Ultra-strong nonlinear optical processes and trigonal warping in MoS2 layers.

Nonlinear optical processes, such as harmonic generation, are of great interest for various applications, e.g., microscopy, therapy, and frequency conversion. However, high-order harmonic conversion is typically much less efficient than low-order, due to the weak intrinsic response of the higher-order nonlinear processes. Here we report ultra-strong optical nonlinearities in monolayer MoS2 (1L-MoS2): the third harmonic is 30 times stronger than the second, and the fourth is comparable to the second. The third harmonic generation efficiency for 1L-MoS2 is approximately three times higher than that for graphene, which was reported to have a large χ (3). We explain this by calculating the nonlinear response functions of 1L-MoS2 with a continuum-model Hamiltonian and quantum mechanical diagrammatic perturbation theory, highlighting the role of trigonal warping. A similar effect is expected in all other transition-metal dichalcogenides. Our results pave the way for efficient harmonic generation based on layered materials for applications such as microscopy and imaging.Harmonic generation is a nonlinear optical process occurring in a variety of materials; the higher orders generation is generally less efficient than lower orders. Here, the authors report that the third-harmonic is thirty times stronger than the second-harmonic in monolayer MoS2.

Nonlinear microscopy using cylindrical vector beams: Applications to three-dimensional imaging of nanostructures.

The three-dimensional (3D) optical fields that arise from the focusing of cylindrical vector beams (CVB) with radial and azimuthal polarizations provide new sources of contrast for optical microscopy of nano-objects. So far, these demonstrations have been restricted to two-dimensional transversal scanning, i.e., along the focal plane of interest, or use of point-like objects, i.e., single molecules and nanoparticles. Here, we demonstrate the first application of CVBs for 3D imaging of 3D nano-objects. This technique is done by acquiring 3D image scans of the second-harmonic generation signal from vertically-aligned semiconductor nanowires, whose second-order response is primarily driven by the longitudinal electric field, i.e., the field component along the nanowire axis. Our technique provides a new way to study individual nano-objects in three dimensions through the unique combination of nonlinear microscopy and CVBs.

Rapid visualization of grain boundaries in monolayer MoS2 by multiphoton microscopy.

Grain boundaries have a major effect on the physical properties of two-dimensional layered materials. Therefore, it is important to develop simple, fast and sensitive characterization methods to visualize grain boundaries. Conventional Raman and photoluminescence methods have been used for detecting grain boundaries; however, these techniques are better suited for detection of grain boundaries with a large crystal axis rotation between neighbouring grains. Here we show rapid visualization of grain boundaries in chemical vapour deposited monolayer MoS2 samples with multiphoton microscopy. In contrast to Raman and photoluminescence imaging, third-harmonic generation microscopy provides excellent sensitivity and high speed for grain boundary visualization regardless of the degree of crystal axis rotation. We find that the contrast associated with grain boundaries in the third-harmonic imaging is considerably enhanced by the solvents commonly used in the transfer process of two-dimensional materials. Our results demonstrate that multiphoton imaging can be used for fast and sensitive characterization of two-dimensional materials.

Rapid and Large-Area Characterization of Exfoliated Black Phosphorus Using Third-Harmonic Generation Microscopy.

Black phosphorus (BP) is a layered semiconductor that recently has been the subject of intense research due to its novel electrical and optical properties, which compare favorably to those of graphene and the transition metal dichalcogenides. In particular, BP has a direct bandgap that is thickness-dependent and highly anisotropic, making BP an interesting material for nanoscale optical and optoelectronic applications. Here, we present a study of the anisotropic third-harmonic generation (THG) in exfoliated BP using a fast scanning multiphoton characterization method. We find that the anisotropic THG arises directly from the crystal structure of BP. We calculate the effective third-order susceptibility of BP to be ∼1.64 × 10-19 m2 V-2. Further, we demonstrate that multiphoton microscopy can be used for rapid, large-area characterization indexing of the crystallographic orientations of many exfoliated BP flakes from one set of multiphoton images. This method is therefore beneficial for samples of areas ∼1 cm2 in future investigations of the properties and growth of BP.

Pyrolytic carbon coated black silicon.

Carbon is the most well-known black material in the history of man. Throughout the centuries, carbon has been used as a black material for paintings, camouflage, and optics. Although, the techniques to make other black surfaces have evolved and become more sophisticated with time, carbon still remains one of the best black materials. Another well-known black surface is black silicon, reflecting less than 0.5% of incident light in visible spectral range but becomes a highly reflecting surface in wavelengths above 1000 nm. On the other hand, carbon absorbs at those and longer wavelengths. Thus, it is possible to combine black silicon with carbon to create an artificial material with very low reflectivity over a wide spectral range. Here we report our results on coating conformally black silicon substrate with amorphous pyrolytic carbon. We present a superior black surface with reflectance of light less than 0.5% in the spectral range of 350 nm to 2000 nm.

Natural Silk as a Photonics Component: a Study on Its Light Guiding and Nonlinear Optical Properties.

Silk fibers are expected to become a pathway to biocompatible and bioresorbable waveguides, which could be used to deliver localized optical power for various applications, e.g., optical therapy or imaging inside living tissue. Here, for the first time, the linear and nonlinear optical properties of natural silk fibers have been studied. The waveguiding properties of silk fibroin of largely unprocessed Bombyx mori silkworm silk are assessed using two complementary methods, and found to be on the average 2.8 dB mm(-1). The waveguide losses of degummed silk are to a large extent due to scattering from debris on fiber surface and helical twisting of the fiber. Nonlinear optical microscopy reveals both configurational defects such as torsional twisting, and strong symmetry breaking at the center of the fiber, which provides potential for various nonlinear applications. Our results show that nonregenerated B. mori silk can be used for delivering optical power over short distances, when the waveguide needs to be biocompatible and bioresorbable, such as embedding the waveguide inside living tissue.

Optical characterization of directly deposited graphene on a dielectric substrate.

By using scanning multiphoton microscopy we compare the nonlinear optical properties of the directly deposited and transferred to the dielectric substrate graphene. The direct deposition of graphene on oxidized silicon wafer was done by utilizing sacrificial copper catalyst film. We demonstrate that the directly deposited graphene and bi-layered transferred graphene produce comparable third harmonic signals and have almost the same damage thresholds. Therefore, we believe directly deposited graphene is suitable for the use of e.g. nanofabricated optical setups.

Polarization and Thickness Dependent Absorption Properties of Black Phosphorus: New Saturable Absorber for Ultrafast Pulse Generation.

Black phosphorus (BP) has recently been rediscovered as a new and interesting two-dimensional material due to its unique electronic and optical properties. Here, we study the linear and nonlinear optical properties of BP flakes. We observe that both the linear and nonlinear optical properties are anisotropic and can be tuned by the film thickness in BP, completely different from other typical two-dimensional layered materials (e.g., graphene and the most studied transition metal dichalcogenides). We then use the nonlinear optical properties of BP for ultrafast (pulse duration down to ~786 fs in mode-locking) and large-energy (pulse energy up to >18 nJ in Q-switching) pulse generation in fiber lasers at the near-infrared telecommunication band ~1.5 µm. We observe that the output of our BP based pulsed lasers is linearly polarized (with a degree-of-polarization ~98% in mode-locking, >99% in Q-switching, respectively) due to the anisotropic optical property of BP. Our results underscore the relatively large optical nonlinearity of BP with unique polarization and thickness dependence, and its potential for polarized optical pulse generation, paving the way to BP based nonlinear and ultrafast photonic applications (e.g., ultrafast all-optical polarization switches/modulators, frequency converters etc.).

Investigation of second- and third-harmonic generation in few-layer gallium selenide by multiphoton microscopy.

Gallium selenide (GaSe) is a layered semiconductor and a well-known nonlinear optical crystal. The discovery of graphene has created a new vast research field focusing on two-dimensional materials. We report on the nonlinear optical properties of few-layer GaSe using multiphoton microscopy. Both second- and third-harmonic generation from few-layer GaSe flakes were observed. Unexpectedly, even the peak at the wavelength of 390 nm, corresponding to the fourth-harmonic generation or the sum frequency generation from third-harmonic generation and pump light, was detected during the spectral measurements in thin GaSe flakes.

Second-harmonic generation imaging of semiconductor nanowires with focused vector beams.

We use second-harmonic generation (SHG) with focused vector beams to investigate individual vertically aligned GaAs nanowires. Our results provide direct evidence that SHG from oriented nanowires is mainly driven by the longitudinal field along the nanowire growth axis. Consequently, focused radial polarization provides a superior tool to characterize such nanowires compared to linear polarization, also allowing this possibility in the native growth environment. We model our experiments by describing the SHG process for zinc-blende structure and dipolar bulk nonlinearity.

High-quality crystallinity controlled ALD TiO2 for waveguiding applications.

We demonstrate a novel atomic layer deposition (ALD) process to make high-quality nanocrystalline titanium dioxide (TiO(2)) with intermediate Al(2)O(3) layers to limit the crystal size. The process is based on titanium chloride (TiCl(4))+water and trimethyl aluminum (TMA)+ozone processes at 250°C deposition temperature. The waveguide losses measured using a prism coupling method for 633 and 1551 nm wavelengths are as low as 0.2±0.1 dB/mm with the smallest crystal size, with losses increasing with crystal size. In comparison, plain TiO(2) deposited at 250°C without the intermediate Al(2)O(3) layers shows high scattering losses and is not viable as waveguide material. The third-order optical nonlinearity decreases with smaller crystal size as verified by third-harmonic generation microscopy but still remains high for all samples. Crystallinity controlled ALD-grown TiO(2) is an excellent candidate for various optical applications, where good thermal stability and high third-order optical nonlinearity are needed.

Rapid large-area multiphoton microscopy for characterization of graphene.

Single- and few-layer graphene was studied with simultaneous third-harmonic and multiphoton-absorption-excited fluorescence microscopy using a compact 1.55 µm mode-locked fiber laser source. Strong third-harmonic generation (THG) and multiphoton-absorption-excited fluorescence (MAEF) signals were observed with high contrast over the signal from the substrate. High contrast was also achieved between single- and bilayer graphene. The measurement is straightforward and very fast compared to typical Raman mapping, which is the conventional method for characterization of graphene. Multiphoton microscopy is also proved to be an extremely efficient method for detecting certain structural features in few-layer graphene. The accuracy and speed of multiphoton microscopy make it a very promising characterization technique for fundamental research as well as large-scale fabrication of graphene. To our knowledge, this is the first time simultaneous THG and MAEF microscopy has been utilized in the characterization of graphene. This is also the first THG microscopy study on graphene using the excitation wavelength of 1.55 µm, which is significant in telecommunications and signal processing.

Horizontal slot waveguide channel for enhanced Raman scattering.

Herein we characterize and experimentally demonstrate a new type of a horizontal slot waveguide structure for remarkably enhanced Raman scattering detection in nanometer-scale void channels. As the measurement sensitivity is one of the key limiting factors in nanofluidic detection, it is essential to search advanced solutions for such detection. Combining an all dielectric resonance waveguide grating and a surface enhanced Raman scattering (SERS) substrate in a close proximity it is possible to create high electromagnetic field energy hot zones within an adjustable slot region. This results in a strong enhancement in Raman scattering. We show the theoretical principles and demonstrate, with rhodamine 6G molecules, an approximately 20-fold enhancement compared to a conventional SERS substrate within the corresponding slot arrangement. We foresee potential applications for the proposed approach in the fields of medical, biological and chemical sensing, where the high detection sensitivity is essential due to integration with nanofluidic devices.