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We present the transformation of electromagnetically induced transparency (EIT) into narrow enhanced absorption with an on-resonant standing-wave coupling field in the 5S1/2-5P1/2 transition of the Λ-type system of 87Rb atoms. When a coupling laser field was changed from a travelling-wave to a standing-wave that was made by adding a counter-propagating LC laser, the transmittance spectrum of the LP laser transformed the typical EIT into dramatically enhanced absorption, and a Bragg reflection signal was generated by the periodic modulation of atomic absorption. The reflected probe laser corresponding to a Bragg reflection was measured to be approximately 11.5% of the power of the incident probe laser. We analyzed the enhanced absorption signal and Bragg reflection spectrum as a function of the power and frequency detuning of the coupling laser.
©2010 Optical Society of America
1. Introduction
The atomic coherence interacting with light and atoms have been attractive issues for a long time. One of the interesting phenomena caused by the two-photon atomic coherence effect is electromagnetically induced transparency (EIT) [1–7]. After the first observation of EIT by Harris et. al., many interesting phenomena of EIT have been reported in various media [3–7] and used in a range of applications, such as atomic clocks, quantum information, optical switching, light storage and atomic magnetometer [8–14]. These applications employ the narrow spectral width of the features due to atomic coherence.
EIT has generally been examined using two travelling-wave laser fields, a probe and a coupling field in a three-level atomic system [1–13]. However, an atomic coherent medium with the standing-wave coupling field has different properties from an EIT medium with a travelling-wave field. When a standing-wave coupling field interacts with a three-level atomic system, the dispersion and absorption of a probe laser in an atomic medium is modulated spatially by the standing-wave coupling field [14–16]. Diffractive phenomena, such as a Bragg reflection, have been observed and understood to be a form of electromagnetically induced grating (EIG) due to spatially modulated absorption. EIG have been observed experimentally in atomic vapor cells, buffer gas cells, and cold atoms [17–19]. Ling et al. calculated the diffractive probe field by EIG in a homogeneous Λ-type atomic system [17]. These studies focused on the reflected signal due to the spatially modulated absorption or atomic coherence with the standing-wave coupling field.
Recently, an absorption phenomenon in the transmittance spectrum of the probe field was observed in the interaction of a three-level atomic vapor with a standing-wave coupling field [20–24]. Babin et al. reported splitting of the EIT peak due to the higher-order spatial harmonics of atomic coherence induced at the probe transition [20]. Brown and Xiao observed a decrease in transmittance of the probe laser due to EIG reflections and spatial interference effects from the modulated absorption in Rb atoms [21]. Although the atomic coherent medium with the standing-wave coupling field has been studied in a variety of situations, there are no reports examining the narrow enhanced absorption signal.
In this paper, we investigate the transformation of EIT into enhanced absorption with a narrow spectral width along with a Bragg reflection of the probe laser in the 5S1/2-5P1/2 lambda-type system of 87Rb atoms. The transmittance spectrum of the LP laser and the backward reflection signal by the periodic modulation of atomic absorption was examined when a coupling laser field was changed from a travelling-wave to a standing-wave. In order to analyze the narrow enhanced absorption spectrum, three signals, such as transmittance, fluorescence and reflection, were measured simultaneously in detail. In addition, the enhanced absorption spectrum and Bragg reflection spectrum were examined according to the frequency detuning of the coupling laser and the power ratio between the co-propagating and counter-propagating coupling lasers.
2. Experimental setup
Figure 1 shows the energy diagram of the D1 line of 87Rb atoms. The separated frequency between the two hyperfine ground states is 6.8 GHz and one between the two excited states is 816.8 MHz. When the frequency of the coupling laser (LC) was fixed on the 5S1/2(F = 2) - 5P1/2(F’ = 2) transition, the frequency of the probe laser (LP) was scanned over the range of the 5S1/2(F = 1) - 5P1/2(F’ = 2) transitions. The Doppler-free EIT spectrum was obtained in a three-level Λ-type atomic system when the LC and LP were co-propagating though the Rb vapor cell. When the LC became a standing-wave by adding another counter-propagating LC, the absorption of the LP was periodically modulated in the nodes and anti-nodes of the standing-wave. At the nodes, the LP was absorbed by the Rb atom due to the absence of the LC. On the other hand, at the anti-nodes, the LP was transmitted in the atomic coherent medium and interacted with the standing-wave coupling field. Since the nodes and anti-nodes appear alternately, the absorption of the LP was spatially modulated in the Rb vapor cell.
Figure 2 shows the experimental setup for observations of the enhanced absorption and the Bragg reflection of the probe laser with the on-resonant standing-wave coupling field [19,21]. Two external cavity diode lasers (ECDL), which are independently controlled and monitored by saturated absorption spectroscopy (SAS), were used in this experiment. To generate the standing-wave LC fields, the LC was split using a 50:50 beam splitter (BS) and the co-propagating and counter-propagating LC fields were then overlapped. Both the LC and LP were passed through a 2 mm diameter aperture and polarized linearly and perpendicular to each other. The power of the LC and the LP was controlled using polarized beam splitters (PBS) and half-wave plates (HWP). In order to detect only the LP, the LC and LP were misaligned with an angle between the LC and LP of approximately 20 μrad. The forward transmission and backward reflection of the LP were detected simultaneously by the PD1 and PD2, respectively. In addition, the fluorescence signal in the Rb vapor cell was observed using a detector on the side of the cell. To reduce the effects of the Earth’s magnetic field, the 2.5 cm diameter, 5 cm long Rb vapor cell was wrapped with three layers of a μ-metal sheet. The cell temperature was controlled using thermally stabilization equipment.
Fig. 2 Experimental setup for the enhanced absorption and Bragg reflection (BS: beam splitter, PD: photo diode, HWP: half-wave plate and FD: fluorescence detector).
3. Experimental Results
Figure 3(a) shows the typical EIT spectrum with the travelling-wave LC laser and the enhanced absorption spectrum with the standing-wave LC field in the 5S1/2-5P1/2 transition of 87Rb atoms. The horizontal axis denotes the detuning of the LP laser from the resonant frequency of the 5S1/2(F = 1) – 5P1/2(F’ = 2) transition. The powers of the LC and LP lasers were 15 mW and 16 μW, respectively. The cell temperature was 90 °C. The EIT with a narrow spectral width could be obtained when the LP laser was co-propagated with the LC laser in the Rb vapor cell. The spectral width of the EIT was measured to be approximately 6 MHz, and included power broadening as well as all the experimental parameters, such as laser line width, collision effect and laser frequency dither.
Fig. 3 The typical EIT spectrum with a travelling-wave coupling field and enhanced absorption spectrum with a standing-wave coupling field in the Λ-type system of the 5S1/2-5P1/2 transition of 87Rb atoms; (a) experimental results and (b) calculated results.
When the standing-wave LC field was generated by adding the counter-propagating LC laser, the spectrum of the LP laser transformed the EIT into dramatically enhanced absorption. The spectral width of the enhanced absorption was measured to be approximately 6 MHz with a coupling power of 15 mW and a sub-natural spectral width of 4.7 MHz with a coupling power of 1 mW. This enhanced absorption was similar to that of the reversed-EIT well-known Lorentzian profile. This enhanced absorption was explained as the effect of the spatial harmonics of atomic coherence [20]. The enhanced absorption spectrum was due only to the atoms in the region of zero velocity to the direction of laser propagation as a result of the velocity selective effect. The laser frequencies of the counter-propagating two coupling fields in the moving atoms were different from each other due to the Doppler Effect. Therefore, the enhanced absorption signal was attributed to an interaction between the standing-wave LC field and slow atoms.
In order to theoretically understand the transformation of EIT into the narrow enhanced absorption, we present the calculated results with an on-resonant standing-wave coupling field, as shown in Fig. 3(b). The density-matrix equations with the standing-wave based on the three-level Λ-type atomic system are solved and the continued-fraction expansion is calculated and integrated with Maxwell-distribution numerically [20,25]. The parameters for the calculation were the decay rate between the ground states 0.5 MHz, the laser dither 2 MHz, the Rabi frequencies of the coupling field 10 MHz and the probe field 0.1 MHz. The most probable velocity in a Maxwell-distribution was 270 m/s. Although there are discrepancies between the experimental and the calculated results, we can see the transformation of EIT into the narrow enhanced absorption from the calculated results. The discrepancy is why the calculated result in the simple three-level Λ-type atomic system is not considered the optical pumping effect, the atom-laser interaction time and the Zeeman sublevels.
The transmission, fluorescence and reflection of the LP laser were measured simultaneously to determine the effect of the enhanced absorption and reflection, as shown in Fig. 4 . The fluorescence in the side of the Rb cell was observed and the enhanced absorption spectrum was measured by the PD1 from Fig. 2. The narrow absorption structure could also be observed from the fluorescence spectrum. The feature of the fluorescence spectrum was similar to one of the enhanced absorption spectrum. However, the magnitude of the narrow absorption structure in the fluorescence spectrum was half that in the enhanced absorption spectrum due to the Bragg reflection. The background absorption spectrum of the fluorescence and transmittance was due to absorption of the LP laser in the nodes of the standing-wave coupling field.
Fig. 4 Three simultaneously measured spectra with the standing-wave coupling field; the enhanced absorption spectrum by measuring the forward transmittance LP signal, the fluorescence spectrum by measuring at the side of the Rb vapor cell, and the Bragg reflection spectrum by measuring the backward reflection LP signal.
As noted above, Bragg reflection of the LP laser by periodic modulation of the atomic absorption is another cause of the narrow structure of the transmittance spectrum of the LP laser with the standing-wave coupling laser [17–19]. Under the condition of Bragg reflection, a fraction of the LP laser was simultaneously reflected and absorbed by the periodic modulation of atomic absorption [17]. The reflected laser was measured to be approximately 11.5% of the incident LP laser. The spectral width of the Bragg reflection was measured to be approximately 11 MHz broader than the spectra of both the narrow absorption structures in the transmittance and fluorescence spectrum. The reason for the broad spectral width of the Bragg reflection is the absorptive property. Since an absorptive Bragg grating was not made by changing the refractive index but by the periodic modulation of LP absorption by induced interference of the two LC laser, the effect of the Bragg grating was not stronger than that due to the periodicity of the refractive index [15]. Therefore, the narrow structure of the transmittance spectrum of Fig. 4 was due to the effects of the spatial harmonics of atomic coherence and the Bragg reflection.
The power disparity between the co-propagating and the counter-propagating LC lasers decreases the contrast of the standing-wave LC field. The additional effects of travelling-wave coupling should be considered because the power of the coupling laser in the nodes is not zero. The additional effects are dependent on the power ratio between the co-propagating and counter-propagating coupling lasers. This study examined the enhanced absorption and Bragg reflection according to the power disparity between the co-propagating and counter-propagating LC lasers, as shown in Figs. 5 and 6 .
Fig. 5 The measured (a) enhanced absorption and (b) Bragg reflection spectra according to the power of the co-propagating LC laser, where the power of the counter-propagating LC laser was 15 mW and one of the co-propagating LC laser was changed from 1 to 15 mW.
Fig. 6 The measured (a) enhanced absorption and (b) Bragg reflection spectra according to the power of the counter-propagating LC laser, where the power of the co-propagating LC laser was 15 mW and the power of the counter-propagating LC laser was changed from 50 μW to 15 mW.
Figures 5(a) and 5(b) show the enhanced absorption and Bragg reflection spectra according to the power of the co-propagating LC laser, where the power of the LP laser was 16 μW and the cell temperature was 90 °C. In this case, the power of the counter-propagating LC laser was fixed to 15 mW and one of the co-propagating LC lasers was changed from 1 mW to 15 mW. The magnitudes of the enhanced absorption peak and Bragg reflection spectra decreased with decreasing power of the co-propagating LC laser, as shown in Figs. 5(a) and 5(b), respectively. Because of the low contrast of the standing-wave LC field, the decrease in the enhanced absorption peak and Bragg reflection spectra can be understood as a decrease in the effect of the spatial harmonics of atomic coherence and reflection resulting from the spatially modulated absorption. The decreasing magnitude of the enhanced absorption peak was the same as that of the Bragg reflection spectrum in the measurement error range.
When the power of the co-propagating LC laser was < 15 mW, the transmittance spectrum was altered by the additional effect by the counter-propagating LC laser, as shown in Fig. 5(a). In the Λ-type atomic system, velocity selective optical pumping is an additional effect of the extra counter-propagating LC laser because the two-photon resonance condition is not Doppler-free. The spectral profile of the Bragg reflection was separated into double peaks (Fig. 5(b)) when the power of the co-propagating LC laser reached approximately 3 mW. Distinct double peaks could be observed in the case of the co-propagating LC laser power of 1 mW, as shown in the insert figure of Fig. 5(b).
Figures 6(a) and 6(b) show the enhanced absorption and Bragg reflection spectra according to the power of the co-propagating LC laser when the power of the co-propagating LC laser was fixed to 15 mW and one of the counter-propagating LC laser was changed from 50 μW to 15 mW. The magnitudes of the enhanced absorption peak and Bragg reflection spectra decreased with decreasing power of the co-propagating LC laser, as shown in Fig. 6(a) and 6(b). A comparison of Fig. 5(a) with Fig. 6(a) showed that the change in the transmittance spectra in Fig. 6(a) is quite different from that shown in Fig. 5(a) due to the different additional effects between the two cases. As is well known, the additional effect by the extra co-propagating LC laser in the Λ-type atomic system was the EIT. When the power of the counter-propagating LC laser reached approximately 3 mW, the transmittance due to EIT could be observed in the enhanced absorption peak, as shown in the insert figure in Fig. 6(a).
In addition, the spectral profile of the Bragg reflection in Figs. 5(b) and 6(b) was separated into double peaks. This suggests that the double peaks are due to the asymmetric ratio of the power between the counter-propagating and co-propagating LC laser. A comparison of Fig. 5(b) with Fig. 6(b) showed that the magnitude and spectral width of the Bragg reflection spectra are similar in the two cases due to spatially modulated absorption.
The frequency detuning dependence of the LC laser was examined. Figures 7(a) and 7(b) show the transmission spectrum and reflection spectrum as a function of the LC laser detuning from the 5S1/2(F = 2) - 5P1/2(F’ = 2) transition, respectively. The transmission and reflection spectra were measured simultaneously as functions of frequency detuning of the LC laser from −97 MHz to 115 MHz. The powers of the LP and LC lasers were 16 μW and 15 mW, respectively. The vapor cell temperature was 90 °C. Moving atoms interact with the opposite detuned LC lasers according to the co-propagating and counter-propagating lasers. The two-photon resonance condition in the Doppler-free atoms was dependent on the LC laser detuning frequency. Therefore, standing-wave LC field detuning affects the magnitude and spectrum profile due to the difference in the two-photon atomic coherence and velocity selective effect.
Fig. 7 The measured (a) transmission and (b) reflection spectra as a function of frequency detuning of the coupling laser.
When the frequency of the coupling laser was detuned from the resonance, we could observe that the transmission spectrum of Fig. 7(a) transformed the enhanced absorption into the EIT through the dispersive-like spectrum. The dispersive-like spectrum was attributed to EIT and enhanced absorption. Here, the resonance of EIT corresponded to the two-photon resonance condition with the LP and co-propagating LC laser in an atomic vapor. However, the effect of the spatial harmonics of atomic coherence was caused by the simultaneous action of both components of the standing-wave LC fields. The resonance of the enhanced absorption was formed by atoms with zero velocity and synchronous with the 5S1/2(F = 1) - 5P1/2(F’ = 2) and 5S1/2(F = 2) - 5P1/2(F’ = 2) transitions.
Figure 7(b) shows the rapid decrease in the Bragg reflection spectra according to LC laser detuning from the 5S1/2(F = 2) - 5P1/2(F’ = 2) transition. When the LC laser detuning frequency was −97 MHz and 115 MHz, the magnitude of the reflection was reduced by approximately 90% and 94% of the resonant reflection, respectively. A comparison of the results from a pure Rb vapor cell and vapor cell with a buffer gas revealed that the decrease in the Bragg reflection spectra according to LC laser detuning was faster in the pure Rb cell than in the Rb cell with a buffer gas [19]. This was attributed to a long interaction with the atom and laser due to collisions with a buffer gas.
4. Conclusion
This study examined the enhanced absorption and Bragg reflection with a travelling-wave probe field and standing-wave coupling field in a pure Rb vapor cell. When the coupling laser field was changed from a travelling-wave to a standing-wave made by adding a counter-propagating LC laser, the transmittance spectrum of the LP laser transformed the typical EIT into dramatically enhanced absorption, and a Bragg reflection signal was generated by the periodic modulation of atomic absorption. The spectral width of the enhanced absorption spectrum was measured to be sub-natural width < 5 MHz with a coupling power of 1 mW. This enhanced absorption was explained as the effect of the higher spatial harmonics of atomic coherence.
Three signals, such as the transmission, fluorescence, and reflection of the LP laser were observed simultaneously to explain the effect of the enhanced absorption and Bragg reflection. The reflected laser was measured to be approximately 11.5% of the incident LP laser. The spectral width of the Bragg reflection was measured to be approximately 11 MHz broader than that of both the narrow absorption structures in the transmittance and fluorescence spectra. The difference between the transmittance and fluorescence spectra, suggested that the narrow absorption structure of the transmittance spectrum was due to both the effect of spatial harmonics of atomic coherence and the Bragg reflection.
The change in the transmittance and the reflection signals was investigated according to the power disparity between the co-propagating and counter-propagating LC lasers. The contrast of the standing-wave LC field decreased with increasing power disparity. Different transmittance spectra were observed in the two cases of both the extra co-propagating LC field and extra counter-propagating LC field due to the two different additional effects. Transmittance due to EIT in the enhanced absorption peak was observed in the case of the extra co-propagating LC field. However, the Bragg reflection spectra were similar in the two cases due to spatially modulated absorption. When the frequency of the coupling laser was detuned from resonance, the transmission spectrum of the LP laser transformed the enhanced absorption into EIT through a dispersive-like spectrum. The dispersive-like spectrum could be explained by EIT and enhanced absorption. Overall, these results are expected to improve the understanding of atomic coherence with a standing-wave coupling field.
Acknowledgement:
This work was supported by the Korea Research Foundation Grant funded by the Korean government(MOEHRD, Basic Research Promotion Fund)(KRF-2008-314-C00075) and by Basic Science Research Program through the National Research Foundation of Korea(NRF) grant funded by the Korea government(MEST)(No. 2009-0073051).
References and links
1. K. J. Boller, A. Imamolu, and S. E. Harris, “Observation of electromagnetically induced transparency,” Phys. Rev. Lett. 66(20), 2593–2596 (1991). [CrossRef] [PubMed]
2. S. E. Harris, J. E. Field, and A. Kasapi, “Dispersive properties of electromagnetically induced transparency,” Phys. Rev. A 46(1), R29–R32 (1992). [CrossRef] [PubMed]
3. D. J. Fulton, S. Shepherd, R. R. Moseley, B. D. Sinclair, and M. H. Dunn, “Continuous-wave electromagnetically induced transparency: A comparison of V, Λ, and cascade systems,” Phys. Rev. A 52(3), 2302–2311 (1995). [CrossRef] [PubMed]
4. M. Xiao, Y. Li, S. Jin, and J. Gea-Banacloche, “Measurement of dispersive properties of electromagnetically induced transparency in rubidium atoms,” Phys. Rev. Lett. 74(5), 666–669 (1995). [CrossRef] [PubMed]
5. Y. Li, S. Jin, and M. Xiao, “Observation of an electromagnetically induced change of absorption in multilevel rubidium atoms,” Phys. Rev. A 51(3), R1754–1757 (1995). [CrossRef] [PubMed]
6. R. R. Moseley, S. Shepherd, D. J. Fulton, B. D. Sinclair, and M. H. Dunn, “Spatial consequences of electromagnetically induced transparency: Observation of electromagnetically induced focusing,” Phys. Rev. Lett. 74(5), 670–673 (1995). [CrossRef] [PubMed]
7. S. A. Hopkins, E. Usadi, H. X. Chen, and A. V. Durrant, “Electromagnetically induced transparency of laser-cooled rubidium atoms in three-level Λ-type systems,” Opt. Commun. 138(1-3), 185–192 (1997). [CrossRef]
8. H. S. Moon, S. E. Park, Y. H. Park, L. Lee, and J. B. Kim, “Passive atomic frequency standard based on coherent population trapping in 87Rb using injection-locked lasers,” J. Opt. Soc. Am. B 23(11), 2393–2397 (2006). [CrossRef]
9. A. Kuzmich, W. P. Bowen, A. D. Boozer, A. Boca, C. W. Chou, L.-M. Duan, and H. J. Kimble, “Generation of nonclassical photon pairs for scalable quantum communication with atomic ensembles,” Nature 423(6941), 731–734 (2003). [CrossRef] [PubMed]
10. D. Budker and M. V. Romalis, “Optical Magnetometry,” Nat. Phys. 3(4), 227–234 (2007). [CrossRef]
11. L. V. Hau, S. E. Harris, Z. Dutton, and C. H. Behroozi, “Light speed reduction to 17 metres per second in an ultracold atomic gas,” Nature 397(6720), 594–598 (1999). [CrossRef]
12. M. D. Lukin and A. Imamoğlu, “Controlling photons using electromagnetically induced transparency,” Nature 413(6853), 273–276 (2001). [CrossRef] [PubMed]
13. D. F. Phillips, A. Fleischauer, A. Mair, R. L. Walsworth, and M. D. Lukin, “Storage of light atomic vapor,” Phys. Rev. Lett. 86(5), 783–786 (2001). [CrossRef] [PubMed]
14. A. André and M. D. Lukin, “Manipulating light pulses via dynamically controlled photonic band gap,” Phys. Rev. Lett. 89(14), 143602 (2002). [CrossRef] [PubMed]
15. M. Bajcsy, A. S. Zibrov, and M. D. Lukin, “Stationary pulses of light in an atomic medium,” Nature 426(6967), 638–641 (2003). [CrossRef] [PubMed]
16. X. M. Su and B. S. Ham, “Dynamic control of the photonic band gap using quantum coherence,” Phys. Rev. A 71(1), 013821 (2005). [CrossRef]
17. H. Y. Ling, Y. Q. Li, and M. Xiao, “Electromagnetically induced grating: Homogeneously broadened medium,” Phys. Rev. A 57(2), 1338–1344 (1998). [CrossRef]
18. M. Mitsunaga and N. Imoto, “Observation of an electromagnetically induced grating in cold sodium atoms,” Phys. Rev. A 59(6), 4773–4776 (1999). [CrossRef]
19. I. H. Bae, H. S. Moon, M. K. Kim, L. Lee, and J. B. Kim, “Electromagnetically induced Bragg reflection with a stationary coupling field in a buffer rubidium vapor cell,” Appl. Opt. 47(27), 4849–4855 (2008). [CrossRef] [PubMed]
20. S. A. Babin, D. V. Churkin, E. V. Podivilov, V. V. Potapov, and D. A. Shapiro, “Splitting of the peak of electromagnetically induced transparency by the higher-order spatial harmonics of the atomic coherence,” Phys. Rev. A 67(4), 043808 (2003). [CrossRef]
21. A. W. Brown and M. Xiao, “All-optical switching and routing based on an electromagnetically induced absorption grating,” Opt. Lett. 30(7), 699–701 (2005). [CrossRef] [PubMed]
22. F. Silva, J. Mompart, V. Ahufinger, and R. Corbalan, “Electromagnetically induced transparency in Doppler-broadened three-level systems with resonant standing-wave drive,” Europhys. Lett. 51(3), 286–292 (2000). [CrossRef]
23. F. Silva, J. Mompart, V. Ahufinger, and R. Corbalan, “Electromagnetically induced transparency with a standing-wave drive in the frequency up-conversion regime,” Phys. Rev. A 64(3), 0033802 (2001). [CrossRef]
24. D. V. Strekalov, A. B. Matsko, and N. Yu, “Electromagnetically induced transparency with a partially standing drive field,” Phys. Rev. A 76(5), 053828 (2007). [CrossRef]
25. B. J. Feldman and M. S. Feld, “Laser-induced line-narrowing effects in coupled Doppler-broadened transition. II. Standing-wave features,” Phys. Rev. A 5(2), 899–918 (1972). [CrossRef]
