Effects of inversionless lasing in two-level media from the point of view of specificities of the spatiotemporal propagation dynamics of radiation

We report the results of computer simulation of the emission of radiation by an extended two-level medium in a ring cavity. The cases of using strong external monochromatic, quasi-monochromatic, and biharmonic radiation for pumping the two-level medium are analyzed. It is shown that the emission of radiation with spectral content different from that of the pump radiation, which is interpreted as the inversionless oscillation, is the result of the spatiotemporal dynamics of light propagation in an extended two-level medium imbedded in a cavity. The appearance of this radiation is not related to known resonances of amplification of a weak probe field in a thin layer of the two-level system (the effect of inversionless oscillation) induced by strong resonance monochromatic or biharmonic field, as was thought before.     

The parametric doppler effect upon oblique radiation incidence in quartz glass

Frequency and reflection angle of probe radiation from a refractive-index inhomogeneity induced by an intense pumping pulse in quartz glass and moving with a relativistic velocity are calculated. Conditions under which the normal component of the wave vector of the reflected wave is directed to the opposite or to the same direction as the same component for the incident wave are determined. Comparison with the case of radiation reflection from a relativistic mirror in vacuum is performed. Conditions of the appearance of the Vavilov-Cherenkov radiation from a relativistic refractive-index inhomogeneity induced in the medium by an intense laser pulse are discussed.     

The parametric Doppler effect in a medium with Lorentz dispersion

The frequency conversion under the parametric Doppler effect, which occurs when weak probe radiation reflects off from a refractive index inhomogeneity induced in a nonlinear medium by intense pumping, has been analyzed. Under these conditions, the frequency dispersion of the medium is assumed to correspond to the single Lorentz oscillator model. It is shown that the presence of resonance and a spectral range forbidden for propagation may lead to a complex Doppler effect, where the incident radiation is single-frequency but the reflected radiation consists of two or three monochromatic waves, one of which has a phaseconjugated wavefront.     

Normal modes of a probe field in the pulsed regime of electromagnetically induced transparency in a Λ-scheme of degenerate quantum transitions

The effect of separation of linearly polarized short probe pulses of electromagnetically induced transparency in the field of linearly polarized coupling radiation is modeled numerically. It is shown that the input-probe pulses polarized parallel or perpendicular to the input-probe field propagate in the medium without changing the state of their polarization. If the input-probe radiation is weak compared to the coupling radiation, then the probe field inside the medium is the sum of two independently propagating linearly polarized normal modes, which are excited by the projections of the input-probe pulse onto the direction of polarization of the coupling radiation and onto the perpendicular direction, respectively. The normal modes have the same phase velocities, but different velocities of their real envelopes. This circumstance leads to the rotation of the plane of polarization of the total probe field at short distances and to its separation into two pulses with mutually perpendicular directions of polarization at long distances. At a high intensity of the probe radiation, the input-probe pulse decays into pulses the planes of polarization of which are not mutually perpendicular. Under these conditions, it is impossible to represent the probe radiation as a sum of normal modes. The modeling is performed in the scheme of degenerate quantum transitions between states of levels ³ P ₀, ³ P ₁ ⁰, and ³ P ₂ of isotope ²⁰⁸Pb taking into account the Doppler broadening of spectral lines.     

Speckle polarization diagnostics of scattering media using partially coherent radiation

The paper discusses a method for probing a randomly inhomogeneous medium using partially coherent radiation and polarization filtering of the radiation scattered by the medium. The method is based on the analysis of the contrast of speckle-modulated images of the object under study as a function of the coherence length of the probe radiation. A theoretical justification of the method is given as applied to systems of discrete scatterers with the subsequent modification of the results obtained for continuously distributed scattering systems. The results of experimental testing of the developed method for scattering media characterized by nondiffusion conditions of propagation of the probe radiation are presented and compared with the results of the theoretical analysis.     

Method of resonance near-field optical microscopy

We have solved the problem in which a thin metal wafer (probe) with a nanohole interacts with the flat surface of a metastructured film consisting of metal nanoparticles in an external optical radiation field. Nanoparticles are considered as two-level atomic systems. This interaction of the wafer-probe and the flat surface in the external optical radiation field gives rise to optical near-field resonance, the frequency of which differs significantly from the natural frequencies of two-level atoms in the medium and the probe. The fields inside and outside the probe and metastructured film are calculated in the near-field and far-field zones. The maximum resolution, which is achievable in the suggested scheme of near-field optical microscopy, can reach about 10 nm. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 4, pp. 499–506, July–August, 2007.     

Polarization decay of pulses of electromagnetically induced transparency on J=0→J=1→J=2 degenerate quantum transitions

The evolution of radiation under conditions of electromagnetically induced transparency in the scheme of degenerate quantum transitions J = 0 → J = 1 → J = 2 in the pulsed interaction regime of the fields and with allowance for the Doppler broadening of spectral lines has been analyzed numerically. It has been shown that, if the input coupling radiation is linearly polarized, the circularly polarized input probe pulse splits in the medium into pulses with mutually perpendicular linear polarizations. The direction of polarization of one of these pulses coincides with the direction of polarization of the input coupling field. The distance that the probe pulse travels in the medium until it completely decays decreases with a decrease in both the duration of the input probe pulse and the intensity of the input coupling radiation. A change in the power of the input probe pulse hardly affects the distance required for the decay and the velocity of propagation of linearly polarized pulses in the medium. An increase in the Doppler broadening of spectral lines leads to a decrease in this distance and, simultaneously, to an increase in the energy losses of the probe radiation. Qualitative considerations that explain the physical reason for the investigated effects have been presented.     

The parametric Doppler effect upon reflection of light from a moving smooth inhomogeneity of a medium

The reflection of probe radiation from a smooth inhomogeneity of characteristics of a medium that propagates with a relativistic velocity is analyzed. Equations that describe the propagation of forward and backward waves in an inhomogeneous medium are derived, conditions of occurrence of a Bragg resonance are formulated, and conditions for increasing the coefficient of reflection from the inhomogeneity are discussed.     

The parametric Doppler effect upon oblique incidence of probe radiation

Transformation of frequency and intensity of probe radiation obliquely incident on a moving inhomogeneity induced by an intense laser radiation in a nonlinear medium is analyzed. The dependences of frequency and intensity of radiation reflected from the inhomogeneity have been found for a variant of frequency dispersion corresponding to collision-free plasma. These dependences possess singularities under conditions corresponding to the Vavilov-Cherenkov effect. The initially weak noise components of probe radiation increase in the reflected radiation up to a considerable value.     

Boundary-value problems in near-field optical microscopy and optical size resonances

We have solved a boundary-value problem for a ball probe interacting with a flat dielectric surface in an external optical radiation field. This interaction gives rise to the optical size resonance at frequencies significantly different from the natural frequencies of two-level atoms both in the medium and in the probe with allowance for the local field corrections. These resonances depend significantly on the distance from the probe center to the surface, on the ball probe size, on the concentration of two-level atoms in the probe and in the medium, on the spectral line width, and on the atomic inversion. The field strengths inside and outside the ball probe and a semiinfinite dielectric medium have been calculated in the near-field and wave zones. It is shown that the proposed electrodynamic theory of optical near-field microscopy agrees with the results of experimental measurements.     

Spectral features of the interaction of femtosecond light pulses of different frequencies near the boundary of a Kerr medium

The interaction of probe ultraviolet (UV) and intense infrared (IR) pump pulses with a duration of 150 fs in a thin 2-mm-long sample of fused silica has been studied theoretically and experimentally. The spectra of UV radiation at the output of the sample have been measured depending on the time delay between the pulses at the sample input. The maximum shifts of the spectrum, attaining up to 0.22 nm for an IR power density of 3 × 10¹¹ W/cm², have been observed under conditions of coincidence of the pulses at the sample input and output, which corresponds to a predominant interaction of the probe radiation with the leading and trailing edges of the pump pulse near the boundary of the medium. The observed dependences are interpreted as a manifestation of the cross-phase modulation due to the Kerr nonlinearity of the medium and the dispersion of the group velocities of the UV and IR pulses. The numerical simulations performed taking into account these effects agree well with the experimental data.     

A Highly Efficient Scheme of Redistribution of Optical Radiation Scattered by Spatial Gratings of Atomic Populations

Optics and Spectroscopy - The radiation intensity redistribution of the probe field upon its scattering by spatially periodic atomic population gratings in a medium with a four-level tripod...     

Complex resonance and complex-frequency spectroscopy

The response of a medium to radiation with a complex frequency corresponding to an exponential time decrease in the radiation amplitude has been analyzed. The effect of complex resonance has been demonstrated when the real and imaginary parts of the complex radiation frequency approach the real and imaginary parts of the complex frequency of damping natural oscillations of medium oscillators. In resonance itself, the absolute value of the refractive index is divergent and radiation is completely reflected from the medium boundary. It has been shown that scanning of not only the real part, but also the imaginary part of the probe radiation frequency expands the capabilities of spectroscopy and makes it possible to, e.g., distinguish resonances even with coinciding (real) frequencies and close (of the same order of magnitude) widths.     

Formation of guided Cherenkov radiation in silicon-based nanocomposites

We use a monochromated 200 keV electron beam as a nanometer-resolution probe of the photonic density of states in bulk and nanoparticle Si/SiO(2) systems, observing infrared to ultraviolet waveguided Cherenkov modes in Si slabs, but none in SiO2. While isolated Si nanoparticles are too small to support Cherenkov radiation, we find high densities of Si nanoparticles in SiO2 support a damped form of the radiation, with the modes determined by the effective medium of the Si/SiO(2) mixture. The guided nature of the radiation is confirmed by the presence of a thickness-dependent cutoff.     

Features of the scattering of focused terahertz radiation from the probe of a terahertz near-field microscope

The scattering of pulsed terahertz radiation from metallic probes in the form of thin cylinders and cones with a small opening angle, which are used in apertureless terahertz near-field microscopes, has been investigated. The extrema of the waveform of pulsed terahertz radiation scattered from a free probe are linearly shifted with a change in the vertical position of the probe, and the spectral distribution is characterized by an inversely proportional frequency dependence. In the presence of a reflecting surface under the probe, when new excitation and detection directions appear, the spectrum of scattered terahertz radiation does not differ from the spectrum of the incident radiation. The experimental data are in mutual agreement with the theoretical results obtained within the model of the generation of diffraction edge waves at the interface of inhomogeneous excitation between the excitation region and shadow region.     

Spectral shifts as a signature of the onset of diffusion of broadband terahertz pulses

We describe measurements of polarization dynamics as a probe of multiple scattering of photons in a random medium by use of single-cycle terahertz pulses. We measure the degree of polarization and correlate it directly with the single-scattering regime in the time domain. We also measure the evolution of the temporal phase of the radiation and show that the average spectral content depends on the state of polarization. In the case of broadband radiation, this effect can be used to distinguish photons that have been scattered a few times from those that are propagating diffusively.     

Effects of transient local heating of spatially and spectrally heterogeneous biotissue by short laser pulses

Summary A new laser method for the spectroscopy of locally absorbing microvolumes much smaller than the radiation wavelength in size is discussed. Such an absorption is characteristic of biotissues and other biological media. The method allows data to be obtained on the size of local microvolumes absorbing at the radiation wavelength. It is based on the possibility of transient overheating of the microvolumes by means of an ultrashort pulse with a duration much shorter than the time it takes to heat to diffuse from the microvolumes. The pulse-heated microvolumes must have an altered refractive index leading to an additional scatter of another probe laser pulse, that is made to irradiate the medium under study after some delay. Besides, the locally heated microvolumes will have temperature-altered fluorescence induced either by the heating laser pulse or by an additional probe pulse at another wavelength more suitable for fluorescence excitation. Due to the relevance of its scientific content, this paper has been given priority by the Journal Direction.     

On the Sub-Doppler Spectroscopy of Optically Excited Atoms in a Thin Gas Cell

This work continues a theoretical investigation of the capabilities of the well-known method based on using a monochromatic probe light beam in combination with optical pumping of atoms (molecules) of a rarefied-gas medium by a broadband radiation in a thin cell the diameter of which is much larger than its internal thickness. In contrast to calculations carried out in the previous publications on this method of spectroscopy, here, we consider the case of arbitrary values of pump intensity and thickness of a cylindrical gas cell. Thus, all the possible mechanisms and specificities of velocity selection of atoms in optically excited levels caused by transit-time relaxation of such atoms in gas cells of this kind are analyzed. Within the framework of this approach, sub-Doppler absorption resonances of the probe light beam corresponding to quantum transitions from the upper level excited by optical pumping are investigated. The obtained results can be used in high-resolution spectroscopy of atoms (molecules), as well as for laser-frequency stabilization to established narrow spectral resonances.     

Near-field scanning optical microscopy based on surface-enhanced Raman scattering

We describe a near-field optical microscopy technique based on the interaction of a probe molecule with the sample surface (e.g., with a flat metal surface) in the field of external optical radiation and consider the spontaneous Raman scattering characterized, in the presence of a metal surface, by the effective polarizability of the probe molecule, depending on the frequency and the distance to the sample surface. At certain distances from the probe molecule to the surface, the effective polarizability of this molecule (determined with allowance for the polarizing influence of the surface of a semi-infinite medium) at the Stokes frequency sharply increases in comparison to the quantum polarizability of an isolated molecule, which is indicative of the formation of optical near-field resonances. It is shown that the proposed method of near-field optical microscopy is characterized by high sensitivity and high spatial resolution (on the order of 1 Å).