Ultrafast intersubband relaxation (⩽150 fs) in AlGaN/GaN multiple quantum wells

The ultrafast intersubband relaxation in GaN quantum wells has been verified. ${\mathrm{Al}}_{\mathrm{0.65}}{\mathrm{Ga}}_{\mathrm{0.35}}\mathrm{N/GaN}$ multiple quantum wells, with as many as 200 wells, were grown by optimizing the barrier thickness and introducing GaN intermediate layers. The intersubband absorption is sufficiently strong for the relaxation time to be measured. A pump–probe measurement is performed to investigate the relaxation. An ultrashort relaxation time of less than 150 fs is obtained at a wavelength of 4.5 μm. The transient time is shorter than that of InGaAs quantum wells by approximately an order of magnitude. This result is promising for realizing ultrafast optical switches.