Ding Ma

Extraordinary Second Harmonic Generation in Tungsten Disulfide Monolayers

We investigate Second Harmonic Generation (SHG) in monolayer WS2 both deposited on a SiO2/Si substrate or suspended using transmission electron microscopy grids. We find unusually large second order nonlinear susceptibility, with an estimated value of deff ~ 4.5 nm/V nearly three orders of magnitude larger than other common nonlinear crystals. In order to quantitatively characterize the nonlinear susceptibility of two-dimensional (2D) materials, we have developed a formalism to model SHG based on the Greens function with a 2D nonlinear sheet source. In addition, polarized SHG is demonstrated as a useful method to probe the structural symmetry and crystal orientation of 2D materials. To understand the large second order nonlinear susceptibility of monolayer WS2, density functional theory based calculation is performed. Our analysis suggests the origin of the large nonlinear susceptibility in resonance enhancement and a large joint density of states, and yields an estimate of the nonlinear susceptibility value deff = 0.77 nm/V for monolayer WS2, which shows good order-of-magnitude agreement with the experimental result.

Broadband and Wide Field-of-view Plasmonic Metasurface-enabled Waveplates

Quasi two-dimensional metasurfaces composed of subwavelength nanoresonator arrays can dramatically alter the properties of light in an ultra-thin planar geometry, enabling new optical functions such as anomalous reflection and refraction, polarization filtering, and wavefront modulation. However, previous metasurface-based nanostructures suffer from low efficiency, narrow bandwidth and/or limited field-of-view due to their operation near the plasmonic resonance. Here we demonstrate plasmonic metasurface-based nanostructures for high-efficiency, angle-insensitive polarization transformation over a broad octave-spanning bandwidth. The structures are realized by optimizing the anisotropic response of an array of strongly coupled nanorod resonators to tailor the interference of light at the subwavelength scale. Nanofabricated reflective half-wave and quarter-wave plates designed using this approach have measured polarization conversion ratios and reflection magnitudes greater than 92% over a broad wavelength range from 640 to 1290 nm and a wide field-of-view up to ±40°. This work outlines a versatile strategy to create metasurface-based photonics with diverse optical functionalities.

Blue-phase liquid crystal cored optical fiber array with photonic bandgaps and nonlinear transmission properties

Blue-phase liquid crystal (BPLC) is introduced into the pores of capillary arrays to fabricate fiber arrays. Owing to the photonic-crystals like properties of BPLC, these fiber arrays exhibit temperature dependent photonic bandgaps in the visible spectrum. With the cores maintained in isotropic as well as the Blue phases, the fiber arrays allow high quality image transmission when inserted in the focal plane of a 1x telescope. Nonlinear transmission and optical limiting action on a cw white-light continuum laser is also observed and is attributed to laser induced self-defocusing and propagation modes changing effects caused by some finite absorption of the broadband laser at the short wavelength regime. These nonlinear and other known electro-optical properties of BPLC, in conjunction with their fabrication ease make these fiber arrays highly promising for imaging, electro-optical or all-optical modulation, switching and passive optical limiting applications.

High-Q silk fibroin whispering gallery microresonator

We have experimentally demonstrated an on-chip all-silk fibroin whispering gallery mode microresonator by using a simple molding and solution-casting technique. The quality factors of the fabricated silk protein microresonators are on the order of 105. A high-sensitivity thermal sensor was realized in this silk fibroin microtoroid with a sensitivity of -1.17 nm/K, that is 8 times higher than previous WGM resonator-based thermal sensors. This opens the way to fabricate biodegradable and biocompatible protein based microresonators on a flexible chip for biophotonics applications.

Dielectric nanoresonator based lossless optical perfect magnetic mirror with near-zero reflection phase

We report an all-dielectric lossless optical mirror for the realization of controllable reflection phase based on an array of isolated dielectric nanoresonators. This dielectric mirror is comprised of a cross-shaped amorphous silicon nanoresonator array that has been designed to achieve a 99.8% reflectivity and zero reflection phase at the wavelength of 0.99 μm. The measured results from the fabricated sample match the theoretical predictions with 99.5% reflectivity and near-zero degree reflection phase at 1 μm, which is very close to the targeted wavelength. This concept and approach pave the way for synthesizing lossless artificial reflecting electromagnetic boundaries with arbitrary phase response and hold great promise in applications ranging from nanocavities to nanowaveguides and nanoantennas.

Incorporating conductive materials into a photonic crystal fiber: toward optoelectronic applications

We report a method to selectively fill arbitrary air holes of microstructured photonic crystal fibers with conductive materials through microsphere-assisted fabrication. A photonic crystal fiber with three of its air holes filled with gallium is fabricated and optically characterized. Further it is shown that nanomaterials such as carbon nanotube can be attached to the tip of the conductive channel through additional optical soldering post-processing.

Vibrational sum frequency generation digital holography

We demonstrated a molecular spectroscopic imaging method by integrating the capabilities of holography and vibrational sum frequency generation (SFG) spectroscopy. SFG is a second-order nonlinear optical process that can probe molecular vibration resonance. SFG also poses stringent requirement of noncentro-symmetry, useful for suppressing interfering contributions in a complex environment. Further, holographic detection captures a wide-field complex SFG signal including both the amplitude and phase information, thus enabling new opportunities for three-dimensional label free imaging.

Flexible zero refractive index optical metamaterial with matched impedance

We demonstrate a flexible thin film zero refractive index optical metamaterial with matched impedance to free space and low absorption loss at 1.55 μm. The metallo-dielectric multilayer structure with fishnet geometry was optimized by a genetic algorithm. The fabrication process and characterization approach are described. The experiment results agree well with the theoretical predictions, showing an effective index of neff = 0.072 + 0.51i and an impedance of Zeff/Z0 = 1.009 – 0.021i.

Sum frequency generation holography (Conference Presentation)

We demonstrate a sum frequency generation (SFG) holographic imaging method by integrating the capabilities of holography and SFG spectroscopy. SFG can probe the molecular vibrational resonance in non-centrosymmetric media. Holographic recording can capture both the amplitude and the phase of the SFG signal, thus leading to label-free spectroscopic three-dimensional imaging.

Harmonic generation in 2D layered materials

Two-Dimensional (2D) layered materials have garnered interest due to their novel optical and electronic properties. In this work, we investigate Second Harmonic Generation (SHG) in Tungsten Disulfide (WS2) monolayers grown on SiO2/Si substrates and suspended on a transmission electron microscopy grid; we find an unusually large second order susceptibility, which is nearly three orders of magnitude larger than common nonlinear crystals. We have also developed a Green’s function based formalism to model the harmonic generation from a 2D layer .