This project is an investigation of laser action in the whispering-gallery modes (WGMs) of individual fused-silica spherical microresonators (100-1000 um in diameter). The high quality factor (Q) of a microsphere WGM enables lasing with a very low threshold. Using a layer-by-layer assembly process, a microsphere can be coated with a thin polyelectrolyte film containing nanoparticle inclusions of HgTe. The composite film then serves as the gain medium, interacting with the evanescent component of a WGM into which pump laser light is coupled by optical tunneling via a tapered fiber. Pumping with a wavelength around 800 nm can produce lasing in the 1400-1700 nm range, with the laser emission coupled out of the microresonator by the same tapered fiber. The objectives of this project include the following: verify that lasing occurs; optimize the laser output; produce stable, tunable single-mode operation; and concurrently develop the theoretical counterpart, for purposes of guiding the experiment and interpreting its results. To meet these objectives, the PIs previously developed experimental techniques for tuning and frequency-locking the microsphere become invaluable. In this project, light from a probe laser around 1550 nm is coupled into the microsphere to measure optical gain. Then an output threshold is sought, and the coincident linewidth narrowing of the output provides further evidence of lasing. Analysis of the laser output spectrum is performed using a scanning monochromator, a scanning confocal optical spectrum analyzer, and a second tuned microsphere acting as a filter. By compression-tuning the lasing microsphere and using a second microsphere as a filter, tunable single-mode emission can be produced. The theoretical analysis includes an evaluation of the laser threshold and relative gain that is produced in different WGMs under various pumping conditions. Possible mode competition and instability are also explored for normal laser operation and also for the microlaser with injected signal. Two exciting new areas of research are combined here: resonant optical enhancement in microsphere WGMs and easily-fabricated nanocomposite films. The potential impact is great, because a single microlaser can act as a source for 100-channel (or more) wave-division multiplexing. In addition, with the future use of conducting and/or electroluminescent polymers in the film, electrical pumping may replace optical pumping. Such a microlaser can be a tremendous asset to the telecommunications industry. This work continues and expands preliminary exploratory studies, the early results from which are promising. The requested funding is to enable a longer-term (3-year) concerted effort in this area, while providing partial support for the educational and professional development of three Ph.D. candidates, and a hands-on introduction to multidisciplinary experimental and theoretical science for two undergraduates each year.
