Mid-infrared Laser Pulses Research Articles

Mid-infrared ultrafast laser pulses induced third harmonic generation in nitrogen molecules on an excited state.

We report on generation of third harmonic from nitrogen molecules on the excited state with a weak driver laser pulse at a mid-infrared wavelength. The excited nitrogen molecules are generated using a circularly polarized intense femtosecond pulse which produces energetic electrons by photoionization to realize collisional excitation of nitrogen molecules. Furthermore, since the third harmonic is generated using a pump-probe scheme, it enables investigation of the excited-state dynamics of nitrogen molecules produced under different conditions. We also perform a comparative investigation in excited argon atoms, revealing different decay dynamics of the molecules and atoms from the excited states in femtosecond laser induced filaments.

Sub-laser-cycle control of coupled electron–nuclear dynamics at a conical intersection

Nonadiabatic processes play a fundamental role in the understanding of photochemical processes in excited polyatomic molecules. A particularly important example is that of radiationless electronic relaxation at conical intersections (CIs). We discuss new opportunities for controlling coupled electron–nuclear dynamics at CIs, offered by the advent of nearly single-cycle, phase-stable, mid-infrared laser pulses. To illustrate the control mechanism, a two-dimensional model of the NO2 molecule is considered. The key idea of the control scheme is to match the time scale of the laser field oscillations to the characteristic time scale of the wave packet transit through the CI. The instantaneous laser field changes the shape and position of the CI as the wave packet passes through. As the CI moves in the laser field, it ‘slices’ through the wave packet, sculpting it in the coordinate and momentum space in a way that is sensitive to the carrier-envelope phase of the control pulse. We find that the electronic coherence imparted on the sub-laser-cycle time scale manifests during much longer nuclear dynamics that follow on the many tens of femtosecond time scale. Control efficiency as a function of molecular orientation is analyzed, showing that modest alignment is sufficient for showing the described effects.

Electromagnetically induced transparency in a cascade-type quantum well subband system under intense picosecond excitation

The coherent light–matter interaction in a 4-level cascade-type subband system of an asymmetric GaAs/AlGaAs quantum well structure is studied in pump-probe transmission experiments with picosecond (ps) time resolution. Coupling two excited subbands by an intense mid-infrared laser pulse at low sample temperatures is found to result in a substantially increased transparency of the fundamental e1–e2 transition. We find a reduction of the absorption coefficient by ~80%, which is one of the most pronounced electromagnetically induced transparency in solid state systems observed so far.

Black phosphorus as saturable absorber for the Q-switched Er:ZBLAN fiber laser at 2.8 μm.

Black phosphorus, a newly emerged two-dimensional material, has attracted wide attention as novel photonic material. Here, multilayer black phosphorus is successfully fabricated by liquid phase exfoliation method. By employing black phosphorus as saturable absorber, we demonstrate a passively Q-switched Er-doped ZBLAN fiber laser at the wavelength of 2.8 μm. The modulation depth and saturation fluence of the black phosphorus saturable absorber are measured to be 15% and 9 μJ/cm(2), respectively. The Q-switched fiber laser delivers a maximum average power of 485 mW with corresponding pulse energy of 7.7 μJ and pulse width of 1.18 μs at repetition rate of 63 kHz. To the best of our knowledge, this is the first time to demonstrate that black phosphorus can realize Q-switching of 2.8-μm fiber laser. Our research results show that black phosphorus is a promising saturable absorber for mid-infrared pulsed lasers.

Carrier-envelope phase dependence of molecular harmonic spectral minima induced by mid-infrared laser pulses.

The spectral minima in harmonic spectra of H2+ induced by mid-infrared laser pulses are numerically investigated based on two models of Born-Oppenheimer (BO) and non-Born-Oppenheimer (NBO) approximations. The simulation results show that, with the variation of the mid-infrared laser's carrier-envelope phase (CEP), the spectral minima positions (SMPs) are fixed for the BO model, while oscillate periodically for the NBO model. This can be understood by the two-center-destructive-interference theory via the detailed investigation to several physical quantities for each CEP case, such as SMPs, effective potential, internuclear separation and the electron's de Broglie wavelength at the time for interference occurring. The fittings to these quantities' CEP-dependent curves demonstrate that they follow a variation law in the form of a sine function.

Scaling laws for high-order-harmonic generation with midinfrared laser pulses

We derive an analytic expression for the wavelength scaling of the high-order-harmonic generation (HHG) yield induced by midinfrared driving laser fields. It is based on a quasiclassical description of the returning electron wave packet, which is shown to be largely independent of atomic properties. The accuracy of this analytic expression is confirmed by comparison with results of numerical solutions of the time-dependent Schr\odinger equation for wavelengths in the range of $1.4\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}\ensuremath{\leqslant}\ensuremath{\lambda}\ensuremath{\leqslant}4\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$. We verify the wavelength scaling of the HHG yield found numerically for midinfrared laser fields in a recent paper by Le et al. [Phys. Rev. Lett. 113, 033001 (2014)].

Optimal generation of high harmonics in the water-window region by synthesizing 800-nm and mid-infrared laser pulses.

We propose a method to optimally synthesize a strong 800-nm Ti:sapphire laser pulse and a relatively weak mid-infrared laser pulse to enhance harmonic yields in the water-window region. The required wavelength of the mid-infrared laser is varied from about 2.0 to 3.2 μm. The optimized waveforms generate comparable harmonic yields as the waveforms proposed in [Sci. Rep.4, 7067 (2014)], but with much weaker intensity for the mid-infrared laser. This method provides an alternative scheme based on the available laser technology to help realize tabletop light source in the water-window region by high-order harmonic generation.

Time-resolved detection of structural change in polyethylene films using mid-infrared laser pulses

Some of the vibrational modes of crystalline organic polymers are known to be sensitive to the structural change from the crystalline phase to the amorphous phase, and vice versa. Using a mid-infrared (mid-IR) pulse from a free-electron laser as a probe, we demonstrate the time-resolved detection of structural change in crystalline polymer (polyethylene) films upon laser heating by a Q-switched Nd:YAG laser. Transmittance of the resonant mid-IR pulse almost instantaneously changes before and after the Nd:YAG laser pulse if its fluence is sufficient to induce the structural change in the film. The developed technique would be useful to study the time-dependent dynamics of the structural change in various materials.

Super high power mid-infrared femtosecond light bullet

Mid-infrared ultrashort high energy laser sources are opening up new opportunities in science, including keV-class high harmonic generation and monoenergetic MeV-class proton acceleration. As new higher energy sources become available, potential applications for atmospheric propagation can dramatically grow to include stand-off detection, laser communications, shock-driven remote terahertz enhancement and extended long-lived thermal waveguides to transport high power microwave and radiofrequency waves. We reveal a new paradigm for long-range, low-loss, ultrahigh power ultrashort pulse propagation at mid-infrared wavelengths in the atmosphere. Before the onset of critical self-focusing, energy in the fundamental wave continually leaks into shock-driven spectrally broadened higher harmonics. A persistent near-invariant solitonic leading edge on the multi-terawatt pulse waveform transports most of the power over hundred-metre-long distances. Such light bullets are resistant to uncontrolled multiple filamentation and are expected to spark extensive research in optics, where the use of mid-infrared lasers is currently much under-utilized. A mechanism for the propagation of mid-infrared femtosecond laser pulses in air is theoretically investigated. A numerical simulation predicts that the propagation of multiple-terawatt pulses is possible over hundreds of metres.

Mid-infrared passively switched pulsed dual wavelength Ho(3+)-doped fluoride fiber laser at 3 μm and 2 μm.

Cascade transitions of rare earth ions involved in infrared host fiber provide the potential to generate dual or multiple wavelength lasing at mid-infrared region. In addition, the fast development of saturable absorber (SA) towards the long wavelengths motivates the realization of passively switched mid-infrared pulsed lasers. In this work, by combing the above two techniques, a new phenomenon of passively Q-switched ~3 μm and gain-switched ~2 μm pulses in a shared cavity was demonstrated with a Ho3+-doped fluoride fiber and a specifically designed semiconductor saturable absorber (SESAM) as the SA. The repetition rate of ~2 μm pulses can be tuned between half and same as that of ~3 μm pulses by changing the pump power. The proposed method here will add new capabilities and more flexibility for generating mid-infrared multiple wavelength pulses simultaneously that has important potential applications for laser surgery, material processing, laser radar, and free-space communications, and other areas.

Co2amp: a software program for modeling the dynamics of ultrashort pulses in optical systems with CO(2) amplifiers.

A computer code for simulating the amplification of ultrashort mid-infrared laser pulses in CO2 amplifiers and their propagation through arbitrary optical systems is described. The code is based on a comprehensive model that includes an accurate consideration of the CO2 active medium and a physical optics propagation algorithm, and takes into account the interaction of the laser pulse with the material of the optical elements. The application of the code for optimizing an isotopic regenerative amplifier is described.

Analytical description of generation of the residual current density in the plasma produced by a few-cycle laser pulse

When a gas is ionized by a few-cycle laser pulse, some residual current density (RCD) of free electrons remains in the produced plasma after the passage of the laser pulse. This quasi-dc RCD is an initial impetus to plasma polarization and excitation of the plasma oscillations which can radiate terahertz (THz) waves. In this work, the analytical model for calculation of RCD excited by a few-cycle laser pulse is developed for the first time. The dependences of the RCD on the carrier-envelope phase (CEP), wavelength, duration, and intensity of the laser pulse are derived. It is shown that maximum RCD corresponding to optimal CEP increases with the laser pulse wavelength, which indicates the prospects of using mid-infrared few-cycle laser pulses in the schemes of generation of high-power THz pulses. Analytical formulas for optimal pulse intensity and maximum efficiency of excitation of the RCD are obtained. Basing on numerical solution of the 3D time-dependent Schrödinger equation for hydrogen atoms, RCD dependence on CEP is calculated in a wide range of wavelengths. High accuracy of analytical formulas is demonstrated at the laser pulse parameters which correspond to the tunneling regime of ionization.

Steering the electron in dissociatingH2+via manipulating two-state population dynamics by a weak low-frequency field

We propose a scheme to steer the electron localization in dissociating ${{\mathrm{H}}_{2}}^{+}$ by using the combination of a phase-stabilized few-cycle mid-infrared laser pulse and a low-frequency field. Via manipulating the population dynamics in the $1s{\ensuremath{\sigma}}_{g}$ and $2p{\ensuremath{\sigma}}_{u}$ states by the weak low-frequency field, the dissociative wave packets on these two degenerate states of opposite parities are managed to share the same kinetic energy and overlap in space. By adjusting the carrier-envelope phase of the mid-infrared pulse or simply changing the intensity of the low-frequency field, the electron localization induced by the spatial interference of the two dissociation pathways can be efficiently controlled.

Photodetachment of Si− by mid-infrared few-cycle femtosecond laser pulses

The Kelydsh model originally developed for infinitely long periodic laser pulses is extended to a spin polarized model to consider direct photodetachment of negative ions with half-filled np3 valence shells by few-cycle laser pulses without taking into account rescattering effects. The theory is applied to study above threshold detachment of Si− negative ions by intense short infrared pulses but is expected to be inherent of any half-filled p subshell negative ion. Photoelectron momentum maps of the direct electrons of Si− are presented. These are found to exhibit a distinctive threshold structure of concentric elliptical rings where the separation of these rings is dependent on the quantized frequency components of the few-cycle laser pulse. Photoelectron angular distributions (PADs) predicted by short duration laser pulses are calculated and threshold effects associated with channel closings are observed as the laser-field frequency and intensity pass through critically induced ponderomotive potential channel closures. Enhanced structures of the PADs in the vicinity near threshold show that the detachment process does not follow the Wigner law. This is a because the photon energy is not conserved in a few-cycle pulse in contrast to a long periodic pulse. In the energy spectra interference effects are observable as multiphoton peaked structures whose positions are dependent on the ponderomotive threshold shifts determined by the nature of the time-dependent vector potential. Additionally we show that carrier envelope phase effects manipulate all emission spectra considered.

Picosecond melting of peptide nanotubes using an infrared laser: a nonequilibrium simulation study.

Self-assembled functional peptide biomaterials are emerging with a wide range of envisioned applications in the field of nanotechnology. Currently, methods and tools have been developed to control and manipulate as well as to explore new properties of self-assembled structures. However, considerably fewer studies are being devoted to developing efficient methods to degrade or recycle such extremely stable biomaterials. With this in mind, here we suggest a theoretical framework, inspired by the recent developed mid-infrared free-electron laser pulse technology, to dissociate peptide nanotubes. Adopting a diphenylalanine channel as a prototypical example, we find that the primary step in the dissociation process occurs due to the strong resonance between the carboxylate bond vibrations of the diphenylalanine peptides and the tuned laser frequencies. The effects of laser irradiation are determined by a balance between tube formation and dissociation. Our work shows a proof of concept and should provide a motivation for future experimental developments with the final aim to open a new and efficient way to cleave or to recycle bio-inspired materials.

Breakdown of the dipole approximation in strong-field ionization.

We report the breakdown of the electric dipole approximation in the long-wavelength limit in strong-field ionization with linearly polarized few-cycle mid-infrared laser pulses at intensities on the order of 10¹³ W/cm². Photoelectron momentum distributions were recorded by velocity map imaging and projected onto the beam propagation axis. We observe an increasing shift of the peak of this projection opposite to the beam propagation direction with increasing laser intensities. From a comparison with semiclassical simulations, we identify the combined action of the magnetic field of the laser pulse and the Coulomb potential as the origin of our observations.

Theoretical Investigation on THz Generation from Optical Rectification with Tilted-Pulse-Front Excitation

The generation of THz pulses from stoichiometric LiNbO3 (sLN) crystal with tilted-pulse-front excitation is studied. In this scheme, THz absorption by sLN is obtained by numerical interpolation based on the measured data. It shows that THz generation can be enhanced by decreasing the temperature of sLN. The refractive indices of the laser from visible to mid-infrared regime are calculated from the Sellmeier dispersion equation. Then the THz generation law pumped by visible and mid-infrared laser pulses is calculated. It is found that the THz amplitude and energy increase in the near-infrared regime and then tread to a constant value when the wavelength of the laser pulses increases.

CEP-controlled supercontinuum generation during filamentation with mid-infrared laser pulse.

With carrier-envelope phase (CEP) stabilized mid-infrared (MIR) laser pulse, the CEP-controlled supercontinuum generation can be distinctly observed in a very small distance range when the focus of the laser pulse closes to the exit surface of the fused silica (FS). This CEP effect will be gradually weakened and finally disappears if the laser focus moves out of this range. With numerical simulation, we find that although the CEP effect starts from the tunneling ionization of the electron, it can be observed only when the supercontinuum mainly comes from the self-phase modulation (SPM) and self-steepening of the laser pulse and too much electrons will make it ambiguous.

Carrier-envelope phase effects on high-harmonic generation driven by mid-infrared laser field

The influence of the carrier-envelope phase on high-harmonic generation is investigated, both experimentally and theoretically, for three different interaction gas media, driven by mid-infrared, few-cycle and CEP-stabilized laser pulses. Different patterns of harmonic spectra with varying CEP for the three interaction gas media are observed. Furthermore, in comparing our experiment results to the previous works driven by near-infrared laser pulses, different phenomena are found. Through numerical simulation, we find that for the two different kinds of driving fields, i.e. mid-infrared and near-infrared laser pulses, different kinds of electron trajectories contribute to the generation of high harmonics.

Fast tuning of Fano resonance in metal/phase-change materials/metal metamaterials

We propose fast tuning of a Fano resonance (FR) in a three dimensional metamaterial (MM). The MM consists of an elliptical nanohole array (ENA) embedded through a metal/ phase-change material (PCM)/metal multilayer. The results show that the interference between the electric and magnetic resonances can be significantly enhanced when the elliptical nanoholes occupy the sites of a rectangular lattice, thus providing a FR with a higher value quality factor (Q) compared to the ENA with a square lattice. By switching the PCM (Ge2Sb2Te5) between its amorphous and crystalline states, the FR peak can be red-shifted by up to 42%. The FR can be tuned with a ultra low energy mid-infrared laser pulse of 0.38 ns duration and an intensity of 3.2μW/μm2.