Ultrashort Mid-infrared Pulses Research Articles

Third- and fifth-harmonic generation by mid-infrared ultrashort pulses: beyond the fifth-order nonlinearity

Third- and fifth-harmonic generation by ultrashort laser pulses in the mid-infrared (mid-IR) reveals nonlinear-optical effects beyond the fifth-order nonlinearity and enables, because of an extraordinarily long coherence length, efficient multiplex frequency upconversion of ultrashort mid-IR pulses. We show that harmonic generation by mid-IR pulses provides an access to the key optical constants of gas media, allowing metrology of linear and nonlinear-optical susceptibilities in the mid-IR and offering a tool for the remote sensing of the atmosphere.

Filamentation in air with ultrashort mid-infrared pulses

We theoretically investigate filamentation of ultrashort laser pulses in air in the mid-infrared regime under conditions in which the group-velocity dispersion (GVD) is anomalous. When a high-power, ultra-short mid-infrared laser beam centered at 3.1-μm forms a filament, a spatial solitary wave is stabilized by the plasma formation and propagates several times its diffraction length. Compared with temporal self-compression in gases due to plasma formation and pulse splitting in the normal-GVD regime, the minimum achievable pulse duration (∼70 fs) is limited by the bandwidth of the anomalous-GVD region in air. For the relatively high powers, multiple pulse splitting due to the plasma effect and shock formation is observed, which is similar to that which occurs in solids. Our simulations show that the energy reservoir also plays a critical role for longer propagation of the air filament in the anomalous-GVD regime.

可调谐超短中红外激光脉冲的参变产生与放大及其最新进展

可调谐超短中红外激光脉冲的参变产生与放大及其最新进展

Generation of intense ultrashort midinfrared pulses by laser-plasma interaction in the bubble regime

As an intense laser pulse propagates through an underdense plasma, the strong ponderomotive force pushes away the electrons and produces a trailing plasma bubble. In the meantime the pulse itself undergoes extreme nonlinear evolution that results in strong spectral broadening toward the long-wavelength side. By experiment we demonstrate that this process can be utilized to generate ultrashort midinfrared pulses with an energy three orders of magnitude larger than that produced by crystal-based nonlinear optics. The infrared pulse is encapsulated in the bubble before exiting the plasma, hence is not absorbed by the plasma. The process is analyzed experimentally with laser-plasma tomographic measurements and numerically with three-dimensional particle-in-cell simulation. Good agreement is found between theoretical estimation, numerical simulation, and experimental results.

Optical parametric generation of nearly transform-limited mid-infrared pulses in dispersion-engineered nonlinear waveguides

Optical parametric generation in dispersion-engineered waveguides fabricated by proton exchange in periodically poled lithium niobate is numerically investigated as a means for producing widely tunable mid-infrared ultrashort pulses starting from low energy pump pulses at a 1.55 microm wavelength and with 100 fs duration, which are typical parameters of amplified Er-fiber oscillators. Numerical evidence is given about the generation of sub-100 fs nearly transform-limited pulses in the 2.4-4.4 microm spectral range.

Ultrafast probing of light-matter interaction in a midinfrared quantum cascade laser

In this work, we study the interaction of ultrashort midinfrared pulses with the active medium of an InGaAs∕InAlAs∕InP quantum cascade laser emitting at an 11.7μm wavelength. Applying an electro-optic sampling technique allowed us to measure the complete phase resolved transmission spectra at operating conditions below and above lasing threshold in a spectral range much broader than the gain band width. Far below threshold, we locate broadband resonant absorption, which spectrally overlaps with the electrically induced gain, forming areas of net absorption and net gain. Above threshold, gain clamping is seen, and it is found that echoes delayed by the round trip time experience spectral pulse shaping converging toward the emission spectrum.

Experimental investigation of parametric transfer in synchronously pumped optical parametric oscillators

We experimentally investigate the parameters affecting parametric transfer in a synchronously pumped optical parametric oscillator for indirect shaping of mid-infrared ultrashort pulses and make comparisons with previously reported numerical modeling. The individual effects of the parameters are discussed in detail. We conclude that signal bandwidth narrowing, minimal signal amplification, large pump depletion, cavity length tuning, and minimal pump and idler temporal walk-off are required for high fidelity transfer, which we are able to demonstrate for a strongly chirped pump pulse.

Generation and complete characterization of intense mid-infrared ultrashort pulses

Intense mid-infrared pulses tunable between 5 and 14 µm with pulse energies of several microjoules were generated by difference-frequency mixing (DFM) in a GaSe crystal. Longer wavelengths (up to 18 µm) were achieved by DFM in a CdSe crystal. The infrared pulses were then characterized using various techniques: The spectrum was measured using a Fourier-transform spectrometer, which was then modified to determine the interferometric second-order autocorrelation. The electric field spectral phase was measured using the same setup, thus leading to a full characterization of the mid-infrared pulses. The spectral phase was measured using the time-domain homodyne optical technique for spectral phase interferometry for direct electric field reconstruction, where spectral interferometry was replaced with time-domain interferometry. The measured pulse duration was 100 fs, nearly transform limited.

Mid-infrared electric field characterization using a visible charge-coupled-device-based spectrometer

We characterize ultrashort mid-infrared pulses through upconversion by using the stretched pulses obtained from the uncompressed output of a chirped-pulse amplifier. The power spectrum thus translated into the visible region can be readily measured with a standard silicon CCD camera-based spectrometer. The spectral phase is also characterized by a variant of zero-added-phase spectral phase interferometry for direct electric field reconstruction. This is a general method that provides a multiplex advantage over conventional infrared detector array-based methods.

Two-photon QWIPs for quadratic detection of weak mid-infrared laser pulses

We report on QWIPs optimized for nonlinear detection using a bandstructure design with three energetically equidistant subbands. The double resonant character of the two-photon transition from the ground state into the second excited state results in quadratic dependence of the photocurrent on the incident power down to excitation densities as low as 0.1 W/cm 2. We have used these two-photon QWIPs as quadratic detectors in second-order interferometric autocorrelation measurements allowing for the separation of coherent and incoherent contributions to the photocurrent. We have studied the carrier dynamics and the influence of the QW doping on both phase and energy relaxation times. The measurements also demonstrate that these devices provide an ultra-sensitive tool with sub-ps response time for the characterization of ultrashort (∼ps) mid-infrared pulses in the pJ-regime such as those generated by modelocked quantum cascade lasers.

Time-domain interferometry for direct electric field reconstruction of mid-infrared femtosecond pulses.

Mid-infrared ultrashort pulses of 9.2-microm center wavelength are characterized in both amplitude and phase. This is achieved by use of a variant of spectral phase interferometry for direct electric field reconstruction in which spectral interferometry has been replaced with time-domain interferometry, a technique that is well suited for infrared pulses. The setup permits simultaneous recording of the second-order interferometric autocorrelation, thus providing an independent check on the retrieved spectral phase.

Correlation function-based finite-difference time-domain method for simulating ultrashort pulse propagation. I. Formalism

A finite-difference time-domain formalism for simulating coherent linear pulse propagation is presented that incorporates a medium response described by any two-time energy gap correlation function. Two algorithms, for real and complex correlation functions, are developed to evaluate the electric polarization through explicit treatment of the density matrix for a two-level system. The coherence relaxation terms in the resulting finite-differenced Maxwell–Liouville equations depend on integrals over the energy gap fluctuation correlation function. The algorithms are used to simulate ultrashort mid-infrared pulse propagation through optically dense samples of HDO in liquid D2O as a demonstration of their performance and flexibility. These algorithms represent a first step toward the goal of incorporating complicated material responses into the full-field simulation of nonlinear pulse propagation and nonlinear optical spectroscopy.

Finite-difference time-domain simulation of ultrashort pulse propagation incorporating quantum-mechanical response functions.

A semiclassical implementation of the finite-difference time-domain method is used to simulate coherent linear propagation of ultrashort mid-infrared pulses through optically dense samples of isotropically diluted liquid water. Bloch equations for the density matrix are used as a simple model of the O--H oscillator relaxation, and the algorithm is extended to other response functions. Sensitivity of the field to the form of the response function is demonstrated, and the results are compared with experimentally determined electric fields in the same media [Rev. Sci. Instrum. 73, 2227 (2002)].

Femtosecond Mid-Infrared Pump–Probe Study of Wave Packet Motion in a Medium-Strong Intramolecular Hydrogen Bond

Abstract We inspect the O–H stretching vibration of the medium-strong hydrogen bond in phthalic acid monomethyl ester in nonpolar solution by use of single-colour spectrally integrated femtosecond mid-infrared spectroscopy. We derive amplitude and phase characteristics of the coherent modulations of the pump-probe signals as a function of excitation wavelength. We exclude the role of Fermi resonances in the oscillatory contribution to the pump–probe signals. The observed periodic feature is due to wave packet motion of an underdamped out-of-plane 100 cm-1 mode that modulates the hydrogen bond distance. This low-frequency mode anharmonically coupled to the O–H stretching vibration is impulsively excited by the ultrashort mid-infrared pump pulse.

Dynamical Studies of Zeolitic Protons and Adsorbates by Picosecond Infrared Spectroscopy

The application of (picosecond) nonlinear infrared spectroscopy to investigate zeolite catalysts and adsorbates is reviewed. In these time-resolved experiments, one specific vibration in the zeolite system (i.e., a zeolite or adsorbate vibration) is selectively excited with an ultrashort (tunable) mid-infrared pulse. The effect of this excitation and the subsequent energy relaxation can be monitored real time, providing information on the structure of the bare zeolite and adsorption complexes. More importantly, with this technique the picosecond energy flow at the catalytic site and the dynamics of the catalyst-adsorbate interaction can be investigated: Short-lived transient species (e.g., reaction intermediates) are observed and the picosecond relaxation rates and pathways at the catalytic site render insights into the dynamics of the interaction between the zeolite catalyst and its adsorbates at a molecular level. This illustrates the potential of time-resolved infrared spectroscopy in the investigations of catalytic systems.

High-repetition-rate femtosecond mid-infrared pulse generation

Ultrashort mid-infrared pulses at a repetition rate of 82 MHz have been generated by difference-frequency mixing at the output of a single-cavity, two-color femtosecond Ti:sapphire laser. The center wavelengths of the midinfrared pulses are tunable from 7.5 to 12.5 microm. Their pulse durations range from 450 to 650 fs, as a function of the center wavelength.

Intense subpicosecond pulses tunable between 4 μm and 20 μm generated by an all-solid-state laser system

The generation of widely tunable ultrashort mid-infrared pulses with intensities sufficient for a substantial excitation of infrared states is discussed. Single pulses of 1 ps duration and an energy of 7 mJ from an APM mode-locked Nd:glass laser with regenerative amplifier (repetition rate 4 Hz) serve as pump pulses. A three-path optical parametric generator pumped by the second harmonic of the Nd-laser produces tunable near infrared pulses. Subsequent down conversion in GaSe provides pulses of 0.7 ps duration tunable between 4 μm and 20 μm. Energies up to 25 μJ are measured.

Simulation of the generation of ultrafast midinfrared laser pulses

A numerical simulation of an ultrafast semiconductor switch for 10.6 mu m CO/sub 2/ radiation is presented. It is based on the solution of the Helmholtz electromagnetic wave equation for p-polarized light and the ambipolar diffusion equation. Calculations are performed in various time dependent inhomogeneous plasma profiles. The analysis shows that the temporal reflectivity depends on the initial visible radiation fluence, duration, and diffusion coefficient of the sample. The results support the feasibility of generating midinfrared ultrashort pulses between 100 fs and 65 ps. By proper choice of the laser fluence and the semiconductor material, longer pulses are also possible. >

External cavity operation of a pulsed InSb spin-flip Raman laser at 10.6 μm

The operation of an external cavity CO 2 pumped InSb spin-flip Raman laser is described. The much lower thresholds observed in external cavity operation combined with the considerably enhanced radiation densities produced by the confinement have enabled us to observe third Stokes and second anti-Stokes outputs for the first time. We have also observed spontaneous mode-locking of the Stokes output which may have significance for the generation of ultra-short mid-infrared pulses.