Microfluidic research summary

Droplet based microfluidics

Published on 26 February 2024

Exploring Novel Optofluidic Lasers: Smectic vs. Soap Bubble Approaches

The present work performed at the Condensed Matter Department at J. Stefan Institute, describes how the researchers demonstrated for the first time that soap bubbles can be used as lasers.

The research summary is based on the peer-reviewed article “Smectic and soap bubble optofluidic lasers” by Zala Korenjak and Matjaž Humar. Their work was published here.

Abstract

Soap bubbles, composed of thin films of water and surfactants, and smectic bubbles made from smectic liquid crystals. Smectic bubbles, with quantized film thickness due to well-defined molecular layers, possess unique stability and infinite lifetime under constant air volume conditions. While whispering gallery modes (WGMs) have been extensively studied in solid microcavities, such as glass microbubbles and capillaries, they have not been explored in soap bubbles until now. This research demonstrates that dye-doped soap and smectic bubbles can support WGM lasing, leveraging their fluid nature for soft, optofluidic resonators with potential unique applications.

Introduction| Whispering gallery modes in soap bubbles

A soap bubble is made of a thin film composed of water and surfactants, which encloses air and forms a spherical shape. However, a bubble can also be made purely from surfactantlike molecules, such as smectic liquid crystals, which offer intriguing properties relevant to various fields, including mathematics, physics, and chemistry [1]. These bubbles have been studied extensively for their interference colors [2], geometry [3], fluid dynamics [4], and mechanical oscillations [5].

The smectic liquid crystal molecules form well-defined layers, which makes their thickness stable and enables virtually infinite bubble lifetime as long as the air volume inside the bubble is kept constant [6].

Optical resonances called whispering gallery modes (WGMs) are formed when the light is trapped in a spherical object due to multiple total internal reflections and circulates near the sphere’s surface. WGMs were studied in various geometries, including solid hollow cavities in the form of glass microbubbles [7] and capillaries [8]. However, WGMs have not been studied in soap bubbles until now.

Aims

To establish the feasibility of utilizing dye-doped soap or smectic liquid crystal bubbles as optical cavities for whispering gallery mode lasing.
To explore the potential of bubble lasers for highly sensitive measurements.
To investigate the versatility of soap bubble lasers as microcavities.

Experiment Setup | Lasing of soap bubbles and smectic bubbles

Approach

Soap bubbles were made using a mixture of water, glycerol, and liquid hand soap or Sodium dodecyl sulfate dissolved in water in a 2:1:1 volume ratio (n=1.364). Doped with 0.1% of fluorescein sodium salt.
Another type of bubbles, smectic bubbles, were created using 8CB liquid crystal doped with 0.2% pyrromethene 597.
Fluorescent dyes were used to generate and amplify the laser light in the bubble. For each type of bubble we used different dye – fluorescin in soap bubbles and pyrromethene 597 in smectic bubbles.
Plastic pipette tips (Eppendorf) of various sizes (10, 20, and 100μl) served as capillaries to inflate the bubbles.
A glass syringe connected to a microfluidic syringe pump or pushed by hand was used to inflate the bubbles.
Electric field tuning was done by placing the bubble between two flat electrodes with an area of 18x20mm and a spacing of 5.46mm.
For pressure measurements, the pipette tip was inserted through a hole into a transparent container connected to a pressure controller (Elveflow, OB1, 0–20kPa).
Bubbles were observed under an inverted microscope through a 4x, 0.13 NA objective.
A nanosecond pulsed optical parametric oscillator pumped the bubbles at specific wavelengths at a repetition rate of 20 Hz.
The resulting fluorescent light was captured by a high-resolution spectrometer at 0.007nm spectral resolution.

Materials

Water
Glycerol
Liquid hand soap (containing sodium laureth sulfate)
Sodium dodecyl sulfate
Fluorescein sodium salt
8CB liquid crystal
Pyrromethene597 (Exiton)
Plastic pipette tips (Eppendorf) of different sizes (10 μl, 20 μl, and 100 μl)
Glass syringe (Hamilton, 1.0ml)
Microfluidic syringe pump (New Era Pump Systems, NE-1002X)
Thin tube
Flat electrodes (with one electrode having a hole with a diameter of 2.6 mm)
Transparent container
Pressure controller (Elveflow, OB1, 0–20kPa)
Inverted microscope (Nikon Ti2) with a 4x, 0.13 NA objective
Nanosecond pulsed optical parametric oscillator (Opotek, Opolette 355)
High-resolution spectrometer (Andor Shamrock SR-500i)

Key Findings | Soap bubbles laser

Lasing of regular soap bubbles

Millimeter-sized soap bubbles containing a fluorescent dye were generated at the tip of a capillary and illuminated with a pulsed laser, resulting in laser emission. The bubbles were created by briefly increasing the air pressure in the capillary and could be adjusted in size between 0.4 and 4 mm. The soap film thickness, typically ranging from 100 to 800 nm, was determined based on interference colors observed in reflection, and the fluorescence intensity was relatively uniform except at the capillary’s end, where the dye concentration was higher.

a-soap-bubble-formed-at-the-end-of-a-capillary — Figure 1: A soap bubble formed at the end of a capillary. (a) Scheme of the experimental configuration. A dye-doped soap bubble is inflated at the end of a horizontal capillary and illuminated by a laser from below. The soap film comprises a layer of water, surfactant molecules, and fluorescent dye molecules. (b) A soap bubble in reflected light. Interference colors are visible. (c) Fluorescence image of a dye-doped bubble.

When a bubble was exposed to a pulsed laser, it emitted laser light due to the circulation of whispering gallery modes (WGMs). Pumping the center of the bubble caused laser light to radiate in all vertical planes except those intersecting with the capillary, creating a bright rim around the bubble, except on the side opposite to the capillary. Alternatively, illuminating the bubble at its edge resulted in the preferential light circulation in one plane, forming a bright narrow ring.

Lasing was successfully demonstrated in soap bubbles floating freely on the surface of CO2 within a tank. Illumination of the bubble with a pump laser, facilitated by an optical fiber and lens setup, resulted in laser light emission. Spectra obtained from the emitted light, collected using the same lens and optical fiber, exhibited distinct spectral lines indicative of lasing.

Free-floating-bubbles — Figure 3: Free-floating bubbles. (a) Experimental setup for the free-floating soap bubbles. (b) A photograph of a larger floating soap bubble, which is illuminated by the pump laser, and emits laser light. (c) Spectrum from a∼2-cm-diameter free-floating bubble.

Lasing of smectic bubbles

Smectic liquid crystal bubbles, made of 4’-octyl-4-biphenylcarbonitrile (8CB), were used to enhance control over film thickness and refractive index. These bubbles appeared uniformly under transmitted light, suggesting consistent molecular layer thickness across the surface. Variations in thickness would manifest as visible islands or holes, implying precise control mechanisms. Typically, bubble thickness ranged from 30 to 120 nm, determined by transmitted light intensity and influenced by the inflation rate.

When pulsed laser pumped the dye-doped smectic bubbles, they emitted laser light characterized by a visible ring and two bright spots. These bubbles exhibited remarkable stability, allowing experiments for up to 30 minutes per bubble. Due to the birefringence of the smectic film, only TM₀ and TE₀ modes existed for typical bubble thicknesses (30–120 nm), with TE₀ mode dominating lasing at the whole observed region (30-120nm) thickness due to its significantly larger effective refractive index (1.049 at 50nm thickness) compared to TM0 mode (1.014 at 50nm thickness).

Lasing-of-smectic-bubbles — Figure 5: Lasing of smectic bubbles. (a) Image of a lasing smectic bubble. The cross indicates the pump laser beam position. (b) Effective refractive indices for different modes calculated for a slab waveguide at the wavelength of 610 nm. The shaded area corresponds to the approximate thickness range of the bubbles used in the experiments. The bulk refractive indices of TE and TM modes are different (dashed lines). (c) Frequency-comb-like spectrum of a lasing smectic bubble 1.75 mm in diameter.

The soft and stretchable nature of smectic bubbles, combined with their resistance to material fatigue, eliminates the need for calibration and mitigates mechanical property issues, highlighting their remarkable potential in sensing applications, particularly in fields requiring precise and reliable sensing capabilities. Control over additional volume in the capillary enhances pressure measurement capabilities, further emphasizing the versatility and performance of smectic bubble lasers in sensing applications.

Conclusion | Soap bubble lasers for sensing applications

Lasing in soap and smectic bubbles was successfully demonstrated, showcasing their unique optical and mechanical properties owing to their thin fluid film structure. These bubbles offer easily adjustable size and stable and uniform thickness down to a molecular level, enabling applications in sensing electric fields and pressure with exceptional sensitivity, as well as potential future applications in studying phenomena like cavity optomechanics and measurement of other quantities affecting bubble shape.

References

Clark, N.A., et al., Realization of hydrodynamic experiments on quasi-2D liquid crystal films in microgravity. Advances in Space Research, 2017. 60(3): p. 737-751.
Afanasyev, Y.D., G.T. Andrews, and C.G. Deacon, Measuring soap bubble thickness with color matching. American Journal of Physics, 2011. 79(10): p. 1079-1082.
Taylor, J.E., The Structure of Singularities in Soap-Bubble-Like and Soap-Film-Like Minimal Surfaces. Annals of Mathematics, 1976. 103(3): p. 489-539.
Meuel, T., et al., Intensity of vortices: from soap bubbles to hurricanes. Scientific Reports, 2013. 3(1): p. 3455.
Kornek, U., et al., Oscillations of soap bubbles. New Journal of Physics, 2010. 12(7): p. 073031.
Stannarius, R. and C. Cramer, Self-supporting bubbles of thermotropic smectic liquid crystals. Europhysics Letters, 1998. 42(1): p. 43.
Watkins, A., et al., Single-input spherical microbubble resonator. Optics Letters, 2011. 36(11): p. 2113-2115
Shopova, S.I., et al., Optofluidic ring resonator based dye laser. Applied Physics Letters, 2007. 90(22).

https://doi.org/10.1103/PhysRevX.14.011002

Want to run a similar experiment? Feel free to contact us at: contact@elveflow.com