the nanoscale via a fast change of Such reconfigurable multilevel optical properties may be phase in their crystalline structure. This large optical exploited as additional degrees of freedom for the design space of multifunctional flat optics with a large num- ber of intermediate states and could be the basis for nu- merous exciting opportunities in applications such as ac- ∗ lotfi[email protected] tively controlled reflectivity modulation, continuous op- 2 tical power limiting, tunable displays, active spectral fil- fraction where 0 and 100 % correspond to the amorphous tering and dynamic wavefront shaping. and crystalline state, respectively. In this work, we design and demonstrate actively re- configurable lithography-free flat optics whose optical properties can be continuously tuned from a strong re- III. THEORETICAL CONSIDERATIONS flection (up to R>80%) to a perfect absorption (A=(1- R)∼99.99%) i.e an extinction of -68 dB that is actively A. Modulation principle controlled by simply adjusting the crystalline fraction of a standard GST thin-film (see Figure1a) for an illustration When a dielectric layer is deposited on a reflective sur- of the concept). Such a modulation depth surpass most face, e.g. a thick gold layer, most of the incident light is of free-space optical modulators reported so far [8, 19{ reflected back. However, depending on the thickness of 21] with the additional advantage of not requiring any this dielectric layer, the reflected light fields from the first complex nanopatterning processes. By precisely exploit- interface and second interfaces may either be in phase ing the non-volatile multilevel states enabled by partial or in opposite phase. This layer can therefore be used crystallization of GST, we provide comprehensive design as a means to adjust the reflectivity of the system via rules to simultaneously tailor the wavelength efficiency constructive or destructive interferences. The reflection range, the angle of operation and the degree of crystal- properties of a generic system comprising a semi-infinite lization leading to perfect absorption. We further pro- reflective substrate and two thin layers on top, can be pose practical implementations of this concept in multi- expressed as follows: layered configurations designed for electrical modulation that conserves all achieved properties. 2iβ1 r01 + r123e r = 2iβ (1) 1 + r01r123e 1 II. OPTICAL PROPERTIES OF GST with r01 the Fresnel coefficient at the first interface and r123 the effective reflection coefficient of the combined 2π Phase change materials such as GST present unique second and third interface. β1= λ n~1d1, withn ~1 and chemical bonding properties (sometimes referred to as d1 the complex refractive index and thickness of layer 1, resonant bonding or "metavalent bonding") with strong respectively. electronic polarizabilities that produce a very large re- From this equation, we can see that there are differ- fractive index in the visible and infrared regions, typically ent ways of tailoring the reflection of such a multilayer in the range of n ∼6-7 [22{25]. This metavalent bonding stack. The simplest one is to adjust the thickness of the requires a long range order between atoms and is there- thin-film until obtaining a desired reflection value at a fore lost when the material is in an amorphous state. In given wavelength. A similar effect can be obtained by that latter state, the refractive index falls back to values choosing a material with appropriate complex refractive typical of a semiconductor i.e. n ∼4. Changing the crys- index so as to adjust the optical path (n:d). Interestingly, talline phase of GST from amorphous to crystalline there- such system can be engineered to reach a perfect optical fore produces an exceptionally large modulation of the absorption via a mechanism called critical coupling. In refractive index. Furthermore, such a structural reorga- equation (1) this regime can be reached when the numer- nization can be driven thermally, electrically or optically. ator equals zero and gives us the following conditions: Figure1b) and c) illustrate the drastic change of refrac- tive index n and absorption coefficient k of a GST layer −4πk1d1/λ of 53 nm deposited on a gold layer upon crystallization. R01 = R123e (2) The dispersion curves n(λ) + ik(λ) have been measured by ellipsometry (more information in the experimental section) as a function of temperature. Between 120 and d1 Φ + 2πn − Φ = 2πm (3) 160 ◦C, the refractive index n values almost double in 123 1 λ 01 the 1200-2000 nm wavelength range upon crystallization, where k is the wavevector in layer 1, λ the wavelength and a transition temperature is found around 130◦C, as 1 and Φ and Φ are the phases of r and r , respec- shown in the figure1d). Simultaneously,
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