Terahertz-wave parametric gain of stimulated polariton scattering

We have experimentally determined the terahertz- (THz-) wave parametric gain of stimulated Raman scattering (SRS) by phonon-polaritons in $\mathrm{LiNb}{\mathrm{O}}_3$. Our approach is based on ultrabright THz-wave generation from SRS under stimulated Brillouin scattering suppression with subnanosecond pump pulses. To obtain the frequency dependence of the parametric gain, we measured the crystal-length dependence of the THz-wave output directly using a surface-coupling configuration. We found that the product of the parametric gain and the threshold crystal length is constant throughout the tuning range. Our result provides a physical basis for the design and performance enhancement of SRS-based ultrabright tabletop THz-wave sources for various applications.

Figure 1

Schematic of the experimental setup for gain measurement. The crystal length is continuously variable through translation of the crystal position as indicated by the dashed arrow. The inset shows the noncollinear phase-matching condition.

Figure 2

Measured THz-wave peak power as a function of frequency and crystal length at $1.25-\mathrm{GW}/\mathrm{c}{\mathrm{m}}^2$ pump intensity.

Figure 3

Crystal-length dependence of THz-wave output. The squares, circles, triangles, and inverted triangles represent the experimental data obtained at 1.0, 1.6, 2.3, and 2.8 THz, respectively. The dashed lines represent the best fit for these data, and the slope efficiencies correspond to the gain coefficient $g_{\mathrm{THz}}$. The solid arrows indicate the threshold crystal length $L_{\mathrm{th}}$. The dotted horizontal line shows the noise level of the pyroelectric detector.

Figure 4

Frequency dependencies of: (a) parametric gain $g_{\mathrm{THz}}$, (b) threshold crystal length $L_{\mathrm{th}}$, and (c) product of $g_{\mathrm{THz}}$ and $L_{\mathrm{th}}$ at $1.25-\mathrm{GW}/\mathrm{c}{\mathrm{m}}^2$ pump intensity. The dashed curve in (a) shows the calculated gain profile based on the constant damping model incorporating the propagation of the THz wave outside the interaction volume. The horizontal dotted line in (c) indicates the constant value of 10.