Metasurfaces, a two-dimensional (2D) form of metamaterials constituted by planar meta-atoms, exhibit exotic abilities to tailor electromagnetic (EM) waves freely. Over the past decade, tremendous efforts have been made to develop various active materials and incorporate them into functional devices for practical applications, pushing the research of tunable metasurfaces to the forefront of nanophotonics. Those active materials include phase change materials (PCMs), semiconductors, transparent conducting oxides (TCOs), ferroelectrics, liquid crystals (LCs), atomically thin material, etc., and enable intriguing performances such as fast switching speed, large modulation depth, ultracompactness, and significant contrast of optical properties under external stimuli. Integration of such materials offers substantial tunability to the conventional passive nanophotonic platforms. Tunable metasurfaces with multifunctionalities triggered by various external stimuli bring in rich degrees of freedom in terms of material choices and device designs to dynamically manipulate and control EM waves on demand. This field has recently flourished with the burgeoning development of physics and design methodologies, particularly those assisted by the emerging machine learning (ML) algorithms. This review outlines recent advances in tunable metasurfaces in terms of the active materials and tuning mechanisms, design methodologies, and practical applications. We conclude this review paper by providing future perspectives in this vibrant and fast-growing research field.

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Recent Advances in Tunable Metasurfaces: Materials, Design, and Applications.


 Photonic integrated microcombs have enabled advanced applications in optical communication, microwave synthesis, and optical metrology, which in nature unveil an optical dissipative soliton pattern under cavity-enhanced nonlinear processes. The most decisive factor of microcombs lies in the photonic material platforms, where materials with high nonlinearity and in capacity of high-quality chip integration are highly demanded. In this work, we present a home-developed chalcogenide glasses-Ge25Sb10S65 (GeSbS) for the nonlinear photonic integration and for the dissipative soliton microcomb generation. Compared with the current integrated nonlinear platforms, the GeSbS features wider transparency from the visible to 11-μm region, stronger nonlinearity, and lower thermo-refractive coefficient, and is CMOS compatible in fabrication. In this platform, we achieve chip-integrated optical microresonators with a quality (Q) factor above 2×10^6, and carry out lithographically controlled dispersion engineering. In particular, we demonstrate that both a bright soliton-based microcomb and a dark-pulsed comb are generated in a single microresonator, in its separated fundamental polarized mode families under different dispersion regimes. The overall pumping power is on the ten-milliwatt level, determined by both the high Q-factor and the high material nonlinearity of the microresonator. Our results may contribute to the field of nonlinear photonics with an alternative material platform for highly compact and high-intensity nonlinear interactions, while on the application aspect, contribute to the development of soliton microcombs at low operation power, which is potentially required for monolithically integrated optical frequency combs.

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Soliton Microcombs in Integrated Chalcogenide Microresonators

Integrated nonlinear photonics, combined nonlinear optics with state‐of‐the‐art photonic integration, play a crucial role in chip‐integrated technologies including optical frequency combs, molecular spectroscopy, and quantum optics. Optical materials with favorable properties are the foundation to promote integrated photonic devices with bandwidth, efficiency, and flexibility in high‐volume chip‐scale fabrication. In this work, a newly developed chalcogenide glass‐Ge25Sb10S65 (GeSbS) is presented for nonlinear photonic integration and for dissipative soliton microcomb generation. The GeSbS features wide transparency (0.5–10 µm), strong nonlinearity (1.3 × 10−18 m2 W−1), and low thermo‐refractive coefficient (3.1 × 10−5 K−1), and is complementary metal oxide semiconductor (CMOS)‐compatible in fabrication. In this platform, chip‐integrated optical microresonators with an average intrinsic quality (Q) factor of ≈1.97 × 106 are implemented, and lithographically controlled dispersion engineering is carried out. In particular, both a bright soliton‐based microcomb with bandwidth of 240 nm (≈1440–1680 nm) and a dark‐pulse comb with bandwidth of 80 nm (≈1510–1590 nm) are generated in a single microresonator in its separated fundamental polarized mode families.The ten‐milliwatt level of soliton microcomb operation power facilitates the monolithically integrated photonic circuits. The results provide a potential material platform for integrated nonlinear photonics for highly compact and high‐intensity nonlinear interactions in visible and infrared regions.

Integrated Chalcogenide Photonics for Microresonator Soliton Combs

This paper reports the effects of ion irradiation on the structural, linear, and nonlinear optical properties of thermally evaporated Bi5In30Se65 thin films. The prepared films were irradiated with 30 keV proton ions with different fluences, such as 5 × 1015 ions per cm2, 1 × 1016 ions per cm2, and 5 × 1016 ions per cm2. Structural analysis via X-ray diffraction (XRD) confirmed the non-crystalline nature of the film after ion irradiation with different doses. However, after the irradiation dose, the surface morphology changed, as shown by atomic force microscopy (AFM) images and field emission scanning electron microscopy (FESEM) images. The compositions of the films were obtained using energy-dispersive X-ray spectroscopy (EDX). Optical analysis via UV-Visible spectroscopy showed a reduction in the transmittance and an increase in the absorption in the higher wavelength region with irradiation. The optical bandgap and Tauc parameter decreased with an increase in the irradiation fluence, which is due to an increase in the irradiation-induced defects and disorder inside the system. The increases in the third order nonlinear susceptibility and the nonlinear refractive index with ion fluence are useful for nonlinear optical applications. The linear refractive index calculated from the transmittance data increased, satisfying Moss's rule. The optical parameters, such as lattice dielectric constant, optical density, skin depth, optical conductivity, real and imaginary dielectric constants, optical conductivity, loss factor, VELF, and SELF, were calculated using several empirical relationships and showed increasing behavior with the ion irradiation dose. The changes obtained in both the linear and nonlinear parameters will be useful for nonlinear optical device applications.

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The impact of fluence dependent proton ion irradiation on the structural and optical properties of Bi5In30Se65 thin films for nonlinear optical devices

The past, present and future of photonic glasses: A review in homage to the United Nations International Year of Glass 2022

Chalcogenide photonics offers unique solutions for a broad range of applications from mid-infrared sensing to integrated, ultrafast, ultrahigh-bandwidth signal processing. However, to date its usage has been limited to the infrared part of the electromagnetic spectrum, thus avoiding ultraviolet and visible ranges due to absorption of chalcogenide glasses. Here, we experimentally demonstrate and report near-infrared to ultraviolet frequency conversion in an As2S3-based metasurface, enabled by a phase locking mechanism between the pump and the inhomogeneous portion of the third harmonic signal. Due to the phase locking, the inhomogeneous component co-propagates with the pump pulse and encounters the same effective dispersion as the infrared pump, and thus experiences little or no absorption, consequently opening previously unexploited spectral range for chalcogenide glass science and applications, despite the presence of strong material absorption in this range. The use of chalcogenide glass in optical science and applications at the UV frequencies has been so far hindered by its absorption in this spectral region. Here the authors demonstrate that a nanostructured chalcogenide glass can efficiently generate third harmonic radiation, leading to a strong UV light source at the nanoscale.

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Near-infrared to ultra-violet frequency conversion in chalcogenide metasurfaces.

Abstract We demonstrate a simple, femtosecond-scale wavelength tunable, subwavelength-thick nanostructure that performs efficient wavelength conversion from the infrared to the ultraviolet. The output wavelength can be tuned by varying the input power of the infrared pump beam and/or relative delay of the control beam with respect to the pump beam, and does not require any external realignment of the system. The nanostructure is made of chalcogenide glass that possesses strong Kerr nonlinearity and high linear refractive index, leading to strong field enhancement at Mie resonances. Although, as many other materials, chalcogenide glasses absorb in the ultraviolet range, fundamental phase-locking mechanism between the pump and the inhomogeneous portion of the third-harmonic signal enables ultraviolet transmission with little or no absorption.

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All-optical tunable wavelength conversion in opaque nonlinear nanostructures

Arsenic triselenide (As2Se3) is of interest for use in dielectric metasurfaces for several reasons: It has a high linear refractive index, n=2.8, enabling high index contrast with its surrounding medium; it has an exceptionally high optical nonlinearity (n2 < 900 × that of silica) making it a good candidate for nonlinear metasurfaces; and its band gap of 1.8 eV and wide transmission window, spanning from approximately 1.5-14 μm, make it useful for applications in the shortwave infrared into the long-wave infrared. We discuss recent results in which we showed that unpassivated As2Se3 films degrade significantly under ambient conditions in the presence of light. We discuss the mechanism of this degradation and show that deposition of a thin (~10 nm) Al2O3 passivation layer, deposited via atomic layer deposition, together with preventing exposure to below-band gap light inhibits degradation. Finally, we fabricate initial As2Se3-based dielectric metasurfaces by writing features via a laser direct write system. For mid-wave to long-wave infrared applications, features with relevant sizes can be written using this technique. We measure resonances for these structures and compare to theoretical results.

Arsenic selenide dielectric metasurfaces

We present a method of post-deposition tuning of the optical properties of thin film dielectric filters and mirrors containing chalcogenide glass (ChG) layers by thermally adjusting their refractive index. A common challenge associated with the use of ChG films in practical applications is that they suffer from slight run-to-run variations in optical properties resulting from hard-to-control changes in source material and deposition conditions. These variations lead to inconsistencies in optical constants, making the fabrication of devices with prescribed optical properties challenging. In this paper, we present new work that takes advantage of the large variation of a ChG films’ refractive index as a function of annealing. We have carried out extensive characterization of the thermal index tuning and thickness change of arsenic selenide (As2Se3) ChG thin films and observed refractive index changes larger than 0.1 in some cases. We show results for refractive index as a function of annealing time and temperature and propose a model to describe this behavior based on bond rearrangement. We apply thermal refractive index tuning to permanently shift the resonance of a Fabry-Perot filter and the cutoff wavelength of a Bragg reflector. The Bragg reflector, consisting of alternating As2Se3 and CaF2 layers, exhibits high reflectance across a ∼550 nm band with only five layers. Modeling results are compared with spectroscopic measurements, demonstrating good agreement.

Thermal tuning of arsenic selenide glass thin films and devices

A common challenge in metastructure fabrication is precisely tuning of the frequency of a device’s resonance. Slight variations in device dimensions or material properties can lead to a deviation in resonance frequency in comparison to design. We present a method of tuning a dielectric metastructure’s resonance by thermally adjusting the refractive index of a chalcogenide glass (ChG) material. Several characteristics of ChGs make them good candidates for use in dielectric metastructures. They exhibit high linear refractive indices, enabling high index contrast devices; they have large optical nonlinearities, making them useful for tunable devices and nonlinear frequency conversion; and they have wide transmission windows extending from the visible through the long-wave infrared. Recently, we have carried out extensive characterization of the index tuning of arsenic selenide (As2Se3) ChG thin films and observed refractive index changes larger than 0.1 in some cases. We use this refractive index change to permanently shift the resonance of a Fabry-Perot filter and the cutoff wavelength for a Bragg reflector. We then demonstrate thermal tuning to shift resonance positions for a metasurface (MS). We compare finite element modeling results with measurement results and show good agreement. This tuning mechanism has potential for use in MS devices where a precise resonance frequency is required.

Anthony Clabeau

Papers

Near-infrared to ultra-violet frequency conversion in chalcogenide metasurfaces.

All-optical tunable wavelength conversion in opaque nonlinear nanostructures

Arsenic selenide dielectric metasurfaces

Thermal tuning of arsenic selenide glass thin films and devices

Thermally tuned resonances in chalcogenide glass dielectric metasurfaces