Ti/TiO2/SiO2 multilayer thin films with enhanced spectral selectivity for optical narrow bandpass filters

Dongju Kim,Kyoung Ryeol Park,Kyu-Bong Jang,Deahyeon Ko,Junho Lee,Sungwook Mhin,Jong Cheol Kim,Kang Min Kim,Jennifer Sue Forrester,Seung Hwan Lee,Dongwon Kim,Hyuksu Han

doi:10.1038/s41598-021-03935-z

Abstract

Thin film-based optical sensors have been attracting increasing interest for use in developing technologies such as biometrics. Multilayered dielectric thin films with different refractive indices have been utilized to modulate the optical properties in specific wavelength bands for spectral selectivity of Thin Film Narrow Bandpass Filters (TFNBFs). Progress in TFNBF design has been made with the incorporation of metallic thin films. Narrower bandwidths with higher transmittance have been achieved in specific spectral bands. In this work, Ti/TiO2/SiO2 based multilayer thin films were prepared using pulsed-DC reactive sputtering. Computer simulations using the Essential Macleod Program allowed the optimal number of layers and thickness of the multilayer thin films to be determined to efficiently tailor the optical path transmitting specific wavelength bands. The addition of Ti metal layers within dielectric (TiO2/SiO2) multilayer thin films significantly changes the cutoff frequency of transmittance at specific wavelengths. Representative 26 multilayer films consisting of Ti, TiO2, and SiO2 show lower transmittance of 10.29% at 400 nm and 10.48% at 680 nm. High transmittance of 80.42% at 485 nm was observed, which is expected to improve the spectral selectivity of the TFNBF. This work provides a contribution to future simulation based design strategy based on experimental thin film engineering for potential industrial development opportunities such as optical biometrics.

Highlights

Thin film-based optical sensors have been attracting increasing interest for use in developing technologies such as biometrics
A light is emitted on a face or fingerprints, and is reflected back to the bandpass filter eliminating ambient light, which transmits the light with high signalto-noise ratio in the desired spectral band to a detector
Surface plasmon polaritons excited by light propagates along the metal surface and decay exponentially at the interface between metal and dielectric thin films, which can lead to transmittance loss of the Thin Film Narrow Bandpass Filters (TFNBFs) at specific wavelengths[13]

Summary

Introduction

Thin film-based optical sensors have been attracting increasing interest for use in developing technologies such as biometrics. Multilayered dielectric thin films with different refractive indices have been utilized to modulate the optical properties in specific wavelength bands for spectral selectivity of Thin Film Narrow Bandpass Filters (TFNBFs). The addition of Ti metal layers within dielectric (TiO2/SiO2) multilayer thin films significantly changes the cutoff frequency of transmittance at specific wavelengths. A light is emitted on a face or fingerprints, and is reflected back to the bandpass filter eliminating ambient light, which transmits the light with high signalto-noise ratio in the desired spectral band to a detector. Stoichiometric and microstructural engineering of the TiO2 and SiO2 layers further increases differences in the refractive index, which may alter transmittance at specific wavelengths[5,6,7,8]. The introduction of a metal layer within dielectric layers results in unique optical properties including high visible transmission, near-infrared heat shielding, and reflective filtering[17,18]

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Results

Conclusion