For numerous applications in science and engineering, the development of technologies for assembling and manipulating macro-, micro-, and nano-objects by means of laser light is of high interest. Most of these attractive forces require dielectric particles they can act on. In recently published papers, a negative light pressure was predicted which acts on metallic bodies if these are separated by a subwavelength slit and which arises from surface plasmon interaction between the metallic bodies1,2. An experimental demonstration of this force has not yet been carried out. First theoretical calculations showed that, for example, a laser power of 100 mW which is fully absorbed in the slit of a laser with a wavelength of 1550 nm would, in a slit having a width of half the wavelength, generate a force of about 1 nN. However, this is roughly three times the force of the radiation pressure of about 0.33 nN.
In this work, we present an experimental setup to measure this force, together with results of refined theoretical calculations and with preliminary results achieved with the experimental setup.
A new method for the crest factor minimization of for multi-tone signal based on the sampling functional is proposed. The minimum of sixth moment is used as an initial value for the minimax search. For high number of tones the optimal values are better than previously reported. Application for fiber Bragg multi-channel filters for telecommunications is discussed.
Two direct numerical methods of soving Gel'fand--Levitan--Marchenko equations are developed based on bordering procedure, hermiscity, Cholesky decomposistion, Toeplitz symmetry and piecewise-linear approximation. The proposed methods are shown to surpass the discrete layer peeling algorithm in accuracy and stability at high reflectance.
The 4-parametric family of exactly solvable profiles of the Bragg reflector is analyzed. The physical meaning of the
parameter set is explained. The solution is proposed to use for approximation of the Gaussian envelope. The high quality
of the approximation within the Bragg window is demonstrated numerically.
Exactly solvable profiles are found for a Bragg grating with amplitude or phase modulation. A simple formulas for reflection coefficient are derived. The shape of reflection spectrum in the case of amplitude modulation is trapezoidal in semi-logarithmic coordinates. In the case of phase modulation the solution is found for grating with the Gaussian chirp.
Continuous-wave generation is demonstrated of the anti- Stokes Raman laser in new (Alpha) scheme ArII 3d'2G9/2 yields 4p'2F7/2 yields 4s'2D5/2 with long- lived start and final levels. Red pump radiation with wavelength (lambda) p equals 611 nm from a dye laser that excites transition 3d'2G9/2 yields 4p'2F7/2 is converted into the blue radiation at (lambda) equals 461 nm (4p'2F7/2 yields 4s'2D5/2) with efficiency of about 30%. The tunability range spans more than +/- 10 GHz around exact resonance, which is five times as wide as the Doppler contour. The output frequency (omega) is found to depend linearly on the frequency (omega) p of the pump field: (omega) approximately equals (omega) p(lambda) p/(lambda) . A sharp peak of output power is observed in the detuning curve at the exact resonance instead of well-known two-photon dip. The model is proposed that includes ionic scattering in plasma and interaction of the running pump and standing output waves. The derived formula offers an interpretation of the observed peak.
Experiments on resonant four-wave mixing demonstrated high- efficiency CW down-conversion of frequency in gas (up to 25%). Meanwhile, the high intensities make the perturbation theory invalid to understand the behavior of output power measured as a function of input powers. The nonlinear susceptibility of optically thin medium is found for difference-sum scheme (omega) 4 equals (omega) 1 - (omega) 2 + (omega) 3, where fields one and three are strong and two and four are weak. In symmetric case (kappa) 2 equals (kappa) 4, the integral over velocity can be calculated analytically for Doppler limits in collinear geometry. The compact explicit formula obtained yields the intensity and frequency dependence. The peak of mixing coefficient as a function of intensity is found around equal Rabi frequencies of fields one and three. The effect is shown to base on resonance between two closed cycles via sublevels of dressed states. The number of peaks in a spectrum varies from three to eight. Explicit formula allows interpreting the measurements and predicting the optimal relation between parameters for high conversion efficiency in gas with large Doppler broadening.
The explicit solution is obtained for four-wave mixing of two strong driving fields at opposite transitions and two weak fields in a four-level system with the large Doppler width. In zero order of the perturbation theory there are two independent two-level systems. A pair of weak fields probe two other allowed transitions. The resonance of the mixing coefficient dependence on intensity is found around equal Rabi splitting in both two level systems. The effect is interpreted as an intersection of quasi-energy levels. Up to 6 peaks appear in the dependence of conversion coefficient on the detuning of the probe field, two of them are a consequence of averaging over velocities and disappear at low temperature. The results allow us to explain the recent experiments on the mixing in sodium vapor.
We investigate the line narrowing in dense plasmas due to ion- ion scattering. Landau term include velocity dependence of the effective transport collision frequency that makes the problem more complicated that the Dicke narrowing in gases. The collision process is analytically described as a diffusion in the velocity space. It is shown that the atomic density matrix can be calculated by decomposition over orthogonal polynomials. The analytical results are obtained in limiting cases of short and long wavelength compared with the mean free path of an ion. For intermediate case a simple interpolation formula, based on numerical calculation, is proposed having an accuracy within 10% both for absorption linewidth and anomalous refractive index. The conditions for experimental observation of the narrowing are discussed.
An analytical expression is found for the profile of Bennett hole with different level lifetimes under strong field and weak collisions. The expression is valid while the diffusion and field broadening being more than the homogeneous one, but less than the thermal width. It is shown that the square of the total width being equal to the sum of the squares of the diffusion and field widths. The shape of saturation curve is calculated including weak collisions. The formula obtained describes the smooth transition from homogenous to inhomogeneous saturation throughout the new intermediate region. Probe-field spectrum of the three-level system is computed in the presence of the strong field at the adjacent transition. The Autler-Townes doublet components are shown to broaden and repel each other under the collisions. The main reason of both the effects occurs to be the diffusion of phase due to quadratic frequency shift.
An absorption coefficient and hence the population density of metastable states 3d4F, 3d2P, 3d'2G were measured by a single-frequency dye laser in gas-discharge plasma. It has been shown that population distribution between levels of multiplet 3d4FJ can be described by Boltzmann's low with temperature close to that for heavy particles. Relaxation constants measured from absorption saturation curves have close values for different metastables and are approximately equal to (Gamma) n approximately equals (2 divided by 6) X 107 s-1. High value of relaxation constants means high excitation rates of metastable levels. Linear dependence of (Gamma) n on electron density was shown for 3d4F7/2 level. It has been demonstrated that electrons are of great importance in the deactivation process. The measured parameters of levels are used for evaluation of output characteristics of three-level Raman laser with metastable start and final levels.
The profile of nonlinear resonance in the probe-field ionic spectrum is shown to narrow down with the saturating field detuning. Physical cause of the effect is the falling velocity dependence of Coulomb collision frequency.
Argon laser lines in the violet-blue spectral region have been studied experimentally. Cw generation has been demonstrated on a set of close lines: 438.4, 437.6, 437.1 and 436.2 nm, in particular, cw lasing on 438.4 and 436.2 nm being observed for the first time. Cw lasing has been also observed on 418.2 and 431.0 nm lines. The highest output appeared on the 438.4 nm line, which corresponds to weak quartet-doublet transition 4p4S3/2 -4 s2P3/2. Determined are the gain 10-4 cm-1, saturation parameter 6.5 kW/cm2, and the Einstein coefficient of this line. High power fundamental TEM00 mode operation has been achieved. It has been shown that a comparatively high output power is extracted due to a high pumping rate of the studied quartet level, comparable to that of the known 514.5 nm argon laser line.
The effect of ion-ion scattering on the generation of plasma-based Raman laser is studied. The ion diffusion in velocity space is shown to broaden the detuning range and to increase the output power. The results are compared with the experiment made in argon laser plasma on anti-Stokes line 437 nm under 648 nm pumping. Agreement between the theory and measured generation spectra confirms that the collisions contribute to lasing processes.