The polariton laser design consists of an active semiconductor (red), sandwiched between a stack of Al-GaAs/AlAs layers acting as a distributed Bragg reflector (gray and white) and a polarization-selective Al-GaAs subwavelength grating (black). Stimulation of the semiconductor layer with laser light creates excitons (blue), which combine with photons to create polaritons within the cavity formed by the DBR and the grating. As the polaritons decay, they emit photons; the grating selects for only a single polarization mode, dramatically improving the coherence of the emitted light. [Image: S. Kim, Univ. of Michigan]
Polariton lasers—which generate laser-like light through spontaneous emission of photons from matter waves, rather than through stimulated emission after substantial optical pumping—hold the prospect of dramatically reducing the input power required for lasing. That makes them a potentially strong fit for certain applications such as high-speed optical switching. But these lasers have shown high fluctuations in intensity and, thus, comparatively low coherence, a property that has limited their real-world usefulness.
Now, a team of researchers from the United States, Germany and the United Kingdom, led by OSA Member Hui Deng at the University of Michigan, has demonstrated that a polariton laser can achieve coherence comparable to that of traditional, purely photonic lasers (Phys. Rev. X, doi: 10.1103/PhysRevX.6.011026). The team accomplished the feat through an unconventional design that throttles back competition between spatial modes in the polariton laser cavity, dramatically decreasing intensity instability and boosting coherence.
The coherence conundrum
Polaritons are “quasiparticles” that form from the coupling of semiconductor excitons (excited electron/hole pairs in condensed-matter systems) and photons in an optical cavity. From a statistical-mechanics viewpoint, the quasiparticles behave as bosons, and can thus form condensates analogous to Bose-Einstein condensates—coherent matter waves made up of collections of polaritons moving in phase.
The condensate thus consists of large numbers of polaritons in the same quantum state. As those excited polaritons decay, the photons emitted as a consequence should be in phase, monochromatic and coherent. That sounds like a good recipe for a laser—and a very energy-efficient one to boot. Because polariton lasers don’t require an electronic population inversion, it’s estimated that the input power required for lasing might be only one percent of that of a typical photon laser.
But though the concept of an energy-efficient polariton laser was first articulated 20 years ago, actually getting reliable, application-grade coherent light from polariton lasers has proved difficult. One possible reason is competition between spatial modes of emitted light in most polariton laser designs, which leads in turn to large intensity fluctuations—more than 50 times the shot noise limit—and to very brief coherence times.
Vanquishing mode competition
To work toward better coherence, the team led by Deng relied on a polariton laser setup specifically designed to minimize competition between modes. Conventional polariton lasers consist of a semiconductor layer to generate the excitons, sandwiched between mirrors, in the form of distributed Bragg reflectors (DBRs), on both sides to create an optical cavity. But Deng’s group replaced one of the two mirrors with a sub-wavelength grating (SWG), a high-index dielectric layer etched with tiny bars spaced 500 nm apart. The grating selects for a single polarization mode, thereby substantially eliminating mode competition in the cavity.
The scientists maintained the device at a temperature of 10 K, and used a Ti:sapphire laser to excite the electron/hole pairs in the semiconductor layer and thus create the polariton condensate in the cavity. The resulting emissions from the condensate, according to the team, sported intensity fluctuations at the shot noise limit—a result on a par with conventional lasers—and coherence times on the order of 60 ps. The laser also showed a long-predicted but previously elusive characteristic of matter-wave lasers: a decay in the coherence time once input power reaches a critical threshold.
The authors suggest that the work represents “the proof of principle of a polariton condensate as a matter-wave-based source of coherent light.” They suggest that the study should “guide the future development of polariton lasers and nonlinear polariton devices.” Applications could include low-energy optical switches, single-photon sources and other devices.