Pump Laser Pulse Research Articles

Nuclear quantum dynamics on the ground electronic state of neutral silver dimer 107Ag109Ag probed by femtosecond NeNePo spectroscopy.

The nuclear quantum dynamics on the ground electronic state of the neutral silver dimer 107Ag109Ag are studied by femtosecond (fs) pump-probe spectroscopy using the 'negative ion - to neutral - to positive ion' (NeNePo) excitation scheme. A vibrational wave packet is prepared on the X1Σ+g state of Ag2via photodetachment of mass-selected, cryogenically cooled Ag2- using a first ultrafast pump laser pulse. The temporal evolution of the wave packet is then probed by an ultrafast probe pulse via resonant multiphoton ionization to Ag2+. Frequency analysis of the fs-NeNePo spectra obtained for a single isotopologue and pump-probe delay times up to 60 ps yields the harmonic (ωe = 192.2 cm-1), quadratic anharmonic (ωexe = 0.637 cm-1) and cubic anharmonic (ωeye = 3 × 10-4 cm-1) constants for the X1Σ+g state of neutral Ag2. The fs-NeNePo spectra obtained at different pump wavelengths provide insight into the excitation mechanism. At a pump wavelength of 510 nm instead of 1010 nm, resonant excitation of a short-lived electronically excited state of the anion followed by autodetachment results in population of higher-energy vibrational levels of the neutral ground state. In contrast, at 1140 nm dynamics with a slightly shorter beating period and different relative phase are observed. The present study demonstrates that isotopologue-specific fs-NeNePo spectroscopy provides accurate vibrational constants of mass-selected neutral clusters in their electronic ground state in the terahertz spectral region, which remains difficult to obtain directly in the frequency domain with any other type of spectroscopy of comparable sensitivity.

Simultaneous formation of two-fold rotation symmetry with charge order in the kagome superconductor CsV3Sb5 by optical polarization rotation measurement

We report the observation of two-fold rotation symmetry in charge density wave (CDW) state in the newly discovered Kagome superconductor CsV$_3$Sb$_5$. Below its CDW transition temperature ($T_{CDW}$), the polarization rotation of the reflected laser beam promptly emerges and increases close to about 1 mrad, and the rotation angle shows two-fold rotation symmetry. With femtosecond laser pulse pumping, the rotation angle can be easily suppressed and then recovers in several picoseconds accompanied with coherent oscillations. Significantly, the oscillations in the signal also experience a 180 degree periodic change. Our investigation provides clear optical evidence for the formation of nematic order with two-fold rotation symmetry just below $T_{CDW}$. The results imply a immediate development of nematicity and possible time-reversal symmetry breaking in CDW state of CsV$_3$Sb$_5$.

Enhancement of Gain Coefficient of Li-Like Ion 3d-4f Soft X-ray Laser Oscillation by a Single Resonator

A recombination plasma soft X-ray laser that is associated with Li-like Aluminum ion Al10+ 3d-4f transition (wavelength: 15.47 nm) was investigated to increase the gain coefficient. By optimizing the time interval of the 16-pulse train, higher-density plasma with a low temperature could be generated, and thus, we obtained the gain coefficient of 9.84 cm−1 at the pump laser pulse energy which was as low as 3 J. To further enhance the X-ray laser output energy, the single resonator which was installed by using a Mo/Si multilayer mirror that has a high reflectivity of 0.45 at 15.5 nm, and the X-ray spectrum after the double pass in the lasing medium was measured. As a result, the output intensity was increased to be 1.5 times higher than t=when it was compared with those that were conducted with no X-ray mirror.

Interference patterns in ionization of Kramers–Henneberger atom

We combine IR pump and XUV probe laser pulses to visualize the Kramers–Henneberger (KH) state of the potassium atom. We demonstrate that ionization of such an atom exhibits some molecular-like features such as low order interference maxima in photoelectron momentum spectra. The locations of these maxima allow to estimate spatial dimensions of the KH atom and can be used for accurate calibration of high intensity laser fields. At the same time, we show that an analogy between the KH atom and a homo-nuclear diatomic molecule cannot be extended too far. In particular, higher order interference maxima are very difficult to observe in the case of the KH state. We attribute this to a particular structure of the KH potential which does not confine electron motion to a well-defined potential well unlike in real diatomic molecules.

Making a case for femto-phono-magnetism with FePt.

In the field of femtomagnetism, magnetic matter is controlled by ultrafast laser pulses; here, we show that coupling phonon excitations of the nuclei to spin and charge leads to femto-phono-magnetism, a powerful route to control magnetic order at ultrafast times. With state-of-the-art theoretical simulations of coupled spin, charge, and lattice dynamics, we identify strong nonadiabatic spin-phonon coupled modes that dominate early time spin dynamics. Activating these phonon modes that we show leads to an additional (up to 40% extra) loss of moment in iron-platinum occurring within 40 femtoseconds of the pump laser pulse. Underpinning this enhanced ultrafast loss of spin moment, we identify a physical mechanism in which minority spin current drives an enhanced intersite minority charge transfer, in turn promoting increased on-site spin flips. Our finding demonstrates that the nuclear system, often assumed to play the role of an energy and angular momentum sink, when selectively preexcited, can play a profound role in controlling femtosecond spin dynamics in materials.

High-Precision Voltage Measurement for Optical Quantum Computation

This paper presents a theoretical study into the use of optical systems for quantum computation. The study results pertain to quantum sampling and quantum communication and provide a basis for further research and the development of a physical implementation. We propose an optical superstructure that can implement specific computation processes and algorithms. The superstructure is composed of nonlinear optical units, such as beta barium borate crystals. The units are positioned in series, powered by a pulse laser pump, and culminate in a beam splitter that generates the output state of a number of entangled photon pairs. Computation is achieved by entanglement propagation via beam splitters and adjustable phase shifters, which set related parameters. Demonstrating a two-component case, we show how a series of cosine-based components can be implemented. The obtained results open a broad front for future research. Future work should investigate the construction of a quantum optimizer using quantum sampling methods and also investigate high-precision temporal voltage measurement, which is a key procedure for the construction of high-fidelity devices.

Unified generation and fast emission of arbitrary single-photon multimode W states

We propose a unified and deterministic scheme to generate arbitrary single-photon multimode $W$ states in circuit QED. A three-level system (qutrit) is driven by a pump-laser pulse and coupled to $N$ spatially separated resonators. The coupling strength for each spatial mode $g_i$ totally decide the generated single-photon N-mode $W$ state $\vert W_N \rangle=\frac{1}{A}\sum_{i=1}^N g_i|0_1 0_2 \cdots 1_i 0_{i+1}\cdots 0_N\rangle$, so arbitrary $\vert W_N \rangle$ can be generated just by tuning $g_i$. We could not only generate $W$ states inside resonators but also release them into transmission lines on demand. The time and fidelity for generating (or emitting) $\vert W_N \rangle$ can both be the same for arbitrary $N$. Remarkably, $\vert W_N\rangle$ can be emitted with probability reaching $98.9\%$ in $20-50$ ns depending on parameters, comparable to the recently reported fastest two-qubit gate ($30-45$ ns). Finally, the time evolution process is convenient to control since only the pump pulse is time-dependent.

Spatially selective excitation of spin dynamics in magneto-photonic crystals by spectrally tunable ultrashort laser pulses

Abstract In this work, we tackle the problem of the spatially selective optical excitation of spin dynamics in structures with multiple magnetic layers. The 120 fs circularly polarized laser pulses were used to launch magnetization precession in an all-dielectric magneto-photonic crystals (MPC) formed by magnetic layers sandwiched between and inside two magnetic Bragg mirrors. Optical pump-probe experiments reveal magnetization precession triggered via ultrafast inverse Faraday effect with an amplitude strongly dependent on the pump central wavelength: maxima of the amplitude are achieved for the wavelength tuned at the cavity resonance and at the edge of the photonic bandgap. The optical impact on the spins caused by the inverse Faraday effect and spectrum of this effect are found to correlate mostly to the direct Faraday effect. We show that even though the pump laser pulses propagate along the whole structure tuning their wavelength allows localization of a larger spin precession either in the cavity layer or in the Bragg mirror layers selectively. The results pave the way to the ultrafast optical control of magnetization dynamics at a sub-wavelength scale that is vital for modern magneto-photonics and magnonics.

A Hybrid Self-Seeded Ti:sapphire Laser with a Pumping Scheme Based on Spectral Beam Combination of Continuous Wave Diode and Pulsed DPSS Lasers

A wide variety of applications require high peak laser intensity in conjunction with a narrow spectral linewidth. Typically, injection-locked amplifiers have been employed for this purpose, where a continuous wave oscillator is amplified in a secondary external resonant amplifier cavity using a pulsed pump laser. In contrast, here we demonstrate a setup that combines a CW Ti:sapphire oscillator and pulsed amplifier in a single optical cavity, resulting in a compact system. Dichroic beam combination of blue wavelength semiconductor diodes and the green wavelength of a Nd:YAG laser allowed the simultaneous excitation of the Ti:sapphire crystal by both continuous wave and pulsed pump sources. A linewidth of <2 MHz is achieved in continuous wave operation, while the linewidth increases to about 10 MHz in the combined CW+pulsed mode with a pulse duration of 73 ns. A peak pulse intensity of 0.2 kW is achieved, which should enable efficient single-pass second harmonic generation in a nonlinear crystal.

Characterization of femtosecond laser-induced grating scattering of a continuous-wave laser light in air.

Nanosecond laser-induced grating scattering/spectroscopy (LIGS) technique has been widely applied for measuring thermodynamic parameters such as temperature and pressure in gaseous and liquid media. Recently, femtosecond (fs) laser was demonstrated to induce the grating and develop the fs-LIGS technique for gas thermometry. In this work, we systematically investigated the fs-LIGS signal generation using 35 fs, 800 nm laser pulses at 1 kHz repetition rate in ambient air by varying the pump laser energies, the probe laser powers and the temporal delays between two pump laser pulses. The stability of single-shot fs-LIGS signal was studied, from which we observed that the signal intensity exhibits a significant fluctuation while the oscillation frequency shows a much better stability. A 4.5% precision of the oscillation frequency was achieved over 100 single-shot signals. By using a previously-developed empirical model, the fs-LIGS signals were fitted using nonlinear least-squares fitting method, by which crucial time constants characterizing the signal decay process were extracted and their dependences on the pump laser energy were studied. From the measured results and theoretical analysis, we found that the appropriate range of the overall pump laser energy for reliable fs-LIGS measurements is approximately located within 80 ∼ 300 μJ. The limitations on the accuracy and precision of the fs-LIGS measurements, the origin of destructive influence of plasma generation on the signal generation as well as the electrostriction contribution were also discussed. Our investigations could contribute to a better understanding of the fs-LIGS process and further applications of the technique in single-shot gas thermometry and pressure measurements in various harsh conditions.

Accurate time zero determination in an ultrafast transmission electron microscope without energy filter

In ultrafast transmission electron microscopy, time zero can be accurately determined by making use of the photon-induced near-field electron microscopy (PINEM) effect, which causes electrons interacting with the near fields of a nanoparticle to coherently gain or lose energy in multiples of the photon energy when the laser pump and electron probe pulse overlap in time. If the instrument is not equipped with an energy filter, which is required to observe the PINEM effect, the response of a sample is frequently monitored instead. However, the gradual or delayed onset of this response can render an accurate measurement as challenging. Here, we demonstrate a simple and accurate method for determining time zero without an energy filter that is based on the observation that the outline of a nanoparticle blurs when the electron and laser pulse overlap in time. We show that this phenomenon arises from the PINEM effect, which causes some electrons to gain a large energy spread, thus blurring the image due to the chromatic aberration of the imaging system. This effect can also be used to characterize the instrument response and determine the laser polarization in situ. Furthermore, it may find applications for mapping out the near fields of a nanoparticle without the help of an energy filter.

Gain-Switched Single-Frequency DBR Pulsed Fiber Laser at 2.0 μm

A gain-switched single-frequency distributed Bragg reflector (DBR) Tm<inline-formula> <tex-math notation="LaTeX">$^{3+}$ </tex-math></inline-formula>-doped fiber pulsed laser operating at 1950 nm is demonstrated experimentally. The gain-switched process is realized by in-band pumping a DBR ultra-short linear cavity via a 1610 nm pulsed fiber laser (pump source). Driven by this pump with varying modulation frequency, repetition rate of the laser output can be flexibly tuned from 1 to 500 kHz. As a result, a maximum peak power of 34.2 W is achieved with a repetition rate of 100 kHz and a pulse width of 19 ns. Moreover, the generated pulses at <inline-formula> <tex-math notation="LaTeX">$2.0~\mu \text{m}$ </tex-math></inline-formula> have excellent optical signal-to-noise ratio of &#x003E;65 dB and exhibit stable single-longitudinal-mode status throughout the experiment. To the best of our knowledge, this is the first demonstration of <inline-formula> <tex-math notation="LaTeX">$2.0~\mu \text{m}$ </tex-math></inline-formula> gain-switched single-frequency pulsed fiber laser based on Tm<inline-formula> <tex-math notation="LaTeX">$^{3+}$ </tex-math></inline-formula>-doped fiber, showing great potential for applications including medical treatment, nonlinear frequency conversion and coherent Doppler lidar.

Nanoscale-Femtosecond Imaging of Evanescent Surface Plasmons on Silver Film by Photon-Induced Near-Field Electron Microscopy.

Surface plasmons on silver nanostructures have a broad range of tunable resonance properties in visible and near-infrared regimes, which possess wide applications in nanophotonics and optoelectronics. Here we use a femtosecond laser to excite surface plasmons on a silver film and trace the subsequent transient dynamics via photon-induced near-field electron microscopy (PINEM). A polarization experiment of PINEM demonstrates a conspicuous polarization dependence of the transient surface plasmon field on the silver film; however, unlike silver nanowires and nanorods, there is no polarization dependence for the PINEM intensity. This compelling finding suggests a thin film platform can be more easily used to identify the temporal and spatial overlaps between the pump laser and probe electron pulses in 4D ultrafast electron microscopy (UEM). Our work illustrates the femtosecond excitation and transient behavior of the surface plasmons on silver film and paves a universal, simple way for identifying the time zero in 4D UEM.

All fiber structured supercontinuum source based on graded-index multimode fiber

We provide an all fiber spectrally flat supercontinuum (SC) source generated by pumping a 20 m graded-index multimode fiber (MMF). The fiber pigtails of the all-fiber pump pulse laser and the MMF fused with a special method. After measuring the spectrum and the power, the output power extending from the visible to the mid-infrared is 2 W. To the best of our knowledge, this is the first watt-class and spectrally flat all fiber structured SC source based on graded-index MMF. Finally, we studied the evolution of the SC with increasing output power, which permitted us to more clearly recognize the nonlinear phenomena in the generation of the SC.

Spectral focusing in picosecond pulsed stimulated Raman scattering microscopy.

We introduce spectral focusing of picosecond laser pulses in stimulated Raman scattering (SRS) microscopy to improve spectral resolution, reduce nonlinear background signals, and decrease nonlinear photodamage. We produce a pair of 14 ps pump and Stokes laser pulses by spectral focusing of a 2 ps laser and achieve a spectral resolution of 2 cm-1. Due to instantaneous narrow-band excitation, we find that the chirped 14 ps laser pulses can be used to improve the signal-to-background ratio in SRS microscopy of various samples such as polymer particles and small molecules in HeLa cells. The lower peak powers produced by chirped picosecond laser pulses also reduce nonlinear photodamage, allowing long-term SRS imaging of living cells with higher SNR.

Separation of interface and substrate carrier dynamics at a heterointerface based on coherent phonons

Transient reflectivity spectroscopy is widely used to study ultrafast carrier- and phonon-dynamics in semiconductors. In their heterostructures, it is often not straightforward to distinguish contributions to the signal from the various layers. In this work, we perform transient reflectivity measurements on lattice-matched GaP/Si(001) using a near infrared pulse, to which GaP is transparent. The pump laser pulse can generate coherent longitudinal optical (LO) phonons both in the GaP overlayer as well as in the Si substrate which have distinct frequencies. This enables us to track the amplitude of the respective signal contributions as a function of GaP layer thickness $d$. The Si phonon amplitude in the signal exhibits an oscillatory behavior with increasing $d$. This can be quantitatively explained by the interference of the probe light reflected at the air/GaP/Si heterointerface. Based on this knowledge, we can then separate the interface- and the substrate-contributions in the carrier-induced non-oscillatory transient reflectivity signal. The obtained interface signal provides evidence for ultrafast carrier injection from the Si substrate into the GaP overlayer. This is also corroborated by examining the deviation of the polarization-dependence of the GaP coherent optical phonon signal from that of the bulk semiconductor.

Investigated coherent anti-Stokes Raman scattering in the process of cascaded stimulated Raman scattering in liquid and ice-Ih D2O.

Stimulated Raman scattering (SRS) of liquid and ice-Ih D2O was investigated using a pulsed Nd:YAG laser with a wavelength of 532nm. The high-order Stokes peaks and corresponding anti-Stokes SRS [Coherent Anti-Stokes Raman Spectroscopy (CARS)] peaks were obtained. Two symmetric and antisymmetric Raman modes of stretching vibrations were observed in liquid D2O, while only a symmetric stretching vibration mode was observed in ice-Ih D2O. Pure Stokes SRS is always collinear with the pump beam along the axial direction. Some ring-like Stokes SRS and CARS shifts, which originate from four-wave mixing processes, can also be observed only in the forward direction along with different angles meeting the phase-matching criteria, respectively. Simultaneously, the temporal behavior of SRS in liquid and ice-Ih D2O was examined, and the temporal waveforms of the pump laser pulse, transmitted pump pulse, and the forward SRS pulse were measured. In both cases, SRS was the dominant contributor to stimulated scattering. However, the efficiency values drastically decreased due to the self-termination behavior of SRS in liquid D2O, which arose from the thermal self-defocusing of both the pump beam and the SRS beam, owing to the Stokes shift-related opto-heating effect. In contrast, for the SRS process in ice-Ih D2O, the thermal self-defocusing influence was negligible, benefitting from a much greater thermal conductivity and a higher conversion efficiency of SRS generation retained under both of the conditions.

Pump pulse characteristics of quasi-continuous-wave diode-side-pumped Nd:YAG laser

The influence of pumping laser pulse on the property of quasi-continuous-wave (QCW) diode-side-pumped Nd:YAG laser is investigated theoretically and experimentally. Under remaining a fixed duty cycle, the average output power increases, and the corresponding thermal focal length shorten with the increase of the pump pulse duration, which attributes to the decrease of the ratio of pulse buildup time to the pulse duration. At a pump power of 146 W, the laser output power changes from 65.1 W to 81.2 W when the pulse duration is adjusted from 150 μs to 1000 μs, confirming a significant enhancement of 24.7%. A laser rate equation model incorporating the amplified spontaneous emission is also utilized and numerically solved, and the simulated results agree well with the experimental data.

NV\u2013 diamond laser

For the first time, lasing at NV− centers in an optically pumped diamond sample is achieved. A nanosecond train of 150-ps 532-nm laser pulses was used to pump the sample. The lasing pulses have central wavelength at 720 nm with a spectrum width of 20 nm, 1-ns duration and total energy around 10 nJ. In a pump-probe scheme, we investigate lasing conditions and gain saturation due to NV− ionization and NV0 concentration growth under high-power laser pulse pumping of diamond crystal.

Acoustic mapping by picosecond ultrasonics for elastic property measurement: Experimental demonstration on a TRISO fuel compact

Picosecond ultrasonics has been demonstrated on a tristructural isotropic (TRISO) fuel compact to measure the elastic properties of each compact layer. This technique utilizes an ultrashort pump laser pulse to excite vibrations in a gold transducer film covering the surface of each component and a second probe laser pulse to record the resulting acoustic strain induced change in optical reflectance. From the damping of this film vibration, the acoustic reflection coefficient, which couples the elastic properties of the transducer film and the sample, can be obtained, enabling a calculation of the sample's acoustic velocity and elastic modulus. Results obtained from this method are consistent with known values of elastic moduli, namely that the SiC coating is the stiffest component of the compact while the carbonaceous matrix is the most compliant. Nanoindentation was conducted as a benchmark technique on the same sample and shows satisfactory agreement with the results of picosecond ultrasonics. Compared to other methods like nanoindentation, picosecond ultrasonics is multimodal with a capability of measuring several key properties simultaneously and has potentials to be coupled into optical fibers for remote sensing. Thus, these demonstration measurements reveal the methodology to be a promising candidate for in-situ and high-throughput optical characterizations of nuclear materials.