Pump Laser Intensity Research Articles

Red-shift law of intense laser-induced electro-absorption in solids

A theoretical study on the red-shift of laser-induced electro-absorption is presented. It is found that laser-induced red-shift scales with the cube root of the pump laser intensity in the optical tunneling regime and has an obvious deviation from this scale in the multi-photon regime. Our results show that in the optical tunneling regime, the laser-induced red shift has the same law as that in the direct current (DC) approximation. Though the scales are the same in the optical tunneling regime, the physical pictures in the two cases are quite different. The electro-absorption in the DC case is a tunneling-assisted transition process, while the laser-induced electro-absorption is a mixed multi-photon process.

Absorption-type terahertz wave switch based on Kerr media

We design an ultra-fast absorption type switch operating in the terahertz wave regime. The device consists of dual metallic layers for allowing near-perfect absorption due to electric and magnetic resonances, and a nonlinear Kerr-media layer for actively manipulating device absorption. In this novel design, the center frequency of the absorption peak can be dynamically shifted by adjusting the incident external pump laser intensity. By using the finite-difference time-domain method, the switch properties of terahertz wave absorption in sub-wavelength period structures on the Kerr-media layer under different incident optical intensities have been investigated. The simulation results demonstrate that device absorption can be flexibly tuned from near unity to zero by adjusting the incident optical intensity. It is extremely versatile to control the ultra-fast terahertz switch by varying the incident optical intensity. These results provide a useful guide for the developments of terahertz wave functional components.

Intense terahertz emission from relativistic circularly polarized laser pulses interaction with overdense plasmas

During the interaction of a relativistic circularly polarized laser pulse with an overdense plasma target, the longitudinal motion of bunches of electrons under the action of light pressure and electrostatic restore force can emit intense terahertz (THz) pulses. This mechanism allows high pump laser intensity and large electron number participating in the emission. Two-dimensional particle-in-cell simulations are carried out to investigate the THz emission. The results suggest that such a source can produce remarkably intense THz pulses with energy of several mJ/sr and power of tens of gigawatts, which could find applications in nonlinear studies and relativistic laser-plasma interaction diagnostics.

Light-Induced Crystallization of Cold Atoms in a 1D Optical Trap

Collective off-resonant scattering of coherent light by a cold gas induces long-range interactions via interference of light scattered by different particles. In a 1D configuration, these interactions grow particularly strong by coupling the particles via an optical nanofiber. Above a threshold pump laser intensity, we predict a phase transition from a homogeneous density to a self-sustained crystalline order. In the dispersive regime, we determine the critical condition for the onset of order as well as the forms of gas density and electric field patterns above threshold. Surprisingly, there can coexist multiple ordered states with distinct appearances.

C- and L-band erbium-doped waveguide lasers with wafer-scale silicon nitride cavities

We report on integrated erbium-doped waveguide lasers designed for silicon photonic systems. The distributed Bragg reflector laser cavities consist of silicon nitride waveguide and grating features defined by wafer-scale immersion lithography and a top erbium-doped aluminum oxide layer deposited as the final step in the fabrication process. The resulting inverted ridge waveguide yields high optical intensity overlap with the active medium for both the 0.98 μm pump (89%) and 1.5 μm laser (87%) wavelengths with a pump-laser intensity overlap of >93%. We obtain output powers of up to 5 mW and show lasing at widely spaced wavelengths within both the C and L bands of the erbium gain spectrum (1536, 1561, and 1596 nm).

Static and dynamic phase-dependence of dark resonances and its dispersion features in double tunneling-coupled quantum wells

In this work we study the coherent population trapping in tunneling-coupled quantum wells interacting with optical radiation. The phase sensitive dependence of dark resonance was investigated. We found that destruction as well as restoration of dark resonance of the coherent population trapping is possible in dependence on algebraic sum of the phases of exciting fields. It is shown that the phase variation of exciting fields influences on the character of the absorption and dispersion of the excited fields in a medium with quantum wells. We found some extreme resonance specify of dispersion relatively phase variation. We obtained analytical expression for the light shift in system with closed contour of excitation. The time-modulation of the laser pumping intensity by phase-time modulation is demonstrated.

All-optical nonlinear frequency—Mixing acoustics of cracks

Recent advances in all-optical evaluation of nonlinear cracks are reviewed. In experiments, the nonlinear acoustic waves are initiated by the absorption of radiation from a pair of laser beams intensity-modulated at two different frequencies. The detection of the acoustic waves at mixed frequencies, absent in the frequency spectrum of the laser intensity, is achieved by optical interferometry or deflectometry. The high contrast in crack imaging achieved by remote optical monitoring of the nonlinear acoustic processes is due to the strong dependence of the optoacoustic conversion efficiency on the state of the crack. The highest acoustic nonlinearity is observed in the transitional state of the crack, which is intermediate between the open and the closed ones. Several crack parameters can be estimated from the measurements of the dependence of the acoustic spectrum on the pump laser intensity. One-dimensional theory of the nonlinear frequency-mixing photo-acoustic crack imaging is presented. The theory relates experimental observation of the large number of mixed frequencies to strong bi-modular nonlinearity of the crack. The theory provides guidelines for the understanding of the dependence of the spatial resolution of this technique on the laser power and on the choice of a particular mixed-frequency component for the crack imaging. [Work supported by ANR project ANL-MEMS ANR-10-BLAN-092302.]

Decoherence of Rabi oscillations in laser-generated microplasmas

Characteristic fringe patterns of spectral interference in dynamic Rabi sidebands are used as a means to investigate decoherence phenomena in a highly nonequilibrium underdense microplasma formed in atmosphericpressure oxygen gas in the wake of a strong-field (∼1013 W/cm2) femtosecond pump pulse. The same transient processes in the nonequilibrium plasma work both to create the effective two-level systems producing coherent sidebands and to destroy the coherence and smear the sideband spectral structure; the observed spectral fringe pattern is a result of the trade-off of these two effects. The sidebands generated on a moderately intense (∼1010 W/cm2) picosecond laser pulse reveal the decoherence rate as a function of the pump-pulse intensity and the pump-probe delay time. The rate increases with the pump laser intensity and decreases with the delay with corresponding decoherence times in the range of 750 fs to 3 ps, in good agreement with theory revealing that electron scattering dominates the dynamics for the subnanosecond relaxation processes.

Nonlinear behavior of vibrating molecules on suspended graphene waveguides

Suspended graphene waveguides over micrometer-scale metal-mesh screens were used as platforms for Raman scattering. Raman signals of B. megaterium spores were found sensitive to in-plane rotations and tilt of the waveguides with respect to the incident linearly polarized pump beam. When at plasmonic resonance for the equivalent long wavelength of the vibration frequency, the Raman signal exhibited an additional quadratic effect.

Bistable behavior of a two-mode Bose–Einstein condensate in an optical cavity

We consider a two-component Bose–Einstein condensate in a one-dimensional optical cavity. Specifically, the condensate atoms are taken to be in two degenerate modes due to their internal hyperfine spin degrees of freedom and they are coupled to the cavity field and an external transverse laser field in a Raman scheme. A parallel laser also excites the cavity mode. When the pump laser is far detuned from its resonance atomic transition frequency, an effective nonlinear optical model of the cavity–condensate system is developed under the discrete mode approximation (DMA), while matter–field coupling has been considered beyond the rotating wave approximation. By analytical and numerical solutions of the nonlinear dynamical equations, we examine the mean cavity field and population difference (magnetization) of the condensate modes. The stationary solutions of both the mean cavity field and normalized magnetization demonstrate bistable behavior under certain conditions for the laser pump intensity and matter–field coupling strength.

Nonlinear optical absorption and stimulated Mie scattering in metallic nanoparticle suspensions

The nonlinear optical properties of four metallic (Au-, Au/Ag-, Ag-, and Pt-) nanoparticle suspensions in toluene have been studied in both femtosecond and nanosecond regimes. Nonlinear transmission measurements in the femtosecond laser regime revealed two-photon absorption (2PA) induced nonlinear attenuation, while in the nanosecond laser regime a stronger nonlinear attenuation is due to both 2PA and 2PA-induced excited-state absorption. In the nanosecond regime, at input pump laser intensities above a certain threshold value, a new type of stimulated (Mie) scattering has been observed. Being essentially different from all other well known molecular (Raman, Brillouin) stimulated scattering effects, the newly observed stimulated Mie scattering from the metallic nanoparticles exhibits the features of no frequency shift and low pump threshold requirement. A physical model of induced Bragg grating initiated by the backward Mie scattering from metallic nanoparticles is proposed to explain the gain mechanism of the observed stimulated scattering effect.

Dynamic exciton-polariton macroscopic coherent phases in a tunable dot lattice

We report on the enhancement of the nonlinear properties of exciton-polariton macroscopic coherent phases created by resonant optical excitation under the spatial modulation of a square acoustic lattice. The confinement increases the local particle density by preventing particle drift due to repulsive interactions and modifies the coupling to the exciting laser field. Both effects lead to the enhancement of the nonlinear properties of the system, which is reflected in a reduction of the pump laser intensity threshold of formation of the coherent phase, pronounced blueshift of the emission energy and screening of the acoustic potential.

A Brief Study on Silicon Crystal Materials as a Mid-IR Raman Amplifier

We consider a bulk silicon crystal as a Mid-IR Raman amplifier and study its Raman amplification. A Raman amplifier is established when an intense pump laser pulse and a Raman laser pulse pass through one silicon simultaneously, with good spatial and temporal overlap. Considering the situation of pumping wavelength at 2.94 μm achievable by using an Er:YAG laser and Raman laser wavelength at 3.47 μm with the 521 cm-1 Raman shift, the properties of the output amplified Raman laser are investigated by numerically solving the coupled transfer equations.

Note: Improved sensitivity of photoreflectance measurements with a combination of dual detection and electronic compensation

A setup is described for performing photoreflectance (PR) measurements using two photodetectors, wherein the photodetectors need not be identical. The second detector monitors the photoluminescence and scattered pump laser background signal in real time. It is then eliminated from the measured PR signal using electronic circuits that compensate for amplitude and phase differences between the background signals from the two detectors. The technique overcomes the adverse effect of short-term fluctuations in the pump laser intensity. The signal-to-noise ratio is shown to improve significantly, enabling measurement of weak PR signals.

Theory of laser-induced demagnetization at high temperatures

Laser-induced demagnetization is theoretically studied by explicitly taking into account interactions among electrons, spins, and lattice. Assuming that the demagnetization processes take place during the thermalization of the subsystems, the temperature dynamics is given by the energy transfer between the thermalized interacting baths. These energy transfers are accounted for explicitly through electron-magnon and electron-phonon interactions, which govern the demagnetization time scale. By properly treating the spin system in a self-consistent random phase approximation, we derive magnetization dynamic equations for a broad range of temperature. The dependence of demagnetization on the temperature and pumping laser intensity is calculated in detail. In particular, we show several salient features for understanding magnetization dynamics near the Curie temperature. While the critical slowdown in dynamics occurs, we find that an external magnetic field can restore the fast dynamics. We discuss the implication of the fast dynamics in the application of heat-assisted magnetic recording.

Shock-wave-based density down ramp for electron injection

We demonstrate a sharp density transition for electron injection in laser wakefield acceleration through numerical study. This density transition is generated by a detached shock wave induced by a cylinder inserted into a supersonic helium gas flow. In a Mach 1.5 flow, the scale length of the density transition ${L}_{\mathrm{grad}}$ can approximately equal to plasma wavelength ${\ensuremath{\lambda}}_{p}$ at the shock front, and can be further reduced with an increase of the flow Mach number. A density down ramp with ${L}_{\mathrm{grad}}\ensuremath{\ge}{\ensuremath{\lambda}}_{p}$ can reduce the phase velocity of the wakefield and lower the energy threshold for the electrons to be trapped. Moreover, the quality of the accelerated beam may be greatly improved by precisely controlling of ${L}_{\mathrm{grad}}$ to be one ${\ensuremath{\lambda}}_{p}$. For an even sharper density down ramp with ${L}_{\mathrm{grad}}\ensuremath{\ll}{\ensuremath{\lambda}}_{p}$, the oscillating electrons in the plasma wave will up shift their phase when crossing the ramp, therefore a fraction of the electrons are injected into the accelerating field. For this injection mechanism, there is no threshold requirement for the pump laser intensity to reach wave breaking, which is a big advantage as compared with other injection mechanisms.

Ultrafast carrier dynamics and terahertz emission in optically pumped graphene at room temperature

We report, within a picosecond time scale, fast relaxation and relatively slow recombination dynamics of photogenerated electrons and holes in an exfoliated graphene under infrared pulse excitation. We conduct time-domain spectroscopic studies using an optical pump and terahertz probe with an optical probe technique and show that graphene sheet amplifies an incoming terahertz field. The graphene emission spectral dependency on laser pumping intensity shows a threshold-like behavior, testifying to the occurrence of the negative conductivity and the population inversion. The phase behavior of the measured terahertz electric field also shows clear Lorentzian-like normal dispersion around the gain peak, testifying to the amplification that can be attributed to stimulated emission of photocarriers in the inverted states. The emission spectra clearly narrow at a longer terahertz probe delay time, giving evidence that the quasi-Fermi energy moves closer to the equilibrium at this longer terahertz probe delay time.

Terahertz light amplification by stimulated emission of radiation in optically pumped graphene

ABSTRACTRecent advances in terahertz light amplification by stimulated emission of radiation in optically pumped graphene are reviewed. We present, within a picosecond time scale, fast relaxation and relatively slow recombination dynamics of photogenerated electrons and holes in an exfoliated graphene under infrared pulse excitation. We conduct time-domain spectroscopic studies using an optical pump and terahertz probe with an optical probe technique and show that graphene sheet amplifies an incoming terahertz field. The graphene emission spectral dependency on laser pumping intensity shows a threshold-like behavior, testifying to the occurrence of the negative conductivity and the population inversion. The emission spectra clearly narrow at a longer terahertz probe delay time, giving an evidence that the quasi-Fermi energy moves closer to the equilibrium at this longer terahertz probe delay time.

Harmonics generation in ultra-thin nanofilms irradiated by intense nonrelativistic laser pulses

The problem of harmonics generation in nanotargets is considered at the range of parameters (a nanotarget diameter and a pump laser intensity) when the oscillation amplitude of an electron in a target is much larger than the target width. Electron motion in charged nanotargets in the presence of a laser field of different (non-relativistic) strength is considered. It is demonstarted that for lasers of infrared frequencies clusters do not posess strong enough potential to bound electrons with large oscillation amplitudes. Opposite to clusters, nanofilms were found to be very perspective targets in the problem considered. A simple analytic model and molecular dynamic simulations showed increased harmonics generation when the oscillation amplitude of electrons in a film becomes much larger, than the film width. Different regimes of generation are briefly discussed.

Modulation Instability of an Intense Laser Beam in an Unmagnetized Plasma

The modulation instability of an intense elliptically polarized laser beam propagating in an unmagnetized plasma is investigated. Adopting a generalized Karpman method, a three-dimensional nonlinear equation is shown to govern the laser fleld. Conditions indicating the occurrence of modulation instability and its temporal growth rate, taking into account the longitudinal and the transverse perturbations, are obtained analytically, and the modulated growth rate is shown to be maximal for any flxed wave number when the initial laser fleld is coplanar with the perturbations. In the weakly relativistic region, the growth rate increases with increasing laser pump intensity and grows signiflcantly when the laser approaches the critical surface.