Time-resolved cyclotron resonance in cuprous oxide

We have demonstrated cyclotron resonance with a temporal resolution of 15 ns by using an electron-spin-resonance cavity. In an undoped direct-gap semiconductor, cuprous oxide, we observe clear low-field shifts of the cyclotron resonance peaks shortly after generation of photoexcited carriers. Based on the plasma shift of the cyclotron resonance, we evaluate the carrier density and quantitatively discuss the interaction between free carriers and excitons. With increasing time delay, the hole resonance asymptotically reaches the constant value corresponding to an effective mass of 0.575 times the free electron mass, providing a definitive answer to the controversy on the effective mass of holes in cuprous oxide.

Figure 1

Temporal change of the cyclotron resonance spectrum of Cu${}_2$O at 10 K. The time intervals for adjacent curves are 40 ns, as shown in the legend. The inset shows the band structure of Cu${}_2$O near the zone center. The arrows indicate energy gaps between the conduction band (c.b.) and the valence bands (v.b.1 and v.b.2).

Figure 2

(a) Temporal change of the effective masses for electrons and holes, which are obtained from the resonance magnetic fields. The inset shows the fitting of the cyclotron resonance spectrum to the sum of two Lorentzian functions. The data (solid line) are taken at a delay time of 160 ns. (b) Fitting of the spectrum, centered at $t=0$ ns and averaged over 20 ns, to the sum of two magnetoplasma resonances.

Figure 3

(a) Scattering rates for electrons and holes, obtained from the linewidth of the cyclotron resonance peaks, as a function of time delay. (b) Scattering rates as a function of the carrier density estimated from the plasma shift of the CR peaks.

Figure 4

(a) Absorption spectra at 2 K, displayed in different vertical scales. The energy positions of the absorption edge of $1s$ excitons, the yellow gap, and the green gap are indicated by vertical arrows. (b) Intensity of the cyclotron resonance peak for holes as a function of incident photon energy. Microwave absorption intensities at three different times are plotted after normalization.

Figure 5

Cyclotron resonance spectra obtained with a vacancy-rich sample at a time delay of (a) 50 ns and (b) 170 ns. The black lines show fitting results with the sum of three Gaussian functions.