Amplitude-modulated Laser Beam Research Articles

Nonlinear Interaction of Amplitude-Modulated Gaussian Laser Beam with Anharmonic Magnetized and Rippled CNTs: THz Generation

Nonlinear Interaction of Amplitude-Modulated Gaussian Laser Beam with Anharmonic Magnetized and Rippled CNTs: THz Generation

Ponderomotive force on an optically levitated sphere in an amplitude-modulated laser beam

Ponderomotive force on an optically levitated sphere in an amplitude-modulated laser beam

Resonant excitation of THz radiations by the interaction of amplitude-modulated laser beams with an anharmonic CNTs in the presence of static D.C. electric and magnetic fields

We proposed an alternative scheme for the generation of terahertz (THz) radiation by using the rectangular array of horizontally aligned hollow anharmonic carbon nanotubes (CNTs) embedded on the base of the dielectric surface. Two transversely amplitude-modulated filamented laser beams interact with this array of CNTs in the presence of static D.C. electric and magnetic fields acting mutually perpendicular to each other as well as to the direction of propagation of the laser beams. Due to the non-uniform density of electrons in CNTs, the restoration force on the different electrons is different and this results in anharmonicity. This anharmonicity broadens the resonance peak. The laser beams also exert space periodic ponderomotive force and beat frequency ponderomotive force on the electrons of CNTs. Space periodic ponderomotive force is well balanced by the pressure gradient force to form a transverse density ripple. Nonlinear coupling between D.C. drift velocity of electrons and electron density in the plasma of CNTs results in enhancement in the generation of THz radiation. We have found that the normalized THz amplitude increases significantly with the increase of applied electric field from 10kV/cm to 20kV/cm and magnetic field from 11.50kG to 24.50kG.

Combined effect of transverse electric and magnetic fields on THz generation by beating of two amplitude-modulated laser beams in the collisional plasma

Combined effect of transverse electric and magnetic fields on THz generation by beating of two amplitude-modulated laser beams in the collisional plasma

Magnetic field generation by amplitude modulated laser pulse in a rippled plasma

The mechanism of a self-generated magnetic field by an amplitude modulated laser beam propagation through a rippled plasma is analyzed. The amplitude modulated laser beam exerts a ponderomotive force on plasma electrons, giving them oscillatory velocities at the modulation frequency. The oscillatory velocity couples with the electron density ripple and drives a nonlinear current. This nonlinear current generates a magnetic field. The magnetic field amplitude shows the resonant enhancement at the laser modulation frequency close to the electron plasma frequency, proportional to the ripple density and the laser intensity. In the relativistic case, it also scales directly with the electron plasma frequency and inversely proportional to the square of laser spot size and can approach the order of several hundred-kilogauss.

Magnetometry using fluorescence of sodium vapor.

Magnetic resonance of sodium fluorescence is studied with varying laser intensity, duty cycle, and field strength. A magnetometer based on a sodium vapor cell filled with He buffer gas is demonstrated, using a single amplitude-modulated laser beam. With a 589nm laser tuned at the D1 or D2 line, the magnetic field is inferred from the variation of fluorescence. A magnetic field sensitivity of 150  pT/Hz is achieved at the D1 line. The work is an important step toward sensitive remote magnetometry with mesospheric sodium.

High-power terahertz emission in magnetized plasma via optical rectification of a super-Gaussian laser beam

The paper discusses the generation of terahertz (THz) wave via the optical rectification of an amplitude-modulated super-Gaussian laser beam in the preformed periodic density plasma when a magnetic field is applied transversely. The non-uniform spatial variation in intensity of the laser beam drives a strong ponderomotive force. Such ponderomotive force generates nonlinear oscillatory drift velocity to plasma electrons that produce a strong transverse nonlinear current at the modulation frequency (the frequency of the radiation lies in the THz domain). Moreover, the presence of the periodic density plasma structure and magnetic field can shift the condition of resonance and can produce the tunable THz frequency radiation. In the current scheme, the THz field amplitude increases with the profile index of the laser, modulation index, ripple parameters, and transversely applied static magnetic field. As a result, the efficiency of the generated THz wave can be achieved of the order of for the exact phase-matching condition.

Self-Inversion of the Image of a Small-Scale Opaque Object in the Process of Focusing of the Illuminating Beam in an Absorbing Medium

We describe the process of adaptive self-inversion of an image (nonlinear switching) of smallscale opaque object, when the amplitude-modulated laser beam, which illuminates it, is focused in a weakly absorbing medium. It is shown that, despite the nonlocal character of the process, which is due to thermal nonlinearity, the brightness-inverse image is characterized by acceptable quality and a high conversion coefficient. It is shown that the coefficient of conversion of the original image to the inverse one depends on the ratio of the object dimensions and the size of the illuminating beam, and decreases sharply for relatively large objects. The obtained experimental data agree with the numerical calculations. Inversion of the images of several model objects and microdefects in a nonlinear KDP crystal is demonstrated.

Thermally induced all-optical inverter and dynamic hysteresis loops in graphene oxide dispersions.

We experimentally study the temporal dynamics of amplitude-modulated laser beams propagating through a water dispersion of graphene oxide sheets in a fiber-to-fiber U-bench. Nonlinear refraction induced in the sample by thermal effects leads to both phase reversing of the transmitted signals and dynamic hysteresis in the input-output power curves. A theoretical model including beam propagation and thermal lensing dynamics reproduces the experimental findings.

Terahertz radiation by self-focused amplitude-modulated Gaussian laser beam in magnetized ripple density plasma

AbstractThis paper presents a theoretical model for efficient terahertz (THz) radiation by self-focused amplitude-modulated laser beam in preformed ripple density plasma. The density of plasma is modified due to ponderomotive nonlinearity which arises because of the nonuniform spatial profile of the laser beam in magnetized plasma and leads to the self-focusing of the laser beam. The rate of self-focusing depends on the intensity of the amplitude-modulated beam as well as on the externally applied magnetic field strength. The electron also experiences time-dependent ponderomotive force by the laser beam at modulated frequency. A nonlinear current at THz frequency arises on account of the coupling between the ripple density plasma and nonlinear oscillatory velocity of the electrons. The yield of the generated THz radiation enhances with enhancement in self-focusing of the laser beam and applied magnetic field.

Thermal diffusivity measurements in solids by photothermal infrared radiometry: Influence of convection–radiation heat losses

This work demonstrates that in photothermal experiments performed in frequency domain the heat losses due to convection and radiation should be taken into account at low frequencies for poor heat conductors. From a model-solution of the heat diffusion equation a dimensionless frequency dependent parameter M = ZH, with sample's thermal impedance Z and H the convection-radiation heat transfer coefficient, turns out to adequately quantify the importance of the effect of those heat losses. A straightforward photothermal infrared radiometry setup was designed to demonstrate the above hypothesis. Disc shaped samples of different test materials were heated by square wave modulated illumination at one of their surfaces at different frequencies using an amplitude modulated laser beam, and the temperature at the rear surfaces was monitored as a function of time using an infrared sensor. The frequency dependence of peak-to-peak values of the temperature signals was found to be consistent with the amplitude spectrum obtained by Fourier transforming the data. The frequency dependence of the peak-to-peak amplitude was compared with a theoretical model with and without taking convection and radiation induced heat losses (CRHL) into consideration. It is found that for poor heat conductors at low modulation frequencies the conventional model without CRHL does not fit well the experimental data, while the extended model leads to good agreement, resulting in reliable values for the thermal diffusivity. For the investigated samples, the contribution to the signal of thermal wave reflection at the back side of the sample turn out to have a minor effect on the signal spectrum.

THz radiation by amplitude-modulated self-focused Gaussian laser beam in ripple density plasma

AbstractThe effect of self-focusing and defocusing on terahertz (THz) generation by amplitude-modulated Gaussian laser beam in rippled density plasma is investigated. A stronger transient transverse current is generated by transverse component of ponderomotive force exerted by laser on electrons that drives radiation at the modulation frequency (which is chosen to be in the THz domain) because of the variation in intensity in the direction transverse to the laser propagation. Numerical simulations indicate the enhancement of THz yield by many folds due to self-focusing of laser beam in comparison with that without self-focusing. The transient focusing of laser beam and its effect on the generated THz amplitude has also been studied.

Phase shift of amplitude-modulated optical signals in graphene oxide water dispersions due to thermal lens focal length oscillation

We analyze the phase shift induced in an amplitude-modulated laser beam propagating through a water dispersion of graphene oxide sheets in a fiber-to-fiber U-bench. This phase shift arises from the thermally induced nonlinear refraction in the sample. The system exhibits strong optical limiting performance for weak continuous-wave signals. A theoretical model including beam propagation and thermal lens focal length oscillation reproduces the experimental findings.

Precise measurement of laser power using an optomechanical system

This paper shows a novel method to precisely measure the laser power using an optomechanical system. By measuring a mirror displacement caused by the reflection of an amplitude modulated laser beam, the number of photons in the incident continuous-wave laser can be precisely measured. We have demonstrated this principle by means of a prototype experiment uses a suspended 25 mg mirror as an mechanical oscillator coupled with the radiation pressure and a Michelson interferometer as the displacement sensor. A measurement of the laser power with an uncertainty of less than one percent (1σ) is achievable.

Optical data encryption using time-dependent dynamics of refractive index changes in LiNbO_3

We present a method for optical encryption of information, based on the time-dependent dynamics of writing and erasure of refractive index changes in a bulk lithium niobate medium. Information is written into the photorefractive crystal with a spatially amplitude-modulated laser beam which when overexposed significantly degrades the stored data making it unrecognizable. We show that the degradation can be reversed and that a one-to-one relationship exists between the degradation and recovery rates. It is shown that this simple relationship can be used to determine the erasure time required for decrypting the scrambled index patterns. In addition, this method could be used as a straightforward general technique for determining characteristic writing and erasure rates in photorefractive media.

Photopotential Imaging on Functionalized Surfaces Dedicated to Label-Free Detection of Biomolecular Interactions

We present a label-free biosensor capable to map local potential by photogenerating charge carriers in a field effect device. It consists in an amplitude-modulated laser beam scanning through an electrolyte/insulator/semiconductor structure at very low illumination around the equilibrium state of semiconductor. A proof of principle is demonstrated by depositing a self-assembled monolayer on the dielectric and by showing a clear photopotential change between naked and covered areas.

Télémétrie laser de haute exactitude. Vers une exactitude sub-micronique dans la mesure des distances kilométriques sans interférométrie

A laser ranging method is described that uses the phase of an amplitude-modulated laser beam: the frequency of the modulation is permanently adjusted to maintain a constant dephasing, and the distance is deduced from its measurement. We show that phase drifts and cyclic errors are suppressed using a fast polarization switch that interchanges the roles of the reference and measurement paths. Preliminary measurements are obtained using this novel design, showing a measurement stability better than 10 nanometres. The comparison of a displacement with an interferometric system shows an agreement of the order of one micron. An analysis of the source of the systematic errors suggests that an accuracy better than one micron can be expected with this very simple set-up.

Terahertz generation by an amplitude-modulated Gaussian laser beam in a rippled density plasma column

The nonlinear interaction of amplitude-modulated two-dimensional and three-dimensional laser beams with a cylindrical plasma column and the generation of terahertz (THz) radiation are studied. The plasma column could be formed by a cylindrically symmetric laser pre-pulse impinging on a gas jet target. A two-dimensional Gaussian laser beam propagating through the plasma exerts a ponderomotive force on electrons, imparting on them a resonant transverse velocity at the modulation frequency, Ω, when , where ωp is the plasma frequency of the column. The current produces THz radiation. In the case of a cylindrically symmetric Gaussian laser beam, the electron response to ponderomotive force misses the resonance, yet the phase matching introduced by a density ripple could efficiently produce THz radiation. The power conversion efficiency of the order of the ratio of THz radiation frequency to laser frequency can be achieved at a laser intensity of ≈1015  W cm−2 at 1 μm wavelength.

周期加熱サーモリフレクタンス法による Cu-Pt 合金薄膜の熱伝導率測定

A novel thermal conductivity measurement technique combining a periodically modulated thermoreflectance method with numerical simulation has been developed to precisely estimate the thermal conductivity of sub-micrometer thickness metal films. Numerical simulation about the surface temperature response was carried out assuming that the sample surface is irradiated by variable heating area in micrometer order using an amplitude modulated laser beam. Analyzed results show that the phase lag of the reflectance signal depends on not only the thermal effusivity but also on the thermal diffusivity of the film in local heating. Hence, the thermal conductivity determined by the thermal effusivity and the thermal diffusivity can be evaluated by the dependence of the phase lags on the heating area. This technique was applied for Cu(1-X)PtX (0≦X≦1.63 at%) films with 300 nm thickness deposited on glass substrates. The evaluated thermal conductivity varies from 340 W/(mK) to 97 W/(mK) with increasing Pt concentration. We found that the thermal conductivity of the Cu thin film becomes less than 90% of that of the bulk value (~400 W/(mK)). Moreover, the evaluated thermal conductivity was confirmed to correspond well to the electric conductivity with the Wiedemann-Franz law.

Optical temporal frequency low-pass filtering and heterodyning with a microchannel plate

A microchannel plate is demonstrated as both an optical low-pass filter and a heterodyning modulator. The frequency response function of the microchannel plate indicates it behaves as a low-pass filter. Antialiasing with the microchannel plate is demonstrated using a two-tone amplitude modulated laser beam, with the higher tone above the cutoff frequency being successfully suppressed while the frequency within the bandwidth of the filter is passed through. Selection of different commercially available phosphors controls the filtering and frequency response characteristics of the optical filter, effectively moving the cutoff frequency. By gating the microchannel plate at different frequencies, the microchannel plate can also be used analogously to a heterodyning spectrum analyzer, supplying all of the required functions of a local oscillator, mixer, and filter.