image: Figure 1 (a) Photograph of a freestanding vacuum-filtrated h-BN thin film. (b) Transmission electron microscopy image of ball-milled h-BN nanosheets. (c) High-resolution transmission electron microscopy image of five-layer h-BN nanosheets. view more
Credit: OEA
A new publication from Opto-Electronic Science; DOI 10.29026/oes.2022.210013 overviews a new approach to tuning the optical third-order nonlinearity of boron nitride.
Hexagonal boron nitride (h-BN) is a two-dimensional (2D) layered wide-bandgap insulating material. Its structure is similar to that of graphene. In addition to the characteristics of ordinary 2D materials, such as planar 2D structure, atomic level flatness, no dangling bonds, etc., h-BN also has excellent mechanical, chemical and thermal stability, so it can be applied in ultraviolet lasers/detectors, near-field optics/imaging, protective coating materials, dielectric layers, tunneling layers. Furthermore, it has a wide range of applications in the field of nonlinear optics.
Currently, although h-BN can be prepared by chemical vapor deposition (CVD), metal-organic vapor phase epitaxy (MOVPE), pulsed laser deposition (PLD), and mechanical peel-off and liquid-phase peel-off. Although the h-BN films and solutions prepared by these methods have been reported to have strong optical nonlinearity, such films deposited on fixed substrates and exfoliated liquid-phase h-BN materials are not suitable of integrated functional devices, which also greatly hinders the application of h-BN in the field of all-optical communications.
Recently, Prof. Baohua Jia's group from the Centre for Translational Atomaterials at Swinburne University of Technology has developed a method for laser-tunable third-order optical nonlinearity of h-BN. Functionalization is an important way to endow materials with new properties and lead to new applications. It is an effective way to enhance the chemical flexibility of h-BN materials by adding atomic-level chemical bonds or functional groups. In this paper, a one-step ball milling method for processing commercially purchased h-BN powders is proposed, and the edge modification of h-BN with -NH2 functional groups during the grinding process can effectively enhance its solubility. The prepared h-BN solution can be filtered by vacuum filtration method to obtain a freestanding h-BN thin film. By controlling the amount and concentration of the solution, the thickness of the h-BN film can be precisely controlled in the order of hundreds of nanometers to micrometers (as shown in Figure 1). This is also the foundation for building integrated optoelectronic devices.
It has been reported that when h-BN material is heated to 800-900 °C at high temperature, h-BN will be oxidized to B2O3. This provides a new idea for the further processing and application of h-BN materials. However, this high-temperature method is obviously not suitable for the fabrication of high-precision, high-reliability optoelectronic devices. The femtosecond laser processing technology that has emerged in recent years is able to solve this challenge. The research group used 800 nm low-repetition-rate femtosecond laser to directly fabricate 50 µm × 50 µm micropatterns on the h-BN film (as shown in Figure 2), creating an ultra-fine low damage method to manufacture arbitrary h-BN structures.
As an emerging photonics material, h-BN has been applied in near-field optical imaging, hyperbolic lenses, and nonlinear optical devices. This article proposes that femtosecond laser direct writing is an effective way to achieve high-precision processing on h-BN materials. On this basis, the changes in chemical properties and optical properties of materials before and after femtosecond laser processing are studied. In the laser irradiation area, the peak position of B-O bond was found on both the Fourier Transform infrared (FTIR) spectrum and Raman spectrum of h-BN, indicating that the oxidation reaction of the material occurred during the laser direct writing process, resulting in the redshift of linear absorption peak of the material from 209 nm to 500 nm. The band gap of the material changes from 3.8 eV to 3.1 eV, the linear refractive index in the visible region also decreases from 2.0 to 1.7 (at 800 nm), and the extinction coefficient also increases to 0.2. The authors also measured the change of the third-order nonlinear optical properties of the material before and after oxidation by Z-scan method. The change of the chemical bond hybridization of h-BN caused by the oxidation reaction, the carrier transport characteristics and thermal lensing effect lead to the increase of its nonlinear refractive index from -0.0143 cm2/GW to 0.1638 cm2/GW, and the third-order nonlinear magnetic susceptibility increases by 20 times (as shown in Figure 3), which promotes the application of h-BN materials in the fields of nonlinear optics such as optical switches, wavelength converters and signal repeaters, and realizes the control and operation of light at the micro/nano scale. At the same time, nonlinear optical components can be processed in one step through laser processing, which has broad application prospects.
Article reference Ren J, Lin H, Zheng XR, Lei WW, Liu D et al. Giant and light modifiable third-order optical nonlinearity in a free-standing h-BN film. Opto-Electron Sci 1, 210013 (2022). doi: 10.29026/oes.2022.210013
Keywords: hexagonal boron nitride / third-order nonlinearity / laser oxidation / optoelectronic device
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The research team is from the Centre of Translational Atomaterials at Swinburne University of Technology. The research fields include the design and optical characterization of novel nanostructures and nanomaterials, fabrication, efficient conversion and storage of light energy, and ultrafast all-optical communication for high-speed communication. In addition, the Centre works on technological developments, such as laser imaging, ultrafast spectral analysis, and ultrafast laser processing for advanced intelligent manufacturing. Professor Baohua Jia is the corresponding author of this research work, and Dr. Jun Ren is the first author of the paper. This research was supported by the Australian Research Council Discovery Project scheme (Grant No. DP190103186 and FT210100806) and Industrial Transformation Training Centres scheme (IC180100005).
https://www.swinburne.edu.au/research/translational-atomaterials/
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Opto-Electronic Science (OES) is a peer-reviewed, open access, interdisciplinary and international journal published by The Institute of Optics and Electronics, Chinese Academy of Sciences as a sister journal of Opto-Electronic Advances (OEA, IF=9.682). OES is dedicated to providing a professional platform to promote academic exchange and accelerate innovation. OES publishes articles, reviews, and letters of the fundamental breakthroughs in basic science of optics and optoelectronics.
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