Highly Efficient Sum‐Frequency Generation in Niobium Oxydichloride NbOCl₂ Nanosheets

Abstract

An alternative approach that can address the limitations of IR detectors is to upconvert the IR photons into the ultraviolet (UV)/visible (vis) domain, where room temperature-operating photon detectors are far more efficient. [2] The generation of high-frequency light quanta is also of great interest for a variety of applications such as coherent light sources, [3] photo-therapy, [4] time-domain fluorescence spectroscopy, [5] and nanolithography. [6] Frequency upconversion through a nonlinear parametric sum frequency generation (SFG) has been widely utilized to upconvert IR and vis light into blue and UV light. [7] SFG is a three-wave mixing process in which two incident photons of frequencies ω 1 and ω 2 are converted into an SFG photon at their sum frequency ω 3 (ω 3 = ω 1 + ω 2 ). [8] A special case of SFG is the second-harmonic generation (SHG), which involves two input photons of equal frequencies and one output SHG photon at the doubled frequency (ω 1 = ω 2 = ω, Parametric infrared (IR) upconversion is a process in which low-frequency IR photons are upconverted into high-frequency ultraviolet/visible photons through a nonlinear optical process. It is of paramount importance for a wide range of security, material science, and healthcare applications. However, in general, the efficiencies of upconversion processes are typically extremely low for nanometer-scale materials due to the short penetration depth of the excitation fields. Here, parametric IR upconversion processes, including frequency doubling and sum-frequency generation, are studied in layered van der Waals NbOCl 2 . An upconversion efficiency of up to 0.004% is attained for the NbOCl 2 nanosheets, orders of magnitude higher than previously reported values for nonlinear layered materials. The upconverted signal is sensitive to layer numbers, crystal orientation, excitation wavelength, and temperature, and it can be utilized as an optical cross-correlator for ultrashort pulse characterization.

Manipulating Peierls Distortion in van der Waals NbOX2 Maximizes Second-Harmonic Generation

“…Niobium oxide dihalides (NbOX 2 , X = Cl, Br, I) are 2D vdW materials with unique distorted structures, where Nb stays away from the octahedral center and forms two short and two long Nb–X bonds. ,,, Among them, the larger size of I – anions increases the degree of distortion in NbOI 2 , resulting in high polarizability . Distinguished from most 2D vdW materials, whose inversion symmetry breaking is thickness-dependent, NbOI 2 exhibits an intrinsic noncentrosymmetric structure with a space group of C 2, which results in a large second-order NLO response in both monolayer and bulk crystals .…”

Ferroelectric Polarization Enhanced Photodetector Based on Layered NbOCl2

“…According to previous reports, NbOCl 2 crystals have the opportunity to obtain few-layers. [24,25] Here, the few-layer NbOCl 2 were obtained through the liquid exfoliation method. Figure S1, Supporting Information, (left) shows a schematic diagram of the typical preparation method for few-layer NbOCl 2 based on liquid exfoliation method.…”

“…An upconversion efficiency of up to 0.004% provides the possibility for designing optical cross‐correlators for ultrashort pulses characterization. [ 25 ] In addition, Su et al reported that NbOCl 2 has high carrier mobility (≈10 4 cm 2 V −1 s −1 ) and strong light absorption in the visible range, implying great application potential in optoelectronics. [ 26 ] More importantly, the positive and negative charge centers of NbOCl 2 do not coincide, causing spontaneous polarization and generating a built‐in electric field.…”

Third Harmonic Generation in Thin NbOI2 and TaOI2

“…Transition metal oxide dihalides MOX 2 (M = V, Nb, Ta, Mo; X = Cl, Br, I) have recently emerged as new members of 2D van der Waals materials, exhibiting unique physical properties [ 26 , 27 , 28 , 29 , 30 , 31 , 32 ]. Among them, NbOI 2 has garnered special interest for its ferroelectricity and robust second-order optical nonlinearity.…”

“…The structural similarity to NbOI 2 suggests that TaOI 2 may also exhibit substantial nonlinearity, though the SHG should be absent. While second-order phenomena, including SHG, sum-frequency generation (SFG), and spontaneous parametric down-conversion (SPDC) [ 28 , 30 , 31 ] in MOX 2 have been widely studied, observations of the third-order nonlinear process are still lacking.…”