Nonlinear optical susceptibility of atomically thin WX2 crystals

Abstract

We have studied tungsten diselenide (WSe2) and tungsten disulfide (WS2) monolayer materials in second harmonic generation spectroscopy and microscopy experiments. Ultra-broadband continuum pulses served as the fundamental beam while its second harmonic spectrum in the visible and ultraviolet (UV) range was detected and analyzed with a better than 0.3 nm spectral resolution (<2 meV). We provide dispersion data and absolute values for χ (2) for the materials within a photon energy range of 2.3-3.2 eV. Fine spectral features that were detected within the dispersion data for the optical nonlinearities indicate the impact of near bandgap exciton transitions. The fundamental bandgap of 2.35 eV and exciton binding energy of 0.38 eV were determined from the measurements for WS2 monolayers while the corresponding values in WSe2 monolayers were 2.22 eV and 0.71 eV. Ranges for the absolute values of the sheet nonlinearity for WS2 and WSe2 are shown to be 0.58-1.65×10-18 m 2 /V and 0.21-0.92×10-18 m 2 /V, correspondingly.

Nonlinear Optical Characterization of 2D Materials

“…For example, in the study of excitonic resonance phenomenon of 2D TMDs by using SHG/THG detection, the characterization of an excitonic resonance phenomenon can be achieved by gradually scanning with a spectrally narrow excitation pulse in a wide spectrum. This experimental technique requires precise control of the pulse duration, pulse energy, and spatial position of the beam on the sample in each adjustment step, which is difficult to achieve in a long detection time [ 111 ]. Therefore, the supercontinuum spectroscopy can be used to overcome these shortcomings.…”

High spectral resolution second harmonic generation microspectroscopy at thin layer interfaces with broadband continuum pulses

“…The detailed analysis of the dispersion data presented in Fig. 4(c), (d) is presented in our recently published work [6]. Briefly, the WSe 2 dispersion data shows just one peak with a maximum χ2Dfalse(2false) value of ˜0.78 × 10 −18 m 2 /V at 2.79 eV.…”

“…Ultimately, this approach has a capability of a single-shot measurement. It does not require point-by-point laser wavelength tuning as well as the associated adjustments and recalibrations that lead to significant signal variations [6] thus limiting ones ability to detect fine and important spectral features. Therefore it significantly enhances precision and sensitivity of the dispersion measurements in the spectral domain.…”

“…We used KTP crystal with a nonlinearity of χBfalse(2false)=1.98 pm/ V and obtained SHG data from it simultaneously. The optical nonlinearity of the single layer sheet can be estimated using the following formula [6]:||χ2D()2=32πcΘ2()nB+13nBωρωξω||χBfalse(2false)

Fig. 1Schematically shown photon diagram for second harmonic generation (SHG) process for a typical bandstructure of a semiconductor with large exciton effect.

…”

“…In the expression above, ρω=S2D/SB, SB represents the SH signal from the reference bulk material at optical frequency 2ω , n B represents the effective refractive index of the bulk material, and Θ∥ ∥ ∥ ∥the numerical aperture of the incident light focusing objective lens, and ξω -factor is explained in Ref. [6]. Thus, obtaining the ratio (ρω) from the experiment provides both absolute scale for the optical nonlinearity measurements and the removal of measurement artifacts.…”