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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Slow and fast light switching in ruby

Rajitha Papukutty Rajan, Hans Riesen

Published May 18, 2015• 4 citations
Publisher

Abstract

Studies about light propagation have been undertaken for more than a century. It is now well established that any material that has normal or anomalous dispersion generates slow or fast light. In this paper, we demonstrate an experimental technique to rapidly switch between slow and fast light in ruby. The experiment utilizes transient holeburning to create drastic variation in refractive index of ruby to produce slow as well as fast light. Transient hole-burning involves the depletion of the ground state leading to a highly populated excited state by single frequency laser excitation. This leads to a hole in the absorption spectrum when readout by a laser. We observed a delay of 29 ns and advancement of -11 ns in an external magnetic field of B║c = 12 mT corresponding to a group velocity of c/961 and negative group velocity of –c/365 respectively.

DOI
10.1117/12.2182363

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Studies about light propagation have been undertaken for more than a century. It is now well established that any material that has normal or anomalous dispersion generates slow or fast light. In this paper, we demonstrate an experimental technique to rapidly switch between slow and fast light in ruby. The experiment utilizes transient holeburning to create drastic variation in refractive index of ruby to produce slow as well as fast light. Transient hole-burning involves the depletion of the ground state leading to a highly populated excited state by single frequency laser excitation. This leads to a hole in the absorption spectrum when readout by a laser. We observed a delay of 29 ns and advancement of -11 ns in an external magnetic field of B║c = 12 mT corresponding to a group velocity of c/961 and negative group velocity of –c/365 respectively.
We report on the observation of slow light induced by transient spectral hole-burning in a solid, that is based on excited-state population storage. Experiments were conducted in the R1-line (2E←4A2 transition) of a 2.3 mm thick pink ruby (Al2O3:Cr(III) 130 ppm). Importantly, the pulse delay can be controlled by the application of a low external magnetic field B||c≤9 mT and delays of up to 11 ns with minimal pulse distortion are observed for ~55 ns Gaussian pulses. The delay corresponds to a group velocity value of ~c/1400. The experiment is very well modelled by linear spectral filter theory and the results indicate the possibility of using transient hole-burning based slow light experiments as a spectroscopic technique.
The generation of slow and fast light in the same transient hole-burning experiment by two laser-frequency modulation methods is reported. The schemes allow for rapid switching between slow and fast light on very short time scales. Experiments were carried out in the R1(±3/2) line of a pale green (0.04 wt. % Cr) emerald, Be3Al2Si6O18:Cr3+ in π polarization (E∥c) at 2.3 K in a low magnetic field B∥c of 12 mT. Maximum delays and advancements for a 115 ns Gaussian pulse were 65 and −25 ns, respectively, corresponding to group indices of 7.8·103 and −3·103. The present schemes point to possible applications in all-optical switches.
Slow light based on transient spectral hole-burning is reported for emerald, Be(3)Al(2)Si(6)O(18):Cr(3+). Experiments were conducted in π polarization on the R(1)(± 3/2) line (E2 ← A(2)4) at 2.2 K in zero field and low magnetic fields B||c. The hole width was strongly dependent on B||c, and this allowed us to smoothly tune the pulse delay from 40 to 154 ns between zero field and B||c = 15.2 mT. The latter corresponds to a group velocity of 16 km/s. Slow light in conjunction with a linear filter theory can be used as a powerful and accurate technique in time-resolved spectroscopy, e.g., to determine spectral hole-widths as a function of time.
Reverse saturation of absorption is a relatively rare phenomenon in light–matter interaction, as it requires a few conditions to be fulfilled. We observe that ruby exhibits a very strong reverse saturation of absorption at 473 nm. Furthermore, we measure the group velocity of a modulated laser beam in ruby and observe that the peaks of the pulses appear more than a hundred microseconds earlier than the reference signal. A theoretical model based on coherent population oscillation would suggest a fast-light effect with an extremely large and negative group index of −(1.7 ± 0.1) × 10 6 in consistency with the observed temporal advancement. We propose that this pulse advancement can also be described by time-dependent absorption of ruby. Our study helps to understand the nature of the fast- and slow-light effects in transition-metal-doped crystals such as ruby and alexandrite with potential applications in optical memories and delay lines.
We have observed slow light propagation with a group velocity as low as 57.5+/-0.5 m/s at room temperature in a ruby crystal. A quantum coherence effect, coherent population oscillations, produces a very narrow spectral "hole" in the homogeneously broadened absorption profile of ruby. The resulting rapid spectral variation of the refractive index leads to a large value of the group index. We observe slow light propagation both for Gaussian-shaped light pulses and for amplitude modulated optical beams in a system that is much simpler than those previously used for generating slow light.