Showing papers on "Slow light published in 2003"

Superluminal and Slow Light Propagation in a Room-Temperature Solid

Abstract: We have observed both superluminal and ultraslow light propagation in an alexandrite crystal at room temperature. Group velocities as slow as 91 meters per second to as fast as -800 meters per second were measured and attributed to the influence of coherent population oscillations involving chromium ions in either mirror or inversion sites within the crystal lattice. Namely, ions in mirror sites are inversely saturable and cause superluminal light propagation, whereas ions in inversion sites experience conventional saturable absorption and produce slow light. This technique for producing large group indices is considerably easier than the existing methods to implement and is therefore suitable for diverse applications.

Observation of ultraslow light propagation in a ruby crystal at room temperature.

Abstract: We have observed slow light propagation with a group velocity as low as $57.5\ifmmode\pm\else\textpm\fi{}0.5\text{ }\mathrm{m}/\mathrm{s}$ 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.

Optical pulse propagation in metal nanoparticle chain waveguides

Abstract: Finite-difference time-domain simulations show direct evidence of optical pulse propagation below the diffraction limit of light along linear arrays of spherical noble metal nanoparticles with group velocities up to 0.06c. The calculated dispersion relation and group velocities correlate remarkably well with predictions from a simple point-dipole model. A change in particle shape to spheroidal particles shows up to a threefold increase in group velocity. Pulses with transverse polarization are shown to propagate with negative phase velocities antiparallel to the energy flow.

Refraction in media with a negative refractive index.

Abstract: We show that an electromagnetic (EM) wave undergoes negative refraction at the interface between a positive and negative refractive index material, the latter being a properly chosen photonic crystal. Finite-difference time-domain (FDTD) simulations are used to study the time evolution of an EM wave as it hits the interface. The wave is trapped temporarily at the interface, reorganizes, and, after a long time, the wave front moves eventually in the negative direction. This particular example shows how causality and speed of light are not violated in spite of the negative refraction always present in a negative index material.

Variable optical buffer using slow light in semiconductor nanostructures

Abstract: A compact variable all-optical buffer using semiconductor quantum dot (QD) structures is proposed and analyzed. The buffering effect is achieved by slowing down the optical signal using an external control light source to vary the dispersion characteristic of the medium via an electromagnetically induced transparency effect. We present a theoretical investigation of the criteria for achieving slow light in semiconductor QDs. A QD structure in the presence of strain is analyzed with the inclusion of polarization-dependent intersubband dipole selection rules. Experimental methods to synthesize and the measurements of coherent properties in state-of-the-art QDs are surveyed. Slow-light effects in uniform and nonuniform QDs are compared. Finally, optical signal propagation through the semiconductor optical buffer is presented to demonstrate the feasibility for practical applications.

Low-light-level nonlinear optics with slow light

Abstract: Electromagnetically induced transparency in an optically thick, cold medium creates a unique system where pulse-propagation velocities may be orders of magnitude less than c and optical nonlinearities become exceedingly large. As a result, nonlinear processes may be efficient at low-light levels. Using an atomic system with three, independent channels, we demonstrate a quantum interference switch where a laser pulse with an energy per area of $\ensuremath{\sim}23$ photons per ${\ensuremath{\lambda}}^{2}/(2\ensuremath{\pi})$ causes a $1/e$ absorption of a second pulse.

On slow light as a black hole analogue

Abstract: Although slow light (electromagnetically induced transparency) would seem an ideal medium in which to institute a ``dumb hole'' (black hole analogue), it suffers from a number of problems. We show that the high phase velocity in the slow light regime ensures that the system cannot be used as an analogue displaying Hawking radiation. Even though an appropriately designed slow-light setup may simulate classical features of black holes---such as horizon, mode mixing, ``Bogoliubov'' coefficients, etc.---it does not reproduce the related quantum effects.

Guided modes in channel waveguides with a negative index of refraction

Abstract: The guided modes of a negative refractive index channel waveguide have been numerically investigated. It has been found that the modes exhibit a number of unusual properties that differ considerably from those of a conventional waveguide. In particular, it has been shown that these waveguides can exhibit low or negative group velocity as well as extraordinarily large group velocity dispersion. Calculation of the Poynting vector reveals that it is possible to support a mode with a zero energy flux motivating a simple design for an optical trap.

Slow light in Doppler-broadened two-level systems

Abstract: We show that the propagation of light in a Doppler-broadened medium can be slowed down considerably even though such medium exhibits very flat dispersion. The slowing down is achieved by the application of a saturating counter propagating beam that produces a hole in the inhomogeneous line shape. In atomic vapors, we calculate group indices of the order of 10 3 . The calculations include all coherence effects.

Group velocity and dispersion model of coupled-cavity waveguides in photonic crystals.

Abstract: A theoretical model of the group velocity, dispersion parameter, and dispersion slope of coupled-cavity waveguides in photonic crystals is reported. Results arising from closed-form expressions show a good agreement with simulation results obtained by employing a plane-wave expansion method. Coupled-cavity waveguides present interesting dispersion properties that may be employed in applications such as optical signal processing, dispersion compensation, and optical delay lines.

Dispersion characteristics of photonic crystal coupled resonator optical waveguides.

Abstract: We investigated group velocities and group velocity dispersion characteristics of photonic crystal waveguides and coupled resonator optical waveguides(CROW's). In photonic circuits comprised of the linear defect waveguides, the insertion of the CROW section suppresses energy flow due to its highly dispersive characteristics. We analyze the change in the group velocity and the group velocity dispersion by varying the radius of the holes in the waveguide channel. Properly designed CROW sections provide a wide range of control in the group velocity and positive/negative group velocity dispersion. They can be used as delay lines or dispersion compensators in photonic integrated circuits comprised of linear defect photonic crystal waveguides.

Slow electromagnetic pulse propagation through a narrow transmission band in a coaxial photonic crystal

Abstract: We demonstrate the slow group-velocity propagation of electromagnetic pulses through a narrow transmission band of a simple coaxial photonic crystal. The narrow transmission band was formed by creating a defect in a periodic coaxial cable filter which resulted in a narrow-frequency passband within an otherwise forbidden band stop region. Strong normal dispersion within this region causes the group velocity of the pulse to be slowed well below the speed of light and below the expected propagation speed in a coaxial cable. This phenomenon is essentially similar to the much-studied slow light propagation observed in atomic vapors through regions of strong dispersion. The simplicity with which this phenomenon can be observed in a one-dimensional coaxial photonic crystal makes this a versatile experiment for such studies. Group velocities of 0.30c, where c is the speed of light in a vacuum, were observed.

Highly nonlinear atomic medium with steep and sign-reversible dispersion*

Abstract: Experimental investigation of the optical properties of atomic media with bright and dark long-lived light-induced Zeeman coherence is reported. We have obtained a small negative value for the group velocity of light pulses in Cs vapour (Vg −c/6000), which agrees with direct anomalous dispersion measurements. The intensity dependent components of the refractive index of an atomic vapour with long-lived Zeeman coherence has been estimated. We have shown that such coherence can significantly enhance four-wave mixing, leading to the appearance of a multi-frequency comb-like spectrum at relatively low light intensity.

Slow light produced by far-off-resonance Raman scattering

Abstract: The authors survey the theoretical and experimental aspects of generation of slow light in a far-off-resonance Raman medium driven by a strong coupling field. When material dispersion is negligible, the propagation of two coupled sidebands can be described in terms of two normal modes that propagate independently at different group velocities, one at the vacuum speed of light and one at a reduced velocity. They use solid hydrogen as a Raman medium to demonstrate the generation of slow light. The numerical calculations and experimental observations show that, due to high density, narrow Raman width, and small two-photon detuning, far-off-resonance Raman scattering in solid hydrogen can slow down the pulse-peak velocity of the Stokes and anti-Stokes fields to the order of c/10000. This velocity reduction affects the amplitudes of the Stokes and anti-Stokes fields via the beating between the normal modes. The double-peak structure observed in the intensity temporal profiles of the sideband fields is a signature of the splitting of the copropagating normal modes.

Signal velocity and group velocity for an optical pulse propagating through a GaAs cavity

Abstract: We present measurements of the signal and group velocities for chirped optical pulses propagating through a GaAs cavity. The signal velocity is based on a specified signal-to-noise ratio at the detector. Under our experimental conditions, the chirp substantially modifies the group velocity of the pulse, but leaves the signal velocity unaltered. At unity transmittance, the velocities are equal. In general, when the transmittance is less than unity, the group velocity is faster than the signal velocity. While the group velocity can be negative, the signal velocity is always less than c/n, where c is the speed of light in vacuum and n is the refractive index of GaAs. To our knowledge, this is the first measurement of both the group velocity and the signal velocity in any system.

In situ velocity imaging of ultracold atoms using slow light

Abstract: The optical response of a moving medium suitably driven into a slow-light propagation regime strongly depends on its velocity. This effect can be used to devise a scheme for imaging ultraslow velocity fields. The scheme turns out to be amenable to study in situ the dynamics of collective and topological excitations of a trapped Bose-Einstein condensate. We illustrate how slow-light imaging works specifically for sloshing oscillations and bent vortices in a stirred condensate.

Slow light propagation in an atomic vapour under conditions of amplification without inversion

Abstract: We report experimental results on slowing a light pulse in a system for amplification without inversion (AWI). We were able to control a subluminal group velocity continuously from Vg = c/2850 to c/7260 by just changing an incoherent pumping beam power from 0 to 12 mW in the AWI system. And several advantages, such as the controllable delay time and the pulse amplification, for slowing of the light in the AWI system compared to in an electromagnetically induced transparency system were found.

Superluminal Light Pulse Propagation at a Negative Group Velocity

Abstract: Anomalous dispersion cannot occur in a transparent, passive medium where electromagnetic radiation is being absorbed at all frequencies, as pointed out by Landau and Lifshitz. Here we show, using gain-assisted linear anomalous dispersion, that superluminal pulse propagation can occur in a transparent medium when gain is present. Superluminal light pulse propagation is observed at a negative group velocity of −c/315 through an atomic medium with little pulse distortion. Experimentally, a pulse propagating through the atomic vapor cell has its peak reaching the exit side before entering it, resulting in a negative transit time. This counterintuitive effect is a direct result of the rephasing process owing to the wave nature of light and is not at odds with causality.

Slow light in Doppler-broadened two-level systems (4 pages)

Abstract: We show that the propagation of light in a Doppler-broadened medium can be slowed down considerably even though such medium exhibits very flat dispersion. The slowing down is achieved by the application of a saturating counter propagating beam that produces a hole in the inhomogeneous line shape. In atomic vapors, we calculate group indices of the order of 10 3 . The calculations include all coherence effects.

Comment on "Total Negative Refraction in Crystals for Ballistic Electrons and Light"

Abstract: In Phys. Rev. Lett. 91, 157404 (2003) Yong Zhang et al. study light propagation through an interface between two identical but differently oriented uniaxial crystals. The authors show that the light refraction at the interface at some region of incident angles is negative. The observed effect of negative refraction resembles the analogous effect in left-handed materials (LHM) although the physics is different. In LHM the negative refraction is caused by a negative group velocity, whereas in the considered case one deals with the special properties of wave propagation in uniaxial anisotropic media.

Observation of superluminal pulse propagation in alexandrite

Abstract: We observe superluminal light propagation in an alexandrite crystal. Negative group velocities result from a narrow anti-hole in the homogeneously broadened absorption spectrum. The anti-hole is formed by coherent population oscillations in an inverse-saturable absorber.

Slow-light in nonuniform quantum dot waveguide

Abstract: We analyzed a new variable all-optical buffer using semiconductor quantum dots (QD) We show that an InAs-InGaAs-GaAs QD waveguide with 20 meV inhomogeneous linewidth can achieve substantial buffering with a novel unbalanced multi-wavelength pumping scheme

Polarized prism made of yttrium vonadate crystal

Abstract: A polarizing prism mad eof YVO4 crystal features that its angle theta must be greater than alpha e but less han alpha o to ensure full reflection of slow light on air gap interface, a high-safety polarizing angle beta must exist to ensure complete linear polarization of outgoing light, and the fast light must have the transmission rate at air gap interface as high as passible. Its advantages are with transmission range, great birefringence, high optical uniformity, low temp coefficient, and high hardness.

Taming light with cold atoms: Light at bicycle speed ... and slower yet!

Abstract: The latest results from our Slow Light/Bose-Einstein condensation experiment will be presented We are using a combination of laser and evaporative cooling to create Bose-Einstein condensates of sodium atoms in a ‘4D’ magnetic bottle We have succeeded in reducing the light speed in a Bose condensate to the speed of a bicycle by using the effect of electromagnetically induced transparency Most recently we have brought light to a complete standstill The results have implications for quantum information processing, for probing and manipulation of Bose-Einstein condensates, and for nonlinear optics

Transfer of information in an anomalous dispersion medium

Abstract: The transfer of information and signal velocity in an anomalous dispersion medium are studied. We propose that the discontinuous points in the envelop and its derivatives of any order are the information carried by a pulse different from others. The signal velocity will not exceed the speed of the shift of these discontinuous points. We study the propagation of pulses with a triangle envelop and with the envelop made up by three pieces of quadratic curve in dilute, anomalous dispersion gas with double gain lines. The discontinuous points of the envelop, its first derivative, and its second derivative are shown to propagate with vacuum speed of light c in the medium. A criterion has been suggested to determine theoretically whether the distortion of a pulse can be ignored.

Dark Resonances in Solids for Quantum Computing and Slow Light

Abstract: Electromagnetically induced transparency and related optical dark-resonance techniques have found many potential applications such as quantum computing, slow light, optical memory, and nonlinear optics. For most of these applications, solid state implementations are preferred. To this end, I will describe our current research and recent experimental results toward the implementation of efficient dark resonance techniques in solids, with emphasis on quantum computing and slow light.

Group velocity manipulation in simple acoustic band gap filters

Abstract: A simple experimental configuration is presented in which the group velocity of acoustic pulses is adjusted from much slower to much faster than the speed of sound. The experiment is an acoustic analog of two much studied optical phenomena: the superluminal group velocity achieved when optical pulses are tunneled through regions of high absorption or attenuation, and slow (or even stopped) light propagation in the vicinity of strong material dispersion, typically realized using electromagnetically induced transparency in atomic vapors. For the acoustic experiments described here, attenuation and dispersion were created using periodically structured waveguides. The periodicity results in one‐dimensional acoustic band gaps, frequency intervals in which propagation of sound is strongly suppressed. The introduction of defects in the perfect periodicity leads to narrow transmission bands within the band gaps. There is strong normal dispersion in the vicinity of these defect modes leading to very slow group vel...

Slow light in a solid without using electromagnetically induced transparency

Abstract: A spectral hole is burnt in a sample of Eu3+ doped Y2SiO5 and measured using a short light pulse. For a 4 mm thick sample the transmission is reduced from 80% to 10% over a bandwidth of 2 kHz and the associated dispersion corresponded to a light speed of 60 m/s.