  The polarization discontinuity across interfaces in polar nitride-based
heterostructures can lead to the formation of two-dimensional electron and hole
gases. In the past, the observation of these electron and hole gases has been
achieved through various experimental techniques, most often by electronic
measurements but occasionally by optical means. However, the occurrence of a
two-dimensional hole gas has never been demonstrated optically. The objective
of this article is to demonstrate, thanks to the combination of various optical
spectroscopy techniques coupled to numerical simulations, the presence of a
two-dimensional hole gas in a GaN/AlGaN/GaN heterostructure. This is made
possible thanks to a GaN/AlGaN/GaN heterostructure displaying a
micrometer-thick AlGaN layer and a GaN cap thicker than in conventional
GaN-based HEMTs structures. The band structure across the whole heterostructure
was established by solving self-consistently the Schr\"odinger and Poisson
equations and by taking into account the experimentally determined strain state
of each layer. Continuous and quasi-continuos photoluminescence reveal the
presence of a broad emission band at an energy around 50 meV below the exciton
emission and whose energy blueshifts with increasing excitation power density,
until it is completely quenched due to the complete screening of the internal
electric field. Time-resolved measurements show that the emission arising from
the two-dimensional hole gas can be assigned to the recombination of holes in
the potential well with electrons located in the top GaN as well as electron
from the bottom AlGaN, each of them displaying different decay times due to
unequal electric fields. Besides the optical demonstration of a two-dimensional
hole gas in a nitride-based heterostructure, our work highlights the optical
recombination processes involved in the emission from such a hole gas.
