Resonant Metasurfaces for Mid-Infrared Photonics

Abstract

The field of photonics offers a robust platform for efficient light-based technologies, directly impacting or serving as competent alternatives to address scientific problems. With the advent and advancement of micro-nano fabrication technologies, sub-domains of photonics like integrated optics (nanophotonics, guided wave optics), optoelectronics etc. are vastly benefited. These technologies resulted in noteworthy contributions in the fields of communication, networking, sensing, and biomedical applications, to name a few. Apart from these general areas of research, photonics played a catalytic role in the rapid growth of two-dimensional material integrated platforms and highly sought after field of quantum technologies. One important aspect in emerging technologies is to identify the limitations and scientific challenges associated with the application domain. The frequency bands (S, C, Ku etc.) of operation for communication and different wavelength windows (visible, near-infrared, mid-infrared etc.) for integrated optics contain useful information which could help the scientific community in filling the gaps in terms of essential technologies or supporting devices. From an integrated optics point of view, the wavelength range spanning 3 to 8 micrometers, referred to as the mid-infrared region stands out as a unique region contributing to a plethora of scientific applications. The wavelength range spanning 3 to 8 micrometers, referred to as the mid-infrared region stands out as a unique region contributing to a plethora of scientific applications. The presence of absorption signatures of most of the molecules (CH4, NO2, N2O, CO, H2O, etc.) or molecular fingerprints ignites a lot of applications including (infrared) spectroscopy, and nanoscale imaging as well as optics and sources. Furthermore, the atmospheric transmission window centered around 3 and 5 μm respectively has direct applications in defense and remote sensing. Conventional and state of the art devices (like photodetectors, sensors), in general, are bulk in nature. From an integrated optics perspective, this would imply larger footprint and power consumption. Existing technologies often require larger interaction volumes to effectively transduce the targeted signals to useful information. We explore the field of resonant metasurfaces for addressing the issues and challenges associated with such devices and eventually realize power-efficient nanoscale devices. By scaling down the device dimensions, interaction volume is largely reduced, but with efficient transduction properties (for e.g. by integrating transition metal dichalcogenide materials (TMDCs)). State of the art devices are based on conventional optical phenomena like refraction, total internal reflection (TIR) etc., which in turn depend on the material properties and bulkiness of the material platform. However, metasurfaces (two-dimensional equivalents of metamaterials) are artificially engineered structures for realizing unconventional photonic applications like optical cloaking, negative refraction, etc. Another challenge is in the choice of material. For conventional devices material properties like index of refraction, absorption or transmission characteristics are of great importance in selecting a compatible platform. Metasurfaces on the other hand depend on the periodic modulation of the refractive indices and most of the functionalities are attributed to the effective index of the structure. Transmission or reflection properties can thus be modified and scaled by using index engineering. We explore the field of resonant metasurfaces for addressing the issues and challenges associated with such devices and eventually realize power-efficient nanoscale devices. Metasurfaces (two-dimensional equivalents of metamaterials) are artificially engineered structures for realizing unconventional photonic applications like optical cloaking, negative refraction, etc. When combined with the well-established CMOS-based fabrication techniques, we can realize ultra-compact, power-efficient devices with metasurfaces as the underlying platform. In our research, we study the scope of such devices in enhancing localized electric fields by energy trapping at the nanoscale via resonance harvesting. Spectral resonances in the mid-infrared region with polarization-independence and angle-tolerance are useful for filtering applications in infrared spectroscopy and imaging systems. Metasurfaces designed to support a special class of resonances known as guided mode resonances (GMRs) using amorphous-Germanium (a-Ge) two-dimensional fully-etched high index contrast gratings (HCGs) on Calcium Fluoride substrate are presented. The resonance centered at 7.42 μm wavelength, exhibits polarization-independent, notch-type characteristics with minimal change across 0 to 30 degrees incidence angle with measured spectral width of 0.56 μm and extinction of 8 dB. These filters find potential applications in imaging applications utilizing un-polarized incident light, while at the same time having moderate field-of-view imaging capability. Next, we experimentally demonstrated a novel quasi-bound state in the continuum (BIC) resonance in the mid-infrared wavelength region with the resonant electric field confined as a slot-mode within a low refractive index medium (silicon nitride) sandwiched between high-index layers (a-Ge). Applications including active photonic functionalities (like non-linear responses) and sensing can greatly get benefited from energy trapping at the nanoscale. The slot-mode profile within the silicon nitride layer with mode field confinement >30% is achieved. Major observations from this study were excitation of quasi-BIC resonance at normal incidence under realistic excitation conditions and subwavelength scale light trapping as small as 10 nm. The highest quality factor of ~400 is experimentally extracted at normal incidence under classical mounting conditions with a resonance peak at 3.41 μm wavelength. We further explored the manifestation of GMRs in metasurfaces as a function of varying device parameters and excitation conditions. We observed that with increasing the duty-cycle of the partially etched amorphous silicon (a-Si) subwavelength gratings, the avoided crossing between the coupled GMR branches underwent band-closure and subsequent band-flip, resulting in the Friedrich-Wintgen type bound-states-in-the-continuum (FW-BICs) transitioning from the upper to the lower GMR branch. An exciting observation here was an interesting electromagnetically induced transparency (EIT)-like resonance. These devices find applications in resonantly enhancing linear absorption from analytes which we demonstrated with the differential enhancement of infrared absorption from a polymethyl methacrylate (PMMA) layer. We also built a simulation model that compares ideal conditions (infinitely periodic resonant metasurfaces under plane-wave excitation) and practical scenarios (finite-size devices under Gaussian-like excitation). Such a model is useful especially when non-standard, off-axis excitation (e.g. Cassegrain-type reflective objective with central obscuration) is employed. The model is not limited to linear responses but also can be extended to non-linear studies. We employed a plane wave expansion (PWE) method here and found good agreement with experimental results of linear (transmission spectra) and non-linear (third-order sum-frequency generation (TSFG)) nature. Apart from these moderately extensive studies, a few devices were also designed and fabricated to experimentally demonstrate enhanced non-linear responses like third harmonic generation (THG) and TSFG. Such devices along with conventional silicon or germanium-based (visible-near IR) photodetectors help in detecting weak mid-IR photons. The incident mid-infrared light is up-converted to the visible wavelength either with harmonic generation (THG) or by mixing (TSFG) with a pump beam (1040 nm here). THG and TSFG processes respectively explored GMRs and quasi-BICs for resonantly enhancing the up-conversion process. THG process up-converted 2.4 μm to 800 nm with an enhancement factor of 900 times whereas TSFG resulted in approximately 300 times enhancement for a 3 μm to 650 nm up-conversion.