Slow-light Medium Research Articles

Rotary Photon Drag Enhanced by a Slow-Light Medium

Transmission through a spinning window slightly rotates the polarization of the light, typically by a microradian. It has been predicted that the same mechanism should also rotate an image. Because this rotary photon drag has a contribution that is inversely proportional to the group velocity, the image rotation is expected to increase in a slow-light medium. Using a ruby window under conditions for coherent population oscillations, we induced an effective group index of about 1 million. The resulting rotation angle was large enough to be observed by the eye. This result shows that rotary photon drag applies to images as well as polarization. The possibility of switching between different rotation states may offer new opportunities for controlled image coding.

Photon lifetime in a cavity containing a slow-light medium

We investigate experimentally the lifetime of the photons in a cavity containing a medium exhibiting strong positive dispersion. This intracavity positive dispersion is provided by a metastable helium gas at room temperature in the electromagnetically induced transparency regime, in which light propagates at a group velocity of the order of 10⁴ m·s⁻¹. The results definitely prove that the lifetime of the cavity photons is governed by the group velocity of light in the cavity and not its phase velocity.

Slowing single photons

The successful integration of a single-photon source with a slow-light medium creates important opportunities for photon synchronization and marks a step towards the development of distributed networks for quantum information processing.

High sensitivity slow light interferometer based on dispersiveproperty of Ⅲ-Ⅴ and Ⅱ-Ⅵ semiconductor materials

We investigate the optical properties of Ⅲ-Ⅴ and Ⅱ-Ⅵ semiconductors. The results show that the sensitivity of the interferometer can be greatly enhanced by the dispersive property of semiconductor. Furthermore, the analyses show that the semiconductor slow light medium has a more wider working spectral range than gas slow light medium. Moreover, we experimentally demonstrate that the sensitivity of the interferometer based on the semiconductor GaAs is 3.2 times higher than those of traditional interferometers.

Storage and retrieval of thermal light in warm atomic vapor

We report slowed propagation and storage and retrieval of thermal light in warm rubidium vapor using the effect of electromagnetically induced transparency (EIT). We first demonstrate slowed propagation of the probe thermal light beam through an EIT medium by measuring the second-order correlation function of the light field using the Hanbury-Brown-Twiss interferometer. We also report an experimental study on the effect of the EIT slow-light medium on the temporal coherence of thermal light. Finally, we demonstrate the storage and retrieval of the thermal light beam in the EIT medium. The direct measurement of the photon number statistics of the retrieved light field shows that the photon number statistics are preserved during the storage and retrieval processes.

Influence of third-order dispersion on delay performance in broadband Brillouin slow light: erratum

We investigate theoretically the delay performance of Brillouin slow light employing a broadband pump with a rectangular spectrum. Analytical expression of delayed Gaussian pulse amplitude is deduced with the second-order gain nonuniformity and the stimulated-Brillouin scattering-induced third-order dispersion effects involved. Calculated results show that the Gaussian pulse with full-width at half maximum of 200 ps is delayed by 80 ps with little pulse broadening in a single-stage Brillouin slow-light medium, and the time delay is extended to multi-hundred ps with low distortion in a cascaded Brillouin slow-light system. Comparing the analytical results with the numerical ones, we confirm that the pulse distortion is mainly attributed to the large third-order dispersion introduced by stimulated-Brillouin scattering.

Qubit transfer between photons at telecom and visible wavelengths in a slow-light atomic medium

We propose a method that enables efficient conversion of quantum information frequency between different regions of spectrum of light based on recently demonstrated strong parametric coupling between two narrow-band single-photon pulses propagating in a slow-light atomic medium [1]. We show that an input qubit at telecom wavelength is transformed into another at visible domain in a lossless and shape-conserving manner while keeping the initial quantum coherence and entanglement. These transformations can be realized with a quantum efficiency close to its maximum value.

All-optical tunable pulse frequency chirp via slow light

We theoretically investigate slow light via stimulated Raman scattering, paying special attention to the picosecond regime where chromatic dispersion and cross-phase modulation must be considered. In addition to the control of the Raman pulse walk-off, we demonstrate that the cross-phase-modulation-induced frequency chirp can also be all-optically tuned via Raman slow light. We further demonstrate that this new implication is a consequence of the fact that the group velocity is significantly more affected than the phase velocity in slow-light media.

A high spectral sensitivity interferometer based on the dispersive property of the semiconductor GaAs

We develop an interferometer which has high spectral sensitivity based on the dispersive property of the semiconductor GaAs in the near-infrared region. Our experiment demonstrates that the spectral sensitivity could be greatly enhanced by adding a slow light medium into the interferometer and is proportional to the group index of the material. Subsequently the factors which influence the spectral sensitivity of the interferometer are analyzed. Moreover, we provide potential applications of such interferometers using the dispersive property of semiconductor in whole infrared region.

Influence of third-order dispersion on delay performance in broadband Brillouin slow light

We investigate theoretically the delay performance of Brillouin slow light employing a broadband pump with a rectangular spectrum. Analytical expression of delayed Gaussian pulse amplitude is deduced with the second-order gain nonuniformity and the stimulated-Brillouin scattering-induced third-order dispersion effects involved. Calculated results show that the Gaussian pulse with full-width at half maximum of 200 ps is delayed by 80 ps with little pulse broadening in a single-stage Brillouin slow-light medium, and the time delay is extended to multi-hundred ps with low distortion in a cascaded Brillouin slow-light system. Comparing the analytical results with the numerical ones, we confirm that the pulse distortion is mainly attributed to the large third-order dispersion introduced by stimulated-Brillouin scattering.

Gravitational redshift and deflection of slow light

We explore the nature of the classical propagation of light through media with strong frequency-dependent dispersion in the presence of a gravitational field. In the weak field limit, gravity causes a redshift of the optical frequency, which the slow-light medium converts into a spatially varying index of refraction. This results in the bending of a light ray in the medium. We further propose experimental techniques to amplify and detect the phenomenon using weak value measurements. Independent heuristic and rigorous derivations of this effect are given.

Increase of the fractional delay of the pulse in an electromagnetically-induced-transparency medium

We propose a nonadiabatic change of the control-field amplitude to restore the temporal shape of a signal pulse, which usually experiences an appreciable temporal broadening in a conventional regime of electromagnetically-induced-transparency. This broadening sets a major limit on the maximum time delay achievable in a slow-light medium. A circumvention of this limit may be useful for the application of a slow-light medium as a practical, controllable delay line.

Multichannel SBS Slow Light Using Spectrally Sliced Incoherent Pumping

We propose and experimentally demonstrate the use of an incoherent spectrally sliced amplified spontaneous emission (ASE) source as the pump for broadband stimulated Brillouin scattering-based slow light. Both 2.5-Gb/s on-off-keyed and differential-phase-shift-keyed signals are continuously delayed by as much as 170 ps, with less than 4-dB power penalty at 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> bit error rate. A 2-GHz and a 4-GHz spectrally sliced pump are compared, emphasizing the need of optimizing the slow-light spectral-width according to the signal bandwidth. Furthermore, independent and simultaneous delay on multiple data channels with different modulation formats within a single slow-light medium is proposed and experimentally demonstrated by using a periodically spectrally sliced ASE source. Our approach might be useful in future heterogeneous multiformat, multibit-rate, and multichannel fiber-optic communication systems and networks.

Slow-light interferometry: practical limitations to spectroscopic performance

We investigate how the use of slow-light methods can enhance the performance of various types of spectroscopic interferometers under practical conditions. We show that, while in ideal cases the enhancement of the spectral resolution is equal to the magnitude of the group index of the slow-light medium, the ratio between the associated gain or loss and the group index of the slow-light medium actually determines the spectral resolution under more-general conditions. Moreover, the dispersion of this ratio leads to frequency-dependent spectral resolution, which limits the useful working bandwidth of the interferometer. We also evaluate the performance of interferometers using three specific slow-light processes in terms of the achievable spectral resolution and the effective working finesse. We show that the spectral resolution is typically limited by the characteristic linewidth of each slow-light process and that there is no fundamental upper limit for the effective working finesse.

Preservation of Energy-Time Entanglement in a Slow Light Medium

We demonstrate the preservation of entanglement of an energy-time entangled biphoton through a slow light medium. Using the D(1) and D(2) fine structure resonances of Rubidium, we delay one photon of the 1.5 THz biphoton by approximately 1.3 correlation lengths and measure the fourth order correlation fringes. After the group delay the fringe visibility is reduced from 97.0+/-4.4% to 80.0+/-4.8%, but is still sufficient to violate a Bell inequality. We show that temporal broadening is the primary mechanism for reducing the fringe visibility and that smaller bandwidths lead to greatly reduced broadening.

Entanglement-preserving frequency conversion in cold atoms

We propose a method that enables efficient frequency conversion of quantum information based on recently demonstrated strong parametric coupling between two single-photon pulses propagating in a slow-light atomic medium at different group velocities. We show that an incoming single-photon state is efficiently converted into another optical mode in a lossless and shape-conserving manner. The persistence of initial quantum coherence and entanglement within frequency conversion is also demonstrated. We first illustrate this result for the case of small frequency difference of converted photons, and then discuss the modified scheme for conversion of photon wavelengths in different spectral ranges. Finally we analyze the generation of a narrow-band single-photon frequency-entangled state.

Spectrally efficient slow light using multilevel phase-modulated formats

A phase-preserving and spectrally efficient slow-light scheme has been proposed and demonstrated by utilizing advanced multilevel phase-modulated formats. A 60 ps symbol delay with error-free demodulation of both I and Q channels for 10 Gbit/s return-to-zero differential-quadrature-phase-shift-keyed (DQPSK) signals via a broadband stimulated Brillouin scattering-based slow-light medium is achieved experimentally. Simulation results on 20 Gbit/s DQPSK and 30 Gbit/s D8PSK propose to transmit very high spectrally efficient multilevel formats through a bandwidth-limited slow-light element.

Slow-Light Fourier Transform Interferometer

We describe a new type of Fourier transform (FT) interferometer in which the tunable optical delay between the two arms is realized by using a continuously variable slow-light medium instead of a moving arm as in a conventional setup. The spectral resolution of such a FT interferometer exceeds that of a conventional setup of comparable size by a factor equal to the maximum group index of the slow-light medium. The scheme is experimentally demonstrated by using a rubidium atomic vapor cell as the tunable slow-light medium, and the spectral resolution is enhanced by a factor of approximately 100.

Design of a tunable time-delay element using multiple gain lines for increased fractional delay with high data fidelity

A slow-light medium based on multiple, closely spaced gain lines is studied. The spacings and relative strengths of the gain lines are optimized by using the criteria of gain penalty and eye-opening penalty to maximize the fractional delay defined in terms of the best decision time for random pulse trains. Both numerical calculations and experiments show that an optimal design of a triple-gain-line medium can achieve a maximal fractional delay about twice that which can be obtained with a single-gain-line medium, at three times higher modulation bandwidth, while high data fidelity is still maintained.

Long Optical Delay Lines Enhanced by Ring Configuration inOptical Fibres

A long optically controlled delay line enhanced by ring configurationis demonstrated by using the group-velocity control of signal pulsesbased on stimulated Brillouin scattering. In experiment, two opticalfibre ring cavities are used: one is used as the Brillouin laser,providing single-mode Stokes wave as probe wave; the other is used asthe Brillouin amplifier, working as slow light medium. We achieve amaximum time delay of 215 ns using the ring Brillouin amplifier, fivetimes larger than the input probe pulse width of 40 ns. In themeantime, a considerable pulse broadening is observed, which agreeswell with the theoretical prediction based on linear theory.