Layered material such as Transition Metal Dichalcogenides (TMDCs) are known to exhibit very high refractive index making them an excellent candidate for building resonant optical structures that facilitate strong nonlinear optical interaction. In particular, third order nonlinear processes such as third harmonic generation and four wave mixing (FWM) can be significantly boosted in thick TMDC based nanodisks by exciting non-radiating Mie resonances such as anapole modes. Here, we report enhanced FWM from isolated MoS2 disks supporting higher-order anapole resonances. The MoS2 disks were fabricated by patterning an MoS2 flake dry-transferred on a 2.2 micron thick thermal SiO2 deposited on a silicon wafer. Scattering cross sectional FDTD simulations were performed to extract the dimension of the MoS2 nanodisk to ensure higher order anapole modes lie within the desired signal wavelength range (1400-1600nm) with a fixed pump wavelength of 1040nm. The final dimensions of MoS2 disk has a diameter of 1.62μm and a height of 108nm. FWM measurements involving two pump photon and one signal photon were performed by varying the input signal wavelength. Maximum experimental enhancement of 150 times at 1470nm when compared with the un-patterned MoS2 flake region obtained. Such emerging optical nanostructure based on layered material has potential use as efficient wavelength converters across widely separated wavelength band.
KEYWORDS: Quantum random number generation, Analog to digital converters, Quantum beats, Quantum numbers, Bandpass filters, Tunable filters, Signal detection, Optical filters, Clocks, Background noise
We report two Quantum Random Number Generator (QRNG) schemes based on Amplified Spontaneous Emission (ASE) to address the problems of previous ASE based QRNGs – (1) costlier and commercially difficult-to-get components, (2) higher electrical noise in the faster detectors. Our ASE-ASE-beating based QRNG, built using commercially available components, clocks in 7.437Gbps random number generation rate. Our second QRNG scheme based on the beating of LASER and ASE of similar power generates random bits at a speed of 7.80Gbps. The minimum entropy values for these schemes are 5.21 and 5.4990 respectively for an 8-bit ADC. The second scheme reports higher entropy because its random numbers has a higher variance than first one. We use the Toeplitz extractor on the raw bits to generate unbiased random numbers and verify them with the help of industry-standard statistical tests.
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.
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.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.