Nick N. Lepeshkin

Nanofabrication of optical structures and devices for photonics and biophotonics

Abstract Nanofabrication offers promise for the design of artificial materials with optical properties unlike those of materials occurring in nature and for the design of new and exotic optical devices. We describe some specific ideas for applications in this area, and present some laboratory results on the development of these applications.

Propagation of smooth and discontinuous pulses through materials with very large or very small group velocities

We investigate the propagation of optical pulses through two different solid- state optical materials, ruby and alexandrite, for which the group velocity can be very small (vgc )o r superluminal (vgc or negative). We find that for smooth pulses the fractional delay or advancement is maximized through the use of pulses with durations comparable to the response time of the physical process—coherent population oscillations—that leads to these extreme group velocities. However, we find that the transmitted pulse shape becomes distorted unless the pulse is much longer or much shorter than this response time. We also investigate the transmission of pulses that possess an abrupt change in pulse amplitude. We find that, to within experimental accuracy, this nearly discontinuous jump propagates at the usual phase velocity of light c/n ,e ven though the smoothly varying portions of the pulse propagate at the group velocity.

Ultra-slow and superluminal light propagation in solids at room temperature

Slow and superluminal group velocities can be observed in any material that has large normal or anomalous dispersion. While this fact has been known for more than a century, recent experiments have shown that the dispersion can be very large without dramatically deforming a pulse. As a result, the significance and nature of pulse velocity is being reevaluated. In this review, we discuss some of the current techniques used for generating ultra-slow, superluminal, and even stopped light. While ultra-slow and superluminal group velocities have been observed in complicated systems, from an applications point of view it is highly desirable to do have this done in a solid that can operate at room temperature. We describe how coherent population oscillations can produce ultra-slow and superluminal light under these conditions.

Enhancement of third-harmonic generation in a polymer-dispersed liquid-crystal grating

We report the observation of significant enhancement of one-step third-harmonic generation in a one-dimensional photonic crystal pumped by a near-infrared laser beam tuned to the low-frequency edge of the first photonic band gap. The third-harmonic phase matching can be controlled by changing the angle of incidence of the fundamental radiation, allowing tunability of the third-harmonic wavelength. The observed phenomenon was modeled theoretically using the transfer-matrix method. The enhancement is attributed to the combined action of phase-matching between the pump and harmonic waves and pump-field localization within the photonic crystal.

Fundamentals and applications of slow light in room temperature solids

An overview of recent research aimed at controlling the propagation velocity of light pulses through material systems is presented. Most of the research involves two basic approaches. One approach is to make use of quantum coherence effects, such as electromagnetically induced transparency or coherent population oscillations, to modify the material response at an atomic level. Another approach is to form artificial material, such as photonic crystals, that possess strong dispersive properties which lead to a large modification to the group velocity. Both of these approaches have been successfully pursued in recent years.

CUMULATIVE BIREFRINGENCE EFFECTS OF NANOSECOND LASER PULSES IN DYE-DOPED PLANAR NEMATIC LIQUID CRYSTAL LAYERS

New cumulative effects in laser-induced birefringence have been observed under 10-Hz-pulse-repetition-rate, nanosecond-duration laser irradiation of azo-dye-doped planar-nematic liquid crystal layers at incident intensities I ~ 1–10 MW/cm2. An irradiation geometry with the incident polarization parallel to the nematic director was used. his geometry does not permit a first-order electric field induced reorientation of the nematic molecules, allowing us to exclude its contribution to the nonlinear response. New laser-induced birefringence effects with a buildup time of several seconds to minutes manifest themselves in: • the appearance of a polarization component perpendicular to the nematic director; • two different modes of spatial pattern formation with different patterns for parallel and perpendicular polarization: (1) At I ~ 1–5 MW/cm2, the perpendicular polarization component forms a four-leaf-clover (a Maltese-like cross) spatial pattern in the far-field from the initial Gaussian spatial intensity distribution. The incident, parallel polarization component forms a round spot with a single ring spatial pattern. (2) At higher incident intensities (I ~ 5–10 MW/cm2), a second regime of pattern formation is observed in the form of high definition patterns and only for the polarization component parallel to the nematic director.

Far-Field Patterns from Dye-Doped Planar-Aligned Nematic Liquid Crystals Under Nanosecond Laser Irradiation

High-definition patterns were observed under 10-Hz-pulse-repetition-rate, nanosecond laser irradiation of azodye-doped planar-nematic liquid crystal layers at incident intensities I ∼ 5–10 MW/cm2 in a single beam configuration and without any feedback involved. An incident polarization parallel to the nematic director was used. Under periodic pulsed laser irradiation, far-field beam patterns at the output of a dye-doped liquid crystal layer changed kaleidoscopically from rings and stripes to multiple hexagons. This pattern-formation regime had a buildup time of several seconds to minutes. We explain the observed effect by diffraction of the laser beam on light-induced micrometer-size inhomogeneities inside the liquid crystal layer with absorption and refraction properties different from the surrounding area. Possible mechanisms of the formation of the inhomogeneities are discussed.

Slow and fast light propagation in erbium-doped fiber

We study propagation of light pulses and modulation in an Er-doped fiber in the regimes of anomalously slow and superluminal group velocities. The pulses experience either delay or advancement depending on the pump power.

Fundamentals and applications of slow and fast light

We review recent research aimed at slowing down or speeding up the group velocity of light by large factors. We are especially interested in techniques based on the use of room-temperature solids. Processes to be described include stimulated Brillouin scattering and coherent pop-ulation oscillations in ruby, alexandrite, and erbium doped fiber. There has recently been great interest in techniques that can be used to exercise great control over the group velocity of light. These techniques include the possibility of ultra-slow (v <<c) and ultra-fast (v >>c or v negative) propagation. The observation of light with these characteristics has led to a re-examination of the meaning of concepts such as group velocity and information velocity. In addition, the ability to exercise such control over the velocity of light suggests important applications of slow and fast light in the field of photonics. The velocity of propagation of a pulse of light is usually associated with the group velocity given by v = c/ng where the group index is given by ng = n + ω dn/dω. In standard slowand fast-light experiments, the second term in the expression for the group index dominates. Slowand fast-light effects are thus maximized through use of highly dispersive material systems. One standard way of inducing large dispersion into a material system is through use of electromagnetically induced transparency. This procedure induces a narrow transparency window into the absorption profile of an otherwise highly absorbing atomic medium. The rapid change in refractive index associated with this absorption feature then leads to a significant slowing down of the group velocity of light. We recently introduced a different but related procedure for slowing down the speed of light based on the use of coherent population oscillations [1]. This method makes use of the narrow dip in absorption that can occur due to the beating of pump and probe fields in a saturable material. This interaction tends to be relatively insensitive to the presence of dephasing transitions, and thus can lead to slow light speeds even in room temperature solids. We have observed slowand fast-light effects in a wide variety of material systems based on the use of coherent population oscillations. In our initial laboratory investigation of these effects, we observed time delays as long as 1.2 ms in propagating through a 7.5-cm-long ruby crystal [1]. This delay time corresponds to a velocity of 57 m/s and to a group index of 5×106 . We have also studied modified group velocities for propagation through alexandrite [2]. Alexandrite is a saturable absorber at certain wavelengths and a reverse saturable absorber at others. We observe slow-light propagation in the former case and fast-light propagation with a group velocity of – 800 m/s in the latter. We have also observed modified propagation velocities at 1550 nm in an erbium-doped optical fiber amplifier. When unpumped, this system acts as an absorber, but at sufficiently high pump intensity acts as an optical amplifier. We thereby can control the sign and magnitude of the group velocity of propagation by changing the pumping level of the amplifier. For many of the proposed applications of slow and fast light, one would want to maximize the magnitude of the “normalized delay,” that is, the time delay measured in units of the duration of the incident light pulses. Most techniques that have been implemented to date produce normalized time delays in the range of one to three. Much longer time delays would be desirable for many applications. We have recently performed a theoretical study of this situation, and have concluded that there are no fundamental limits to how large a time delay can be implemented using slow-light techniques [3]. Large fractional time delays can be obtained by ensuring that the transparency window leads to complete (100%) transmission. There is then no QThL1-1-INV

Ultra‐Slow and Superluminal Light Propagation in a Room‐Temperature Solid

We review some of the current techniques used for generating ultra‐slow, superluminal, and even stopped light. While ultra‐slow and superluminal group velocities have been observed in complicated systems, from an applications point of view it is highly desirable to do have this done in a solid that can operate at room temperature. We describe how coherent population oscillations can produce ultra‐slow and superluminal light under these conditions.