Efficient Two-Photon Light Amplification by a Coherent Biexciton Wave

A reversible coupling between photon pair states and a long-lived, highly coherent biexciton wave in CuCl allows efficient phase-sensitive two-photon amplification or attenuation of ultrashort light pulses. We demonstrate a gain of $350{\mathrm{c}\mathrm{m}}^{-1}$ for a pump intensity of $1\mathrm{M}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$, nearly 2 orders of magnitude higher than that achievable with conventional parametric crystal amplifiers. We develop a theoretical model that describes this new type of parametric converter where the light pump is replaced by a coherent biexciton wave and show that it is well suited for the generation of entangled photons and the squeezing of an optical beam with ultrafast time gating.

Figure 1

Schematic illustration of the parametric amplification and deamplification of the optical pulses by a coherent biexciton wave. A coherent biexciton wave prepared by two-photon absorption of a laser pulse, shown in the inset, stores the second order coherence at frequency $2\hslash {\omega }_0=6.372\mathrm{e}\mathrm{V}$ within the radiative dephasing time of 50 psec. With pulses of 12 meV spectral width, the momentum distribution of created biexcitons is $\Delta k={10}^3{\mathrm{c}\mathrm{m}}^{-1}$. A subsequent pulse, tuned to half the biexciton energy, interacts with the biexciton wave and is parametrically amplified (in phase condition) or deamplified (out of phase condition).

Figure 2

Bottom: transmittance of the second pulse as a function of the fine time delay $\delta \tau$ between the first and second pulse. The respective intensities are 700 and $46\mathrm{k}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$. The transmittance shows parametric amplification and deamplification with an oscillation period corresponding to the biexciton frequency $2{\omega }_0$. A sketch of the experimental configuration is also shown. Top: simultaneously measured four wave mixing intensity, showing the squared biexciton wave amplitude $|\psi {|}^2$. Sinusoidal wave fits are shown by solid lines.

Figure 3

Bottom: dependence of the maximum (filled circles) and minimum (empty circles) of the second pulse transmittance on the first pulse intensity. The second pulse intensity is kept constant, at $46\mathrm{k}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$. Top: single beam transmission versus incident pulse intensity, showing two photon absorption due to the biexciton. Theoretical fits are shown by solid lines.