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Quantum Electronics and Laser Science Conference

First observation of ultraslow group velocity of light in a solid

Alexey V. Turukhin, Venkatapura S. Sudarshanam, M. S. Shahriar, J. A. Musser, Philip Hemmer

Published Nov 13, 2002• 8 citations
Publisher

Abstract

Summary form only given. Electromagnetically induced transparency (EIT) has been the subject of many experimental and theoretical studies over recent years. Most recently, substantial experimental and theoretical work in this area has been focused on the use of EIT to give ultraslow group velocities for light in both ultracold atomic gas and hot atomic vapors. Much of this interest is based on the fact that slowing the group velocity of a laser pulse down to the speed of sound in the same material can produce strong coupling between acoustic waves and the electromagnetic field. It has been suggested that a giant nonlinearity obtained by this method might be utilized for efficient multi-wave mixing and quantum nondemolition measurements. Slow light might also allow a very efficient nonlinear interaction between laser pulses of extremely low (down to a single photon) energies. Here, we report the first direct observation of an ultraslow group velocity of light down to 45 m/s in a solid, well below the speed of sound in the medium. This group velocity is fundamentally limited only by the inhomogeneous broadening of the ground state transition. At low coupling beam intensities, the fraction of centers that are pumped into the "dark state" increases with coupling laser power, so that the observed group delay is increasing with coupling beam intensity. At higher intensities, power broadening reduces the sharpness of the dispersive feature and the observed group delay decreases.

DOI
10.1109/qels.2001.961775

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The group velocity at which light pulses propagate through a dispersive material system is very different from the vacuum speed of light c, One refers to light as being "slow" for v g << c (Boyd & Gauthier, 2009) or "fast" for v g > c or v g <0 (Stenner et al, 2003 ).For v g <0, the pulse envelope appears to travel backward in the material (Gehring et al, 2006), and hence it is sometimes referred to as "backward light."The subject of slow light has caused keen interest in the past decade or more, and it is possible to control the group velocity of light pulses in the dispersive materials.Interest in slow and fast light dates back to the early days of the 20th century.Sommerfeld and Brillouin (Sommerfeld & Brillouin, 1960) were intrigued by the fact that theory predicts that v g can exceed c, which leads to apparent inconsistencies with Einstein's special theory of relativity.Experimental investigations of extreme propagation velocities were performed soon after the invention of the laser (Faxvog and et al, 1970).In 1999, Harris's group research work greatly stimulated researchers' interests, which showed that light could be slowed down to 17m/s.The result was obtained in ultra cold atom clouds with the use of electromagnetically induced transparency (EIT), which induces transparency in a material while allowing it to retain strong linear and nonlinear optical properties (Hau et al, 1999).Slow light can also be obtained through the use of the optical response of hot atomic vapors (Philips et al, 2001).These early research works require hard conditions and the slow light cannot operate in room temperature.Recently, researchers found ways to realize slow light operating in room temperature and solid-state materials, which are more suited for many practical applications, namely slow light via stimulated Brillouin scattering(SBS), slow light via coherent population oscillations (CPO), tunable time delays based on group velocity dispersion or conversion/ dispersion(C/D), slow light in fiber Bragg gratings and so on.In this chapter, we describe some of the physical mechanisms that can be used to induce slow and fast light effects in room-temperature solids (Bigelow et al, 2003) and some of the exotic propagation effects that can thereby be observed.We also survey some applications of slow and fast light within the fields of quantum electronics and photonics. Fundamentals of slow and fast lightSlow and fast light refer to the group velocity of a light wave.The group velocity is the velocity most closely related to the velocity at which the peak of a light pulse moves through an optical dispersive material (Milonni, 2005), and is given by the standard result www.intechopen.comRecent Progress in Optical Fiber Research 82where n is the refractive (phase) index and ω is the angular frequency of the carrier wave of the light field.One refers to light as being slow light for v g << c, fast light for for v g > c, and backwards light for v g <0 or v g is negative.Extreme values of the group velocity invariably rely on the dominance of the second contribution to the group index of Equation ( 1).This contribution of course results from the frequency dependence of the refractive index, and for this reason extreme values of the group velocity are usually associated with the resonant or near-resonant response of material systems.According to this theory, slow light is expected in the wings of an absorption line and fast light is expected near line center, and the spatial dispersion, that is, the non-locality in space of the medium response, is another mechanism that can lead to slow light, as has been predicted (Kocharovskaya et al,2001) and observed (Strekalov et al, 2001).We catalogue the main methods to realize slow light in room temperature solid, namely slow light via stimulated Brillouin scattering(SBS), slow light via coherent population oscillations (CPO), tunable time delays based on group velocity dispersion or conversion/ dispersion(C/D), slow light in fiber Bragg gratings and so on.We will describe CPO and SBS slow light in more details.

InTech eBooks•Jan 25, 2012•0 citations
Summary form only given. Electromagnetically induced transparency (EIT) has been the subject of many experimental and theoretical studies over recent years. Most recently, substantial experimental and theoretical work in this area has been focused on the use of EIT to give ultraslow group velocities for light in both ultracold atomic gas and hot atomic vapors. Much of this interest is based on the fact that slowing the group velocity of a laser pulse down to the speed of sound in the same material can produce strong coupling between acoustic waves and the electromagnetic field. It has been suggested that a giant nonlinearity obtained by this method might be utilized for efficient multi-wave mixing and quantum nondemolition measurements. Slow light might also allow a very efficient nonlinear interaction between laser pulses of extremely low (down to a single photon) energies. Here, we report the first direct observation of an ultraslow group velocity of light down to 45 m/s in a solid, well below the speed of sound in the medium. This group velocity is fundamentally limited only by the inhomogeneous broadening of the ground state transition. At low coupling beam intensities, the fraction of centers that are pumped into the "dark state" increases with coupling laser power, so that the observed group delay is increasing with coupling beam intensity. At higher intensities, power broadening reduces the sharpness of the dispersive feature and the observed group delay decreases.
We report ultraslow group velocities of light in a solid. Light speeds as slow as 45 m/s were observed, corresponding to a group delay of 66 microsecond(s) in a 3-mm thick, optically dense crystal of Pr doped Y2SiO5. Reduction of the group velocity is accomplished by using a sharp spectral feature in absorption and dispersion that is produced by resonance Raman excitation of a ground-state spin coherence. Potential applications of slow and stopped light for the highly efficient storage and recall of optical data are discussed.
We prove that it is possible to freeze a light pulse (i.e., to bring it to a full stop) or even to make its group velocity negative in a coherently driven Doppler broadened atomic medium via electromagnetically induced transparency (EIT). This remarkable phenomenon of the ultraslow EIT polariton is based on the spatial dispersion of the refraction index n(omega,k), i.e., its wave number dependence, which is due to atomic motion and provides a negative contribution to the group velocity. This is related to, but qualitatively different from, the recently observed light slowing caused by large temporal (frequency) dispersion.
Summary form only given. The recent observation of slow light propagation by Harris and Hau (1999) motivates the study of nonlinear optical processes under conditions where slow group velocity, or equivalently large nonlinearity, is the dominant feature of the problem. By combining electromagnetically induced transparency (EIT) and cold atom technology, a sharp resonance is created with a transmission linewidth much less than the natural linewidth of the atoms. A pulse of light propagating in this medium has a phase velocity of c and a group velocity that can be less than 10/sup -7/ c. As a light pulse of sufficiently wide bandwidth enters a cold-atom, EIT medium, the pulse shape, electric field strength, and power density remain nominally unchanged. Concurrently, because the group velocity is decreased, the stored energy per volume must increase. This increase occurs by spatial compression of the travelling pulse. A pulse measuring 300 m in free space compresses to 30 /spl mu/m within the cold-atom medium.
We show that the group velocities of light pulses can be decelerated dramatically by the use of a dispersive phase-coupling effect through a wave mixing process. We have observed experimentally such a phase-coupling-induced ultraslow light propagation with a group velocity as low as 0.05  m/s0.05\text{ }\text{ }\mathrm{m}/\mathrm{s}0.05  m/s in a photorefractive Bi12SiO20{\mathrm{B}\mathrm{i}}_{12}{\mathrm{S}\mathrm{i}\mathrm{O}}_{20}Bi12​SiO20​ crystal at room temperature. Moreover, the ultraslow light is amplified in the Bi12SiO20{\mathrm{B}\mathrm{i}}_{12}{\mathrm{S}\mathrm{i}\mathrm{O}}_{20}Bi12​SiO20​ crystal because of the unidirectional energy transfer from a coupling beam to the ultraslow light. This technique to produce ultraslow light propagation is valid for all nonlinear wave mixing processes with a dispersive phase-coupling effect.
The group velocity at which light pulses propagate through a dispersive material system is very different from the vacuum speed of light c, One refers to light as being "slow" for v g << c (Boyd & Gauthier, 2009) or "fast" for v g > c or v g <0 (Stenner et al, 2003 ).For v g <0, the pulse envelope appears to travel backward in the material (Gehring et al, 2006), and hence it is sometimes referred to as "backward light."The subject of slow light has caused keen interest in the past decade or more, and it is possible to control the group velocity of light pulses in the dispersive materials.Interest in slow and fast light dates back to the early days of the 20th century.Sommerfeld and Brillouin (Sommerfeld & Brillouin, 1960) were intrigued by the fact that theory predicts that v g can exceed c, which leads to apparent inconsistencies with Einstein's special theory of relativity.Experimental investigations of extreme propagation velocities were performed soon after the invention of the laser (Faxvog and et al, 1970).In 1999, Harris's group research work greatly stimulated researchers' interests, which showed that light could be slowed down to 17m/s.The result was obtained in ultra cold atom clouds with the use of electromagnetically induced transparency (EIT), which induces transparency in a material while allowing it to retain strong linear and nonlinear optical properties (Hau et al, 1999).Slow light can also be obtained through the use of the optical response of hot atomic vapors (Philips et al, 2001).These early research works require hard conditions and the slow light cannot operate in room temperature.Recently, researchers found ways to realize slow light operating in room temperature and solid-state materials, which are more suited for many practical applications, namely slow light via stimulated Brillouin scattering(SBS), slow light via coherent population oscillations (CPO), tunable time delays based on group velocity dispersion or conversion/ dispersion(C/D), slow light in fiber Bragg gratings and so on.In this chapter, we describe some of the physical mechanisms that can be used to induce slow and fast light effects in room-temperature solids (Bigelow et al, 2003) and some of the exotic propagation effects that can thereby be observed.We also survey some applications of slow and fast light within the fields of quantum electronics and photonics. Fundamentals of slow and fast lightSlow and fast light refer to the group velocity of a light wave.The group velocity is the velocity most closely related to the velocity at which the peak of a light pulse moves through an optical dispersive material (Milonni, 2005), and is given by the standard result www.intechopen.comRecent Progress in Optical Fiber Research 82where n is the refractive (phase) index and ω is the angular frequency of the carrier wave of the light field.One refers to light as being slow light for v g << c, fast light for for v g > c, and backwards light for v g <0 or v g is negative.Extreme values of the group velocity invariably rely on the dominance of the second contribution to the group index of Equation ( 1).This contribution of course results from the frequency dependence of the refractive index, and for this reason extreme values of the group velocity are usually associated with the resonant or near-resonant response of material systems.According to this theory, slow light is expected in the wings of an absorption line and fast light is expected near line center, and the spatial dispersion, that is, the non-locality in space of the medium response, is another mechanism that can lead to slow light, as has been predicted (Kocharovskaya et al,2001) and observed (Strekalov et al, 2001).We catalogue the main methods to realize slow light in room temperature solid, namely slow light via stimulated Brillouin scattering(SBS), slow light via coherent population oscillations (CPO), tunable time delays based on group velocity dispersion or conversion/ dispersion(C/D), slow light in fiber Bragg gratings and so on.We will describe CPO and SBS slow light in more details.