# Characterization of the Second Harmonic Generation of Boron Nitride Nanotube Macroscopic Assemblies

## TL;DR

Researchers found that boron nitride nanotube assemblies can produce strong second harmonic light, which could be useful for optical applications.

## Contribution

This study experimentally demonstrates SHG in bulk BNNT assemblies and identifies dip-coating as a preferred method for high SH response.

## Key findings

- SHG was observed in free-standing BNNT sheets and silica-coated BNNTs using 800 nm laser pulses.
- SHG intensity varied with assembly thickness and nanotube alignment.
- Dip-coated BNNTs showed the strongest SH response and polarization dependence.

## Abstract

Boron nitride nanotubes (BNNTs) are predicted to be promising one-dimensional nonlinear optical materials, but to date, only one experimental observation has been made using individual nanotubes. In this work, second harmonic generation (SHG) was achieved from free-standing bulk BNNT sheets and BNNT coatings on silica substrates. Focusing femtosecond infrared (fs-IR) laser pulses with a wavelength of 800 nm onto the BNNT assemblies resulted in strong SHG at a wavelength of 400 nm. It was observed that due to the thickness variation of the BNNT assemblies and orientational alignment of BNNTs in the assemblies, the intensity of the second-harmonic (SH) radiation changed dramatically when different locations on the samples were investigated. Among all the BNNT assemblies tested, the localized SH response and its dependence on the polarization of the excitation fs-IR pulses were the strongest in BNNT coatings produced by a dip-coating process. By measuring the SH response, the uniformity, reproducibility, and efficiency of BNNT deposition processes could be assessed. For applications requiring a high SH response from BNNT assemblies, the process of dip coating is preferred.