Revealing the properties of nanolasers with quantum optics
The laser is one of the most seminal inventions of the second half of the 20th century with applications ranging from telecommunications to eye surgery. But although it was invented in 1960, the properties of the laser are still investigated by scientists.
Among the various types of lasers, semiconductor-based nanolasers form a fascinating class on their own. These lasers are made from dielectric materials and can be engineered using techniques available in clean room facilities and reach a size where the active volume responsible for light emission is of the order of one cubic wavelength.
Using the nanofabrication tools available at EPFL’s Center of MicroNanoTechnology (CMi) and Institute of Physics, the laboratory of Nicolas Grandjean teamed up with researchers from the Technical University of Berlin (optical spectroscopy) and the University of Bremen (microscopic theory) to investigate nanolasers made from gallium nitride (GaN) material: the semiconductor behind white light-emitting diodes (LEDs) used for solid-state lighting. This work unveils the emission features of nanolasers.
The scientists found that the compact, low-threshold nanolasers that emit in the blue spectral range under continuous wave optical pumping are ideally suited for exploring the ultimate properties of lasing in dielectric media thanks to the efficient photon funneling offered by their geometry. These are referred to as “high-β nanolasers”.
Beyond engineering light-matter interactions at the nanoscale and the emergence of coherent light at the few-photon level, the study illustrates the potential of such a platform for a number of practical uses, such as small-footprint, low-threshold coherent emitters for photonic integrated circuits, lab-on-a-chip light sources, and biocompatible light emitters that are suitable for single-cell optogenetic experiments.
Experimentally, the lasing threshold was assessed by monitoring the evolution of the second-order intensity autocorrelation function at zero-time delay —the so-called g(2)(0)— as a function of pump power. This quantity exhibits a transition from chaotic photon emission, for which bunches of photons are emitted as a function of time—corresponding to g(2)(0) > 1 — to coherent (Poissonian) light emission, for which photons are randomly emitted as a function of time, i.e., g(2)(0) = 1.
Remarkably, the closer to one the β value (the present GaN nanocavities exhibit a β value of 0.7 knowing that β can take any value between 0 and 1 and that most lasers have a β value less than 10-3), the smoother the lasing transition, and hence the less defined the threshold. Lasing in the GaN nanocavities was confirmed through power-dependent g(2)(0) measurements in a Hanbury-Brown and Twiss interferometer showing a progressive transition from chaotic emission to the Poisson limit.