Semiconductor Waveguide Source of Counterpropagating Twin Photons

We experimentally demonstrate an integrated semiconductor source of counterpropagating twin photons in the telecom range. A pump beam impinging on top of an AlGaAs waveguide generates parametrically two counterpropagating, orthogonally polarized signal/idler guided modes. A 2 mm long waveguide emits at room temperature one average photon pair per pump pulse, with a spectral linewidth of 0.15 nm. The twin character of the emitted photons is ascertained through a time-correlation measurement. This work opens a route towards new guided-wave semiconductor quantum devices.

Figure 1

Parametric generation of counterpropagating twin photons in a multilayer waveguide. (a) A pump photon of frequency ${\omega }_p$ is converted in two counterpropagating twin photons with frequencies ${\omega }_s$ and ${\omega }_i$, with ${\omega }_s+{\omega }_i={\omega }_p$ (energy conservation). (b) Momentum conservation is fulfilled in the longitudinal direction ($z$) for appropriate values of ${\mathbf{k}}_{\mathbf{s}}$ and ${\mathbf{k}}_{\mathbf{i}}$. (c) Calculated tuning curves for a pump wavelength 768.2 nm. Thick lines: additive interaction (+). Thin lines: subtractive interaction (−). The solid lines represent the photons copropagating with the $z$ component of the pump beam, while the dashed lines represent the counterpropagating ones. The intersections with the vertical bar at $\theta =3°$ are associated with the spectral measurements shown in Fig. 3.

Figure 2

Experimental setup for spectral analysis and time-correlation measurements. The generated photons are collected by two microscope objectives, sent through two monochromators, and detected by single-photon avalanche photodiodes. A time-interval analyzer records temporal correlations between the corresponding current pulses.

Figure 3

Emission spectra for an angle of incidence of 3°. Spectrum (a) is acquired by the detector A of Fig. 2, and spectrum (b) is acquired by the detector B of Fig. 2. Photons copropagating with the $z$ component of the pump (i.e., impinging on the detector B) carry a supplementary momentum, and therefore have a lower wavelength. In both spectra TM photons have a lower wavelength than the TE ones, corresponding to their higher propagation velocity in the multilayer waveguide. The small secondary peaks result from generated photons that undergo a reflection at a waveguide facet before impinging on the detector A or B.

Figure 4

Time-correlation histogram between counterpropagating photons. The peak at $t_s=t_i$ corresponds to twin photons, simultaneously generated in the additive parametric down-conversion process. The flat background corresponds to coincidences between uncorrelated counts.