Two-color Laser Beam Research Articles

Vortex harmonic generation in indium tin oxide thin film irradiated by a two-color field.

When a two-color Laguerre-Gaussian laser beam propagates through an indium tin oxide (ITO) material, the spatial distributions of odd- and even-order vortex harmonics carrying orbital angular momentum (OAM) are studied. The origin of vortex harmonics can be directly clarified by investigating their dependence on the incident laser field amplitude and frequency. In addition, it is shown that the spectral intensities of vortex harmonics are sensitive to the epsilon-near-zero nonlinear enhancing effects and the thickness of ITO materials. Thus the vortex harmonics can be conveniently tunable, which provides a wider potential application in optical communications based on high-order OAM coherent vortex beams.

Generation of High-Power Terahertz Waves in a Collisional and Magnetized Plasma by Two-Color Femtosecond Laser Beams

Terahertz (THz) radiation generation based on a nonlinear induced electron current density in the interaction of two-color femtosecond laser beams with collisional and magnetized plasma is investigated. The nonlinear electron current density that is responsible for THz radiation includes a share from nonlinear ponderomotive force and a part from relativistic term. In this work, relativistic effects along with the effects of variation of interacting plasma density, external magnetic field, and electron collision frequency on THz wave generation have been considered. The reduction of electron collision frequency along with rising in the magnitude of the external magnetic field generates more power without any deformation of radiation patterns. The angular distribution showing the radiation pattern of THz radiation in the forward direction is obtained and the effect of laser pulse second harmonic and variation of electric field on the directivity of generated waves is examined. The change in two-color laser beam intensities both in relativistic and nonrelativistic regimes leads to the generation of THz wave electric fields with different orders of magnitude. The numerical analysis indicated that the utilization of laser pulse second harmonic in a two-color scheme enhances the THz electric field leading to more intense radiation compared to single-color mechanism. The model also shows the effect of external magnetic field and electron collision frequency on the radiation efficiency of THz generation for linear and circular polarizations of two-color laser pulses.

Generation of elliptically polarized high-order harmonic radiation with bi-elliptical two-color laser beams

High-harmonic generation (HHG) from an atomic gas target provides a valuable tool to generate high-energy radiation on the tabletop. The polarization properties of this radiation can be manipulated if the HHG process is driven by a bi-elliptical orthogonal two-color laser beam (BEOTC beam). Here we theoretically consider this setup within the so-called strong-field approximation and a quantum-orbit approach. We demonstrate that this approach allows the complete description of the geometry of the outgoing elliptically polarized harmonic radiation. In particular, we identify and classify the relevant quantum orbits that contribute to the process and analyze the harmonic spectrum, ellipticities, and offset angles in detail for $\ensuremath{\omega}\text{\ensuremath{-}}2\ensuremath{\omega}$ BEOTC beams with a fundamental wavelength of $\ensuremath{\lambda}=800\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$. For these specific beam parameters, we find that the ellipticity as well as the offset angle of the polarization ellipse with respect to the BEOTC driving beam changes rapidly for consecutive harmonic orders. We show that this oscillatory behavior of the harmonic ellipticity can be explained via the superposition of the dipole radiation emitted from the target atom during different time intervals within one optical cycle of the BEOTC beam. This understanding of the HHG process driven by BEOTC beams will facilitate the generation of highly energetic radiation with complex polarization properties on the tabletop.

Delay times for ablation rate suppression by femtosecond laser irradiation with a two-color double-pulse beam

We investigated the delay time Δt for suppressing the ablation rate of titanium with a two-color double-pulse laser beam at Δt = 0–600 ps. The double-pulse laser beam consisted of cross-polarized pulses with 150 and >150-fs pulse widths and 800 and 400-nm wavelengths, respectively. The ablation rate was suppressed most at Δt ≈ 60 ps for a 400-nm second pulse but at Δt ≈ 200 ps for an 800-nm second pulse. This difference suggests that the ablation rate was effectively suppressed when the expanding surface plasma produced by the first pulse was near the critical density for the second pulse.

Spectral control of terahertz radiation from inhomogeneous plasma filaments by tailoring two-color laser beams.

Terahertz (THz) radiation from an inhomogeneous plasma filament generated by focusing two-color femtosecond laser pulses into argon gas filled in a chamber is investigated experimentally by tailoring the Gaussian pump laser beams with an iris, where broadband THz emission over 10 THz is produced. It is found that the collected far-field THz radiation includes not only coherent but also partial-coherent components of the THz waves, which are emitted from the different parts of the inhomogeneous plasma filament with different plasma densities, contributing correspondingly to the different frequencies of the THz spectrum. Our results suggest that the THz spectrum can be manipulated by controlling the plasma density distribution of the filaments.

A multi-terawatt two-color beam for high-power field-controlled nonlinear optics.

Two-color laser beams are instrumental in light-field control and enhancement of high-order harmonic, spectral supercontinuum, and terahertz radiation generated in gases, plasmas, and solids. We demonstrate a multi-terawatt two-color beam produced using a relativistic plasma mirror, with 110 mJ at 800 nm and 30 mJ at 400 nm. Both color components have high spatial quality and can be simultaneously focused, provided that the plasma mirror lies within a Rayleigh range of the driving fundamental beam. Favorable scaling of second-harmonic generation by plasma mirrors at relativistic intensities suggests them as an excellent tool for multi-color waveform synthesis beyond the petawatt level.

Waveform control in generations of intense water window attosecond pulses via multi-color combined field

The waveform control in the improvements of high-order harmonic generation (HHG) spectra and attosecond pulse signals driven by the two-color and three-color combined fields has been theoretically investigated. (a) The results show that by properly controlling the [Formula: see text]–2[Formula: see text] two-color laser beam (including the modulations of chirps, carrier envelope phases and delay time), either the harmonic cutoff can be extended, showing a water window spectral continuum, or the selective enhancement of the single-order and two-order harmonics can be found. Further, with the introduction of a third controlling field, the efficiency of spectral continuum can be enhanced by two orders of magnitude compared with that from the two-color field. Moreover, the enhancement of HHG is not very sensitive to the frequency of the third field (i.e., the frequency of the third field is chosen to be 3[Formula: see text], 4[Formula: see text] and 6[Formula: see text]). Thus, some water window attosecond pulses with the durations of 60 as can be obtained. (b) Furthermore, the harmonic cutoff can be further extended when using a half-cycle controlling pulse or introducing the inhomogeneous effect of the laser field. Moreover, the efficiency of HHG can be further improved when the initial state is prepared as the superposition state of the ground state and some excited state of He atom. Consequently, a much broader spectral continuum with an intensity enhancement of another two orders of magnitude can be found. Finally, through the Fourier transformation of some spectral continuum, the intense water window attosecond pulses with the durations of 60 as can be produced.

Coupled quantum molecular cavity optomechanics with surface plasmon enhancement

Cavity optomechanics is applied to study the coupling behavior of interacting molecules in surface plasmon systems driven by two-color laser beams. Different from the traditional force–distance measurement, due to a resonant frequency shift or a peak splitting on the probe spectrum, we have proposed a convenient method to measure the van der Waals force strength and interaction energy via nonlinear spectroscopy. The minimum force value can reach approximately 10−15 N, which is 3 to 4 orders of magnitude smaller than the widely applied atomic force microscope (AFM). It is also shown that two adjacent molecules with similar chemical structures and nearly equal vibrational frequencies can be easily distinguished by the splitting of the transparency peak. Based on this coupled optomechanical system, we also conceptually design a tunable optical switch by van der Waals interaction. Our results will provide new approaches for understanding the complex and dynamic interactions in molecule–plasmon systems.

Enhanced harmonic generation by propagation of two-color p-polarized laser beams in plasma

AbstractA theoretical model is developed for studying the generation of third-harmonic radiation by the interaction of obliquely incident, two-color p-polarized laser beams with spatially varying plasma density. The ratio of the fundamental frequencies of the two laser beams are considered to be an arbitrary integer. The amplitude of harmonic radiation obtained by oblique incidence of two-color laser fields propagating in homogeneous plasma is enhanced in comparison with that obtained by normal incidence of two-color laser beams. The periodicity of the plasma density allows the harmonic radiation to be phase-matched, leading to further increase in the amplitude of phase-matched harmonic radiation by an order of magnitude. The amplitude of the generated harmonic radiation increases with the increase in angle of incidence.

Observation of the field-free orientation of a symmetric-top molecule by terahertz laser pulses at high temperature

We investigate experimentally and numerically the field-free orientation of the symmetric top molecule of methyl-iodide at high temperature using a terahertz radiation generated by a plasma induced by a two-color laser beam. The degree of orientation is measured from the free-induction decay emitted by the sample. The observed experimental signal is reproduced with a good accuracy by numerical simulations.

Origin of ellipticity of high-order harmonics generated by a two-color laser field in the cross-polarized configuration

Recently several techniques demonstrated the production of elliptically polarized high harmonics. One of these techniques consists of the interaction in a noble gas of two-color laser beams having orthogonal linear polarizations. Here we present the theoretical explanation of such a result observed in Lambert et al. [Nat. Commun. 6, 6167 (2015)]. Numerical calculations based on the nonperturbative light-atom interaction theory reproduce well the experimental data. The degree of polarization is analyzed for different harmonic orders and found to be high. With the help of a simplified theoretical model it is shown that the degree of harmonic ellipticity depends mainly on the population of atomic state sublevels with different angular momentum projections.

Enhanced two-color high-harmonic generation achieved by adding an extra gas medium

The high-harmonic generation in a two-color laser field, consisting of a 30-fs Ti:Sapphire laser pulse and its second harmonic, could be enhanced by installing an extra gas medium of Ar in front of the harmonic generation medium of He. The effects of the extra medium were examined by analyzing harmonic spectra taken under different experimental conditions. The observed characteristics of enhanced harmonic generation were explained in terms of the profile modification of the two-color laser beam. The 34th harmonic at 23.5 nm and 38th harmonic at 21.5 nm were enhanced 2.5 and 2 times, respectively.

Fabricating subwavelength dot-matrix surface structures of molybdenum by transient correlated actions of two-color femtosecond laser beams.

Two-dimensional matrix of subwavelength dot structures are directly generated on molybdenum surfaces by focusing two-color femtosecond laser pulses through an optical lens. In contrast to the traditional fabrication approaches, the spatial periodicities of such surface structures are demonstrated to rely on the time delay of two laser beams through transient correlation between their ultrafast dynamic processes, in spite of their different colors and polarizations. The structure orientations can be tuned effectively via the laser polarization. Discussion suggests a new possible flexible way towards the nanoscale sophisticated material processing for many potential applications.

Cold atomic beam from a two-dimensional magneto-optical trap with two-color pushing laser beams

We present an intense source of cold rubidium atoms from a two-dimensional magneto-optical trap (2D MOT) with a two-color pushing laser that generates an adjustable mean velocity ranging from 20 to 45m/s. The dependence of the flux and velocity distribution on the pushing beam parameters such as laser detuning and laser power was investigated in detail. We obtained a flux Φ=4.5×1010atoms/s with the total laser power of 60mW at a vapor pressure of 1.2×10−7torr. By adding a red-detuned pushing beam (Δ−=−4.5Γ) into a blue-detuned one (Δ+=+4.5Γ) the flux was increased by more than a factor of 2. Furthermore this modified 2D MOT system improved 3D MOT loading rate to greater than 3×1010atoms/s keeping the background pressure below 10−11torr in the ultra-high-vacuum chamber.

Generation of a broadband soft X-ray supercontinuum by quantum path control

数值研究了双色激光脉冲与一维氩原子相互作用的谐波发射功率谱.双色激光脉冲由800 nm和1600 nm的两束激光组成，脉冲持续时间为9.7 fs.计算结果表明，相对相位为π时，谐波截止频率从单色场情形的58 eV大幅度地展宽到316 eV，获得频宽为93 eV的软X射线连续辐射谱.此外，还采用短程的普薛耳－特勒势和长程的软化库仑势进行比对，数值结果表明该连续辐射谱的存在具有普适性.通过半经典的“三步”模型和时频分析小波变换方法对该连续辐射谱进行分析，发现其主要由单一的“短”量子路径生成，直接叠加频率从22

Three-Dimensional Writing of Copper Nanoparticles in a Polymer Matrix with Two-Color Laser Beams

The multicolor laser processing is a choice for the nondestructive direct three-dimensional photoprocessing. We achieved the three-dimensional writing of copper nanoparticles in a polymer matrix wi...

Quasi 2-D Measurements of Gaseous and Liquid Fuel Concentrations Using Two-Color Laser Beam Scanning Technique

Planer Laser Induced Fluorescence (PLIF) has been employed to measure the spatial liquid and vapor fuel concentration distributions, although it is generally difficult to achieve quantitatively accurate measurement. The authors devised a 2-D fuel spray concentration distribution measurement method which combines three optical principles; absorption, fluorescence, and scatter. NO2 gas was used as a fluorescence dopant while Ar+ laser was employed as a light source by scanning across the cylinder in order to measure the spatial fuel concentration distribution. This technique was applied to gaseous jets and swirl sprays. Quantitative measurement was successfully achieved in both tests. Also, the characteristics of liquid and vapor fuel concentration distribution using a swirl type injector were confirmed.

Control of the plasmon absorption of gold nanoparticles with a two-color excitation

Substantial photoinduced changes of ultraviolet (UV) -visible (UV-vis) optical spectra were observed for the Au nanoparticles prepared by a “touch” seed-mediated growth technique on the surface of indium-tin-oxide substrate. The phototreatment was performed by two-color coherent laser beams with a wavelength of 1060 nm (fundamental beam) and double frequency 530 nm (writing beam). The maximal photoinduced changes of the UV-vis spectra were observed for the surfaces possessing low resistivity (about 4Ω∕sq). Those of higher resistivity (about 50Ω∕sq) demonstrate only slight shifts of the main Au particle surface-plasmon resonance maxima at λ=570nm. The effect observed is a consequence of a superposition of optically induced surface plasmon resonance, scattering on optically induced grating and electron-phonon relaxation.

Resonance-enhanced Multiphoton Ionization Mass Spectrometry Using a Two-color Laser Beam Generated by Stimulated Raman Scattering.

Two-color (1+1') resonance-enhanced multiphoton ionization/time-of-flight mass spectrometry (REMPI/TOF-MS) combined with supersonic jet (SSJ) spectrometry has been demonstrated. The methodology uses a two-color laser beam consisting of a Stokes beam generated by stimulated Raman scattering (SRS) and a fundamental beam for the excitation and subsequent ionization of p-dichlorobenzene. The MPI signal was found to be substantially increased when a two-color laser beam was employed. This approach greatly simplifies the analytical system and reduces the cost of the instrumentation required for two-color ionization. The potential advantage of applying this method for the analysis of polychlorinated dibenzo-p-dioxin (PCDD) and polychlorinated dibenzofuran (PCDF) emitted from an incinerator is discussed.

Super-resolution Fluorescence Microscopy in Nano-meter Scale Region Using Two-color Laser Beams

A super-resolution fluorescence microscopy using two-color laser beams was proposed. The microscopy is based on the combination of two-color fluorescence dip spectroscopy and a phase modulation technique for the laser beam. By applying the proposed technique to a laser-scanning microscope, a fluorescence image of a sample can be observed with a spatial resolution overcoming the optical diffraction limit. To demonstrate validity of the microscopy, we constructed a scanning microscope system using commercial nano-second pulse lasers. An image of micro beads containing dye molecules was observed by the microscopy. We succeeded in obtaining the image with a resolution overcoming the diffraction limit in nano-meter scale region. The experimental data showed that the resolution was improved three times at least. The microscopy is expected to be an appropriate analysis method for the samples with nano-meter scale structures.