Guided mode resonances (GMR) can be made angle insensitive using conical mounting of the grating relative to the external illumination. Conical mounting has been previously utilized for linear optical applications, such as optical filtering, and sensing. Here we present Third Harmonic Generation (THG) enhancement from 10 nm amorphous silicon overlayer on silicon nitride-based one-dimensional sub-wavelength grating GMR structures under conical-mounting illumination. The designed structure comprising of 70 nm deep silicon dioxide gratings of 1054 nm pitch, 50% duty cycle over which 160 nm thick silicon nitride and 10 nm a-Si layers are deposited is resonant at 1580 nm for TE polarized excitation. With increased angular spread of the incident excitation, the GMR spectral width and contrast are known to degrade. The angular aperture of the GMR structures studied here, which is defined as the angular spread across which the resonance drops to 50% of its peak value is calculated as 0.46° and 5.2° for classical and full-conical illumination respectively, highlighting the angular insensitivity of the full-conical mounting condition. Rectangular aperture masks placed in the back focal plane of the objective lens are used to limit the illumination angle along the grating wave-vector direction when compared to the grating line direction, thus achieving conical mounting condition. Experimentally, we observe the THG enhancement, defined as the ratio of on- to off-grating THG, improves from 2860 to 4742 and 1.7x104 by utilizing 0.06 NA objective and illuminating in classical configuration (no aperture) with rectangular apertures of size 3x13 mm and 1.5x13 mm respectively.
Resonant metasurfaces supporting quasi bound-states-in-continuum (BICs) resonances are particularly attractive for achieving high quality factor resonances which can be used to enhance nonlinear optical processes. In this work we first experimentally demonstrate quasi-BIC resonances in the mid infrared wavelength range using amorphous Germanium (a- Ge) based one-dimensional (1D) sub-wavelength grating structures with vertical asymmetry. The vertical asymmetric structures studied here consist of 1D a-Ge partially etched zero-contrast gratings (ZCG) on quartz substrate with the addition of a second asymmetric step-profile creating in-plane asymmetry. The vertical asymmetry added to the grating structure through the second etching step creates an open channel for accessing symmetry-protected guided mode resonances (GMR) which are otherwise experimentally inert to outgoing radiation. The fabricated device dimensions are: total height of 424 nm, ZCG etch depth of 190 nm, asymmetric step etch depth of 100 nm, pitch of 2.0 μm, and grating (asymmetric step) duty-cycles of 75% (32.5 %). FTIR measurements show clear resonance peak in the transmission spectra at ~3.2 μm wavelength for transverse magnetic (TM) polarized incidence with quality factor of ~50. We also characterize the incidence angle dependence of the measured resonance peak under classical and conical mounting condition and observe close to angle insensitive response for the latter. Next, we demonstrate third-order sum-frequency generation (TSFG) based up-conversion of the resonant mid infrared light to visible wavelengths in the presence of an additional 1040 nm pump excitation. Maximum TSFG enhancement of ~32 is obtained close to the mid infrared resonance.
We report Second Harmonic Generation (SHG) enhancement in a custom-designed vertically stacked multilayer Gallium Selenide with a low-index PMMA spacer layer. The structure obtained using an evolutionary optimization algorithm consists of two 40 nm GaSe layers separated by a 195 nm PMMA layer, with a 130 nm Silicon-dioxide layer on a Silicon substrate. This results in 11x and 303x enhancement in SHG signal when compared to a single 40nm GaSe on a 130nm and 300nm SiO2/Si substrate, respectively. This work underscores the importance of micro-cavity engineering in choosing appropriate 2D material and interspacer thickness to enhance SHG emission from popular 2D materials.
There is interest in studying nonlinear optical properties of monolayer and few-layer 2D materials due to the inherently strong nonlinear optical properties, interesting symmetry properties and polarization dependence. However, the inherent ultrathin 2D material limits the interaction length and efficiency of the nonlinear optical process studied. To overcome this limitation, 2D materials are integrated with resonant photonic structures to increase the overall nonlinear optical interaction strength. Such heterogeneously integrated structures offer the advantage of combining a range of 2D materials exhibiting diverse linear and nonlinear optical properties with prefabricated photonic structures using simple dry-transfer or chemical vapor deposition techniques. Here we will discuss some of the recent progress made in the area of heterogenous integration of 2D materials with dielectric resonant structures. We will also describe some of our recent effort in this direction in the resonant enhancement of second harmonic generation (SHG) from multi-layer Gallium Selenide coupled to silicon twodimensional resonant metasurface to achieve polarization independent SHG enhancement. We find that the designed 2D silicon resonant structures exhibit field depolarization at the resonance wavelength which needs to be accounted for when analyzing the nonlinear polarization. Furthermore, the second-harmonic signal radiated from the structure exhibits higher order diffraction effects with strong incident polarization angle dependence for the higher order diffraction components. Experimental studies on the above structures are also discussed with the observation of resonant enhancement of SHG and similar polarization dependence of the SHG on-/ off- the resonant metasurface when restricting the collection angles to the zeroth order diffracted nonlinear signal.
We discuss the design of partially etched 1D silicon nitride medium-index contrast sub-wavelength grating structures for generating strong longitudinally polarized resonant field and compare this with focal-field simulations obtained for radially and linearly polarized incident light. Normal incident plane-wave polarized perpendicular to the silicon nitride grating lines (TM polarization) generates both in-plane and out-of-plane field components relative to the grating plane, also here called as field depolarization, the ratio of which can be engineered by varying the grating dimensions. To have maximum depolarization above the structure, the etch depth of the gratings was fixed low at 30 nm, keeping the pitch and duty cycle fixed at 1 𝜇m and 50% respectively and the unetched thickness was varied. Maximum depolarization ratio, defined as the ratio of maximum longitudinal to transverse electric field intensity above the structure was observed to be 1.2 for an unetched thickness of 300 nm at resonant wavelength of 1489 nm. With thicker unetched thickness of 1000 nm, the depolarization within the structure can be maximized to 4.8 with resonance at 1826 nm. Often the generation of strong longitudinally polarized focal-fields relies on the use of tightly focused radially polarized incident light and imposes restrictions on the specimen due to the use of high-index immersion media. Such depolarization ratios are typically achieved with high numerical aperture (NA) focusing objective lens with NA greater than 1.25. Furthermore, we also report a simulation-based study of these structures for enhancing dark excitonic photoluminescence from Tungsten Diselenide(WSe2) monolayer integrated with these structures and observed the photoluminescence in presence of grating to be 20 times enhanced than that off grating.
In this paper, we report demonstration of sub-wavelength high-index contrast gratings which exhibit guided mode resonances for enhancing nonlinear optical effects from 2D materials transferred on top of the structures. Twodimensional hexagonal arrays of c-Si nanodisks on a silicon-on-insulator wafer have been designed to have normal incidence resonance in the 1550-1650 nm wavelength region. Numerical simulations were performed to show resonance variations with structural parameters and corresponding field enhancements outside the structure to aid nonlinear optical response from materials placed on top of the structures. The fabricated structures were characterized for linear reflection using an external cavity tunable laser as the incident light source at close to normal incidence and compared with simulated reflection. As a proof-of-concept, we transferred few-layer Gallium Selenide (GaSe) flake on to the grating using a dry transfer method to examine second harmonic generation response of GaSe in presence of the grating. Second harmonic generation measurements showed strong SHG signal from the GaSe on top of the grating structure, with enhancement of ~ 15x observed at 1645 nm close to fundamental resonance wavelength. No SHG emission was observed from the silicon nanodisks withput the GaSe overlayer. Spectral and power of the SHG were also characterized. This work shows that the potential of heterogeneous integration of high nonlinearity 2D materials on to silicon based resonant optical structures to realize high efficiency nonlinear metasurfaces.
Dielectric nanostructures designed in sub-wavelength scale can be tuned to achieve high-Q resonances in the wavelength region of interest with a high concentration of field in and around the structure, which can be used to achieve enhanced light-matter interaction. Such dielectric metasurfaces are potentially conducive platforms for exploiting nonlinear photonic devices at lower input power levels. In this work, we design, fabricate and experimentally demonstrate one-dimensional silicon nitride based guided-mode resonant structure, which exhibits inherently low nonlinear optical effects for enhancing third harmonic signals from a conformal layer of ultra-thin amorphous silicon coated over the gratings. The GMR structures studied here consist of an etched silicon dioxide layer deposited on top of a glass substrate, followed by the deposition of a silicon nitride layer. The thickness of the silicon nitride layer is chosen (~ 160 nm) to achieve GMR resonances around 1550 nm wavelength. The resonance is found to redshift to 1580 nm in presence of the 10 nm amorphous silicon layer. THG studies on the above amorphous-silicon deposited GMR structures shows resonant enhancement of ~ 18x on-grating when compared to off-grating at the peak of the GMR resonance. The present work demonstrates the use of a silicon-processing compatible material stack to realize separately GMR resonances and nonlinear medium to achieve resonant nonlinear enhancement, thus paving the way for silicon-compatible layered nonlinear metasurfaces.
We report spatially resolved measurement of third-harmonic generation (THG) emission from a Tin diselenide (SnSe2) multi-layer flake at a fundamental excitation wavelength of 1550 nm using a nonlinear optical microscopy system and study its thickness dependence. We also estimate the magnitude of the real part of the electronic nonlinearity susceptibility (χ(3) coefficient) by analyzing the thickness-dependence and found to be approximately 1.6×10-19 m2/V2, which is around 1500 times higher than that of the glass when measured with the same settings. We find excellent agreement between the measured THG thickness dependence and the analytical model considering absorption of harmonic emission in SnSe2 medium, phase mismatch and the multipath interference due to the underlying oxide/Si substrate. We also measure the second harmonic generation from same flake and find this to be maximum for thickness in the range of 10-12nm.
In this work, we have designed, fabricated and characterized silicon nitride sub-wavelength gratings on glass substrate to enhance the fluorescence in the green-red wavelength range. Silicon nitride was chosen as the material to fabricate the gratings as it exhibits low absorption losses and negligible fluorescence at visible wavelengths. Due to lower refractive index contrast, the structures were designed such that the medium index contrast gratings still achieve good quality factor resonances by using higher duty cycles (~ 70%) which clearly distinguishes two-mode region from higher order diffraction regime. The designed structure (Duty cycle: ~70%, thickness: 290nm, pitch: 370nm) supports resonant modes at 542nm for TE and at 548nm and 568nm for TM polarization. Rhodamine B dye was attached to the grating through an intermediate polymer layer PAH (Polyallylamine hydrochloride) by dip coating method. Using a fluorescence microscope with suitable excitation (510-550nm) and emission (>590nm) filters, we observed fluorescence enhancement of 5.4x and 5.8x in TE and TM modes respectively.
Water quality monitoring has become important in today’s scenario due to severe chemical and bacterial contamination in urban and rural water bodies. However, current monitoring methods do not provide fast and reliable results. By using intrinsic fluorescence, microbial contamination and industrial pollutants in water can be monitored in real-time, continuously and at very low concentrations. Intrinsic fluorescence can be enhanced by using High Contrast Gratings (HCGs) spectrally tuned to the fluorescence signatures of pollutants. Compared to metallic gratings which suffer from higher losses especially at lower wavelengths and are easily prone to oxidation, an all dielectric approach can overcome these limitations. HCGs using silicon nitride as grating material on a glass substrate are optimized to detect the presence of tryptophan (a bio-chemical marker for bacterial contamination) and phenanthrene (chemical contaminant). Tryptophan and phenanthrene have a fluorescence emission wavelength of 410 nm and 420 nm respectively. HCGs are optimized to enhance fluorescence emission at both of these wavelengths, therefore the optimized grating parameters for tryptophan (period: 255 nm, duty cycle: 0.8 and thickness: 260 nm) and phenanthrene (period: 282 nm, duty cycle: 0.8 and thickness: 289 nm) resulted in Q factor of 683 and 709 respectively. The optimized HCGs show an electric field enhancement of eight times concentrated in the air region between the gratings which would result in enhanced fluorescence.
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