Probe Laser Pulse Research Articles

Investigation on response function of terahertz air coherent detection technique

Terahertz (THz) air coherent detection is a broadband coherent detection technique, which has been used widely in the THz science and technology. In this paper, a frequency-dependent response function of THz air coherent detection is defined based on the nonlinear polarization of gas molecules induced by the ultrashort laser pulses and the THz pulses. Calculations show that the response function of this technique has a half of Gaussian-like profile, which has a high sensitivity in the low frequency range and low sensitivity in the high frequency range of THz waves. Two fitted functions are given based on the calculated results, which can provide a good reference bandwidth when using the air coherent detection with a given probe laser pulse. The distortions of THz pulses induced by the air coherent detection in the detection process are discussed as well. This study will help to understand the response bandwidth and the limitation of detection bandwidth of the air coherent detection technique.

Optical generation and detection of gigahertz shear acoustic waves in solids assisted by a metallic diffraction grating

Absorption of ultrashort laser pulses in a metallic grating deposited on a transparent sample launches in the material both compression/dilatation (longitudinal) and shear coherent acoustic pulses in directions of different orders of acoustic diffraction. The propagation of the emitted acoustic pulses can be monitored by measuring the variation of the optical reflectivity of the time-delayed ultrashort probe laser pulses. The direction of probe light incidence and its polarization relative to the sample surface as well as the orientation of the metallic grating should be specifically chosen for efficient Brillouin scattering of the probe light from shear phonons propagating in the elastically isotropic materials. As theoretically predicted, the obtained experimental data contain multiple frequency components which are due to a variety of possible Brillouin scattering angles for both shear and compression/dilatation coherent acoustic waves. All these different frequency components are explained through multiplexing the propagation directions of probe light and coherent sound by the metallic diffraction grating. Our experimental scheme of time-domain Brillouin scattering with metallic gratings operating in reflection mode provides access to monitoring the shear acoustic waves launched in the direction of the first diffraction order by backward Brillouin scattering process. Applications include simultaneous determination of several different acoustic mode velocities and optical refractive index and, potentially, measurements of the acoustic dispersion of samples with a single direction of possible optical access.

Tablet Coating Thickness Measurements by Combined Raman Spectrometry and Laser Induced Breakdown Spectrometry Techniques

For the first time, pharmaceutical tablet coating thickness is measured by combined Raman spectrometry (RS) and laser induced breakdown spectrometry (LIBS) techniques. Measurements by two methods (RS and LIBS) are performed during a single pump lamp flash of a pulsed Nd:YAG laser in the double pulse lasing mode. This approach combining a probe laser pulse (RS) followed by an ablating laser pulse (LIBS) allows depth profiling analysis. An additional advantage is a possible comparison of the results of measurements of molecular and elemental compositions within the same spot. It is shown that the published method for measuring the coating thickness by LIBS depth profiling leads to systematic errors. An approach for improving the accuracy of coating thickness data measurements by comparing the RS and LIBS is proposed.

1000-fps consecutive ultrafast 2D-burst imaging with a sub-nanosecond temporal resolution by a frequency-time encoding of SF-STAMP

We demonstrated real-time, in-situ operation of a single-shot, frequency-time-encoding burst imaging method of sequentially timed all-optical mapping photography utilizing spectral filtering (SF-STAMP) by employing a high-speed camera with the frame rate of a 1000 fps as a detector. We obtained single-shot burst images of a pulse-by-pulse femtosecond laser ablation of a glass by a chirped probe laser pulse with a 1–2-ps frame interval or by a spectrally sweeping burst laser pulse train with a 300-ps interval. We observed burst images of plasma generation during the early stage of laser ablation in a glass with a short frame interval by a chirped probe laser pulse. We also captured burst images of plume and shock-wave generation in air with a 1.5-ns observation time window by a spectrally sweeping probe laser pulse train.

Effects of ring-strain on the ultrafast photochemistry of cyclic ketones.

Ring-strain in cyclic organic molecules is well-known to influence their chemical reactivity. Here, we examine the consequence of ring-strain for competing photochemical pathways that occur on picosecond timescales. The significance of Norrish Type-I photochemistry is explored for three cyclic ketones in cyclohexane solutions at ultraviolet (UV) excitation wavelengths from 255–312 nm, corresponding to an π* ← n excitation to the lowest excited singlet state (S1). Ultrafast transient absorption spectroscopy with broadband UV/visible probe laser pulses reveals processes common to cyclobutanone, cyclopentanone and cyclohexanone, occurring on timescales of ≤1 ps, 7–9 ps and >500 ps. These kinetic components are respectively assigned to prompt cleavage of an α C–C bond in the internally excited S1-state molecules prepared by UV absorption, vibrational cooling of these hot-S1 molecules to energies below the barrier to C–C bond cleavage on the S1 state potential energy surface (with commensurate reductions in the energy-dependent α-cleavage rate), and slower loss of thermalized S1-state population. The thermalized S1-state molecules may competitively decay by activated reaction over the barrier to α C–C bond fission on the S1-state potential energy surface, internal conversion to the ground (S0) electronic state, or intersystem crossing to the lowest lying triplet state (T1) and subsequent C–C bond breaking. The α C–C bond fission barrier height in the S1 state is significantly reduced by the ring-strain in cyclobutanone, affecting the relative contributions of the three decay time components which depend systematically on the excitation energy above the S1-state energy barrier. Transient infra-red absorption spectra obtained after UV excitation identify ring-opened ketene photoproducts of cyclobutanone and their timescales for formation.

Time-resolved dynamics in iodide-uracil-water clusters upon excitation of the nucleobase.

The dynamics of iodide-uracil-water (I-·U·H2O) clusters following π-π* excitation of the nucleobase are probed using time-resolved photoelectron spectroscopy. Photoexcitation of this cluster at 4.77 eV results in electron transfer from the iodide moiety to the uracil, creating a valence-bound anion within the cross correlation of the pump and probe laser pulses. This species can decay by a number of channels, including autodetachment and dissociation to I- or larger anion fragments. Comparison of the energetics of the photoexcited cluster and its decay dynamics with those of the bare iodide-uracil (I-·U) complex provides a sensitive probe of the effects of microhydration on these species.

Quantum state preparation of an atomic ensemble via cavity-assisted homodyne measurement

The quantum spin states of an atomic ensemble are of special interest to both fundamental studies and precision measurement applications. Here, we propose a scheme to prepare collective quantum states of an atomic ensemble placed in an optical cavity via homodyne measurement of a probing light field. The effective interactions of atoms mediated by photons are enhanced by the optical cavity, and the output probe light could also be entangled with the collective spin states. By selectively measuring the quadrature of the output light, we can prepare various quantum states, including superposition states of Dicke states and Dicke squeezed states. It is also demonstrated that the fidelity of the prepared quantum state can be enhanced by repetitive homodyne detection and using longer probe laser pulses. With the experimental parameters, we discuss some imperfections, including background noise, fluctuation of the measurement outcomes, and free-space scattering.We believe that our scheme should be feasible for experimental realization with current technologies, which may be used in the future study of quantum mechanics and quantum metrology.

Capturing the photo-induced dynamics of nano-molecules by X-ray free electron laser induced Coulomb explosion

We performed reaction dynamics simulations to demonstrate that the vibrational dynamics of C60 induced by infrared (IR) pulses can be traced by triggering Coulomb explosion with intense femtosecond X-ray free electron laser (XFEL) probe pulses. The time series of the angular anisotropy β(t) of fast C+ and C2+ fragments of C60 60+ produced by such an XFEL pulse reflects the instantaneous structure of C60 vibrationally excited by IR pulses. The phases and amplitudes of excited vibrational modes and the coupling between excited modes can be successfully extracted from the expansion of β(t) in terms of vibrational modes. This proof-of-principle simulation clearly demonstrates that various information of the structures and reaction dynamics of large clusters or biomolecules can be retrieved by decomposing the experimentally determined β(t) into vibrational modes.

Propagation of a laser pulse under electromagnetically induced transparency conditions

We investigate the propagation dynamics of a pair of probe and coupling laser pulses in a three-level type-V atomic medium under the condition of electromagnetically induced transparency (EIT) by numerical solving the Maxwell-Bloch equations for atoms and fields. Influences of the intensity and pulse area of the coupling laser on the EIT formation of probe laser pulse are studied in a wide region from micro- to pico-second of pulse duration. It is found that a nearly solition pulse can be established at the smallest pulse area in the pico-second region and with the largest pulse area in the micro-second region. These results can be important for applications in all-optical switching, and quantum information processing and transmission.

Resonance-enhanced harmonics in mixed laser-produced plasmas

The enhancement of high-order harmonics of ultrashort pulses is crucial for application of coherent extreme ultraviolet (EUV) pulses in optoelectronics. Strong resonances of ionic species are useful for enhancement of the yield of single harmonic in EUV region. Another attractive concept is the quasi-phase matching (QPM) allowing the growth of conversion efficiency of the groups of harmonics in different ranges of EUV. Here we demonstrate simultaneous enhancement of 9th, 13th and 17th harmonics of 800 nm, 30 fs pulses in laser-produced plasmas (LPP) created on the surfaces of Zn, In and Sn targets. The enhancement of harmonics is attributed to the resonance transitions of singly charged particles of those materials. Strong 9th harmonic is generated in Zn plasma in a broad range of the delays between heating and probe laser pulses. We analyze strong even harmonics enhanced in Zn + In and Sn + In LPP at the conditions of two-color pump. We also show the concurrence of micro- and macroscopic processes in indium plasma by analyzing the resonance conditions for 13th harmonic and QPM for 21st and 31st harmonic using different configurations of LPP. We find the phase differences of harmonics generated in indium and zinc plasma, and show the pronounced effect of the resonances on the attosecond timing of the harmonic emission.

Alignment of the CS2 dimer embedded in helium droplets induced by a circularly polarized laser pulse

Dimers of carbon disulfide (CS$_2$) molecules embedded in helium nanodroplets are aligned using a moderately intense, 160ps, non-resonant, circularly polarized laser pulse. It is shown that the intermolecular carbon-carbon (C-C) axis aligns along the axis perpendicular to the polarization plane of the alignment laser pulse. The degree of alignment, quantified by $\langle \cos^2(\theta_\text{2D}) \rangle$, is determined from the emission directions of recoiling CS$_2$$^+$ fragment ions, created when an intense 40fs probe laser pulse doubly ionizes the dimers. Here, $\theta_\text{2D}$ is the projection of the angle between the C-C axis on the 2D ion detector and the normal to the polarization plane. $\langle \cos^2(\theta_\text{2D}) \rangle$ is measured as a function of the alignment laser intensity and the results agree well with $\langle \cos^2(\theta_\text{2D}) \rangle$ calculated for gas-phase CS$_2$ dimers with a rotational temperature of 0.4K.

The photoionization dynamics based on molecular pre-orientation.

We study theoretically the photoionization dynamics of pre-oriented NaK molecule. Firstly, a THz laser pulse is utilized to orient the ground state molecule. And then the pump and probe laser pulses are used to excite and ionize the molecule, respectively. We study the influence of molecular orientation duration and degree on the ionization probability, angle-resolved photoelectron spectrum and photoelectron angular distribution (PAD). It is shown that we could obtain more stable ionization signal and PAD when the molecules are ionized in molecular orientation duration. We could increase the ionization probability and obtain more concentrated ionization signal and photoelectron distribution by increasing the orientation degree in the ground state. Moreover, we discuss the splitting pattern in the angle-resolved photoelectron spectrum.

Real-time optoacoustic temperature determination on cell cultures during heat exposure: a feasibility study

Objective/Purpose: In order to study the effects of hyperthermia and other temperature-related effects on cells and tissues, determining the precise time/temperature course is crucial. Here we present a non-contact optoacoustic technique, which provides temperatures during heating of cultured cells with scalable temporal and spatial resolution.Methods: A thulium laser (1.94 µm) with a maximum power of 15 W quickly and efficiently heats cells in a culture dish because of low penetration depth (1/e penetration depths of 78 µm) of the radiation in water. A repetitively Q-switched holmium laser (2.1 µm) is used simultaneously to probe temperatures at different locations in the dish by using the photoacoustic effect. Due to thermoelastic expansion of water, pressure waves are emitted and measured with an ultrasonic hydrophone at the side of the dish. The amplitudes of the waves are temperature dependent and can be used to calculate the temperature/time course at any location of probing.Results: We measured temperatures of up to 55 °C with a heating power of 6 W after 10 s, and subsequent lateral temperature profiles over time. Within this profile, temperature fluctuations were found, likely owing to thermal convection and water circulation. By using cultured retinal pigment epithelial cells, it is shown that the probe laser pulses alone cause no biological damage, while immediate cell damage occurs when heating for 10 s at temperatures exceeding 45 °C.Conclusions: This method shows great potential not only as a noninvasive, non-contact method to determine temperature/time responses of cells in culture, but also for complex tissue and other materials.

Effective dielectric function of laser-pumped anatase nanoparticles: influence of free carriers trapping and depletion of valence band.

Anatase nanoparticles were examined using the z-scan technique with simultaneous measurements of the intensity of the scattered laser light. In the course of the z-scan experiments, the average intensity of probing laser pulses varied between 1.0·106 W/cm2 and 1.08·1011 W/cm2 at the wavelength equal to 355 nm and between 1.0·107 W/cm2 and 1.0·1012 W/cm2 at 532 nm. The pulse duration was equal to 10 ns in all cases. A method for recovery of an effective dielectric function of nanoparticles is suggested. It was found that, in the case of the interband transition, the recovered dielectric functions for a short duration of the laser pulse sequences can be fitted by parametric dependencies corresponding to the Lorentz model. A kinetic model describing the changes in the population of mobile carriers is considered. It was found that the efficiency of the charge recombination is considerably less than the efficiency of the trapping. The dwell time of the mobile charge carriers before being captured was estimated as ≈13 ms.

Strong Third-order Optical Nonlinearities of the Ag, Ni, Ti, and Co Nanoparticles Synthesized During Laser Ablation of Bulk Metals in Liquids

Laser ablation of the materials immersed in liquid environment has been established as an advanced method for formation of the stable nanoparticles, especially those which cannot be synthesized by chemical methods. We review the studies of four metal (Ag, Ti, Co, Ni) nanoparticles synthesized during laser ablation in different liquids. The correlation between strong nonlinear optical response of silver nanoparticles out of plasmon resonance and efficient third harmonic generation in the plasmas containing silver nanoparticles is discussed. The change of sign of the nonlinear refraction with variation of the wavelength and duration of probe laser pulses is analyzed. The studies of third-order nonlinear optical processes (saturable absorption, reverse saturable absorption, two-photon absorption and nonlinear refraction) in the titanium and cobalt nanoparticles synthesized by laser ablation of bulk materials in water, toluene and ethylene glycol are discussed. The concurrence of different nonlinear absorption processes in nanoparticle-containing suspensions is analyzed. We discuss the two-photon absorption and nonlinear refraction studies of the nickel nanoparticles in water using femtosecond laser pulses at 400 and 800 nm. Third harmonic generation from the Ni nanoparticles contained plasma using picosecond and femtosecond pulses as heating radiation and femtosecond probe pulses is analyzed.

Tunneling-induced ultraslow solitons in triangular quantum dot molecules

We theoretically investigate the propagation of a weak probe laser pulse in a triangular quantum dot molecules scheme based on the tunneling induced transparency. We find that the ultraslow optical solitons can be realized due to the destructive quantum interference induced by the interdot tunneling coupling which can be adjusted by the gate voltage appropriately. This work may provide practical applications such as electro-optic modulated devices and other information processes in semiconductor quantum dots structure.

Strong third-order optical nonlinearities of Ag nanoparticles synthesized by laser ablation of bulk silver in water and air

We demonstrate the correlation between strong nonlinear optical response of silver nanoparticles (Ag NPs) out of plasmon resonance and efficient third harmonic generation in the plasmas containing Ag NPs. The dynamics of nonlinear optical response of the 8 nm and 50 nm Ag NPs prepared by laser ablation of bulk silver in deionized water using 6 ns, 200 ps, and 60 fs laser pulses is systematically analyzed. Their optical limiting properties are studied at the wavelengths of 800 and 355 nm, using femtosecond and nanosecond laser pulses. Nonlinear absorption coefficient of 8 nm Ag NPs at the wavelength of 1064 nm was measured to be as high as 3 × 10−5 cm W−1. Nonlinear refraction shows the change of sign with variation of the wavelength and duration of probe laser pulses. The theoretical calculation of silver ablation shows the formation of a few nanometer sized particles. We also analyze third harmonic generation in the laser-produced plasmas containing Ag NPs and attribute the enhancement of this process to the influence of small silver clusters. The conversion efficiency of 800 nm towards the third harmonic was measured to be 4 × 10−3, which was a few times larger compared with similar process in air.

Full Ab Initio Many-Electron Simulation of Attosecond Molecular Pump-Probe Spectroscopy.

Here, we present an ab initio approach to full simulation of an attosecond molecular pump-probe experiment. Sequential molecular double ionization by the pump and probe laser pulses with controlled delay is described from first-principles with a full account of the continuum dynamics of the photoelectrons. Many-electron bound-continuum dynamics is simulated using the time-dependent (TD) molecular B-spline algebraic diagrammatic construction (ADC) method. Our calculations give a quantitative prediction about the creation of a coherent superposition of molecular ionic states in the photoionization process and simulate the probe of the ensuing attosecond dynamics by a second ionizing pulse within a single first-principles many-electron framework. We therefore demonstrate the capability to simulate and interpret the results of a prototypical molecular pump-probe experiment of interest in attoscience. As a particular example, we simulate and elucidate the interpretation of a pump-probe experiment in CO2 aimed at measuring strong field-induced hole dynamics via photoionization yields.

Single-Shot Terahertz Time Profiling Using Curved Wavefront

We report an improved single-shot electro-optic sampling (EOS) based detection scheme to measure the time profile of a terahertz (THz) pulse using inherent curvature of a converging wavefront. A simple addition to conventional multishot EOS technique enables single-shot recording of THz time profile. While the temporal resolution of the proposed method is limited only by the duration of the probe laser pulse, the spatial resolution can be externally controlled with ease. Our method is easy to implement and has the potential to provide better signal-to-noise ratio compared to other single-shot methods. An interesting spin-off of the proposed technique is that the quintessential space-to-time calibration process itself reveals the spatial profile of the THz radiation. The technique is simple yet versatile and capable of characterizing THz radiation emitted by high-power THz sources employing ultrashort intense lasers and modern-day particle accelerators. As a proof of principle demonstration, we provide measurement and characterization of the THz pulse emitted by two-color laser-produced plasma in ambient atmosphere using 45 fs Ti:sapphire laser pulses. Our method faithfully reproduces the THz waveform measured by conventional scanning EOS technique.

Pulse propagation in an atomic medium under spontaneously generated coherence, incoherent pumping, and relative laser phase

Influences of spontaneously generated coherence (SGC) and relative phase of laser fields on a probe laser pulse propagating in a three-level cascade atomic medium under incoherent pumping and electromagnetically induced transparency (EIT) are studied theoretically. It is shown that the leading edge of the pulse oscillates during its propagation. Furthermore, the oscillating magnitude is enhanced by growing SGC, however, it can be depressed by choosing a suitable relative phase between the laser fields. Indeed, the probe pulse depends sensitively on the relative phase with a period of 2 π in which it oscillates strongest at ϕ=0, π and 2 π; whereas it is unchanged at ϕ=π∕2 and 3π/2. On the other hand, the influences of SGC and the relative phase on the pulse envelope are more effective as growing the incoherent pumping rate.