Design, Demonstration, and Simulation of the Diode-Pumped Thulium/Holmium (Tm/Ho) Composite Fiber 2.1-μm Laser
How using a designable hybrid photonic crystal cavity (HPCC) polariton system can contribute to advancing the fundamental understanding and technological innovation in semiconductor photonics.
Coherence properties are what distinguish a laser from other sources of light and make it useful. This research demonstrates for the first time a polariton laser with coherence reaching the intrinsic limit of single-mode matter-wave lasers. It further demonstrates intensity coherence at the shot-noise limit expected of an ideal coherent state (Figure 1, left), and phase coherence revealing interactions within the polariton condensate due to its matter-wave nature (Figure 1, right).
This research also demonstrates a new mechanism of frequency comb generation using coupled polariton condensates. It results from the dynamic interplay between on-site nonlinearity and inter-site couplings. This is distinct from other systems, such as microtoroidal resonators and quantum cascade lasers that are based on cascaded four-wave mixing process. A unique feature of this form of frequency comb generation is that the comb spacing is not given by the cavity transverse mode spacing and can be engineered from GHz to THz without having to drastically change the physical dimension of the system. Furthermore, since it is based on polariton condensation, the polariton comb allows a very low threshold and an incoherent pump, including electrical injection.
The matter-wave frequency comb is created using the designable HPCC cavity with a suspended high-contrast grating mirror, which allows the creation of two coupled condensate, as well as engineering their interaction and coupling via deformation of the suspended mirrors. Extensive experimental and theoretical evidence was obtained to support the results, including spectroscopy and interferometric measurements showing equidistant frequency lines and characteristic phase relations between the two coupled condensates (Figure 2).
This work paves the way for future development and optimization of low threshold optical frequency comb sources and will open doors to other novel phenomena and device concepts based on dissipatively-coupled nonlinear cavity systems.
This research also demonstrated the first polariton laser in the Bardeen-Coo-per-Schrieffer (BCS) regime, which is also the first demonstration of the particle-hole type BCS state as well as matter-light hybrid BCS state. It validates and pushes forward many decades of theoretical efforts on this subject.
Both the BEC and BCS states were postulated in excitonic systems half a century ago. Research on exciton- and especially polariton-BEC has exploded recently, with a large part of the community accepting spectral features as experimental evidence of a polariton BEC despite persistent skepticism. BCS has been considered much more difficult to achieve, and no experimentally accessible hallmark signatures have been identified.
In the HPCC polariton system, both the weakly and strongly cavity-cou-pled excitons coexist and share the same hot carrier reservoir, enabling access to the electronic media at the presence of polariton lasing. In what would have been commonly accepted as a polariton BEC, fermionic gain and population inversion (Figure 3) were measured directly, unambiguously distinguishing it from a polariton BEC. Stark contrast in spectral properties between the BCS polariton laser and a conventional photon laser were also shown, thereby unambiguously distinguishing it from the commonly known photon laser. A theory based on extended semiconductor Bloch equation reproduces the experimental results and in turn reveals the BCS nature of the new many-body phase.
These findings challenge the previous understanding and experimental interpretation of polariton BEC while at the same time reconfirming that strong electronic correlations underline phenomena so far associated with polariton BEC.
This work was performed by Hui Deng of the Regents of the University of Michigan for the Air Force Research Laboratory. For more information, download the Technical Support Package (free white paper) at mobilityengineeringtech.com/tsp under the Optics, Photonics & Lasers category. AFRL-0308
This Brief includes a Technical Support Package (TSP).
Coherent, Efficient and Practical Polariton Lasers Using a Designable Cavity
(reference AFRL-0308) is currently available for download from the TSP library.
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