Laser oxidation – a new approach for tuning the optical third-order nonlinearity of boron nitride

Image: Figure 1 (a) Photograph of a freestanding vacuum-filtered 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.
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Credit: OEA

A new publication from Optoelectronic Science† DOI 10.29026/oes.2022.210013 provides an overview of a new approach for tuning the optical third-order nonlinearity of boron nitride.

Hexagonal boron nitride (h-BN) is a two-dimensional (2D) wide bandgap layered insulating material. Its structure is similar to that of graphene. In addition to the characteristics of common 2D materials, such as 2D planar 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, tunnel layers. In addition, it has a wide range of applications in the field of nonlinear optics.

Currently, though, 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 solid substrates and exfoliated h-BN materials in the liquid phase are not suitable for integrated functional devices, which also greatly hinders the application. of h-BN in the field of all-optical communication.

Recently, the group of Prof. Baohua Jia of the Center for Translational Atomaterials at Swinburne University of Technology developed a method for laser-tunable third-order optical nonlinearity of h-BN. Functionalization is an important way to give materials new properties and lead to new applications. It is an effective way to improve the chemical flexibility of h-BN materials by adding chemical bonds or functional groups at the atomic level. In this article, a one-step ball milling method for processing commercially purchased h-BN powders is presented, and the edge modification of h-BN with -NH2 functional groups during the milling process can effectively improve its solubility. The prepared h-BN solution can be filtered by vacuum filtration to obtain a free-standing h-BN thin film. By controlling the amount and concentration of the solution, the thickness of the h-BN film can be precisely controlled on the order of hundreds of nanometers to micrometers (as shown in Figure 1). This is also the basis 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 clearly not suitable for the fabrication of high precision and high reliability optoelectronic devices. The femtosecond laser processing technology that has emerged in recent years can solve this challenge. The research group used a femtosecond laser with a low repetition rate of 800 nm to fabricate 50 m × 50 m micro-patterns directly on the h-BN film (as shown in Figure 2), creating a low-damage ultrafine method to randomize h-BN structures to be manufactured.

As an emerging photonics material, h-BN has been applied in near field optical imaging, hyperbolic lenses and nonlinear optical devices. This paper proposes that femtosecond laser direct writing is an effective way to achieve high-precision processing on h-BN materials. Based on this, the changes in chemical properties and optical properties of materials before and after femtosecond laser processing are studied. In the laser irradiation region, the peak position of BO binding was found on both the Fourier Transform infrared (FTIR) spectrum and the 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 the material’s linear absorption peak from 209 nm to 500 nm. The bandgap 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 in the third-order nonlinear optical properties of the material before and after oxidation with the Z-scan method. The change of the chemical bond hybridization of h-BN caused by the oxidation reaction, the carrier transport characteristics and the thermal lens effect lead to an increase in the nonlinear refractive index of -0.0143 cm2/GW to 0.1638 cm2/GW, and the third-order nonlinear magnetic sensitivity increases by 20 times (as shown in Figure 3), proving the application of h-BN materials in the fields of nonlinear optics such as optical switches, wavelength converters and promotes signal amplifiers, and realizes the control and operation of light on a micro/nano scale. At the same time, nonlinear optical components can be processed in one step through laser processing, which offers wide application possibilities.

Item reference Ren J, Lin H, Zheng XR, Lei WW, Liu D et al. Gigantic and slightly adaptive third-order optical nonlinearity in a freestanding h-BN film. Optoelectron Science 1, 210013 (2022). bye: 10.29026/oes.2022.210013

keywords: hexagonal boron nitride / third order nonlinearity / laser oxidation / optoelectronic device

The research team is from the Center of Translational Atomaterials at Swinburne University of Technology. Research areas include design and optical characterization of new nanostructures and nanomaterials, fabrication, efficient conversion and storage of light energy, and ultra-fast all-optical communications for high-speed communication. In addition, the center is working on technological developments such as laser imaging, ultra-fast spectral analysis and ultra-fast laser processing for advanced intelligent manufacturing. Professor Baohua Jia is the corresponding author of this research work and Dr. Jun Ren is the lead author of the article. This research was supported by the Australian Research Council Discovery Project scheme (Grant No. DP190103186 and FT210100806) and the Industrial Transformation Training Centers scheme (IC180100005).

Optoelectronic 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 Optoelectronic Advances (OEA, IF=9.682). OES is committed to providing a professional platform to foster academic exchange and accelerate innovation. OES publishes articles, reviews and letters on the fundamental breakthroughs in the fundamental science of optics and optoelectronics.

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