Fabrication of High-Quality Whispering Gallery Mode Microbubble Resonators

Riepilogo

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.

Introduzione

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^10,11. Under optical resonance conditions, light is trapped within the device, leading to a significant power amplification^12,13. Any localized change to the resonator (such as the binding of a biomolecule or changes in the refractive index of the surrounding media) induces changes in the local optical environment, therefore shifting the resonant frequency or wavelength. By monitoring the shifts in resonance wavelength or frequency, one can detect and characterize analytes in real time.

WGM microresonators can be designed in a variety of geometries. Common geometries include but are not limited to, microtoroids¹⁴, microrings¹⁵, and microbubble¹⁶ resonators (MBR). Here, we focus on MBRs due to their great potential in optofluidic sensing applications. A key advantage of MBRs is their fluidic integration^17,18,19,20, which is enabled by the fabrication of the device from a microcapillary. In this design, the inline capillary facilitates the easy delivery of small volumes (i.e., microliters) of analytes in solution to the sensing area without the need for external fluidic channels, as shown in Figure 1. With their unique fluidic handling capabilities, MBRs are well suited for a wide range of sensing applications that are not easily achievable with other WGM platforms. For example, MBRs have been filled with magnetic fluids, thereby imbuing sensitivity to external magnetic fields²¹. Additionally, MBRs have also been used to control the specific orientation of gold nanorods in solution through optical torques²².

The fabrication of MBRs can be summarized as follows: Aerostatic pressure is applied inside the capillary while a small area of the capillary is locally heated. The combination of localized heating and internal pressure inflates the heated section into a spherical geometry capable of supporting high-Q WGMs, as illustrated in Figure 2. Various methods can be employed to achieve localized heating of the capillary, such as using a CO₂ laser²³, a fiber optic splicer²⁴, a hydrogen flame source²⁵, and a precision glass processing machine (PGP). The methods presented here can be expanded to other heating sources, including a CO₂ laser. The PGP is similar to an optical fiber splicer but offers enhanced control over heating time, power setting, and the positioning of fibers or capillaries²⁶. PGPs often include built-in microscopes adjacent to the heating elements, enabling real-time monitoring of the fabrication process. Typically, light from a tunable diode laser is coupled into the MBR via a tapered optical fiber that is in contact with the equator of the MBR. The fiber is tapered (to ~1 μm) to enable efficient coupling of light into and out of the MBR. The resulting transmission spectra from the MBR are then captured by a photodetector through the optical fiber and visualized on an oscilloscope.

Sensing with WGM MBRs relies on the interaction of the WGM field with the target analyte. The strength of this interaction is directly proportional to the fraction of the WGM field that penetrates the MBR's hollow cavity where liquid or gas phase samples can flow through²⁷. As shown in Figure 3, COMSOL simulations illustrate how the penetration of the WGM field into the inner cavity varies with the MBR's wall thickness. Maximum field penetration of the WGM field occurs as the wall thickness is reduced to less than 1 μm, with these simulations conducted using light in the 780 nm band. Achieving such reduced wall thickness through the standard heat-and-inflate fabrication protocol alone is challenging. To further thin the walls of the MBR and to make the device more sensitive, we incorporate additional wet etching steps using Hydrofluoric (HF) acid.

Using a PGP, we will focus on the fabrication of MBRs in line with a silica capillary. A detailed description of the fabrication process and methods to enhance sensitivity to refractive index changes through wet etching will also be presented.

Protocollo

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 capillary. Set the heating time and duration with a right-click on the splice only button.
4. To ensure a proper seal, examine the sealed capillary end, Figure 1B.
5. At ~25 cm from the sealed end (or 1/3 the capillary length from the sealed end), burn off a ~2.5 cm long strip of polymer coating from the capillary and clean the resulting area with IPA.
   NOTE: The position described above is not critical as pressure is evenly distributed along the capillary length. If the user's experimental setup warrants a different placement of the MBR, this should not affect the rest of the procedure.
6. Repeat this step until no polymer is visible under the imaging system, which displays capillary image with sub-micron resolution.
7. Place this newly cleaned section of the capillary in the glass processor instrument over the heating element and under the microscope described earlier.
8. Using a gas-tight syringe and a syringe pump, inject air into the capillary such that the internal pressure reaches 10 bar (i.e., decreasing the volume in a syringe from 5 mL to 0.5 mL, with corresponding pressures calculated with Boyle's Law). Fit the capillary to a syringe with a luer lock to a 360 μm adapter. Alternatively, apply pressure via a pressure regulator and compressed air/inert gas.
9. Heat the capillary with the PGP using a filament power of ~100-110 W for 1.5-2 s.
10. Conduct additional heating steps with 90-100 W and 0.1-1 s firings of the filament. This method will slowly and controllably increase the MBR's diameter to the desired range. However, this process may result in asymmetric MBRs. If an asymmetric MBR is observed (the sphere is not symmetric about the capillary axis), rotate it about the capillary axis between successive heating stepsto promote symmetry.
11. Inspect the MBR under a microscope for quality control, looking for things such as dust, cracks, or deformations. Additionally, use the microscope here to estimate the wall thickness of the MBR.

2. Wet etching with hydrofluoric acid

CAUTION: Hydrofluoric acid is very dangerous, toxic, and corrosive. Calcium gluconate should be kept nearby as this chemical can neutralize hydrofluoric acid. Wear appropriate personal protective equipment and follow all safety precautions in the Material Safety Data Sheet (MSDS).

1. Remove the sealed end of the capillary by cutting 2 cm below the sealed end.
2. Place one capillary end inside a blunt tip to the luer lock adapter. Make sure the blunt tip's inner diameter is as close as possible to the outer diameter of the capillary while making sure the capillary can still fit inside.
   NOTE: The adapter material must not be reactive with HF acid.
3. Apply UV curable epoxy to the end of the blunt tip with the capillary inside to join the two, then cure with UV light (50-100 W UV light source; higher wattage will speed up the curing).
   CAUTION: Appropriate eye protection must be worn when operating a UV light source²⁸.
4. Once the epoxy is cured, attach the luer lock tip to a 25 mL syringe that is inert to HF.
5. Place the syringe in a syringe pump with a withdrawal rate of 50 μL/min.
6. With one end of the capillary connected to the syringe, place the other end of the capillary into deionized (DI) water.
7. Set the syringe pump to withdraw water through the capillary into the syringe to check that proper flow has been established (i.e., the system is free from air bubbles with liquid consistently flowing through the capillary).
8. Transfer the capillary end from the DI water to a container of HF acid once proper flow is established.
9. Calculate an approximate etching time based on the MBR wall thickness before etching, considering a measured etching rate of 8.18 μm/h with 12% (w/v) hydrofluoric acid. Wall thickness can be measured under an imaging system that displays MBRs with sub-micron resolution.
   NOTE: The etching rate for acids of other concentrations can be empirically determined. This can be done by measuring the capillary wall thickness before and after a set etching time.
10. Run the syringe pump at 50 μL/min for the calculated etching time.
11. Rinse the capillary with DI water for 10-15 min to remove all acid from the capillary after the etching time has elapsed. Calculate the rinsing time to rinse the entire capillary volume 3-5 times.
12. Measure the MBR wall thickness after etching. Use this information to update the etching rate for the acid.
13. Repeat this process can be repeated until the desired wall thickness is achieved.

Risultati

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 achiev...

Discussione

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...

Materiali

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