1. Introduction

Electromagnetically induced transparency (EIT) [1] is a quantum interference effect that allows for the transmission of light through an otherwise opaque medium. EIT has led to a variety of novel physics phenomena. For example, by using EIT, researchers have experimentally demonstrated the storage and retrieval of a classical light pulse [2, 3]. Also nonclassical states of light have been stored and retrieved in EIT atomic ensembles in a series of impressive experiments [4, 5, 6]. This light-storage technique is based on the quantum state transfer between light and matter, which has a promising prospect for application in both classical and quantum information processing. Currently, most experimental studies on light storage have been carried out in atomic gases. For practical applications, a solid medium is preferred. Compared with atomic gases, solid mediums have obvious advantages, such as compactness, absence of atomic diffusion, and high density of atoms. So the experimental studies in solids are more valuable than that in atomic gases. Recently, EIT [7], quantum switching [8], enhanced four-wave mixing [9], light storage [10, 11, 12, 13], and stimulated Raman adiabatic passage [14] have been experimentally reported in solids.

All-optical routing is very useful in the future all-optical network and quantum information. As the bandwidth increases in optical communications, the switching speed of the network node becomes a major bottleneck. The electronic routing device with limited switching speed can not meet the requirement of the future communication, and then all-optical routing technique is required. Recently, all-optical routing based on atomic coherence has been proposed and demonstrated experimentally [15, 16, 17]. In this paper, we experimentally demonstrate a three-channel all-optical routing based on light storage in a Pr³⁺:Y₂SiO₅ (Pr:YSO) crystal. By switching off the control field under EIT condition, the optical information of the probe pulse is stored in the crystal. By switching on three retrieve control fields in the release process, the original optical information carried by one light channel is transferred (or distributed) into three different light channels. Also we show that this all-optical routing can be time-delayed. This multichannel all-optical routing may have practical application in the quantum information processing and all-optical network.

2. Experimental results and discussion

Figure 1 shows an energy-level diagram of Pr:YSO. The system consists of 0.05% Pr-doped YSO. The relevant optical transition is ³ H ₄→¹ _{D 2}, which has a resonant wavelength of 605.977 nm. The ground (3 H ₄) and the excited (¹ D ₂) states each have three degenerate hyperfine states. The inhomogeneous width of the optical transition is about 10 GHz at 1.4 K, which is much wider than the hyperfine splitting. The spin inhomogeneous width for the 10.2 MHz transition is 30 kHz at 1.6 K. We call ω _p1, ω _c1, ω _c2, ω _c3 and ω_r the probe, control-1, control-2, control-3 and repump field, respectively. The probe field ω _p1 is resonant with the transition of ³ H ₄(±3/2)↔¹ D ₂(±3/2). The control-1 field ω _c1 is resonant with the transition of ³ H ₄(±1/2)↔¹ D ₂(±3/2). The control-2 field ω _c2 is resonant with the transition of ³ H ₄(±1/2)↔¹ D ₂(±1/2). The control-3 field ω _c3 is resonant with the transition of ³ H ₄(±1/2)↔¹ D ₂(±5/2). The repump field ωr is resonant with the transition of ³ H ₄(±5/2)↔¹ D ₂(±5/2).

 

Fig. 1. The related energy level diagram for Pr:YSO.

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Fig. 2. Schematic diagram of light propagation

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The experimental setup is similar to that of Ref. 9. A Coherent-899 ring laser (R6G dye) is used as the light source. The dye laser output is split into five beams ω _p1, ω _c1, ω _c2, ω _c3 and ω_r. The applied cw laser powers of ω _p1, ω _c1 and ω_r are 1.0 mW, 5.5 mW and 0.3 mW, respectively. By using acousto-optic modulators (AOMs), the laser beams ω _p1, ω _c1, ω _c2, ω _c3 and ω_r are upshifted 205.4 MHz, 195.2 MHz, 190.6 MHz, 200 MHz and 227.5 MHz from the dye laser frequency, respectively. All five beams are linearly polarized and focused into the sample by a 30 cm focal-length lens. The angle between the beams is about 10 mrad. The generation of ω _p2 and ω _p3 satisfy the phase-matching condition (K⃗_p2=K⃗_p1+K⃗_c2−K⃗_c1,K⃗_p3=K⃗_p1+K⃗_c3−K⃗_c1), as shown in Fig. 2. The Pr:YSO crystal 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.

For state preparation, we switch on the control-1 and repump fields for 12 ms each, followed by a 20 µs dark time. After this step, the populations are pumped into ³ H ₄(±3/2) level. The control-1 field is again switched on 20 µs prior to the probe pulse. A gauss probe pulse is used to demonstrate slow light and light storage, and its 1/e full width is 43 µs. In the process of slow light demonstration, the control-2 and control-3 fields are not applied to the crystal. The probe pulse is slowed because of EIT effect, as shown in Fig. 3. We observed a time delay of about 30 µs from the center of the input pulse to the center of the slowed pulse.

 

Fig. 3. Slow light demonstration.

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When slow light is obtained, the storage and release of the light pulse is realized by switching off and on the control-1 field. When the control-1 field is switched off, the probe pulse disappears, and a weak ground-state spin coherence is created, which inherits the coherent optical information of the probe pulse. When the retrieve control field is switched on, the spin coherence is converted into the laser excitation, which passes through the crystal as the slow light. Figure 4(a) shows a typical light storage based on EIT. The peak-1 is the portion of the probe pulse ω _p1 that has left the crystal before the control-1 field ω _c1 is switched off, which is not affected by the storage operation. The peak-2 is the portion of the probe pulse ω _p1 that is stored in and subsequently released from the crystal. Because only one retrieve control field ω _c1 is switched on in the release process, the spin coherence is converted into the light pulse with the original light frequency ω _p1, and no signal of the frequency ω _p2 and ω _p3 is observed. In order to transfer the stored optical information into other light channels, we switch on the retrieve control field ω _c2 or ω _c3 in stead of ω _c1 in the release process, as shown in Fig. 4(b) and (c). It is found that the stored optical information is released at the frequency ω _p2 or ω _p3, which depends on the selective switch-on of the corresponding retrieve control field. Note that the released signal ω _p2 (ω _p3) has a different propagation direction and different carrying frequency compared with the released signal ωp1. Thus the original optical information is transferred into other light channels by selectively switching on the corresponding retrieve control field. The distribution of the retrieved optical information between three light channels is studied. We simultaneously switch on three retrieve control fields (ωc1, ωc2 and ωc3) in the release process, and then the spin coherence is converted into three laser excitations. As shown in Fig. 4(d), the stored probe optical information is simultaneously released at three different frequencies (ωp1, ωp2 and ωp3). Thus the original optical information from one channel is distributed into three different light channels by simultaneously switching on three retrieve control fields. The all-optical transfer and distribution of the optical information, i.e. all-optical routing, is useful for all-optical network and quantum information. If the atomic system has multiple excited states, multichannel all-optical routing can be implemented by the method reported in this paper.

 

Fig. 4. All-optical routing by light storage. (a), (b) and (c): ω _c1, ω _c2 and ω _c3 is respectively switched on in the release process. (d) Three retrieve control fields are simultaneously switched on in the release process. The intensity of ω _c2 and ω _c3 are 6.5 mW and 8.0 mW, respectively.

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Three released signals (ω _p1, ω _p2 and ω _p3) are detected by three photodiodes respectively, and the photodiodes have nearly equal collection efficiencies. The intensity of the released signal is proportional to that of the associated retrieve control field [16]. From Fig. 4, we can see that the released signal ω _p3 is weaker than the released signals ω _p1 and ω _p2. Now we give the following explanation. The retrieve control field ω _c3 not only is resonant with the transition ³ H ₄(±1/2)↔¹ D ₂(±5/2), but also is applied to the transition ³ H ₄(±3/2)↔¹ D ₂(±3/2) with a 0.8 MHz detuning. Due to four-wave mixing (FWM), the part of ω _c3 acting with ³ H ₄(±3/2)↔¹ D ₂(±3/2) scatters the ground-state spin coherence to generate a FWM signal ω_g corresponding to the transition of ³ H ₄(±1/2)↔¹ D ₂(±3/2). The generation of the signal ω_g meets the phase-matching condition (K⃗_g=K⃗_c1+K⃗_c3−K⃗_p1), as shown in Fig. 2. We have experimentally observed the signal ω_g, which is stronger than the released signal ω _p3. So, there are two processes, FWM and the release of stored optical information, when the control field ω _c3 is applied to the crystal. The competition of these two processes leads to that the released signal ω _p3 has a weak intensity. A detailed discussion of the competition process is beyond the scope of this paper, and will be discussed in a separate paper.

We study that this all-optical routing can be time-delayed as shown in Fig. 5. In this part, we use a series of short retrieve control pulses to release the stored optical information. Firstly, a short retrieve control pulse ω _c2 is applied to the crystal in the release process, then the stored optical information is released at the frequency ω _p2. In this step, only a part of the spin coherence is converted to the laser excitation because the retrieve control pulse ω _c2 is too short to deplete spin coherence completely. Secondly, a short retrieve control pulse ω _c1 is applied to the crystal, then the stored optical information is released at the frequency ω _p1. Also the spin coherence is not depleted completely. Thirdly, a short retrieve control pulse ω _c3 is applied to the crystal, the stored optical information is released at the frequency ω _p3. Three signals ω _p1, ω _p2 and ω _p3 are released at different times, which depend on the switch-on of the corresponding retrieve control fields. Each of the released signals contains part of the stored coherent optical information. So time-delayed all-optical routing is obtained in this system.

 

Fig. 5. Time-delayed all-optical routing by switching on a series of short retrieve control pulses in the release process.

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3. Conclusions

In summary, we have experimentally demonstrated a three-channel all-optical routing by light storage in a Pr:YSO crystal. By switching on three retrieve control fields to release the stored optical information, the original optical information is transferred or distributed into three light channels. Also we realize time-delayed all-optical routing by applying retrieve control pulses at different times. This all-optical routing by light storage may have many applications in quantum information and all-optical network.