Flow rate based control of wavelength emission in a multicolor microfluidic dye laser
Introduction
Optical diagnostics are of great interest for biochemical analysis: absorption, fluorescence, Raman spectroscopy, surface plasmon resonance, interferometry. These techniques are successfully applied to microsystems, however even if the samples are manipulated on the microscale, there is an increasing demand for miniaturizing elements such as optical sources and detectors. So an effort has been made to develop microfluidic lasers [1]. Microfluidic lasers fit well for such applications mainly due to their easy integration with other components and their tunability. Linear cavities, ring cavities, distributed feed back structures have been investigated for single mode lasers, multicolor lasers, tunable lasers and their integration into micrototal analysis systems [2], [3], [4].
The selection of the emission wavelength is a crucial issue for laser based applications such as bandwidth selection for absorption or excitation of fluorophores, or probing resonances. This way, the optical source may be adapted to analyze different samples. Considering only one dye and one cavity, wavelength tunability can be achieved using different concentrations [5]. However the range is limited to 10 nm and reaching larger tuning range requires using several dyes, multiple laser cavities [6] or an external activator [7]. Such systems imply a higher degree of complexity in their fabrication process or in their manipulation.
In our group, we have demonstrated a microlaser that delivers a simultaneous collinear dual-color emission [8]. This is based on a mixture of two dyes that generates laser effects at two discreet wavelengths. Presently, we further investigate the dye flow rate dependence of the lasing emission. We show that the output wavelength can switch from one value to the other depending on the dye mixture flow rate. It provides for an alternative way to control the laser emission and results in a wavelength selective laser system.
Section snippets
Microfabrication and experimental set up
Poly(dimethylsiloxane) (PDMS) and a glass substrate created the microfluidic channels, optical fibers were used to define the optical cavity. The optical fibers were cleaved, both metalized on the ends with 2 nm Titane then covered with 30 nm Gold for the fiber acting as the output mirror and 100 nm Gold for the fiber acting as a back mirror. The channels’ design was patterned in a 125 μm thick photoresist layer (Microchem SU8 2100). Another pair of optical fibers, non-metalized sacrificial fibers,
Results and discussion
Fig. 2a gives the spectra of the lasing effects when individual dye solutions are successively introduced in the “dye channel”. Rh6G in ethanol at a concentration of 0.005 mol L−1 generates a lasing effect at 568 nm while Su640 in ethanol at the same concentration has a peak at 612 nm. Their bandwidths are 3 and 4 nm respectively, due to the spectrometer resolution and multimode regime in reason of cavity length. Indeed the cavity length is 550 μm while a monomode regime requires a cavity length less
Conclusion
We have investigated here a new way to control the wavelength emission of a microfluidic dye laser. Without changing of dye solution or optical cavity, we have shown that it is possible to monitor the output wavelength using a dye mixture of Rh6G and Su640. By controlling the dye flow rate, the peak wavelength can be selectively oriented towards 566 or 600 nm. This is a really simple way that does not require any actuation valve or additional external mechanism. We have shown the underlying
Acknowledgment
The authors would like to thank the LPN clean-room staff and the LPPM optical instrumental group for technical assistance. G. Aubry is financially supported by the French Foundation RTRA-Triangle de la Physique under Contract No. 68.
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