Integrated Optical Memory Based on Laser-Written Waveguides

We propose and demonstrate a physical platform for the realization of integrated photonic memories based on laser-written waveguides in rare-earth-doped crystals. Using femtosecond-laser micromachining, we fabricate waveguides in ${\mathrm{Pr}}^{3+}:{\mathrm{Y}}_2{\mathrm{SiO}}_5$ crystal. We demonstrate that the waveguide inscription does not affect the coherence properties of the material and that the light confinement in the waveguide increases the interaction with the active ions by a factor of 6. We also demonstrate that analogous to the bulk crystals, we can operate the optical pumping protocols necessary to prepare the population in atomic-frequency combs that we use to demonstrate light storage in excited and spin states of the Praseodymium ions. Our results represent a realization of laser-written waveguides in a ${\mathrm{Pr}}^{3+}:{\mathrm{Y}}_2{\mathrm{SiO}}_5$ crystal and an implementation of an integrated on-demand spin-wave optical memory. They open perspectives for integrated quantum memories.

Figure 1

(a) Microscope picture of the waveguide cross section. The distance between the damage tracks is $25\mu \mathrm{m}$. The red dashed ellipse indicates the $e^{-2}$ contour of the guided mode. Scale bar is $15\mu \mathrm{m}$. (b) CCD-acquired near-field intensity profile of the guided mode. Scale bar is $15\mu \mathrm{m}$. (c),(d) Normalized intensity profiles of the waveguide mode along the $x$ and $y$ sections indicated in panel (b). The resulting full widths at half maximum (FWHM) are ${\mathrm{\Delta }}_x=11.3\mu \mathrm{m}$ and ${\mathrm{\Delta }}_y=8.6\mu \mathrm{m}$ (green solid level). The measured $e^{-2}$ diameters are ${\tau }_x=18.5\mu \mathrm{m}$ and ${\tau }_y=15.8\mu \mathrm{m}$ (red dashed level).

Figure 2

(a) Pulse sequence for the two-pulse photon-echo experiment. (b) Echo efficiency as a function of the time delay $2\tau$. The fitting of the experimental data to an exponential decay is also shown (solid line). For this set of data, an optical coherence time $T_2=49.9\mu \mathrm{s}$ is extrapolated. (c) Optical coherence times $T_2$ measured in the waveguide (circles) and in an equivalent bulk sample (squares) as a function of the quantity $[(P_p\times \mathrm{OD})/t_p]$. The circled data point in panel (c) refers to the decay shown in panel (b).

Figure 3

(a) Intensity of a long light pulse $P_p\approx 2\mathrm{mW}$ transmitted by a single-class absorption feature with $\mathrm{OD}=2.35$. The Rabi frequency ${\mathrm{\Omega }}_R^{\mathrm{WG}}\approx 2\pi \times 1.6\mathrm{MHz}$ is estimated from the rising time $t_{\pi }$ (indicated in the figure with vertical solid lines). (b) Rabi frequency as a function of the pulse power as calculated from optical nutation measurements performed on the waveguide (circles) and on the bulk (squares). The circled data point in panel (b) refers to the pulse reported in panel (a).

Figure 4

(a) Energy-level scheme reporting the hyperfine separation of the lowest electronic sublevels (0) of the ${}^3{H}_4$ ground and ${}^1{D}_2$ excited manifolds of ${\mathrm{Pr}}^{3+}$ in ${\mathrm{Y}}_2{\mathrm{SiO}}_5$. The $\mathrm{\Lambda }$ scheme chosen for the storage is indicated by arrows. (b) AFC prepared at the frequency of the $1/2_g\to 3/2_e$ transition.

Figure 5

Light-storage experiments using the AFC protocol. (a) Storage in excited state. Dash-dotted line: 345-ns-long input pulse transmitted through a transparency window (divided by 10 for clarity) in the absence of AFC, polarized parallel to the ${\mathrm{Y}}_2{\mathrm{SiO}}_5$ crystalline $D_2$ axis. Dotted line: Input pulse polarized perpendicular to the $D_2$ axis. Solid lines: AFC echoes for different AFC spacings $\mathrm{\Delta }$. (b) Spin-wave storage. Black dash-dotted curve: Similar as in (a) but 260 ns long (divided by 20 for clarity). Red dashed line: AFC echo in the absence of control pulse, at a storage time of $\tau =2.5\mu \mathrm{s}$. The internal efficiency is ${\eta }_{\mathrm{AFC}}=8.3\%$. The green plain pulses represent the control pulses, as detected before the sample by a reference detector. The blue solid line is the output when control pulses are applied, with a time difference of $T_S=3.6\mu \mathrm{s}$. The AFC echo is partially suppressed, and a spin-wave echo appears $6.1\mu \mathrm{s}$ after the input. The internal efficiency of the spin-wave echo is 2%. The technical noise due to the detector is subtracted from the spin-wave storage trace. (c) Normalized spin-wave echo intensities as a function of the storage time $T_S$. The experimental data (dots) are fitted to a Gaussian decay to account for the inhomogeneous spin broadening from which we obtain ${\gamma }_{\mathrm{inh}}=(23.6\pm 7.7)\mathrm{kHz}$.