  Refrigeration of a solid-state system with light has potential applications
for cooling small-scale electronics and photonics. We show theoretically that
two coupled semiconductor quantum wells are efficient cooling media for optical
refrigeration because they support long-lived indirect electron-hole pairs.
Thermal excitation of these pairs to distinct higher-energy states with faster
radiative recombination allows an efficient escape channel to remove thermal
energy from the system. This allows reaching much higher cooling efficiencies
than with single quantum wells. From band-diagram calculations along with an
experimentally realistic level scheme we calculate the cooling efficiency and
cooling yield of different devices with coupled quantum wells embedded in a
suspended nanomembrane. The dimension and composition of the quantum wells
allow optimizing either of these quantities, which cannot, however, be
maximized simultaneously. Quantum-well structures with electrical control allow
tunability of carrier lifetimes and energy levels so that the cooling
efficiency can be optimized over time as the thermal population decreases due
to the cooling.
