Femtosecond Pump Pulses Research Articles

Absolute Cross Sections of Liquids from Broadband Stimulated Raman Scattering with Femtosecond and Picosecond Pulses.

Broadband stimulated Raman scattering (SRS) is often observed in applications that use nonlinear spectroscopy to probe the composition or dynamics of complex systems. Whether the SRS response is measured intentionally or unintentionally, as a background signal, the relative scattering intensities provide a quantitative measure of the population profile of target molecules. Solvent scattering is a valuable internal reference for determining absolute concentrations in these applications, but accurate cross sections have been reported for only a limited number of transitions in select solvents and were measured using spontaneous Raman scattering with narrowband continuous wave or nanosecond light sources. This work reports the measurement and analysis of absolute Raman scattering cross sections spanning the frequency range of 500-4000 cm-1 for cyclohexane, DMSO, acetonitrile, methanol, water, benzene, and toluene using broadband SRS with femtosecond and picosecond Raman pump pulses at 488 nm. Varying the duration of the Raman pump pulses from ∼80 fs to >1 ps confirms that the cross sections are independent of the spectral bandwidth across the range of ∼250 to <20 cm-1. The cross sections and depolarization ratios measured using broadband SRS agree with the limited number of previously reported values, after accounting for overlapping transitions in the lower-resolution femtosecond and picosecond spectra. The SRS cross sections reported here can be used with confidence as internal reference standards for a wide range of applications, including nonlinear spectroscopy and coherent microscopy measurements using ultrafast lasers.

Bandwidth analysis of optical parametric amplifier pumped by broadband pulses

We present analytical solutions to nonlinear coupling equations for parametric down-conversion of a broadband pump at large gain. The feasibility of analytical expressions is demonstrated for the case of a collinear type II parametric interaction in beta-barium borate crystal and group-velocity matching of signal and pump waves. The correlation function of amplified signal wave is obtained, and it is revealed that the bandwidth of a noise-seeded optical parametric amplifier increases with a decrease in pump pulse duration, especially for femtosecond pump pulses.

All-Optical Multipulse Pump-Probe Method for Selective Control of Magnetization Precessions in Magnetic Multilayers

A method to all-optically selectively excite and probe individual resonances within a multimodal system is demonstrated. As an example, the method is applied to control spin waves within a multilayer magnetic structure. By tuning the timing of two femtosecond pump pulses, the magnetization precession of a particular spin wave resonance can be selectively quenched while maintaining the precession of the other. This control depends on the relative positions of the magnetization vectors within their precessional trajectory and on the power ratio of the pump pulses. A representation of the position of the vectors in the complex plane is introduced, which serves as a convenient means for manipulation of the systems.

GLS and GLSSe ultrafast laser inscribed waveguides for mid-IR supercontinuum generation

Using the ultrafast laser inscription technique, buried channel waveguides have been fabricated in gallium lanthanum sulfide and gallium lanthanum sulfide selenide glasses to demonstrate the suitability of the materials for supercontinuum generation in the mid-IR. Supercontinuum generation was performed using 100 femtosecond pump pulses with micro-Joule pulse energies and a center wavelength of 4.6 µm, which is in the anomalous dispersion regime for these waveguides. Under such pump conditions, supercontinuum was obtained covering a 25-dB-bandwidth of up to 6.1 um with a long-wavelength edge of 8 µm. To our knowledge, this represents the broadest and the longest-wavelength IR supercontinuum generated from an ultrafast laser inscribed waveguide to date.

Terahertz emission from laser-driven gas plasmas: a plasmonic point of view

We disclose an unanticipated link between plasmonics and nonlinear frequency down-conversion in laser-induced gas-plasmas. For two-color femtosecond pump pulses, a plasmonic resonance is shown to broaden the terahertz emission spectra significantly. We identify the resonance as a leaky mode, which contributes to the emission spectra whenever electrons are excited along a direction where the plasma size is smaller than the plasma wavelength. As a direct consequence, such resonances can be controlled by changing the polarization properties of elliptically-shaped driving laser pulses. Both, experimental results and 3D Maxwell consistent simulations confirm that a significant terahertz pulse shortening and spectral broadening can be achieved by exploiting the transverse driving laser beam shape as an additional degree of freedom.

Dispersion-Engineered Step-Index Tellurite Fibers for Mid-Infrared Coherent Supercontinuum Generation from 1.5 to 4.5 μm with Sub-Nanojoule Femtosecond Pump Pulses

Mid-infrared supercontinuum generation from 1.5 to 4.5 µm with sub-nanojoule femtosecond pump pulses is demonstrated by using a short segment of dispersion-engineered step-index tellurite fiber with very low OH content. Distinct group-velocity dispersion regimes in a simple design of step-index tellurite fiber are also reported, which allows to choose the nonlinear pulse propagation regime according to the required tailoring of the supercontinuum source. Numerical simulations based on the generalized nonlinear Schrödinger equation are used to determine optimized fiber parameters before experimental demonstrations. We also analyse the coherence properties of the resulting supercontinuum sources.

Ultrafast double magnetization switching in GdFeCo with two picosecond-delayed femtosecond pump pulses

The recently discovered thermally induced magnetization switching (TIMS) induced by single femtosecond laser pulses in ferrimagnetic GdFeCo alloys proceeds on the picosecond time-scale. The rate at which data can be changed for use of TIMS in technological devices is limited by the processes leading to thermal equilibrium. In the present work, we address the question of whether it is possible to further excite switching via TIMS well before thermal equilibrium between subsystems is reached. In particular, we investigate the conditions for double thermally induced magnetic switching by the application of two shortly delayed laser pulses. These conditions become relevant for potential applications as it sets both a limit to rewrite data and demonstrates the importance of spatial confinement of a heat pulse to bit size, as neighboring bits may be accidentally re-switched for spatially extended pulse spots. To demonstrate this effect, we theoretically study the switching behavior in a prototypical ferrimagnetic GdFeCo alloy as a function of composition. We use computer simulations based on thermal atomistic spin dynamics and demonstrate the possibility of inducing a second switching event well before thermal equilibrium is reached and define the conditions under which it can occur. Our theoretical findings could serve as a guidance for further understanding of TIMS as well as to act as a guide for future applications.

Ultraviolet to visible continuum generation in a silica photonic crystal fiber

The ultraviolet to visible continuum generations are experimentally demonstrated by coupling femtosecond pump pulses around 800 nm into the normal dispersion region close to the zero-dispersion wavelength in the fundamental mode of a silica photonic crystal fiber (PCF). The ultraviolet to visible continuum is generated through a combination of nonlinear effects including degenerate four-wave mixing, cross-phase modulation, and stimulated Raman scattering. When pump pulses with the wavelength of 800 nm and the input average power of 500 mW are used, more than 40% of the incident pump power is converted to the ultraviolet to visible spectral region. The evolution of the output spectra for different PCF lengths is studied to understand the nonlinear propagation dynamics during the process of the continuum generation. The ultraviolet to visible continuum generation will find important applications in the ultraviolet and visible-based biophotonics and spectroscopy.

Temperature effect on the internal conversion dynamics following different stimulated absorptions in a conjugated polymer

Abstract By applying a femtosecond electric pump pulse to a conjugated polymer, we theoretically study the temperature effect on the internal conversion dynamics following different stimulated absorptions. The simulations are performed within the framework of an extended version of the tight-binding Su-Schrieffer-Heeger model modified to include temperature effect. Firstly, temperature effect on different stimulated absorptions is focused, by which we speculate that the absorption spectra of a single polymer should be nearly temperature independent. However, following stimulated absorptions, the internal conversion (IC) from a higher-lying excited state to the lowest-lying exciton state will be remarkably accelerated by increasing temperature. From this point of view, temperature effect is not favorable for charge separation in polymer solar cells (PSCs), since charge separation from a higher-lying excited state can be effectively suppressed by its IC relaxation. In addition, considering that these IC relaxations are lattice vibration assisted, effects of the vibrational damping and the electron-lattice interaction on them are separately discussed. These findings are crucial for further understanding the primary photovoltaic process in PSCs, especially the role of a higher-lying excited state in charge separation.

Experimental generation of discrete ultraviolet wavelength by cascaded intermodal four-wave mixing in a multimode photonic crystal fiber.

In this Letter, we demonstrate experimentally for the first time, to the best of our knowledge, discrete ultraviolet (UV) wavelength generation by cascaded intermodal FWM when femtosecond pump pulses at 800nm are launched into the deeply normal dispersion region of the fundamental guided mode of a multimode photonic crystal fiber (MPCF). For pump pulses at average input powers of Pav=450, 550, and 650mW, the first anti-Stokes waves are generated at the visible wavelength of 538.1nm through intermodal phase matching between the fundamental and second-order guided mode of the MPCF. The first anti-Stokes waves generated then serve as the secondary pump for the next intermodal FWM process. The second anti-Stokes waves in the form of the third-order guided mode are generated at the UV wavelength of 375.8nm. The maximum output power is above 10mW for Pav=650 mW. We also confirm that the influences of fiber bending and intermodal walk-offs on the cascaded intermodal FWM-based frequency conversion process are negligible.

Deep-ultraviolet second-harmonic generation by combined degenerate four-wave mixing and surface nonlinearity polarization in photonic crystal fiber

Deep-ultraviolet (UV) second-harmonics (SHs) have important applications in basic physics and applied sciences. However, it still remains challenging to generate deep-UV SHs especially in optical fibers. Here, for the first time, we experimentally demonstrate the deep-UV SH generations (SHGs) by combined degenerate four-wave mixing (FWM) and surface nonlinearity polarization in an in-house designed and fabricated air-silica photonic crystal fiber (PCF). When femtosecond pump pulses with average input power Pav of 650 mW and center wavelength λp of 810, 820, 830, and 840 nm are coupled into the normal dispersion region close to the zero-dispersion wavelength of the fundamental mode of the PCF, the anti-Stokes waves induced by degenerate FWM process are tunable from 669 to 612 nm. Then, they serve as the secondary pump, and deep-UV SHs are generated within the wavelength range of 334.5 to 306 nm as a result of surface nonlinearity polarization at the core-cladding interface of the PCF. The physical mechanism of the SHGs is confirmed by studying the dependences of the output power PSH of the SHs on the PCF length and time. Finally, we also establish a theoretical model to analyze the SHGs.

Real-time visualization of the vibrational wavepacket dynamics in electronically excited pyrimidine via femtosecond time-resolved photoelectron imaging.

The vibrational wavepacket dynamics at the very early stages of the S1-T1 intersystem crossing in photoexcited pyrimidine is visualized in real time by femtosecond time-resolved photoelectron imaging and time-resolved mass spectroscopy. A coherent superposition of the vibrational states is prepared by the femtosecond pump pulse at 315.3 nm, resulting in a vibrational wavepacket. The composition of the prepared wavepacket is directly identified by a sustained quantum beat superimposed on the parent-ion transient, possessing a frequency in accord with the energy separation between the 6a1 and 6b2 states. The dephasing time of the vibrational wavepacket is determined to be 82 ps. More importantly, the variable Franck-Condon factors between the wavepacket components and the dispersed cation vibrational levels are experimentally illustrated to identify the dark state and follow the energy-flow dynamics on the femtosecond time scale. The time-dependent intensities of the photoelectron peaks originated from the 6a1 vibrational state exhibit a clear quantum beating pattern with similar periodicity but a phase shift of π rad with respect to those from the 6b2 state, offering an unambiguous picture of the restricted intramolecular vibrational energy redistribution dynamics in the 6a1/6b2 Fermi resonance.

Dynamically reconfigurable metal-semiconductor Yagi-Uda nanoantenna

We propose a novel type of tunable Yagi-Uda nanoantenna composed of metal-dielectric (Ag-Ge) core-shell nanoparticles. We show that, due to the combination of two types of resonances in each nanoparticle, such hybrid Yagi-Uda nanoantenna can operate in two different regimes. Besides the conventional nonresonant operation regime at low frequencies, characterized by highly directive emission in the forward direction, there is another one at higher frequencies caused by hybrid magneto-electric response of the core-shell nanoparticles. This regime is based on the excitation of the van Hove singularity, and emission in this regime is accompanied by high values of directivity and Purcell factor within the same narrow frequency range. Our analysis reveals the possibility of flexible dynamical tuning of the hybrid nanoantenna emission pattern via electron-hole plasma excitation by 100 femtosecond pump pulse with relatively low peak intensities $\sim$200 MW/cm$^2$.

Polarization-dependent intermodal four-wave mixing in a birefringent multimode photonic crystal fiber.

In this Letter, polarization-dependent intermodal four-wave mixing (FWM) is demonstrated experimentally in a birefringent multimode photonic crystal fiber (BM-PCF) designed and fabricated in-house. Femtosecond pump pulses at wavelengths ∼800 nm polarized along one of the principal axes of the BM-PCF are coupled into a normal dispersion region away from the zero-dispersion wavelengths of the fundamental guided mode of the BM-PCF. Anti-Stokes and Stokes waves are generated in the 2nd guided mode at visible and near-infrared wavelengths, respectively. For pump pulses at an average input power of 500 mW polarized along the slow axis, the conversion efficiencies ηas and ηs of the anti-Stokes and Stokes waves generated at wavelengths 579.7 and 1290.4 nm are 19% and 14%, respectively. For pump pulses polarized along the fast axis, the corresponding ηas and ηs at 530.4 and 1627 nm are 23% and 18%, respectively. We also observed that fiber bending and intermodal walk-off have a small effect on the polarization-dependent intermodal FWM-based frequency conversion process.

Optical nonlinearity enhancement with graphene-decorated silicon waveguides

Broadband on-chip optical frequency combs (OFCs) are important for expanding the functionality of photonic integrated circuits. Here, we demonstrate a huge local optical nonlinearity enhancement using graphene. A waveguide is decorated with graphene by precisely manipulating graphene’s area and position. Our approach simultaneously achieves both an extremely efficient supercontinuum and ultra-short pulse generation. With our graphene-decorated silicon waveguide (G-SWG), we have achieved enhanced spectral broadening of femtosecond pump pulses, along with an eightfold increase in the output optical intensity at a wavelength approximately 200 nm shorter than that of the pump pulses. We also found that this huge nonlinearity works as a compressor that effectively compresses pulse width from 80 to 15.7 fs. Our results clearly show the potential for our G-SWG to greatly boost the speed and capacity of future communications with lower power consumption, and our method will further decrease the required pump laser power because it can be applied to decorate various kinds of waveguides with various two-dimensional materials.

The influence of molecular pre-orientation on the resonance-enhanced multi-photon ionization dynamics

We investigate theoretically the influence of molecular pre-orientation on the resonance-enhanced multi-photon ionization (REMPI) dynamics, taking the LiH molecule for example. The LiH molecule is first pre-oriented by a single-cycle pulse (SCP) in terahertz (THz) region, and then excited by the femtosecond pump pulse, and finally ionized by the femtosecond probe pulse. We focus on the impact of the pre-orientation on the ionization probability, energy- and angle-resolved photoelectron spectra and photoelectron angular distribution (PAD). It is found that the ionization probability and peak intensity of energy-resolved photoelectron spectra are significantly affected by molecular orientation. The angle-resolved photoelectron spectra are related to the molecular orientation. The PAD can be changed by varying the delay time between the THz SCP and pump pulse. We also investigate the effect of temperature on excitation and ionization dynamics.

Manipulation of lattice vibration by ultrafast spectroscopy

The ultrafast pump-probe spectroscopy allows us to make movies of the dynamics of the carriers and vibrational excitations on the timescales shorter than the typical scattering time. In general, the temporal evolution of the reflectivity change is comprised of the oscillatory and the non-oscillatory components. The former corresponds to the coherent lattice vibration, while the latter is related to the complex cooling process of the hot carriers. To investigate the dynamics of the hot carrier and the lattice vibration, it is necessary to decouple the two parts in the detected signal. Comparatively, the manipulation of the coherent lattice vibration is easier in spite of its super-high frequency and subatomic vibration amplitude. In this work, the behavior of the coherent lattice vibration in Bi2Te3 single crystalline film with a thickness of 100 nm is studied by using the double pump-single probe ultrafast spectroscopy. Firstly, the coherent lattice vibration with the subatomic amplitude and a frequency of about 1.856 THz is simulated by a femtosecond pump pulse, and its damped oscillation signal is detected by the reflectivity change of a probe pulse. Compared with the Raman spectrum, this vibration is confirmed to be the coherent optical phonon with A1g1 symmetric vibration mode. To manipulate this lattice vibration, a pulse shaper is then installed in the pump-beam arm to generate double pump pulses with the different separation times and the intensity ratios. The resulting reflectivity change is found to be a superposition of the pulse train: the oscillation amplitude is enhanced when the separation time is matched to the period of the oscillation; if the separation time is the odd times the half-period of the oscillation, the A1g1 vibration mode can be completely cancelled out after adjusting the intensity ratio. Finally, by maintaining the same intensity ratio, the amplitudes of the oscillation signals after the second pump pulse are measured with different separation times. The results agree well with the theoretical predictions: the amplitude of the oscillation after the second pump pulse shows a cosine function of separation time with a period of about 1080 fs, which is the twice the period of the oscillation illuminated by a single pump pulse. This work suggests that the lattice vibration can be optically manipulated, thus provides an effective way to disentangle the lifetimes of the phonons and the interactions with the excited carriers in the ultrafast energy relaxation process in semiconductor, which is extremely important for a number of interesting phenomena such as the non-thermal melting and the insulator-to-metal transition.

Field-free molecular alignment control of filamentation

With an approach of controlling the nonlinearity of medium rather than the light field, the effect of field-free molecular alignment on filamentation and resulting white-light generation is studied. This is done by measuring the rotational wavepacket evolution of nitrogen molecules after passing of a femtosecond laser pump pulse by observing the nonlinear propagation dynamics of a variably delayed filament-producing probe pulse.

Double-pulse femtosecond laser peening of aluminum alloy AA5038: Effect of inter-pulse delay on transient optical plume emission and final surface micro-hardness

Double-pulse ablative femtosecond laser peening of the AA5038 aluminum alloy surface in the phase explosion regime results in its enhanced microhardness, which monotonously decreases till the initial value versus inter-pulse delay, increasing on a sub-nanosecond timescale. Optical emission spectroscopy of the double-pulse ablative plume reveals the same trend in the yield of the corresponding atomic and ion emission versus inter-pulse delay, enlightening the interaction of the second femtosecond laser pump pulse with the surface and the resulting plume.

Intervalley scattering in MoS2 imaged by two-photon photoemission with a high-harmonic probe

We report on the direct mapping of electron transfer in the momentum space of bulk MoS2 by means of time- and angle-resolved two-photon photoemission with a high-harmonic probe. For this purpose, we have combined a high-repetition rate high-harmonic source with tunable femtosecond pump pulses and a 3D (kx,ky,E) electron spectrometer. We show that optical excitation slightly above the A exciton resonance results in an immediate occupation of the conduction band at K¯ followed by an ultrafast transfer (&amp;lt;50 fs) to the conduction band minimum at Σ¯. Both signals, at K¯ and Σ¯, do not vanish over the observed period of 400 fs. The technique described here enables direct access to the charge transfer dynamics in k-space and allows the study of decay times and decay channels in various systems with dependence on the excess energy or helicity of the excitation.