MTL Annual Research Report 2011

Cascaded Energy Transfer for Efficient Broadband Pumping of High-quality Micro-lasers

Figure 1: Illustration of ray tracing under (a) normal (spontaneous) and (b) lasing (stimulated) conditions within an LSC. (c) Typical laser transfer characteristic, showing the dramatic change in efficiency above the threshold.

Many on-chip optical applications, including spectroscopy ^[1] ; sensing ^{[2] [3]} ; nonlinear optics ^{[4] [5] [6]} ; and optical communications require high-finesse ^{[7] [8]} , high-quality factor (high-Q) micro-lasers. Such lasers must be exceptionally transparent at their emission wavelength. But if high-Q micro-lasers exhibit correspondingly weak absorption at the pump wavelengths, they are challenging to excite. Here we demonstrate micro-ring lasers that exhibit Q > 5.2 × 10⁶ and a finesse of > 1.8 × 10⁴ with a direct-illumination, non-resonant pump.  The micro-rings are coated with a combination of three organic dyes. This cascaded combination of near and ultimately far field energy transfer reduces material-losses by a factor of more than 10⁴, transforming incoherent light to coherent light with high quantum-efficiency. The operating principle established here is general and can enable fully integrated on-chip, high-finesse micro-lasers without the complications of coupled pump and emitter resonators ^[9] .

We are now working on lasing luminance solar concentrators (LSC) or solar powered lasers based on the cascaded energy concept. Above threshold, all the fundamental properties of an LSC improve. Specifically, (i) the brightness of the lasing LSC can be orders of magnitude larger than conventional solar concentrators; (ii) stimulated emission enhances the photoluminescent efficiency; (iii) it increases the trapping efficiency of the LSC; (iv) and stimulated emission decreases self-absorption. A solar powered laser also, in essence, converts a portion of the incoherent solar spectrum into a coherent source. This conversion enables the solar powered laser light to be frequency converted in nonlinear crystals, allowing harvesting of more of the solar spectrum via efficient upconversion and downconversion for high efficiency photovoltaics. Figure 1 shows ray tracing of a below- and an above-threshold LSC.

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9. Rotschild, C., Tomes, M., Mendoza, H., Andrew, T. L., Swager, T. M., Carmon, T. and Baldo, M. A., “Cascaded Energy Transfer for Efficient Broad-Band Pumping of High-Quality, Micro-Lasers,” Advanced Materials, vol. 23, 2011 [↩]