Pump Laser Beam Research Articles

Fast and slow light propagations in a four-level cascade atomic medium

Under weak probe field, rotating wave and electric dipole approximations, we have derived analytic expressions for absorption and dispersion coefficients, group index and group velocity in a four-level cascade atomic medium interacting with the probe and pump laser fields. The model is applied to 85Rb atomic system to study the control of the fast and slow light propagation of the probe beam according to the intensity and frequency of the pump laser beam. With this excitation model, we find two transparent domains and thus the light group velocity is also controlled at these two frequency domains. In particular, the amplitude and the sign of the group index are easily changed by adjusting the intensity or frequency of the pump laser. That is, the probe beam propagation can be changed between fast and slow light modes by adjusting the intensity or frequency of the pump laser. Our analytical results can be useful for related experimental studies.

High-power near-CW Raman lasing in mm-sized glass disks

We experimentally demonstrate Raman lasing in a mm-sized CaF2 disk, with an optical power output of the first Stokes sideband exceeding 100 mW in a near-continuous wave (CW) operation. The pump beam at a wavelength of 1.06 μ m is coupled to the whispering gallery modes of the disk with the use of a tapered glass fiber. With the pump beam coupled to one of the resonator modes, we observe the generation of the first and the second Stokes sidebands at wavelengths of 1.1 μ m and 1.14 μ m, respectively. The conversion efficiency from the pump laser beam to the generated sidebands is ∼10%. These results open up the prospect of using such mm-sized disks as compact, highly efficient ultrafast modulators with modulation frequencies as high as 10 THz.

Spatially enhanced electric field induced second harmonic (SEEFISH) generation for measurements of electric field distributions in high-pressure plasmas

The spatial resolution of the ps electric field induced second harmonic (EFISH) generation has been enhanced by using non-collinear pump laser beam arrangements. The pump laser beam (1064 nm, nominal pulse duration 150 ps, pulse energy 20–40 mJ) is separated into two coaxial or crossing beams, overlapping only near the focal point. The spatially enhanced EFISH (SEEFISH) signal is generated over a shorter beam overlap region compared to the collinear beam arrangement. Blocking of either of the two beams results in a complete suppression of the signal. The signal is spatially isolated from the ‘conventional’ EFISH signal and measured by a photomultiplier detector. Measurements of a known Laplacian field generated between two parallel cylinder electrodes in ambient air shows that SEEFISH improves the spatial resolution of the measurements by up to a factor of 2, such that the measurement results agree with the Laplacian field distribution. The spatial resolution is improved further by reducing the focal distance of the lens. The magnitude of the SEEFISH signal is significantly lower compared to that of the single-beam EFISH and decreases rapidly as the beam crossing angle is increased, due to the phase mismatch. This approach has a significant potential for measurements of electric field distributions in high-pressure plasmas, with an additional benefit of removing the stray second harmonic signal from the optical access windows.

Measurement of the temperature dependency of the linewidths and amplitudes of the sub-Doppler resonances of the 87Rb D2 line with π (π) polarized pump (probe) laser beams

Measurement of the temperature dependency of the linewidths and amplitudes of the sub-Doppler resonances of the 87Rb D2 line with π (π) polarized pump (probe) laser beams

Index-leveling for forced-flow turbulent face-cooling of laser amplifiers.

Direct laser slab face-cooling by a fluid crossing the main and pump laser beams is an important method to reach high average laser powers. However, the flow regime is usually maintained at low Reynolds numbers to prevent the onset of turbulence features in the flow that would degrade the wavefront quality. We show here how bringing the fluid temperature to the thermo-optical null point, close to the water/ice transition in the case of water, allows one to mitigate the optical consequences of hydrodynamic instabilities, by bleaching optically the temperature inhomogeneities within the flow. This optical process, dubbed index-leveling, opens the door to a highly efficient forced-flow, weakly turbulent face-cooling regime that should be instrumental to boost the kilowatt capabilities of next-generation high-power lasers.

Three-axis closed-loop optically pumped magnetometer operated in the SERF regime.

We propose a three-axis closed-loop optically pumped magnetometer with high sensitivity. The closed-loop magnetometer has a three-axis sensitivity of approximately 30fT/Hz1/2 using two orthogonal laser beams for pumping and probing the alkali metal atoms. In the closed-loop mode, the dynamic range is improved from ±5 nT to ±150 nT. The bandwidth is increased from about 100 Hz to over 2 kHz with 10 kHz modulation fields in x- and y-axes and another 6 kHz modulation field along the z-axis. Compared with single-axis or dual-axis magnetometers, the proposed magnetometer not only provides the direction and magnitude of the magnetic field but also has high robustness in a challenging environment. The magnetometer has applications in biomagnetic measurements, magnetic resonance imaging, and fundamental physics.

Effect of self-focusing of an intense cosh-Gaussian laser beam on second harmonic generation in a collisionless plasma: Higher-order paraxial theory

This paper presents the study of second harmonic generation in a collisionless plasma by an intense self-focused cosh-Gaussian laser beam under the combined effect of relativistic and ponderomotive nonlinearities. The second harmonics of a cosh-Gaussian laser beam is generated by the excitation of an electron plasma wave at the pump frequency. The excited electron plasma wave coupled with the pump laser beam and generate the radiation of double frequency. This study is carried out in a higher-order paraxial ray approximation where higher order terms are taken into account in the expansion of the dielectric function and the eikonal. Nonlinear differential equations have been established for the self-focusing of cosh-Gaussian laser beam in the plasma, the generation of electron plasma wave at the pump frequency, and the second harmonic generation. Numerical simulations have been carried out to investigate the effects of laser and plasma parameters such as decentred parameter (b), incident laser intensity (a), and relative plasma density (ωp0/ω0) on the self-focusing of the cosh-Gaussian laser beam and the electric field associated with electron plasma wave and second harmonic wave. A well-established set of laser and plasma parameters have been used in numerical analysis. The results show that the focusing of the cosh-Gaussian laser beam leads to an increase in the electric field of the electron plasma wave and the second harmonic wave. The results are also compared to the paraxial-ray approximation. The focusing of the cosh-Gaussian laser beam in the plasma is strengthened in the higher-order paraxial region, which increases the electric field of both the electron plasma wave and the second harmonic wave at high values of b, a, and ωp0/ω0.

Generation and detection of axions using guided structures

We propose a new experimental technique to generate and detect axions in the lab with a good experimental sensitivity over a broad axion mass range. The scheme relies on using laser-based four-wave mixing, which is mediated by the hypothetical axion field. Intense pump and Stokes laser beams that are confined to a waveguide (i.e., for example, an optical fiber) with appropriately chosen frequencies resonantly drive axion generation. Under such a geometry, we predict the existence of guided axion waves, which we refer to as "axitons". These are solutions of the axion Klein-Gordon field equation that are spatially guided by the profiles of the driving pump and Stokes laser beams. These guided axitons can then couple to a nearby fiber and mix with another laser, affecting the propagation of a probe laser beam. A key advantage of the scheme is that the mass range of the hypothetical axion can be scanned by varying the frequencies of the pump and the Stokes laser beams. We predict that, using reasonable parameters, the technique will be able to detect axions in the mass range $10^{-6} $eV $< m<$ $10^{-2} $eV with a sensitivity at the level of $10^{-12}$ GeV$^{-1}$ for the axion-photon coupling constant.

A highly drift-stable atomic magnetometer for fundamental physics experiments

We report the design and performance of a nonmagnetic drift stable optically pumped cesium magnetometer with a measured sensitivity of 35 fT at 200 s integration time and stability below 50 fT between 70 and 600 s. The sensor is based on the nonlinear magneto-optical rotation effect: in a Bell–Bloom configuration, a higher order polarization moment (alignment) of Cs atoms is created with a pump laser beam in an anti-relaxation coated Pyrex cell under vacuum, filled with Cs vapor at room temperature. The polarization plane of light passing through the cell is modulated due the precession of the atoms in an external magnetic field of 2.1 μT, used to optically determine the Larmor precession frequency. Operation is based on a sequence of optical pumping and observation of freely precessing spins at a repetition rate of 8 Hz. This free precession decay readout scheme separates optical pumping and probing and, thus, ensures a systematically highly clean measurement. Due to the residual offset of the sensor of &amp;lt;15 pT together with negligible crosstalk of adjacent sensors, this device is uniquely suitable for a variety of experiments in low-energy particle physics with extreme precision, here as a highly stable and systematically clean reference probe in search for time-reversal symmetry violating electric dipole moments.

First uncertainty evaluation of the cesium fountain primary frequency standard NMIJ-F2

We report the first uncertainty evaluation of NMIJ-F2, the second atomic fountain primary frequency standard at the National Metrology Institute of Japan. To improve the frequency stability, we increase the number of detected atoms to 9 × 105 using high-power cooling laser beams for vapor-loaded optical molasses and optical pumping into the Zeeman sublevel m F = 0. We also employ an ultra-stable cryogenic sapphire oscillator as a local oscillator to prevent the degradation of frequency stability due to the Dick effect. After correcting the collisional frequency shift by alternating atom densities, the frequency stability typically reaches 2.5 × 10−13(τ/s)−1/2. Its value is 1.9 × 10−16 after 20 days of measurement. Type B uncertainty is typically evaluated at 4.7 × 10−16; the largest contribution is from a distributed cavity phase shift, followed by a microwave leakage shift. In long-term comparison, the frequency of NMIJ-F2 is found to be consistent with that of the other primary and secondary frequency standards within the uncertainty.

Dynamics of laser-induced tunable focusing in silicon

We report here on focusing of a probe IR (λ = 1.55 μm) laser beam in silicon. The focusing is done by a second pump laser beam, at λ = 0.775 μm and 30 ps pulse width, with a donut shape that is launched collinearly and simultaneously (with some delay time) with the IR beam pulse. The pump beam pulse is absorbed in the silicon and creates, temporally, a free charge carriers (FCCs) donut pattern in the silicon. Following the plasma dispersion effect, the donut FCCs shapes a complex index of refraction pattern in the silicon that serves as a sort of dynamic GRIN lens for the probe beam due to the diffusion of the FCCs towards the donut center. This lens can be tuned to its focal point by the pump-probe delay time to reduce the point spread function (PSF) of the IR probe beam. We start seeing the focusing of the probe beam at pump-probe delay time of mathrm{Delta t }approx 100mathrm{ ps}. The best focusing (results in PSF < 2mathrm{ mu m}) was observed at mathrm{Delta t}approx 350mathrm{ ps} and it slowly degrades before the FCCs full recombination at mathrm{Delta t }sim 12mathrm{ ns}. We propose this beam shaping method to overcome the diffraction resolution limit in silicon microscopy on and deep under the silicon surface dependent on the pump wavelength and the pulse width. We also proposed this technique for direct measurement of the FCCs dynamics.

Efficient Emission Outcoupling from Perovskite Lasers into Highly Directional and Long‐Propagation‐Length Bloch Surface Waves

AbstractHalide perovskite micro‐ and nanolasers have become a novel platform for efficient generation of coherent emission in the whole visible range. However, the laser emission outcoupling from the perovskite microcrystals faces a number of problems related to large divergence angle, light‐soaking by a substrate, and high losses in the case of coupling with nanowaveguides. Here the perovskite nano‐ and microlasers are proposed to be integrated with a photonic crystal supporting Bloch surface waves (BSWs), which are coupled directly from the perovskite lasers with the efficiency of over 16%. The BSWs exciting at 535‐nm wavelength show high in‐plane directivity, down to 9, and long‐propagation length, up to 50 m. Moreover, a pronounced beam steering effect for the generated laser emission is demonstrated by varying the pumping laser beam position, which provides an additional degree of freedom for in‐plane control of the lasing mode outcoupling.

Superradiant atomic recoil lasing with orbital-angular-momentum light

We analyze the collective light scattering by cold atoms in free space of a pump-laser beam possessing orbital angular momentum. We derive a set of coupled multiparticle equations for the atomic motion in which the vacuum-mode field is adiabatically eliminated. The resulting equations describe collective recoil as due to either transfer of linear momentum or orbital angular momentum. For a transverse annular atomic distribution the initial equilibrium with uniform atomic phases and no scattered field is unstable. The atoms are set in rotation and bunched in phase at different harmonics depending on the pump azimuthal index $\ensuremath{\ell}$ and on the ring radius.

Spectroscopy and laser cooling of SiBr+: A computational perspective

In the previous work, the SiBr+ cation is suggested as a candidate molecule for laser cooling, and in this work, on the basis of high-level ab initio calculation, the possibility of SiBr+ cation laser cooling is studied by considering the core-valence electrons correlations and spin–orbit coupling (SOC) effect. The potential energy curves of 12 Λ-S states and 36 Ω states considering SOC were obtained by using the multi-reference configuration interaction method and Davidson correction. Our computed results show that the Franck-Condon factor f00 for SiBr+ is only 0.619, which cannot guarantee that the number of the leaked photons is enough small, and more pumping laser beams are needed. Based on the calculated molecular properties, we discuss in detail the feasibility of laser cooling of SiBr+ and the results indicate that it is a challenge for laser cooling of SiBr+ .

Detection of sub-micrometer thermomechanical and thermochemical failure mechanisms in titanium with a laser-based thermoreflectance technique

This work reports on a pump–probe laser-based heating and sensing metrology to study the failure mechanisms of materials during extreme heat fluxes localized near surfaces, the localization of which is controlled by the focus of the laser beam and sensed by the reflection of a secondary probe laser. We focus the demonstration of these power density at failure tests on the damage mechanisms of commercially pure titanium metal during and after high heat fluxes induced from the absorbed laser energy. Using this steady-state thermoreflectance pump–probe metrology, a localized region of the material was irradiated at a low modulated frequency, while the average change in the thermoreflectance signal was monitored. We observe surface and cross-sectional oxidation of the titanium, revealing correlations between microstructural evolution events and shifts in thermoreflectance trends as a function of absorbed power density. Furthermore, the damage morphology was shown to be heavily influenced by the size of the heater (dictated by the radius of the pump laser beam), which controlled the relative degree of thermomechanical, melting, and oxidative decohesion failure mechanisms in the samples. The analysis of the temperature distribution coupled with the observed microstructural damage gives rise to a high-throughput experimental technique to induce desired deformation modes through cyclic thermal testing.

Influence of Laser Modulation Frequency on the Performance of Terahertz Photoconductive Switches on Semi-Insulating GaAs Exhibiting Negative Differential Conductance

In typical terahertz time-domain spectroscopy systems, the use of the lock-in technique is necessary because of the low current induced at the receiver so that the laser pump beam must be modulated (chopped) at a frequency much lower than the laser repetition rate. This work shows that, in the case of semi-insulating GaAs (SI-GaAs) antennas, this modulation has an important effect on the antenna current and consequently, on the radiated electromagnetic pulse. There exists a threshold bias (whose value depends on the chopping frequency) where an abrupt increase in the current and consequently, in the terahertz emission takes place. The calculated energy of the pulse below and above the threshold shows that the energy doubles. The exact bias voltage at which this occurs changes with the laser modulation frequency when this is below 350 Hz, but at higher frequencies, the threshold remains almost constant. The experiments show that the responsibility for this behavior is the S-shape negative differential conductance exhibited by SI-GaAs originated by a slow field-enhanced charge trapping mechanism, which is also an important source of noise at the receiver of the system.

Partially coherent spontaneous parametric downconversion: Twisted Gaussian biphotons

Partially coherent optical beams present phenomena that may not appear in perfectly coherent beams. In this regard, spontaneous parametric downconversion is an intriguing physical process, since the optical properties of the pump laser beam are transferred to the quantum properties of the down-converted photon pairs. Here, the authors study the transfer of twist phase, a novel property of the so-called twisted Gaussian Schell model beams, from a pump beam to down-converted photons. The authors show that a non-zero twist phase can lead to highly entangled yet highly mixed states. The correlations that appear in near/far-field intensity measurements, in the two-photon covariance matrix as well as in orbital angular momentum modes, are explored. The authors derive expressions for the purity of the two-photon state and relate it to the transverse spatial coherence of the pump beam. Their main results utilize a Wigner function formalism that can be applied to a number of scenarios involving partially coherent downconversion sources and contribute to the recent interest in spatially correlated photon pairs produced in mixed states, with possible applications in quantum communication and imaging.

Stimulated Rayleigh–Bragg scattering study in a two-photon absorbing liquid and a solid

Stimulated Rayleigh–Bragg scattering (SRBS) in highly two-photon active chromophores’ solutions and in a doped polymer rod has been investigated. Under optimized experimental conditions, the energy conversion efficiency from the input 816-nm and ∼ 10 - n s pump pulses to the backward SRBS pulses was up to η = 58 % in the solution of AF450 chromophore in methyl methacrylate (MMA) liquid of 0.001 M concentration, while for the AF450 doped poly(methyl methacrylate) rod of the same concentration the measured value was only η = 0.16 % . Considering that the two-photon absorption properties of the chromophore molecules are basically the same in both cases, the above-mentioned results strongly suggested that the formation of the induced Bragg grating in a solution is mainly due to spatial redistribution of the dopant molecules driven by two-photon resonance enhanced gradient force. In contrast, the same dopant molecules are “frozen” in a solid matrix which prohibits the spatial redistribution of these molecules from forming an effective Bragg grating. Based on the high efficiency of backward SRBS generation as well as the coexisting two-photon absorption, the intense input laser beam will undergo a much stronger nonlinear attenuation. For the tested AF450/MMA solution sample of 0.004 M concentration, when the input laser pulse energy changed from 0.06 mJ to 3.7 mJ, the measured nonlinear transmissivity decreased from 0.82 to 0.11, showing a superior nonlinear attenuation performance with regard to the input pump laser beam.

Quantum dot membrane external-cavity surface-emitting laser at 1.5 μm

A membrane external-cavity surface-emitting laser (MECSEL) with an InAs/InP quantum dot (QD) based gain region is demonstrated. The pumping scheme employs a 90° off-axis parabolic mirror to focus the diode laser pump beam to a nearly circular pump spot. With this pump arrangement, the QD MECSEL with SiC heat spreaders produced 320 mW output power at room temperature with direct emission in the near-infrared at 1.5 μm. We report a record value of 86 nm for the tuning range at this wavelength region, owing to a broad QD gain bandwidth and wide tunability in MECSELs.

Multidimensional Entanglement Generation with Multicore Optical Fibers

Trends in photonic quantum information follow closely the technical progress in classical optics and telecommunications. In this regard, advances in multiplexing optical communications channels have also been pursued for the generation of multi-dimensional quantum states (qudits), since their use is advantageous for several quantum information tasks. One current path leading in this direction is through the use of space-division multiplexing multi-core optical fibers, which provides a new platform for efficiently controlling path-encoded qudit states. Here we report on a parametric down-conversion source of entangled qudits that is fully based on (and therefore compatible with) state-of-the-art multi-core fiber technology. The source design uses modern multi-core fiber beam splitters to prepare the pump laser beam as well as measure the generated entangled state, achieving high spectral brightness while providing a stable architecture. In addition, it can be readily used with any core geometry, which is crucial since widespread standards for multi-core fibers in telecommunications have yet to be established. Our source represents an important step towards the compatibility of quantum communications with the next-generation optical networks.