Group Velocity Of Light Research Articles

Dependence of extrinsic loss on group velocity in photonic crystal waveguides

We examine the effects of disorder on propagation loss as a function of group velocity for W1 photonic crystal (PhC) waveguides. Disorder is deliberately and controllably introduced into the photonic crystal by pseudo-randomly displacing the holes of the photonic lattice. This allows us to clearly distinguish two types of loss. Away from the band-edge and for moderately slow light (group velocity c/20-c/30) loss scales sub-linearly with group velocity, whereas near the band-edge, reflection loss increases dramatically due to the random and local shift of the band-edge. The optical analysis also shows that the random fabrication errors of our structures, made on a standard e-beam lithography system, are below 1 nm root mean square.

Coupled photonic crystal heterostructure nanocavities

We show the first experimental demonstration of multiple heterostructure photonic crystal cavities being coupled together to form a chain of coupled resonators with up to ten cavities. This system allows us to engineer the group velocity of light over a wide range. Devices were fabricated using 193 nm deep UV lithography and standard silicon processing technology. Structures were analysed using both coupled resonator and photonic bandstructure theory, and we highlight the discrepancies arising from subtle imperfections of the fabricated structure.

Low-threshold bistability of slow light in photonic-crystal waveguides

We analyze the resonant transmission of light through a photonic-crystal waveguide side coupled to a Kerr nonlinear cavity, and demonstrate how to design the structure geometry for achieving bistability and all-optical switching at ultralow powers in the slow-light regime. We show that the resonance quality factor in such structures scales inversely proportional to the group velocity of light at the resonant frequency and thus grows indefinitely in the slow-light regime. Accordingly, the power threshold required for all-optical switching in such structures scales as a square of the group velocity, rapidly vanishing in the slow-light regime.

Tunable slow light in Bragg-spaced quantum wells

The group velocity of light is continuously varied in the intermediate band of a Bragg-spaced quantum well structure by tuning the pulse frequency. Delays of 0–0.4bit, without significant pulse distortion, are measured. The high group index is found to lead to large Fresnel reflection coupling losses and Fabry-Pérot fringing. Antireflection (AR) coatings deposited on both sides of the Bragg-spaced quantum well structure are shown to improve coupling of light into the intermediate band but to be sensitive to small errors (∼1%) in the AR coating layer thicknesses.

Phase dependences of optical dispersion and group velocity in an Er3+-doped yttrium aluminium garnet crystal

A new method to control the dispersion property and furthermore the group velocity of a weak probe light by the use of the total closed interaction phase is proposed in erbium-doped yttrium aluminium garnet crystal. From numerical results, it is found that the flat dispersion, the normal dispersion and the anomalous dispersion can be achieved separately by controlling the phases of the applied fields. On this basis, the corresponding variations of the group velocity are discussed and we demonstrate that the subluminal group velocity and the superluminal group velocity can be separately achieved with different light phases and intensities.

All Semiconductor Low-Δ Photonic Crystal Waveguide for Semiconductor Optical Amplifier

An ultralow group velocity of light in a III–V semiconductor photonic crystal (PC) waveguide is expected to enhance the optical gain per unit length of the waveguide. We discuss the possibility of a compact semiconductor optical amplifier that utilizes a single-line-defect PC waveguide fabricated from an all semiconductor low-Δ slab with deep airholes. From photonic band and traveling-wave rate equation analysis, a gain of larger than 20 dB is expected in a short active PC waveguide of 10 µm length, although the waveguide mode has a large radiation loss. We experimentally demonstrate such a waveguide in the passive wavelength regime. The waveguide is fabricated into a GaInAsP/InP wafer with high-mesa input/output waveguides. We show that the observed transmission characteristics of light correspond well to the photonic band theory.

Wide-band transmission of nondistorted slow waves in one-dimensional optical superlattices

Few micron-thick one-dimensional optical superlattices were designed and grown, in which an optimized choice of external dielectric layers allows the formation of a wide and high transmission miniband of coupled cavity states. In such structures a reduction in light group velocity and minimal line shape distortion of propagating optical signal was observed. Group velocity reduction by a factor of 5, obtained both from phase (white-light interferometry) and from time-resolved measurements, is in reasonably good agreement with those calculated through a transfer matrix approach. Time-resolved experiments confirm the minimal line shape distortion for optical pulses of 1.8THz bandwidth at λ=1.5μm wavelength.

Simultaneous Negative Phase and Group Velocity of Light in a Metamaterial

We investigated the propagation of femtosecond laser pulses through a metamaterial that has a negative index of refraction for wavelengths around 1.5 micrometers. From the interference fringes of a Michelson interferometer with and without the sample, we directly inferred the phase time delay. From the pulse-envelope shift, we determined the group time delay. In a spectral region, phase and group velocity are negative simultaneously. This means that both the carrier wave and the pulse envelope peak of the output pulse appear at the rear side of the sample before their input pulse counterparts have entered the front side of the sample.

Photonic crystals slow light for better sensing

Photonic-crystal (PC) structures offer an important platform for optofluidic devices, photonic circuits, and dispersionengineered metamaterials. These devices promise lab-on-a-chip capabilities with considerable flexibility for managing light. They are also likely to reduce the cost of integrated optical circuits and devices. PC circuits have primarily been defined by their periodicity and the architecture of the holes or columns fabricated in the transparent material. This has encouraged the development of stamping and printing technologies that can produce prototypes quickly and make large quantities of sensors inexpensively. In addition to being manufacturable, PC platforms have two attractive attributes for sensing applications. First, a gas or liquid analyte can flow directly through the nanoscopic holes within the photonic structure. Second, ’slow light’ can boost the device’s sensitivity. Optical sensors that exploit spectroscopic analysis—such as surface-enhanced Raman phenomena—are already theoretically capable of molecular sensitivity: PC sensors using slow light could increase even this sensitivity. Because controlling the speed of light has a host of important applications, engineers have learned to manage the dispersion characteristics of a material to control it or, more specifically, to control the group velocity of light. In an isotropic linear medium, the velocity of light is the same in all directions and is defined by the angular frequency (ω) divided by the wavenumber (k). The plot of ω versus k provides the phase velocity of the light. An optical pulse—which one can consider as the envelope or constructive interference arising from a number of frequencies— has a group velocity that is the derivative of ω with respect to k. If this derivative is zero, then at that frequency and for the small band of frequencies in its neighborhood, a pulse has a group velocity that is close to zero. This interests us because slow-light pulse propagation can enhance the energy density of the electromagnetic field within Figure 1. A carefully designed periodic stack with sub-wavelength features can slow certain wavelengths to a crawl and increase the resonant field intensity. The form-birefringent structure in the photo is made of Fullcure 720, a UV-cured material with a refractive index of 1.9 at 8GHz.

V pogone za "medlennim svetom"

A critical review of experimental studies on so-called 'slow light' arising due to the anomalously high steepness of the refractive index dispersion under conditions of electromagnetically induced transparency or coherent population oscillations is presented. It is shown that a considerable amount of experimental evidence of the observation of 'slow light' is not related to the low group velocity of light and can be easily interpreted in terms of a standard model of interaction of light with a saturable absorber.

Controllable Snail-Paced Light in Biological Bacteriorhodopsin Thin Film

We observe that the group velocity of light is reduced to an extremely low value of 0.091 mm/s in a biological thin film of bacteriorhodopsin at room temperature. By exploiting unique features of a flexible photoisomerization process for coherent population oscillation, the velocity is all-optically controlled over an enormous span, from snail-paced to normal light speed, with no need of modifying the characteristics of the incident pulse. Because of the large quantum yield for the photoreaction in this biochemical system, the ultraslow light is observed even at low light levels of microwatts, indicating high energy efficiency.

Controllable group velocity of the probe light in a Λ-type system with two fold levels

The group velocities of the probe laser field are studied in a Λ-type system where one lower state has two fold levels coupled by a control field. It is found that the interaction of double dark states leads to controllable group velocity of the probe field in this system. It can be easily realized, due to the interacting double dark resonances, that one of the group velocities at transparency positions is much slower than the other by tuning the control field to be off resonance. In particular, when the control field is on resonance, we can obtain two equal slow group velocities with a broader EIT width, which provides potential applications in quantum storage and retrieval of light.

Ultraslow Light Propagation in an Inhomogeneously Broadened Rare-Earth Ion-Doped Crystal

We show that coherent population oscillations effect allows us to burn a narrow spectral hole (26 Hz) within the homogeneous absorption line of the optical transition of an erbium ion-doped crystal. The large dispersion of the index of refraction associated with this hole permits us to achieve a group velocity as low as 2.7 m/s with a transmission of 40%. We especially benefit from the inhomogeneous absorption broadening of the ions to tune both the transmission coefficient, from 40% to 90%, and the light group velocity from 2.7 m/s to 100 m/s.

Observation of superluminal and slowdown light propagation in doped lithium niobate crystals

We investigate the group velocity of light in a one-dimensional volume grating inside lithium niobate crystals doped with different impurities. The superluminal and slowdown light propagations are both observed in the crystals. The relationships between the group refractive index and the grating amplitude and phase shift are presented and discussed.

Experimental demonstration of the slow group velocity of light in two-dimensional coupled photonic crystal microcavity arrays

We recently proposed two-dimensional coupled photonic crystal microcavity arrays as a route to achieve a slow-group velocity of light (flat band) in all crystal directions. In this letter we present the experimental demonstration of such structures with a measured group velocity below 0.008c and discuss the feasibility of applications such as low-threshold photonic crystal lasers with increased output powers, optical delay components, and sensors.

Optical-acoustic soliton under the conditions of slow light and stimulated Mandelstam-Brillouin scattering

The possibility of forming a stable optical-acoustic soliton in the regime of electromagnetically induced transparency has been analyzed under the condition that the group velocity of light in a medium with stimulated Mandelstam-Brillouin scattering decreases to the speed of sound. This possibility exists because the forward Mandelstam-Brillouin scattering, which is forbidden in a nondispersive medium, is allowed under this condition. The optical component is an envelope pulse, and the acoustic component has no carrier frequency. It has been shown that such a soliton can be formed for anomalously low input intensities of the optical pulse.

Slow light in photonic crystals

In this work we demonstrate an interesting opportunity to decrease drastically the group velocity of light in two-dimensional photonic crystals constructed form materials with large dielectric constant without dispersion (εd∼100). The group velocity of light in the crystals can be lowered up to 10−5–10−4c (c is velocity of light in free space) in a wide range of the Brillouin zone. We propose a simple and general explanation why that should appear in photonic crystals with large dielectric constants.

Effect of one-photon detuning on light speed reduction in a three-level Λ-type atomic system

We have observed the reduction of group velocity of light via electromagnetically induced transparency (EIT) and measured the dependence of pulse delay on the one-photon detuning in a three-level Λ-type atomic system. The results show that due to the effect of Doppler broadening the light speed reduction is signif icant in a range of ±600MHz one-photon detuning. Under the condition of consid ering the Doppler shift of frequency, the pulse delay as a function of the one-p hoton detuning is numerically calculated. The experimental measurements are in g ood agreement with the theoretical results. These researches provide theoretical and experimental references for controlling the group velocity of light by mean s of one-photon detuning.

Light amplification from Cantor and asymmetric multilayer resonators

The light amplification properties of fractal and asymmetric multilayer resonator structures which contain a light amplifying medium in the middle layer are studied theoretically. The amplitude gain of transmitted light is analyzed by use of two-dimensional plots which we denote gain maps. The gain map, which is plotted against two different structural parameters, illustrates some regularities in the location of gain peaks and bandgaps. Cantor structures exhibit gain peaks at the edges of multiple bandgaps, and those peaks are higher and sharper than those of periodic structures. Asymmetric resonator structures are found to provide more gain than symmetric ones when the gain length is short. A comparison between the gain and the group velocity of light is also presented, together with the electromagnetic energy distributions in the multilayers.

Mode-locking of monolithic laser diodes incorporating coupled-resonator optical waveguides

We investigate the operational principle of mode-locking in monolithic semiconductor lasers incorporating coupled-resonator optical waveguides. The size of mode-locked lasers operating at tens of GHz repetition frequencies can be drastically reduced owing to the significantly decreased group velocity of light. The dynamics of such devices are analyzed numerically based on a coupled-oscillator model with the gain, loss, spontaneous emission, nearest-neighbor coupling and amplitude phase coupling (as described by the linewidth enhancement factor alpha) taken into account. It is demonstrated that active mode-locking can be achieved for moderate alpha parameter values. Simulations also indicate that large alpha parameters may destabilize the mode-locking behavior and result in irregular pulsations, which nevertheless can be effectively suppressed by incorporating detuning of individual cavity resonant frequencies in the device design.