Microfluidic Dye Laser Generates Two Coaxial Wavelengths

Breck Hitz

The lab-on-a-chip concept entails integrating the components of one or more analytic instruments on a single, monolithic chip. Such devices are of enormous value in chemical — and, especially, biological and medical — analysis. They are portable, can be operated in remote locations and require only minute samples. Photonic techniques are well suited for integration into these tiny instruments, and micro-fluidic lasers have proved to be especially useful.

Microfluidic lasers that can produce two or more collinear wavelengths would enable simultaneous sample analysis in two spectral regions, enhancing the utility of the microlaboratories. Recently, scientists at Université Paris-Sud XI in Orsay and at Laboratoire de Photonique et de Nanostructures of the Centre National de la Recherche Scientifique in Marcoussis, both in France, demonstrated what they believe is the first microfluidic laser capable of generating two collinear wavelengths. Moreover, they believe that the principle embodied in the laser would make it easy to expand to three or more wavelengths.

They produced microfluidic channels in polydimethylsiloxane by soft lithography and fabricated the laser resonator by applying gold coatings to the cleaved faces of two multimode optical fibers (Figure 1). The fibers and cores were 125 and 85 µm in diameter, respectively. The two gold-coated faces were separated by 140 µm, resulting in an effective volume of the dye region between the mirrors of ~0.8 nl.

A mixture of ethanol solutions of two dyes served as the laser’s gain medium. The researchers created a 0.001-mol/l solution of rhodamine 6G and a 0.01-mol/l solution of sulforhodamine 101, and combined the two. Optically pumped with ~0.5-ns, ~1.6-µJ pulses of light from a frequency-doubled, Q-switched Nd:YAG laser operating at ~5 kHz, the injected dyes lased simultaneously and produced the dual-wavelength collinear output.

Figure 2. The spectra for different pump energy densities obtained with a USB2000 spectrometer from Ocean Optics Inc. of Dunedin, Fla., show that the sulforhodamine wavelength one (~600 nm) was more sensitive to photobleaching than the rhodamine one (~570 nm). The two upper traces are offset for clarity; the laser wavelengths do not change with pump energy, as noted in the figure.

In most cases, the scientists focused the green pump pulses into the dye resonator using a bulk external lens, but they also coupled the pump pulses into the chip using an optical fiber. The results essentially were the same in both approaches.

The output signals strengthened as the energy density of the pump increased (Figure 2). Although there was no intracavity device to reduce the oscillating bandwidth, the nat-ural bandwidth was ~4 nm, corresponding to five longitudinal resonator modes.

Figure 3. The slope efficiencies for either dye alone were similar. A significantly higher slope efficiency was observed for the mixture of the dyes.

For comparison, the investigators also ran the laser with each dye separately. The slope efficiencies for rhodamine and sulforhodamine alone essentially were the same (Figure 3). The slope efficiency for the mixture of the two dyes was significantly greater than for that of either dye alone. They are investigating the mechanism behind this effect.

Applied Physics Letters, Feb. 27, 2006, 091101.