Ultraslow Light Propagation in an Inhomogeneously Broadened Rare-Earth Ion-Doped Crystal

We show that coherent population oscillations effect allows us to burn a narrow spectral hole (26 Hz) within the homogeneous absorption line of the optical transition of an erbium ion-doped crystal. The large dispersion of the index of refraction associated with this hole permits us to achieve a group velocity as low as $2.7\mathrm{m}/\mathrm{s}$ with a transmission of 40%. We especially benefit from the inhomogeneous absorption broadening of the ions to tune both the transmission coefficient, from 40% to 90%, and the light group velocity from $2.7\mathrm{m}/\mathrm{s}$ to $100\mathrm{m}/\mathrm{s}$.

Figure 1

Scheme of the experimental setup: HWP1 and HWP2 are half-wavelength plates; PBS, polarizing beam splitter; AOM, acousto-optical modulator driven by a sinusoidal wave function oscillating at $\delta$; BS, beam splitter; L, lens; ac, alternating current; dc, direct current.

Figure 2

Experimental (points) and theoretical (continuous lines) delay $\tau$ of the probe as a function of the pump power for different modulation frequencies $\delta$. Inset: zoom at low pump powers.

Figure 3

Experimental (squares) and theoretical (continuous lines) delay $\tau$ of the probe as a function of the modulation frequency $\delta$ and for a fixed pump power ($12\mu \mathrm{W}$) with $S=1/2$. Inset: the corresponding normalized transmission.

Figure 4

The experimental group velocity (continuous line), pump (dashed line), and probe (dotted line) transmissions as a function of the pump frequency. The modulation frequency $\delta$ is fixed at 10 Hz and the pump power is such that $S=1/2$.