Group Velocity Of Light Research Articles

Slowdown of Group Velocity of Light Using Coherent Population Oscillation

We investigate the reduction of the group velocity propagation resulting from the steep change of the refractive index by the coherent population oscillation in erbium-doped optical fibre. The largest delay is measured to be about 8.75ms corresponding to a group index of 1.312 × 106. The time delay or advancement depends on the pump intensity. Influences of the ion density on the fractional delay and the group velocity reduction of light propagation are studied. Based on our discussion the optimization parameters should be selected in order to obtain more appropriate time delay and the slowdown of group velocity.

Controllable ultraslow light propagation in highly-doped erbium fiber

We report on slow light propagation induced by the coherent population oscillation in an erbium-doped optical fiber (EDOF). The slowdown of group velocity of light is demonstrated in a solid-state material at room temperature. We observe a maximum fractional delay of 0.129 and a maximum delay of 8.75ms corresponding to a group velocity as low as 228.57m/s in a sinusoid-like modulated waveform. We study in details the influences of the erbium ion density and the length of fiber on the fractional delay and the slow light propagation. The data show that the fractional delay can be increased using the fiber with high erbium ions density or long interaction length.

Slow light with cavity electromagnetically induced transparency

We report an experimental observation of slow light propagation in cold Rb atoms exhibiting cavity electromagnetically induced transparency (EIT). The steep slope of the atomic dispersion manifested by EIT reduces the light group velocity. The cavity filtering and feedback further contribute to the slowdown and delay of the light pulse propagation. A combination of the cavity and the EIT atomic system significantly improves the performance of the slow light propagation. A propagation time delay of approximately 200 ns was observed in the cavity and Rb EIT system, which is approximately 70 times greater than the time delay calculated for the light pulse propagation through the same Rb EIT system without the cavity.

Local field effects on phonon-induced transparency in quantum dots embedded in a semiconductor medium

We study theoretically the influence of local fields on phonon-induced transparency (PIT) in quantum-dot systems embedded in a semiconductor matrix. As compared with our previous work without local field effects, we present analytical and numerical results from solution of the generalized optical Bloch equations including the local field effects. It is shown that the local field effects broaden the transparency window due to PIT and reduce the group velocity of light. For some specific parameters of the light and quantum dots, fast light can be obtained in such systems. The results also demonstrate that Kerr nonlinearity is enhanced greatly due to the local field effects.

Polarization rotation of slow light with orbital angular momentum in ultracold atomic gases

We consider the propagation of slow light with an orbital angular momentum (OAM) in a moving atomic medium. We have derived a general equation of motion and applied it in analyzing propagation of slow light with an OAM in a rotating medium, such as a vortex lattice. We have shown that the OAM of slow light manifests itself in a rotation of the polarization plane of linearly polarized light. To extract a pure rotational phase shift, we suggest to measure a difference in the angle of the polarization plane rotation by two consecutive light beams with opposite OAM. The differential angle {delta}{alpha}{sub l} is proportional to the rotation frequency of the medium {omega}{sub rot} and the winding number l of light, and is inversely proportional to the group velocity of light. For slow light the angle {delta}{alpha}{sub l} should be large enough to be detectable. The effect can be used as a tool for measuring the rotation frequency {omega}{sub rot} of the medium.

Control of group velocity in an annular chain of microring coupled-resonator optical waveguides

In order to control group velocity, an annular chain of microring coupled-resonator optical waveguides was analyzed, and its transmission spectra were obtained using the transfer matrix method. The result showed that the curvature of the chain caused the dispersion curves to shift and the resonance frequency got fixed. It is therefore concluded that the group velocity of light in the annular chain of microring coupled-resonator optical waveguides can be controlled by adjusting the curvature of the chain. We believe that such an annular chain can be used as an optical buffer or signal retarder for optical communication.

Bloch modes and group velocity delay in coupled resonator chains

AbstractCoherently coupled optical microresonators are studied as a concept for guiding and slowing down light. So‐called coupled‐resonator optical waveguides (CROWs) composed of nanocrystal doped microspheres are studied using mode mapping spectroscopy at the diffraction limit and microphotoluminescence spectroscopy. We demonstate coherent photon coupling in one‐dimensional multiresonator chains and show the evolution of individual cavity modes into waveguide modes. A coupled‐oscillator model is developed, which explains the resulting coupled‐resonator mode patterns in finite systems. The potential to slow down the group velocity of light by orders of magnitude is explored. Experimentally, a slowing‐factor S = 31 has been observed. A model for disorder in CROW structures is developed, by which size tuning of individual resonators is found to result in anticrossing relations. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

On the non-existence of the micro-causal sector of Lorentz non-invariant quantum field theory on a continuum

We study a model for a Lorentz non-invariant quantum field theory. In this model Lorentz violation is due to a non-linear dispersion relation admitting signals in form of wave packets propagating with super-luminal (faster than the low-energy speed of light) group velocities. We characterize the causal sector of the theory by a cutoff condition protecting the theory against such signals and study the question as to whether the causal sector exhibits micro-causality (locality in all frames), in which case we call the causal sector the micro-causal sector. It is argued that in the causal sector the canonical commutation relation are changed in such a way that the imposition of locality postulate becomes incompatible with the continuity of space. The conclusion therefore is that there is no micro-causal sector of the theory on the continuum space underlying the model and in this respect it suggests the sensitivity of an effective Lorentz non-invariant quantum field theory to a short-distance cutoff.

The manipulating of electromagnetically induced transparency in Pr 3+: Y 2SiO 5 crystal by a rf-driving field

We studied theoretically the effect of an additional rf-driving field on electromagnetically induced transparency (EIT) and group velocity of a probe light in Pr 3+:Y 2SiO 5 crystal. The results show that more than one EIT widnows could be opened and their position, intensity can be controlled by the rf-driving field. The depth and position of the minimum of group velocity could also be manipulated by the rf field.

Modeling of single- and multimode photonic-crystal planar waveguides with the plane-wave admittance method

In this paper we present the plane-wave admittance method (PWAM) in application to modeling light propagation in photonic crystal waveguides. We determine out-of-plane losses in single- and multimode waveguides and analyze the coupling of fundamental and higher-order modes in the latter case. Finally we compute the group velocity of light and show the possibilities and costs of achieving a very slow light regime.

Millimeter wave localization: Slow light and enhanced absorption in random dielectric media

We exploit millimeter wave technology to measure the reflection and transmission response of random dielectric media. Our samples are easily constructed from random stacks of identical subwavelength quartz and Teflon wafers. The measurement allows us to observe the characteristic transmission resonances associated with localization. We show that these resonances give rise to enhanced attenuation even though the attenuation of homogeneous quartz and Teflon is quite low. We provide experimental evidence of disorder-induced slow light and superluminal group velocities, which, in contrast to photonic crystals, are not associated with any periodicity in the system. Furthermore, we observe localization even though the sample is only about four times the localization length, interpreting our data in terms of an effective cavity model. An algorithm for the retrieval of the internal parameters of random samples (localization length and average absorption rate) from the external measurements of the reflection and transmission coefficients is presented and applied to a particular random sample. The retrieved value of the absorption is in agreement with the directly measured value within the accuracy of the experiment.

Quantum control and manipulation of multi-color light fields

Quantum control of stationary multi-color (MC) light fields in resonant medium with multi-Λ scheme of atomic transition is proposed. We have found the general analytical solution in the adiabatic limit of quantum evolution resulting from the interaction of the slow probe light with the new fields generated in the nondegenerate multi-wave mixing scheme. We have found a critical condition for the stopping and quantum manipulation of the MC-light fields where the united light group velocity can be reduced down to zero with optimal spectral parameters while preserving the delicate quantum correlations of the initial probe light pulse. The manipulations, which provide the effective transference of quantum probe light to the new multifrequency light fields have been analyzed.

Improving coherent atomic vapor optical buffers

We report on a theoretical study of the influence of incoherent optical pumping and beam profiling on slow light propagation in three-level atomic vapor. The pumping reduces the residual unwanted absorption of the signal while destroying the atomic coherence and increasing the group velocity of light. We show that the pumping enhances the resulting group delay for a certain range of parameters. We also examine changing the cross section of the beam of light along the propagation direction by, e.g., focusing the beam, for the purpose of maintaining the drive intensity to effectively counter its absorption.

Design of photonic crystal directional couplers for electro-optical wavelength switching in Si technology

Electro-optical wavelength switch in Si technology operating around = 1.55 µm and based on a photonic crystal directional coupler configuration was studied using Plane Wave Method and Finite Difference Time Domain simulations. Owing to the exploitation of slow wave supermodes, ultra-compact active devices (~ 20 µm) exploiting free carrier plasma dispersion effect in Si can be designed. Injection of input/output strip access waveguides with the photonic core area is optimized using tapering stages to gradually reduce light group velocity. With carrier injection around 1018 cm-3, the coupler configuration can be used as an optical switch of individual wavelengths or a multiplexer/demultiplexer device with active routing of wavelengths.

Control of group velocity of light via magnetic field

We have theoretically studied the effects of quantum coherence in a driven quasi-degenerate two-level atomic system. We have shown that the quantum interference, which can be destructive or constructive, can be controlled by an externally applied magnetic field allowing one to implement both electromagnetically induced transparency and electromagnetically induced absorption in the same atomic system. Determined by frequency dispersion of the index of refraction of the system, the group velocity of light pulses ranges from ultra-slow to superluminal with changing of the magnitude of the magnetic field.

Bloch modes and disorder phenomena in coupled resonator chains

Coherent photon coupling and disorder effects in chains of coupled resonators are studied both theoretically and experimentally, aiming at applications in the visible spectral range. Coupling effects have been explored utilizing the Bloch-mode formation in linear structures of quantum dot doped microsphere resonators. Coherent coupling in chains of 14 resonators is evidenced experimentally by spectrally and spatially resolved mode spectroscopy. Realizing a coupling constant of $\ensuremath{\kappa}=1.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ ensures spectral distinction of the collective resonances. Experimentally, a slowing factor of $S=31$ is derived for the group velocity of light. A coupled oscillator model for arbitrarily disordered chains of coupled resonator delay lines is presented. The transition between order and disorder effects is investigated with a formalism combining Bloch-mode formation with mutual anticrossing relations.

High speed silicon photonic crystal waveguide modulator for low voltage operation

A high speed compact silicon modulator is experimentally demonstrated to work at a low driving voltage desirable for on-chip applications. As carrier injection is the only practical option for optical modulation in silicon, a lower limit of current density (∼104A∕cm2) exists for achieving gigahertz modulation in the p-i-n diode configuration. Exploiting the slow group velocity of light in photonic crystal waveguides, the interaction length of this Mach-Zehnder interferometer-type silicon modulator is reduced significantly compared to conventional modulators. The required high current density is achieved with a low voltage (2V) by scaling down the interaction length to 80μm.

Long and short light pulses propagation in a long free electron laser: Effects and possible applications in high-field physics

We have studied propagation of Gaussian electromagnetic pulses through a long undulator free electron laser (FEL). Our analysis takes into account both the active and dispersive properties of the laser active medium. We have introduced the definitions of the FEL operating bandwidth and characteristic time. With the last definition a natural time scale has been established, and this allowed us to differentiate the input signals as long and short pulses. We have established that the results of the conventional theories, based on the model of a monochromatic wave, are applicable for pulses of finite but long duration. The propagation of pulses of finite lengths has been examined analytically in the (a) small spectral gain and (b) high spectral gain regimes. We have discovered a stabilization effect in the case (a). Namely, short pulses, which have random initial lengths at the laser entrance, will have the same duration at laser exit. On the other hand, the propagation of short pulses results in a nearly harmonic modulation of their envelopes. In the case (b) we have calculated the light phase and group velocities and ascertained that the laser active medium is like a plasma medium in the amplification regime; whereas it is simultaneously similar to plasma and resonant atom medium in the absorption regime. In the amplification regime, the pulse propagation is accompanied by a smooth increasing of its duration. On the contrary, the FEL can operate as a slight compressor or explosive stretcher in the absorption regime. Besides, the amplification and absorption regimes provide a linear chirping of light pulses and controlling the pulse peak amplitude. Thus the long FELs might be in principle employed to control the amplitude, phase, duration and profile of superintense short pulses, which damage ordinary atom media.

Recursion formula for reflectance and the enhanced effect on the light group velocity control of the stratified and phase-shifted volume index gratings

We derived a recursion formula for the reflectance of the stratified and phase-shifted volume index gratings. The characteristics of the reflectance spectra of the stratified and phase-shifted volume index gratings were studied based on the recursion formula. It is shown that narrow bandwidth transparency peaks appear within the stop-band of the reflectance spectrum of the volume index gratings due to the intervention of the homogeneous buffer layers that induce the phase-shifts between neighboring volume index gratings. The spectral positions of the transparency peaks can be shifted within the stop-band by controlling the phase-shift, i.e., the buffer layer thickness. The described properties may find applications in addressable band-pass filter, switching, wavelength division multiplexing, and de-multiplexing. The dispersion near the transparency peaks of the stratified and phase-shifted volume index grating is found to be sharply enhanced as compared to the uniform volume index gratings. Significantly enhanced control on the group velocity of light by several orders of magnitude while keeping high transmittance is demonstrated in the stratified and phase-shifted volume index grating.

Slow to superluminal light waves in thin 3D photonic crystals

Phase measurements on self-assembled three-dimensional photonic crystals show that the group velocity of light can flip from small positive (slow) to negative (superluminal) values in samples of a few mum size. This phenomenon takes place in a narrow spectral range around the second-order stop band and follows from coupling to weakly dispersive photonic bands associated with multiple Bragg diffraction. The observations are well accounted for by theoretical calculations of the phase delay and of photonic states in the finite-sized systems.