Slow Light Pulses Research Articles

Producing slow light in warm alkali vapor using electromagnetically induced transparency

We present undergraduate-friendly instructions on how to produce light pulses propagating through warm Rubidium vapor with speeds less than 400 m/s, i.e., nearly a million times slower than c. We elucidate the role played by electromagnetically induced transparency (EIT) in producing slow light pulses and discuss how to achieve the required experimental conditions. The optical setup is presented, and details provided for preparation of pump, probe, and reference pulses of the required size, frequency, intensity, temporal width, and polarization purity. EIT-based slow light pulses provide the most widely studied architecture for creating quantum memories. Therefore, the basic concepts presented here are useful for physics and engineering majors who wish to get involved in the development of cutting-edge quantum technologies.

Slow light pulse propagation through spherical quantum dot with on-center hydrogen impurity in magnetic field

Abstract The probe pulse propagation through the spherical quantum dot with an on-center hydrogen impurity in an external magnetic field in the regime of the electromagnetically induced transparency is studied in detail. The interaction of this quantum system with the weak pulse probe and strong continuous wave control laser is described by the three-level ladder-type configuration. By solving Maxwell-Bloch equations with the help of the Fourier transform method, the approximate analytic expressions for the probe pulse envelope and probe pulse group velocity are obtained. We investigate the influence of the magnetic field strength and dephasing rates on slowing down the probe pulse and on its shape during the propagation. We found that the magnetic field can additionally reduce the group velocity of the probe pulse. It has also been found that reducing dephasing rates enables the weak probe pulse propagation with almost negligible losses. The obtained analytic expressions are in the good agreement with the numerical calculations.

Fast and slow light generated by surface plasmon wave and gold grating coupling effects

We present here the results of a simulation of the effect of gold and graphene coatings on silicon micro-ring resonators. We studied the effect of different radii of graphene on the time delay, from which one an interesting aspect of light pulse behaviors, such as fast light, was numerically investigated. The obtained results indicate that the time delay can be varied, which is in good agreement with theoretical predictions. Fast and slow light pulse trains can be obtained by modifying the throughput port, which forms the gold grating length. The temporal gaps between the fast and slow light in the used graphene and gold are 140 and 168 fs, respectively, which can be tuned by varying the radius or grating length. The obtained results show that such a device may be useful in applications requiring fast and slow light pulse train pairs, such as optical switching, sensors, communications, and security applications.

Light Stops at Exceptional Points.

Almost twenty years ago, light was slowed down to less than 10^{-7} of its vacuum speed in a cloud of ultracold atoms of sodium. Upon a sudden turn-off of the coupling laser, a slow light pulse can be imprinted on cold atoms such that it can be read out and converted into a photon again. In this process, the light is stopped by absorbing it and storing its shape within the atomic ensemble. Alternatively, the light can be stopped at the band edge in photonic-crystal waveguides, where the group speed vanishes. Here, we extend the phenomenon of stopped light to the new field of parity-time (PT) symmetric systems. We show that zero group speed in PT symmetric optical waveguides can be achieved if the system is prepared at an exceptional point, where two optical modes coalesce. This effect can be tuned for optical pulses in a wide range of frequencies and bandwidths, as we demonstrate in a system of coupled waveguides with gain and loss.

On-chip autocorrelator using counter-propagating slow light in a photonic crystal with two-photon absorption photodiodes

We propose an ultra-compact solid-state autocorrelator fabricated using Si photonics technology that obtains correlation by detecting the overlap of two slow light pulses counter-propagating in a photonic crystal waveguide integrated with a two-photon absorption photodiode array. As the device does not require a mechanical delay scanner, it can be integrated onto a chip and operated without alignment. Using the correlator, we successfully acquired the autocorrelation of picosecond pulses. The nonlinear enhancement of slow light improves sensitivity, resulting in an evaluated detection limit in terms of the product of the peak and average power on the order of 10−7 W2, which is equal to or even better than that of commercial scanning autocorrelators and 107-fold better than that of conventional single-shot autocorrelators.

Dirac-graphene quasiparticles in strong slow-light pulse

An analytical Volkov's solution of the massless Dirac equation for graphene in the field of slow-light pulse with arbitrary time dependence is obtained. Exact solutions are presented for special cases of monochromatic field and a single-cycle pulse. Following the Fock-Schwinger proper time method, the Green's function for quasiparticles is derived with the account of the influence an external classical electromagnetic wave field.

Synchronized Ion Acceleration by Ultraintense Slow Light.

An effective scheme of synchronized laser-triggered ion acceleration and the corresponding theoretical model are proposed for a slow light pulse of relativistic intensity, which penetrates into a near-critical-density plasma, strongly slows, and then increases its group velocity during propagation within a target. The 3D particle-in-cell simulations confirm this concept for proton acceleration by a femtosecond petawatt-class laser pulse experiencing relativistic self-focusing, quantify the characteristics of the generated protons, and demonstrate a significant increase of their energy compared with the proton energy generated from optimized ultrathin solid dense foils.

Slow-light-induced Doppler shift in photonic-crystal waveguides

In this Rapid Communication, we theoretically discuss a large Doppler shift in a signal slow-light pulse in a photonic-crystal waveguide by considering its reflection at a quasilight speed mirror. The mirror is formed by the photonic band-gap shift induced by the high nonlinearity of a control slow-light pulse, which could be possible in a realistic device. In the simulation, the Doppler shift appears at multiple frequencies due to the Bloch nature of the photonic lattice. Larger but inefficient Doppler shifts occur through nonadiabatic processes, whereas the smallest but more efficient shift (i.e., the intraband Doppler shift) occurs through an adiabatic process. The occurrence of the intraband shift depends on whether the adiabatic process produces a complete reflection of the incident pulse, despite the fact that the pulse penetrates the mirror. A large band-gap shift and a moderately slow mirror satisfy this condition; otherwise, the shift ends at the halfway point.

Theoretical analysis of dipole-induced electromagnetic transparency

A detailed theory describing linear optics of vapors comprised of interacting multi-level quantum emitters is proposed. It is shown both by direct integration of Maxwell-Bloch equations and using a simple analytical model that at large densities narrow transparency windows appear in otherwise completely opaque spectra. The existence of such windows is attributed to overlapping resonances. This effect, first introduced for three-level systems in [R. Puthumpally-Joseph, M. Sukharev, O. Atabek and E. Charron, Phys. Rev. Lett. 113, 163603 (2014)], is due to strongly enhanced dipole-dipole interactions at high emitters' densities. The presented theory extends this effect to the case of multilevel systems. The theory is applied to the D1 transitions of interacting Rb-85 atoms. It is shown that at high atomic densities, Rb-85 atoms can behave as three-level emitters exhibiting all the properties of dipole induced electromagnetic transparency. Applications including slow light and laser pulse shaping are also proposed.

Estimation of temporal separation of slow light pulses in atomic vapors by weak measurement

We show how two circular polarization components of a linearly polarized pulse, propagating through a coherently driven dilute atomic vapor, can be well resolved in time domain by weak measurement. Slower group velocity of one of the components due to electromagnetically induced transparency leads to a differential group delay between the two components. For low number density, this delay may not be large enough to temporally resolve the two components. We show how this can be enhanced in terms of mean time of arrival of the output pulse through a post-selected polarizer. We demonstrate the idea with all the analytical and numerical results, with a specific example of alkali atoms.

Temporal pulse compression by dynamic slow-light tuning in photonic-crystal waveguides

We demonstrate on-chip pulse compression based on the dynamic tuning in a copropagating slow-light system. Low-dispersion slow-light control pulse in lattice-shifted photonic-crystal waveguides enhances two-photon absorption. Generating free carriers dynamically through the two-photon absorption so as to sweep a signal slow-light pulse, the spectral broadening occurs with a monotonic chirp. The signal pulse is then compressed through the compensation of the chirp by means of integrated heaters. An input pulse of 13.9 ps length was compressed to 1.4 ps, corresponding to a compression factor of 9.9.

Slow light propagation in tunable nanoscale photonic crystal cavity filled with nematic liquid crystal

Slow light propagated through a photonic crystal with a nematic liquid crystal-filled cavity has been simulated and presented. Both slow and fast modes propagate in the waveguide. Design efforts were made to adjust the group velocities of the propagating modes. Numerical studies show that the nematic liquid crystal provides designers an additional degree of freedom to tune the device by using external perturbations such as applying heat or electric field. Comparative studies have also been done to see the performance of the devices built in two different material platforms (silicon and InP). The device can be used as an economic and efficient functional materials system for building robust integrated photonic devices that have the ability to slow, store, and process light pulses.

Dynamic Wavelength Conversion in Copropagating Slow-Light Pulses

Dynamic wavelength conversion (DWC) is obtained by controlling copropagating slow-light signal and control pulse trajectories. Our method is based on the understanding that conventional resonator-based DWC can be generalized, and is linked to cross-phase modulation. Dispersion-engineered Si photonic crystal waveguides produce such slow-light pulses. Free carriers generated by two-photon absorption of the control pulse dynamically shift the signal wavelength. Matching the group velocities of the two pulses enhances the shift, elongating the interaction length. We demonstrate an extremely large wavelength shift in DWC (4.9nm blueshift) for the signal wavelength. Although DWC is similar to the Doppler effect, we highlight their essential differences.

Stationary and quasistationary light pulses in three-level cold atomic systems

We have studied stationary and quasistationary signal light pulses in cold \ensuremath{\Lambda}-type atomic media driven by counterpropagating control laser fields at the condition of electromagnetically induced transparency. By deriving a dispersion relation we present spectral and temporal properties of the signal light pulse and a significant influence of atomic decoherence on the coupled stationary light pulses for spatial splitting. Finally we discuss quasistationary light pulse evolution characterized by frozen spatial spreading for a robust coherent control of slow light pulses.

SLOW LIGHT AND DISPERSION COMPENSATION BY CASCADING TWO PHOTONIC CRYSTAL WAVEGUIDES

In this paper, we propose a structure with cascaded two photonic crystal line-defect waveguides to reduce group velocity dispersion (GVD) of slow light. The width of the line-defect waveguides is tuned to obtain the two matched dispersion relations, where one of the dispersion relations has a maximum point with zero group velocity and large positive GVD; the other has a minimum point with zero group velocity and large negative GVD. The waveguides have ultra slow light with the group velocity 0.0012c. Finite-difference time-domain simulation demonstrates that the spreading of the slow light pulse in the first waveguide can be recovered by the dispersion compensation of the second waveguide with positive GVD.

The effect of Kerr nonlinearity and Doppler broadening on slow light propagation

The influence of Kerr nonlinearity and Doppler broadening on the slow light propagation of a dispersive atomic medium is revisited using a four-level Λ-type (87Rb) atomic system under the condition of electromagnetically induced transparency (EIT). The medium exhibits an EIT process which is accompanied by normal dispersion, i.e., sub-luminal (slow light) pulse propagation. In the presence of Kerr nonlinearity and the Doppler broadening effect the dispersion property of the proposed atomic medium is modified, which leads to enhancement of the positive group index of the medium. The combined effect of Kerr nonlinearity and Doppler broadening on slow light propagation inside the medium is the major motivation for this work.

High-speed delay tuning of slow light in pin-diode-incorporated photonic crystal waveguide

We demonstrate the high-speed electrical delay tuning of slow light pulses using Si photonic crystal waveguides. The device has an i-region-chirped pin diode, within which thermo-optic and carrier plasma effects are generated by forward bias. The former changes the delay up to 62 ps for the DC bias. The latter changes the delay for 1 Gbps pseudo random bit sequence tuning signals, which will be applicable to advanced time-domain optical signal processing.

Slow and stored light pulses in the presence of magnetic fields

We systematically studied slow light (SL) and stored light in the presence of a static magnetic field, either parallel to or transverse to the direction of light propagation. The theoretical calculations are in good agreement with the measured data, and paves the way for determining the information of optical density, coupling Rabi frequency, and ground state population distribution of a system. We also observed an unexpected double-peak profile in the output of SL and the retrieved signal of stored light under a large transverse magnetic field. Our work provides a better understanding in the studies of SL and storage of light.

Multi-mode cross-Kerr interaction and three-qubit all-optical logic gates in a triple electromagnetically induced transparency system

We investigate the nonlinear interaction of weak optical fields in triple electromagnetically induced transparency (EIT) where EIT is induced simultaneously for three weak slow light pulses. Different kinds of nonlinear interaction including two-mode cross-Kerr interaction, three-mode cross-Kerr interaction, high order two-mode cross-Kerr interaction and hybrid high order two-mode cross-Kerr interaction can be obtained under certain conditions. These nonlinear interactions have promise for a realistic scheme of photon logic gates necessary for practical implementation of quantum information processing.

Ultrafast Slow-Light Tuning Beyond the Carrier Lifetime Using Photonic Crystal Waveguides

We demonstrate ultrafast delay tuning of a slow-light pulse with a response time <10 ps. This is achieved using two types of slow light: dispersion-compensated slow light for the signal pulse, and low-dispersion slow light to enhance nonlinear effects of the control pulse. These two types of slow light are generated simultaneously in Si lattice-shifted photonic crystal waveguides, arising from flat and straight photonic bands, respectively. The control pulse blueshifts the signal pulse spectrum, through dynamic tuning caused by the plasma effect of two-photon-absorption-induced carriers. This changes the delay by up to 10 ps only when the two pulses overlap within the waveguide and enables ultrafast tuning that is not limited by the carrier lifetime. Using this, we succeeded in tuning the delay of one target pulse within a pulse train with 12 ps intervals.