We experimentally demonstrate light storage and release in a four-level double-lambda atomic system of a Pr 3+:Y2SiO5 crystal. Based on the technique of light storage, we realize optical information transfer between two light channels. The coherent optical information of a probe pulse stored in the crystal can be selectively released into two different light channels by varying the frequency and propagation direction of the switch-on control field.
©2007 Optical Society of America
Light is an excellent carrier of information, but it is difficult to localize and store. The ability to manipulate the behavior of light becomes very important in optical communications. Recently, storage and release of light pulse in an atomic ensemble by using the effect of electromagnetically induced transparency (EIT)  have been proposed theoretically  and demonstrated experimentally [3, 4]. The light-storage technique is based on the quantum state transfer between light and matter. In such a process, a weak probe pulse can be completely halted in the atoms by adiabatically switching off the control field, and then can be subsequently released by the reverse process. Furthermore, phase coherence of stored photonic information , storing light of arbitrary polarization , cross-phase-modulation based on stored light pulses , controlled release of stored light pulses [8, 9], and quantum destructive interference in inelastic two-wave mixing , have been demonstrated experimentally.
However, most of experimental studies on light storage and release have been carried out in atomic gases. For many practical applications of light storage, the corresponding processes are very valuable in a solid. The obvious advantages of solids are high density of atoms, compactness, and absence of atomic diffusion. Most solid materials have relatively broad optical linewidths and fast decoherence rate, which limit the achievable light storage. A notable exception to this general rule is a class of materials consisting of rare-earth ions that exhibit spectral hole-burning. The rare-earth doped crystals, such as Pr3+:Y2SiO5 (Pr:YSO) [11, 12], have sharp spectrum structure and long spin coherence time. EIT , quantum switching , light storage [15, 16], and stimulated Raman adiabatic passages (STIRAP) [17, 18, 19] have been experimentally reported with Pr:YSO.
In this letter, we report an experimental implementation of light storage and release in a Pr:YSO crystal which exhibits a four-level double-lambda atomic system. Under EIT condition, a weak probe pulse is slowed and compressed in the crystal. By switching off the control field, the coherent optical information of the probe pulse is stored in the crystal. The stored coherent optical information can be selectively released into two different light channels, by changing the frequency and propagation direction of the switch-on control field. Then we realize optical information transfer between two different light frequencies in a Pr:YSO crystal. Such an optical information transfer has practical applications in quantum information processing and all-optical network.
2. Ionic levels and pulse sequences
Figure 1 shows an energy-level diagram of Pr:YSO. The system consists of 0.05% Pr-doped YSO in which Pr3+ substitutes Y3+. The relevant optical transition is 3H4→1D2, which has a resonant frequency of 605.977 nm at site 1 [11, 12]. The inhomogeneous width of the optical transition is about 10 GHz at 1.4 K, which is much wider than the hyperfine splitting. The ground (3 H 4) and the excited (1 D 2) states each have three degenerate hyperfine states. The optical population decay time T 1 and transverse decay time T 2 are 164 µs and 111 µs, respectively. The ground state population decay time Ts 1 is about 100 s and spin transverse decay time Ts 2 for the 10.2 MHz transition is 500 µs at 6 K. The spin inhomogeneous width for the 10.2 MHz transition is 30 kHz at 1.6 K [11, 12].
we call ωp, ωc, ωc 2 and ωr the probe, control, control-2, and repump field, respectively. The probe field ωp is at resonance with the transition of 3 H 4(±3/2)↔1 D 2(±3/2). The control field ωc is at resonance with the transition of 3 H 4(±1/2)↔1 D 2(±3/2). The control-2 field ωc2 is at resonance with the transition of 3 H 4(±1/2)↔1 D 2(±1/2). The repump field ωr is at resonance with the transition of 3 H 4(±5/2)↔1 D 2(±5/2). As mentioned in Ref. [13, 15], the four light fields apply to a small subset of Pr ions only. The optical inhomogeneous width in this system is modified by the laser jitter due to the persistent spectral hole burning. Since effective optical inhomogeneous width is much smaller than the 10 GHz intrinsic inhomogeneous width of optical transition, effective atomic coherence can be established at a much lower coupling intensity than would otherwise be predicted.
The waveform and the pulse sequence of the four fields are shown in Fig. 2. The process of the experiment consists of the following three steps: step-1 is the optical pump; step-2 is the preparation of the ionic state 3 H 4(±3/2); step-3 is the storage and release of the light pulse. Because of the large inhomogeneous broadening, there are many useless ions which do not simultaneously couple the four fields to all three ground states, but have at least a transition interacting with a field. By the optical pump of step-1, the population of the useless ions is concentrated on the ground states not interacting with any laser field. In this step, the fields of ωp and ωc are applied as square pulse sequences so that coherent population trapping does not prevent the optical pump. In step-2, the control field ωc and the repump field ωr are applied to the crystal in the absence of the other fields, so Pr ions are populated into the ionic state 3H4(±3/2) due to the optical pump of these two fields. In step-3, the control pulse ωc with the duration of 58 µs is turned on 8 µs prior to the probe pulse ωp with the duration of 50 µs. After 10 µs (20 µs) storage time, the control field ωc or the control-2 field ω c2 is switched on.
3. Experimental setup and results
The experimental arrangement is illustrated in Fig. 3. A Coherent-899 ring laser (R6G dye) is used as the light source, which has about 0.5 MHz laser jitter. The dye laser output is split into four beams ωp, ωc, ωc2 and ωr by four acousto-optic modulators (AOM). To match the setup in Fig. 1, the laser beams ωp, ωc, ωc2 and ωr are upshifted 178.1 MHz, 167.9 MHz, 163.1 MHz and 200 MHz from the dye laser frequency by AOM-P, AOM-C, AOM-C2, and AOM-R, respectively. The applied cw laser powers of ωp, ωc, ωc2 and ωr are 0.5 mW, 3 mW, 6.2 mW and 4.5 mW, respectively. All four beams are linearly polarized and focused into the sample by a 30 cm focal-length lens. The beam diameters are about 150 µm in the crystal. The angle between the beams is about 10 mrad. The alignments of laser beams ωp, ωc and ω c2 satisfy the phase-matching condition (K⃗p2=K⃗p+K⃗c2-K⃗c) for the generation of ωp 2 at the position indicated on L2. The persistent spectral hole-burning crystal of Pr:YSO is inside a cryostat (Cryomech PT407) and the temperature is kept at 3.5 K. The size of the crystal is 4 mm×4 mm×3 mm, and optical B-axis is along 3 mm. The laser propagation direction is almost parallel to the optical axis. The optical signal passing through the sample is detected by a photodiode connected to a fast oscilloscope.
The slowing of the probe pulse is shown in Fig. 4. The control field ωc is turned on 8 µs prior to the probe pulse ωp. The dash line corresponds to the input probe pulse in the absence of the control field and the repump field. We can see that both slow and fast light components of the probe pulse pass through the crystal. The fast light emerges first, and is overlapped with the slow light. The presence of the fast light is due to the fact that a portion of the probe pulse does not interact with the ions and the control field, and this portion passes through the crystal freely without absorption, as shown in Ref. . The rise and fall edges of the input pulse are fast, and some of their Fourier frequency components exceed the EIT width. The exceeded components are absorbed by Pr ions, which leads a result that the rise and fall edges of the slow light are attenuated. A time delay of about 40 µs is measured from the center of the input pulse to the center of the delayed pulse, which corresponds to the group velocity of Vg≈75 m/s. So the probe pulse is spatially compressed by more than 6 orders of magnitude in 3-mm crystal.
Figure 5 shows the storage and release of a probe pulse. When most part of the probe pulse ωp is contained in the crystal, we switch off the control field ωc to store the probe pulse ωp. The ground-state coherence or atomic spin excitation inherits the coherent optical information of the probe pulse ωp. As shown in Fig. 5(a), the peak-1 is the portion of the probe pulse ωp which has left the crystal before the control field ωc is switched off, which resulted in an observed signal unaffected by the storage operation. The gap of the signal shows the storage time of 10 µs. We switch on the control field ωc in the release process. The stored optical information is released into a light pulse with the original probe frequency ωp and propagation direction. The observed peak-2 is the portion of the probe pulse ωp that was stored in and subsequently released from the crystal. The released peak-2 maintains the same temporal profile as the back portion of the slow light, but its amplitude decreases due to the decoherence between the hyperfine levels of Pr ions. Note that no output signal ωp2 is observed because the control-2 field ωc2 is not switched on, as shown in Fig. 5(b). Similarly, we switch on the control-2 field ωc2 instead of the control field ωc in the release process. As shown in Fig. 5(c) and Fig. 5(d), the stored coherent optical information is released into a light pulse with the frequency ωp2, but the light pulse with the frequency ωp is not released. The generated light pulse ωp2 carries the coherent optical information of the probe pulse ωp. We emphasize that the restored pulse ωp2 has a different frequency and propagation direction from the probe pulse ωp, thus this system realizes optical information transfer between two light channels.
Due to the coherence decay between hyperfine levels 3 H 4(±1/2) and 3 H 4(±3/2), the amplitudes of the restored pulses decrease with increasing storage time. In Fig. 6, we show the restored signal pulses which are stored in the crystal for a time interval of 20 µs. The amplitudes of the restored signal pulses are smaller than that in Fig. 5. From Fig. 5 and Fig. 6, it is seen that the restored pulse ωp2 has a larger amplitude and a smaller temporal width than that of the restored pulse ωp. This is because that the control-2 field ωc2 has a larger intensity than that of the original control field ωc. The actual amplitude of the restored pulse ωp2 should be higher, because it propagates in a new direction, and the crystal is optically dense. The shape of the restored pulse ωp2 depends on the intensity of the control-2 field. In Fig. 7, we show the shape of the restored pulse ωp2 under similar conditions except that we vary the intensity of the control-2 field ωc2 when it is switched on. When the intensity of the control-2 field becomes larger, the amplitude of the restored pulse ωp2 increases and its temporal width decreases. The stored atomic coherence dictates the ratio of the intensity of the control and restored probe field, as well as the temporal width of the regenerated pulse.
In our experiment, the peak powers of the input probe pulse and the slowed pulse are 0.5mW and 0.05 mW, respectively. The peak power of the restored pulse ωp is 0.04 mW for 10 µs storage time, and 0.02 mW for 20 µs storage time. In Ref. , the storage efficiency is 1%, which is defined by the ratio of the intensity of the stored pulse and the input probe pulse. In our experiment, the storage efficiency is about 5%, which is higher than that in Ref. . The laser linewidth is 200 Hz in Ref. , and 0.5 MHz in our experiment. Due to the spectral selection, more Pr ions can participate in the interaction with the laser fields in our experiment, which leads to relative higher storage efficiency.
In summary, we have experimentally demonstrated the storage and release of a light pulse in four-level double-lambda atomic system of a Pr:YSO crystal. We realize the optical information transfer between two light channels by changing the frequency and propagation direction of the switch-on control field. Such optical information transfer between different light channels has potential applications in quantum information processing and all-optical network.
The authors acknowledge the financial support from the NSFC (Grant No.10334010), from the doctoral program foundation of institution of High Education of China, and from the National Basic Research Program (Grant No.2006CB921103).
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