KEYWORDS: Luminescence, Near field scanning optical microscopy, Transition metals, Signal generators, Signal detection, Second-harmonic generation, Near field, Mode locking, Harmonic generation, Femtosecond phenomena
The combination of far-field and near-field scanning optical microscopy (NSOM) with a position (line) and wavelength dependent detection allow us to observe the fluorescence form points away from the excitation spot. This contrasts with confocal illumination/detection and with fluorescent imaging of uniform illumination, the two commonly used fluorescence measurement modes. We discuss the origins of the nonlocal emission and argue that the results can be used to measure the exciton diffusion length in these two-dimensional materials. In particular, we study transition metal dichalcogenides. We use near-field second harmonic generation (SHG) with NSOM detection of the mode-locked femtosecond laser pulse generated signals.
Ultrashort pulse characterization and measurement is critical in the field of ultrafast and nonlinear optics. Here we present a method to reconstruct the complex pulse profile using a colinear frequency resolved optical gating (CFROG) acquisition combined with a convolutional neural network (CNN). The CFROG approach can be implemented with nonlinear nanoprobes for probing complex ultrafast optical fields. Typically, a CFROG trace is filtered and converted to a standard FROG trace which can then be processed by using the FROG retrieval algorithm to reconstruct both the amplitude and the phase profiles of the pulse. In this method, however, the reconstruction is often dependent on the subjective filtering step. In our approach, a CNN is trained with simulated unfiltered CFROG traces. Furthermore, we customize the CNN architecture to mitigate the ambiguity in the solution space and minimizes the error between the predicted and the input
We study the second-order nonlinear optical properties of several 2D materials through second harmonic generation (SHG) and sum frequency generation (SFG). SHG signals from 2D transition metal dichalcogenides (TMD) pumped at multiple fundamental wavelengths are measured and compared with theoretical analysis. We also use polarization-resolved second harmonic generation to characterize 2D materials and explore their biological applications. Using a narrow-band femtosecond laser beam and a supercontinuum, we measure the SFG of TMDs to characterize their second-order nonlinear susceptibility over a range of wavelengths.
Two-dimensional transition metal dichalcogenides (TMD), such as WS2 and MoS2, have been shown to exhibit large second order optical nonlinearity due to their non-centrosymmetric crystalline symmetry in few odd- and mono-layers, and resonance enhancement. Here we study the second-order nonlinear susceptibility of 2D TMDs through second harmonic generation (SHG) and sum frequency generation (SFG). Using a wavelength-tunable femtosecond laser, we can characterize SHG of TMDs to obtain the second-order nonlinear susceptibility at multiple wavelengths. Along with the experimental studies, theoretical investigation of the second-order nonlinear susceptibility is also performed. With this we explore the estimation of the second-order nonlinear susceptibility of 2D TMD layered materials based on their first-order susceptibility through the experimental and theoretical verification of Miller’s Rule for these materials. Additionally, we characterize the second-order nonlinear susceptibility of 2D TMD alloys through the SFG spectroscopy.
Two-dimensional materials have attracted significant interest recently for their unique optical properties compared to their bulk counterparts. Specifically, the family of transition metal dichalcogenides (TMD), such as MoS2 and WS2, have large second order nonlinear susceptibility. Extraordinary second harmonic generation and sum frequency generation have been observed. Here we investigate the second order nonlinearity of 2D materials, including TMD layered materials with dopants and defects. Experimental results and preliminary theoretical analysis will be discussed.
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