Fabrication of High-Quality Whispering Gallery Mode Microbubble Resonators

Summary

Here, we demonstrate a robust and standardized protocol for fabricating high-quality factor (Q-factor) Whispering Gallery Mode (WGM) Microbubble resonators (MBRs) with a precision glass processing machine (PGP).

Abstract

We demonstrate a robust and standardized method for the fabrication of high-quality factor (Q-factor) Whispering Gallery Mode (WGM) Microbubble resonators (MBRs) with a precision glass processing machine (PGP). Microbubble resonators are a unique class of WGM devices with integrated fluidic channels, making them ideal for diverse sensing applications. Herein, we show a standardized protocol to fabricate high-Q microbubble resonators through the optimization of key performance metrics, such as Q-factor and wall thickness. We also show methods to improve the sensitivity of the platform to refractive index changes and other sensing targets through Hydrofluoric acid (HF) wet etching. Lastly, a brief analysis of the resistance of microbubbles to fluid flow is discussed, showing that smaller-diameter microbubbles exhibit greater resistance to flow for analyte delivery - a factor that should be considered for analyte delivery. The implementation of this refined fabrication protocol not only increases the success rate of device production but also reduces fabrication time. Moreover, the protocol can be expanded to other techniques used to produce MBRs, such as CO₂ laser-based methods.

Introduction

Whispering Gallery Mode (WGM) microresonators are a class of optical sensors that have demonstrated enormous potential not only for the detection of single molecules and nano-particles^1,2,3,4,5,6 but also for sensing a wide range of physical phenomena such as magnetic⁷ and electric fields⁸, temperature⁹, and ultrasonic waves^10,1....

Protocol

1. Microbubble fabrication

1. Start with a polymer-coated silica glass capillary (250 μm inner diameter and 360 μm outer diameter) that is ~75 cm in length. The length of the capillary can vary by user needs; ensure that the pressure described below is reached at longer capillary lengths.
2. Burn off ~2.5 cm of the polymer coating at one end of the capillary with a butane torch and clean the end with a delicate task wipe and isopropyl alcohol (IPA).
3. Place the clean end of the capillary in the PGP and click the splice only button on the software to heat for 5 s at 180 W to seal the end of the capilla....

Results

A representative MBR fabricated with the PGP machine is shown in Figure 1C. Given our starting capillary outer diameter (OD) of 360 μm, we expand the capillary ~2x in the fabrication process. Expanding the capillary to ~700 μm results in wall thicknesses between 5 μm and 15 μm. It has been shown that the optimal wall thickness for biosensing with MBRs is on the order of the wavelength of light used to excite the WGM²⁷. MBRs can theoretically achieve a quality factor o.......

Discussion

Here, we described the protocol to fabricate high-quality whispering gallery mode (WGM) microbubble resonators (MBRs) using a precision glass processor. We present critical steps in the fabrication protocol, including the heat and expand steps. Here, a combination of overheating, heating too long, or injecting too much internal air pressure can lead to unsuccessful fabrication. To address these issues, adjustments such as lowering the heating power or heating duration in the software user interface of the PGP machine can.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Blunt tip to luer lock adapter	Ellsworth Adhesives	8001286
Gas-tight syringe	Hamilton	81520
Luer Lock to 360 µm adapter	IDEX	p-662
Silica Capillary	BGB Analytik	TSP250350
Syringe Pump	Universal	na
UV Glue	Amazon	B09H7BJKT1
Vytran Glass Processor	Thorlabs/Vytran	GPX3000	PGP instrument with software