The growing interest in layered transition metal dichalcogenide (TMD) materials stems from their potential applications in valleytronics devices and the ability to control valley polarization. Monolayer molybdenum disulfide (MoS2), with its optical bandgap of 1.8eV and modulatable valley degree of freedom through circularly polarized light or strain engineering, exhibits distinct characteristics. In this study, we investigate the interaction between structured light possessing orbital angular momentum (OAM) and layered MoS2. The interaction between optical orbital angular momentum (OAM) and materials has led to the discovery of novel and intriguing physical phenomena, with significant advancements in recent years. In this study, we investigated the resonant Raman spectroscopy of monolayer molybdenum disulfide (MoS2) under the illumination of light with different orbital angular momentum. Excitation lasers with wavelengths of 633 nm (1.96 eV) and 532 nm (2.33 eV) were utilized, and a spatial light modulator (SLM) was employed to generate light with optical OAM, which was then used to illuminate the monolayer MoS2 and observe the resulting Raman spectra. The experimental results revealed that, under resonant excitation, an increase in orbital angular momentum caused a blue-shift in the peak positions of the Raman spectra. This finding indicates a strong coupling between excitons and phonons in the material, where the transfer of orbital angular momentum to the layered material results in compressive strain, altering the characteristic Raman peak positions. The insights gained from this study have the potential to enhance the manipulation of spin properties in TMD materials, opening up new possibilities for spin-based optoelectronics.
Extensive research has been conducted on the negative differential resistance (NDR) behavior in various electronic applications. Theoretical simulations suggest that defects in monolayer 2D materials could impact the NDR phenomenon. In this study, we experimentally validated this theoretical prediction using straightforward fabrication methods on monolayer MoS2. To create MoS2 transistors with a specific amount of sulfur vacancy, we employed techniques such as KOH solution treatment, electron beam irradiation, and chemical vapor deposition (CVD) using low sulfur supply. Through comprehensive analysis of the devices' electrical characteristics and spectroscopic examination, we successfully observed the NDR in the defective monolayer MoS2 field-effect transistors (FETs) with approximately 5% sulfur vacancy, as confirmed by x-ray photoelectron spectroscopy (XPS). Moreover, this NDR effect remains stable and can be controlled by the gate electric field or light intensity at room temperature. This discovery suggests that the NDR effect in monolayer MoS2 transistors holds promising potential for future electronic applications.
We study here the amount of helicity exchange of scattered light from the monolayer molybdenum disulfide (MoS2) or graphene on different structures by using spin angular momentum of light in Raman spectroscopy. Our in-house modified Raman spectroscopy setup is used for analyzing the optical helicity by calculating the degree of circular polarization (DoCP) of circularly polarized incident light when scattered from the 2D materials on different substrates. We observe Raman intensity variation when controlling phonon vibration of the 2D materials under the polarized excitation. The experimental results show that the layered materials deposited on SiO2/Si substrates has maximum DoCP as compared to other substrates with different structures. All the findings are analyzed by numerical simulation that incorporate the related Raman tensors and optical Jones calculus, which is worth exploring the phonon-electron interaction.
The effects of the peculiar in-plane lattice vibrations in monolayer molybdenum disulfide (MoS2) are oftentimes ignored in the analysis of the material’s lattice behaviors due to the lack of variation of polarization for the excitation light. In this work, we have observed variations in the relative intensity of the two most dominant Raman peaks of MoS2 via polarized micro-Raman spectroscopy using elliptically polarized incident light. The asymmetry of the incident excitation light gives an additional degree of freedom affecting the relationship between the x- (E12gx) and y- (E12gy) components of the material’s in-plane lattice vibrations. Different ratio of the magnitudes for E12gx and E12gy in the lattice vibrations can be induced by changing the polarization state of the incident light. This work investigates the material’s unexplored fundamental phonon property which may enlighten past and future studies involving phonon behaviors.
A laser cavity composed of a cylindrical mirror and a gain medium is used to generate scalar flower modes possessing orbital angular momentum. We proposed a new complex modes by using a hemi-cylindrical cavity or spatial light modulator to generate superposition of vector Laguerre-Gaussian modes and vector flower modes. The generated vector fields possessing not only orbital angular momentum but polarization properties which correspond to the specific point on a higher-order Poincaré sphere. This work paves the way to cavity laser in several applications.
We employ a large-Fresnel-number laser system to demonstrate the three-dimensional optical coherent waves localized
on Lissajous and trochoidal parametric surfaces with Lissajous and trochoidal transverse patterns in degenerate cavities.
The coherent structured beams are verified to be composed of degenerate Hermite-Gaussian and Laguerre-Gaussian
modes with different longitudinal indices resulted from longitudinal-transverse coupling. As well known, the Hermite-
Gaussian modes can be converted into Laguerre-Gaussian modes possessing orbital angular momentum by use of a pair
of cylindrical lens. Consequently, we make use of cylindrical lenses to transform the Lissajous structured beams
superposed of degenerate Hermite-Gaussian modes into the intriguing trochoidal structured beam possessing optical
orbital angular momentum.
The dynamical degradation process of operating organic light-emitting diode (OLED) was proposed and investigated by
non-destructive reflectivity measurements using a p-polarized He-Ne laser as probing tool. The intrinsic OLED
degradation mechanism mainly depends on intermediate layers at organic/electrode interfaces. The optical behavior of
these interfacial dielectric layers and corresponding optical parameters may be capable of representing OLED
degradation in macroscopic aspect. Optical parameters defined as optical constants (n, k) and thickness (d) were obtained
from fitting our experimental data to a theoretical model including interfacial dielectric layers with (n, k, d) as adjustable
parameters. Our experimental results revealed that the change of the reflectivity spectra obtained from static and operated
OLED was observable. The tendency of change in reflectivity spectra can be used as qualitative and dynamical aspect of
degradation of operating OLED. The dynamical degradation process can be quantitatively modeled by inspecting the
variation of optical parameters. This dynamical aspect may also include time-dependent information of degradation as
operating OLED. Our data-fitting results indicated that optical constants of intermediate layers have a trend to increase
as OLED from static to turn-on status. The thickness of intermediate layers obtained from data-fitting ranged from 0.2 to
14nm, satisfying our expectation. The reflective spectra obtained from basic OLED with ITO/Alq3/LiF/Al structure
revealed a clear dip located around 61.5° incident angle from ITO glass side as OLED in turn-on status. It indicated that
surface plasma resonance may occur even in OLED layer structure.
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