Optical Sensitivity of Waveguides Inscribed in Nanoporous Silicate Framework

Zhong Lijing,Alexander P Alodjants,Vadim P Veiko,Maksim M Sergeev,Andrey B Petrov,Roman A Zakoldaev

doi:10.3390/nano11010123

Zhong Lijing, Alexander P Alodjants + Show 4 more

Open Access

https://doi.org/10.3390/nano11010123

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Abstract

Laser direct writing technique in glass is a powerful tool for various waveguides’ fabrication that highly develop the element base for designing photonic devices. We apply this technique to fabricate waveguides in porous glass (PG). Nanoporous optical materials for the inscription can elevate the sensing ability of such waveguides to higher standards. The waveguides were fabricated by a single-scan approach with femtosecond laser pulses in the densification mode, which resulted in the formation of a core and cladding. Experimental studies revealed three types of waveguides and quantified the refractive index contrast (up to Δn = 1.2·10−2) accompanied with ~1.2 dB/cm insertion losses. The waveguides demonstrated the sensitivity to small objects captured by the nanoporous framework. We noticed that the deposited ethanol molecules (3 µL) on the PG surface influence the waveguide optical properties indicating the penetration of the molecule to its cladding. Continuous monitoring of the output near field intensity distribution allowed us to determine the response time (6 s) of the waveguide buried at 400 µm below the glass surface. We found that the minimum distinguishable change of the refractive index contrast is 2 × 10−4. The results obtained pave the way to consider the waveguides inscribed into PG as primary transducers for sensor applications.

Highlights

We demonstrated that the transmitted laser radiation through the waveguide has a response to ethanol molecules deposited on the porous glass (PG) surface
The femtosecond laser-induced densification approach [32,33] was applied for the inscription of waveguides in PG with an average pore size ~10 nm, and total porosity 26%
The waveguides observed by optical microscopy possess the shape of an elongated ellipse in the cross-section, which is the direct evidence of a filament structure appearance [38]

Summary

Introduction

A nanoporous silicate framework of porous glass (PG) with multiple buried hollow channels and pores with a well-controlled size in the range of 2–20 nm [1,2] represents a promising matrix, which captures, stores, and transports molecules absorbed from the environment [3,4,5,6,7]. For a more accurate analysis of chemical reactions in such a nanoporous medium, it is necessary to inscribe a chip-scale optical channel, namely, a bulk waveguide, which typically consists of a core and cladding to obtain internal reflection of coupled light resulting in a good balance between light localization and optical losses [9]. The combination of a nanoporous silicate matrix and bulk waveguide may provide unique sensing abilities for a nanopore sensor, which operates with single molecules [10,11].

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