Terahertz generation from coherent optical phonons in a biased GaAs photoconductive emitter

Terahertz radiation, generated from a biased and asymmetrically excited low-temperature-grown GaAs photoconductive emitter, has been characterized with a $20\text{−}\mu \mathrm{m}$-thick ZnTe crystal using free-space electro-optic sampling. Pronounced coherent emission, originating from longitudinal optical phonon oscillations, has been observed, with a characteristic frequency of $8.7\mathrm{THz}$ and a decay time of $2.1\mathrm{ps}$. In the frequency domain, the sharp spectral features from phonon oscillations superimpose on an ultrabroad (over $20\mathrm{THz}$) response from the initial transient. Furthermore, a broad feature around $16\mathrm{THz}$ is observed, and interpreted as originating from the coherent plasmon-phonon coupled modes.

Figure 1

(a) The temporal THz wave form and (b) its corresponding Fourier-transform amplitude spectrum (upper trace), together with the spectrum measured after being transmitted through a $0.53\text{−}\mathrm{mm}$-thick GaAs wafer (lower trace). The arrows mark spectral features corresponding to the TO and LO phonons of ZnTe $({\nu }_{\mathrm{TO}}\equiv 5.3\mathrm{THz};{\nu }_{\mathrm{LO}}\equiv 6.2\mathrm{THz})$ and GaAs $({\nu }_{\mathrm{TO}}\equiv 8.06\mathrm{THz};{\nu }_{\mathrm{LO}}\equiv 8.76\mathrm{THz})$, together with the coupled plasmon-phonon mode of GaAs (P1). Inset of (a): Schematic geometry of the electrodes ($0.4\mathrm{mm}$ gap) used in the biased GaAs photoconductive emitter. The white spot ($\sim 40\mu \mathrm{m}$ diameter) indicates the position of the asymmetric laser excitation. Inset of (b): Density dependence of coupled plasmon-phonon mode frequencies (upper branch: P1; lower branch: P2) calculated for GaAs parameters (${\epsilon }_0=12.9$, ${\epsilon }_{\infty }=10.9$, ${\nu }_{\mathrm{LO}}=8.76\mathrm{THz}$, ${\nu }_{\mathrm{TO}}=8.06\mathrm{THz}$, and $\mu =0.06m_0$). Dashed and dotted lines represent GaAs TO phonon, LO-phonon and plasmon (PL) frequencies, respectively.

Figure 2

The time-partitioned Fourier transform of the THz data from Fig. 1. The time interval between two adjacent curves is $0.3\mathrm{ps}$, with the time on the right-hand axis being the start of the $2\mathrm{ps}$ partitioning interval. Two dotted lines correspond to frequencies 6.2 and $8.7\mathrm{THz}$, and the arrow (P1) indicates the broad feature originating from coupled plasmon-phonon modes.

Figure 3

The measured (points) and calculated (curve) temporal THz wave form between $2.3–6.3\mathrm{ps}$. Note that the phonon oscillations can be seen immediately after the initial transient [see Fig. 1a], and extend over $8.0\mathrm{ps}$, although for clarity, only a restricted interval is shown here.

Figure 4

The time-partitioned Fourier transform of the THz signal measured after being transmitted through a $0.53\text{−}\mathrm{mm}$-thick GaAs wafer. The time interval between two adjacent curves is $0.3\mathrm{ps}$, with the time on the righthand axis being the start of the $2\mathrm{ps}$ partitioning interval. Two dotted lines correspond to frequencies 6.2 and $8.7\mathrm{THz}$.