# A multi-color fast-switching microfluidic droplet dye laser

Birgit Hausmann

## Overview

A multi-color microfluidic dye laser operating in whispering gallery mode based on a train of alternating droplets containing solutions of different dyes is presented. The laser is capable of switching the wavelength of its emission between 580 nm and 680 nm at frequencies up to 3.6 kHz.

## Results and Discussion

The different dyes are emitting coherent light at different wavelengths in the visible. They are solute in droplets (20–40 mm in diameter) which are suspended in a fluorocarbon carrier liquid flowing in a polydimethylsiloxane (PDMS) microchannel. The dyes are an excellent gain medium which can be optically excited with a frequency-doubled Nd:YAG laser at 532nm. Since the droplets have a higher refractive index than the carrier liquid the spherical drop can provide a high-Q microcavity if the scattering losses due to surface roughness is minimized and the index contrast is high enough. The relaxation of excited dyes leads to the emission of photons which can couple to whispering gallery modes in the droplets which leads to stimulated amplification of light (lasing). High-speed generation and switching of droplets containing different dyes (frequencies up to 100 kHz, or switching times of 10 microseconds) over a large spectral range (>100 nm) can be performed. Microfluidic channels in PDMS were fabricated via soft lithography. The microfluidic system can be described via two functional parts: droplet generators and region for optical excitation (Fig. 1). Two T-junctions sharing the same main channel for the generation of droplets were used. The alternation of droplets from these two opposing T-junctions was spontaneous at the rates of flow. The main channel widened downstream to ensure that the drops were close to spherical in shape, and to avoid contact with the PDMS wall. Frequencies of generation between 25 Hz and 3600 Hz were used. While the dyes both had an absorption spectra overlapping with the excitation wavelength their emission spectra differ significantly: rhodamine 560 and rhodamine 640. Benzyl alcohol (refractive index n=1.54, viscosity $\mu=$8mPa-s) was used as solvent and HFE-7500 (refractive index n=1.29, viscosity $\mu=$1.24 mPa-s) as the continuous phase. The index contrast between the microsphere drop cavity and the surrounding liquid guides light inside the drop due to total internal reflection coupling to the traveling Whispering gallery cavity modes (WGMs). The number and position of discrete modes is governed by geometry. To characterize the laser quality of drops containing solutions of dyes, we varied the pump pulse energy from 0.01 to 100 $\mathrm{J cm^2}$. The pulse width was about 20 ns. Lasing light from the drops emitted in all directions and was collected with 10X objective normal to the plane of the microchannel (in the z-direction). The drops were actively synchronized with the pump pulses to avoiding drops being unevenly excited at a fixed repetition rate. Fig. 2a shows the output intensities from 36 mm-droplets containing 5-mM solutions of rhodamine 560 (‘‘R560’’) and rhodamine 640 (‘‘R640’’) in benzyl alcohol as a function of input pulse energy. The threshold for lasing was about 1 mJ per pulse. Fig. 2b,c show the lasing spectra from these drops at an excitation pulse energy of 0.25 mJ, compared with their broad fluorescence spectra at input energy below the lasing threshold. Lasing occurred in the longer wavelength regions of the emission spectra of the dyes, where the overlap between absorption and emission spectra of the dyes was smaller. The drops supported multi-mode lasing at the excitation power we used. The emission from droplets of R560 ranged from 582 nm to 594 nm; while that from droplets of R640 ranged from 670 nm to 690 nm. Each mode had a full width at half maximum around 0.3 nm. Fig. 3a shows the switching of lasing wavelengths as a function of time. Lasing was indicated in blue color for R560 (570–595 nm), and in red color for R640 (665–695 nm). The switching time between the two colors was about 30ms. The shift in emission wavelength was approximated as 6 nm for a wavelength of 600 nm. Fig. 4a presents the output from the two photodiodes normalized to the excitation intensity as a function of time. The switching time between the colors was about 0.85 ms, corresponding to a switching speed of 1.18 kHz while Fig. 4b shows a histogram for the distribution of different switching times for data recorded over 1.3 seconds (1558 drops). In movies taken with a fast camera (10 000 frames per second) (Fig. 4b insets), it was observed that the drops traveled in groups of two. The variation in the intensity of lasing (normalized to the pump intensity) was about 3.9% for R560, and 9% for R640, for data recorded for 1558 drops. The switching time between the lasing wavelengths is determined by the speed at which the drops pass through the optical excitation region. The alternation in the generation of the two types of drops from two opposing T-junctions has been characterized using the capillary number, Ca. A maximum switching rate of 2.2 kHz (at Ca=0.05) or 5.7 kHz (at Ca= 0.13) for 20-mm drops was estimated. Experimentally, a maximum switching rate between the two types of drops at 3.63 kHz was achieved, but the generation of the drops was already in the dripping regime. The variation in the lasing intensity from the drops was larger (12% for R560, and 16% for R640).