Oscillations of intersubband electron relaxation in a ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ wide single quantum well near the single- to double-layer transition

We present the photoluminescence (PL) and transport study of an electron system confined in a high-quality wide single quantum well at $T>~60$ mK and $B<~14$ T. Near the single- to double-layer transition, if only one electron layer/subband is occupied in equilibrium, strong magneto-oscillations of the second-subband PL intensity with maxima around odd electron filling factors are observed at low temperatures $\lesssim 600$ mK. The shape of the oscillation maxima depends on intersubband energy spacing. The observed oscillations are shown to originate from the electron intersubband relaxation determined by the relative spin orientation of the initial and final electron states.