S. A. Magnitskii

Compression of ultrashort light pulses in photonic crystals: when envelopes cease to be slow

A modification of the transfer-matrix method and the nonlinear finite-difference time-domain technique are applied to simulate the propagation of ultrashort laser pulses in linear and nonlinear one-dimensional photonic band gap (PBG) structures. Dispersion properties of photonic crystals allow the chirp of a short light pulse to be compensated in a controlled fashion. It is demonstrated that photonic crystals with embedded optical nonlinearity provide an opportunity to efficiently compress laser pulses to a duration of several optical cycles on a submillimeter spatial scale, providing new opportunities for the miniaturization of femtosecond solid-state laser systems. Physical factors limiting the minimum duration of compressed pulses are studied for such structures and the ways to optimize pulse compression are discussed. Examples of PBG pulse compressors that can be fabricated by means of currently existing technologies are considered.

Evolution of ultrashort light pulses in a two-level medium visualized with the finite-difference time domain technique

The finite-difference time-domain technique is employed to examine the evolution of the amplitude, duration, waveform, and phase of ultrashort light pulses propagating in a medium of two-level atoms or molecules. The results of these numerical simulations agree reasonably well with predictions of the McCall-Hahn analysis for the evolution of the amplitude and the phase of short pulses in a two-level medium until the pulse duration becomes less than the duration of a single optical cycle. Noticeable deviations from the McCall-Hahn scenario were observed for pulses with durations shorter than the duration of a single field cycle.

Matched second-harmonic generation of ultrashort laser pulses in photonic crystals

It is shown that one-dimensional photonic bandgap structures are capable of simultaneously satisfying the phase and group-velocity matching conditions for second-harmonic generation involving extremely short light pulses. When these conditions are satisfied, an optical frequency doubler utilizing photonic bandgap structures provides a means for increasing the rate of growth of the second-harmonic signal as a function of the nonlinear-optical interaction length relative to structures designed for quasi-matched interactions and affords possibilities for enhancing the frequency doubling efficiencies independently of the matching length in the bulk nonlinear material.

Generation of bandwidth-limited tunable picosecond pulses by injection-locked optical parametric oscillators

We report a new Nd:YAG-pumped picosecond optical parametric oscillator that generates bandwidth-limited pulses. Using two LiNbO3 crystals, it produces tunable, near-1.4-μm (signal-wave) pulses of 18-psec duration and Δν = 1.2 cm−1 FWHM (Δντ = 0.7). The output energy of the optical parametric oscillator in a signal wave is no less than 2 mJ with 10% energy stability. The key to this device is the injection of cw single-frequency GaAs diode-laser radiation. Using the injection of diode-laser radiation, we have measured the spectral intensity of a quantum noise at λ = 0.85 μm. The intensity was found to be 6 ± 2 W/cm2 cm−1 sr (theoretical value, 4.7 W/cm2 cm−1 sr).

Phase and group synchronization in second-harmonic generation of ultrashort light pulses in one-dimensional photonic crystals

It is shown, on the basis of an analysis of the dispersion relation for an infinite one-dimensional periodic multilayer structure and direct numerical integration of Maxwell’s equations by the finite-difference method, that structures with photonic band gaps (PBGs) make it possible to provide simultaneously conditions for phase and group synchronizations for second-harmonic generation with participation of extremely short light pulses. The phase and group detunings, which arise as a result of the dispersion of the nonlinear medium, are compensated by the dispersion of the PBG structure. The use of this regime of nonlinearly optical interactions opens up the possibility of attaining high frequency conversion efficiencies irrespective of the synchronization length in the interior volume of a nonlinear material.

Photonic-crystal fibers with a photonic band gap tunable within the range of 930–1030 nm

The physical principles of photonic-crystal fibers with a photonic band gap tunable in the visible and near-IR spectral ranges are demonstrated. Direct numerical integration of the Maxwell equations with the use of the finite-difference time-domain technique reveals the possibility of creating holey fibers with a photonic-crystal cladding whose photonic band gap lies within the frequency range characteristic of widespread solid-state femtosecond lasers. The fabrication of holey fibers with a pitch of the two-dimensional periodic structure of the cladding less than 500 nm allowed us to experimentally observe a photonic band gap in transmission spectra of holey fibers tunable within the range of 930–1030 nm. This photonic band gap is satisfactorily described within the framework of the proposed numerical approach based on the finite-difference time-domain method.

SNOM INVESTIGATION OF THE ELECTROMAGNETIC FIELD INTENSITY AND POLARIZATION DISTRIBUTION IN THE VICINITY OF NANOSTRUCTURES

The experimental and calculated results of the investigation of electromagnetic field distribution including its polarization characteristics in the vicinity of the nanostructures are presented. The experimental investigation was realized by aperture type scanning near field optical microscopes (SNOMs) which operated in collection mode. Normal resolution which allows us to image down to 0.3 nm height surface steps was demonstrated for the shear force probe to surface gap control system of the SNOM. Theoretical computation of the electromagnetic field distribution was realized by finite-difference time-domain (FDTD) method. The experimental three-dimensional maps of intensity and polarization distribution as a result of light diffraction at nanoaperture in the metal screen, dielectric and metallized nanocylinders were obtained. The qualitative difference between the orthogonal polarized component distributions near nanoaperture was experimentally shown. The electromagnetic field concentration in the proximity of the dielectric nanocylinders was observed. This observation gives a good fit with the results of FDTD computations. A spiral type electromagnetic field distribution pattern was experimentally observed in the proximity of metallized nanocylinders, which is unexpected from both experimental and theoretical points of view.

Localization and channeling of light in defect modes of two-dimensional photonic crystals

The spatial distribution of the electromagnetic field in a two-dimensional photonic crystal with a lattice defect is investigated. It is shown that in such a structure the field can be localized in a region smaller than one wavelength in size. The dependence of the spectrum of defect modes on the parameters of a two-dimensional photonic crystal is investigated. The light field at the exit of the photonic crystal possesses properties of a nonradiative mode, making it possible to achieve spatial resolution in the near-field much higher than the radiation wavelength. The possibilities of using this phenomenon in optical near-field microscopy to produce optical memory devices and to increase the efficiency of nonlinear optical interactions are discussed.

Surface-plasmon vortices in nanostructured metallic films

Light scattering by a small protrusion on a metal surface is analyzed within the framework of perturbation theory. Upon normal incidence of a linearly polarized monochromatic wave, slight deviations of the protrusion’s shape from a circularly symmetric one lead to the formation of optical vortices in the near-field region due to resonant excitation of circular surface plasmons. This agrees with the results of scanning near-field optical microscopy experiments revealing distinct spiral patterns in the in-plane near-field intensity distribution for metallized nanostructured polymer substrates.

Constructing a light-field distribution for the laser guiding of atoms in photonic crystals

Abstract Numerical analysis based on the finite-difference time-domain simulation of the light-field distribution in a defect mode of a two-dimensional photonic crystal demonstrates that, with an appropriate geometry of the defect in the photonic-crystal lattice, a potential permitting the laser guiding of atoms with blue-detuned radiation can be produced. The light field in a defect mode of a photonic crystal may decrease by nearly five orders of magnitude on the subwavelength spatial scale, providing a very high localization degree of atoms, which are pushed to the center of the defect due to the dipole force. Simulations of atomic-field distribution and trajectories of atoms in a defect mode of a photonic crystal show that the temperatures of atoms guided by a light field in photonic crystals may be much higher than temperatures characteristic of atoms guided in hollow-core fibers.