Smectic and Soap Bubble Optofluidic Lasers

Soap bubbles are simple, yet very unique and marvelous objects. They exhibit a number of interesting properties such as beautiful interference colors and the formation of minimal surfaces. Various optical phenomena have been studied in soap films and bubbles, but so far they have not been employed as optical cavities. Here we demonstrate that dye doped soap or smectic liquid crystal bubbles can support whispering gallery mode lasing, which is observed in the spectrum as hundreds of regularly spaced peaks, resembling a frequency comb. The lasing enabled the measurement of size changes as small as 10 nm in a millimeter-sized, $\sim 100\text{−}\mathrm{nm}$-thick bubble. Bubble lasers were used as extremely sensitive electric field sensors with a smallest measurable electric field of $110{\mathrm{Vm}}^{-1}{\mathrm{Hz}}^{-1/2}$. They also enable the measurement of pressures up to a 100 bar with a resolution of 1.5 Pa, resulting in a dynamic range of almost ${10}^7$. By connecting the bubble to a reservoir of air, almost arbitrarily low pressure changes can be measured while maintaining an outstanding dynamic range. The demonstrated soap bubble lasers are a very unique type of microcavities which are one of the best electric field and pressure microsensors to date and could in the future also be employed to study thin films and cavity optomechanics.

Popular Summary

A soap bubble is a thin film composed of water and surfactants, which encloses air and forms a spherical shape. Soap bubbles exhibit many interesting properties such as beautiful interference colors and the formation of minimal surfaces. But one aspect that has not yet been explored is their potential ability to act as a type of optical cavity, circulating light that is trapped within. Here, we do just that and demonstrate for the first time that soap bubbles can be used as lasers.

In our experiments, we dope soap bubbles—attached to a tube or free floating in a container—with a fluorescent dye and pump them with an external laser. The generated light circulates in the wall of the bubble, which consequently works as a laser cavity. We also make bubbles from surfactantlike molecules without any water present. Such bubbles have a completely uniform thickness with an integer number of molecular layers. Most importantly, they are extremely stable and can in principle survive indefinitely. In both types of bubbles, we observe whispering gallery mode lasing as sharp peaks in the spectrum of the emitted light. Shifts in the lasing wavelengths reveal subtle changes in the bubble size as small as 10 nm. This incredible precision allows us to use the bubbles as one of the best pressure and electric field sensors.

Because of their unique properties, soap bubble lasers could in the future be used as tunable laser sources and extremely sensitive sensors.

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 is composed of 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.

Figure 2

Soap bubbles emitting laser light. (a) A soap bubble illuminated in the center (red cross) by a pump laser. The drawing shows the gain regions (red) which are formed at the positions where the laser beam (green) passes through the soap film. The WGMs circulating in any vertical plane (black lines) pass through the two gain regions and are therefore excited. (b) Another bubble but illuminated at its rim, which generates a bright ring of circulating light. The gain region in this case is in the shape of a vertical patch. WGMs circulating in a narrow band in a vertical plane experience the most gain and are therefore observed as a bright ring. (c) A typical emission spectrum when a bubble is pumped above the lasing threshold. (d) Lasing intensity summed over a 5-nm-wide spectrum range as the input laser pulse energy was increased shows a typical threshold behavior. The dashed lines are a guide to the eye. (e) Spectrum emitted by a soap bubble attached to the end of a capillary when the pulse energy of the pump laser is increased. At lower energies only fluorescence is observed, while at higher energies sharp peaks appear. (f) Slab waveguide modes for a soap film at the wavelength of 555 nm. The shaded area corresponds to the approximate thickness range of the bubbles used in the experiments. (g) Spectra of consecutive pump laser pulses. Displaying a smaller wavelength range reveals a shift of the modes toward longer wavelengths.

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 $\sim 2\text{−}\mathrm{cm}$-diameter free-floating bubble.

Figure 4

Smectic bubbles. (a) A smectic bubble in transmitted light. (b) Scheme of the molecular structure of the film. Drawn is an example of a three layer smectic film composed of ordered liquid crystal and dye molecules. (c) Same bubble under crossed polarizers. The typical bright cross is observed indicating uniform molecular orientation either parallel or perpendicular to the bubble surface. (d) Same bubble with an additional wave plate inserted between the polarizers. From the resulting colors it can be deduced that the molecules are oriented perpendicular to the surface, that is, in the bubble radial direction.

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.

Figure 6

Lasing of smectic islands. (a) Two smectic islands (indicated by the arrows) on a bubble. (b) Image of a lasing island observed as a red ring (white arrow) on the surface of the bubble (indicated by the red arrows). (c) Lasing spectra of two smectic islands of a different diameter $d$.

Figure 7

Measurement of smectic bubble size. (a) Lasing spectrum from a 1.75-mm smectic bubble in time. The color represents the intensity. The white line highlights the shifting of a single lasing mode in time due to the decreasing size of the bubble. (b) Change in the diameter of a bubble in time measured by following the lasing peaks in (a). The initial size was measured from the FSR. The bubble size was slowly decreasing due to air diffusion through the thin wall. Since the size measurement is very precise, the plot looks like a perfectly smooth line. (c) When the decrease in size is subtracted from the data, only an extremely small noise remains.

Figure 8

Electric field measurement with smectic bubble lasers. Shift of the lasing modes when an electric field is increased continuously to $30\mathrm{V}/\mathrm{mm}$ and back to zero. The continuous decrease in the size due to diffusion was subtracted from the data.

Figure 9

Pressure measurement with smectic bubble lasers. (a) Change of the bubble size measured from the lasing spectrum as the pressure outside the bubble was increased by 400 Pa above the atmospheric pressure. The volume of the capillary in this experiment was $80\mathrm{\mu }\mathrm{l}$ and the initial volume of the bubble was $4.2\mathrm{\mu }\mathrm{l}$, resulting in $V_c/V_b+1\approx 20$ for this particular case. (b) The calculated pressure sensitivity of a 2-mm bubble as a function of total volume relative to the volume of the bubble ($V_c/V_b+1$). The dotted line would be for a bubble with zero surface tension. The red circle in all panels represents the maximum $V_c$ used in the experiments. (c) Critical additional volume relative to the volume of the bubble, at which the bubble becomes unstable. Above this additional volume the bubble collapses due to the Laplace pressure. (d) Maximum measurable positive and negative pressure. (e) The minimum measurable pressure change. (f) The dynamic range for positive pressure change, that is, the ratio between the highest and the lowest measurable pressure changes.