5 μm few-cycle pulses with multi-gigawatt peak power at a 1 kHz repetition rate

Generation and Implementation of Continuum Infrared Pulses for Broadband Detection in 2D IR Spectroscopy

In recent years, new methods of generating continuum mid-infrared pulses through filamentation in gases have been developed for ultrafast time-resolved infrared vibrational spectroscopy. The generated infrared pulses can have thousands of wavenumbers of bandwidth, spanning the entire mid-IR region while retaining pulse length below 100 fs. This technology has had a significant impact on problems involving ultrafast structural dynamics in congested spectra with broad features, such as those found in aqueous solutions and molecules with strong intermolecular interactions. This study describes the recent advances in generating and characterizing these pulses and the practical aspects of implementing these sources for broadband detection in transient absorption and 2D IR spectroscopy.

Bending of Lloyd's mirror to eliminate the period chirp in the fabrication of diffraction gratings

We present a new technique to prevent the detrimental period chirp that appears in optical gratings fabricated by laser interference lithography (LIL). The idea is to bend the Lloyd's mirror in the lithographic setup to eliminate the period chirp already at the step of the grating's exposure. A new mathematical model was developed to describe the required bending geometry of the mirror. It is shown that this geometry can be described by multiple cross-sections of the mirror, each obtained by the solution of an implicit first-order differential equation. The proposed approach is illustrated on the basis of a concrete example. By slightly bending the Lloyd's mirror (by ≈ 3.5 mm of maximum deflection over an area of 142 mm × 215 mm) the period chirp of the exposed grating can be eliminated completely.

Nonlinear compression of few-cycle multi-mJ 5 µm pulses in ZnSe around zero-dispersion

We present a compact nonlinear compression scheme for the generation of millijoule few-cycle pulses beyond 4 µm wavelength. For this purpose 95 fs pulses at 5 µm from a 1 kHz midwave-IR optical parametric chirped pulse amplifier (OPCPA) are spectrally broadened due to a self-phase modulation in ZnSe. The subsequent compression in a bulk material yields 53 fs pulses with 1.9 mJ energy. The compression succeeds efficiently with only slight beam distortions and an energy throughput of 85%, which results in a peak power of 34 GW. The nonlinear refractive index of ZnSe was derived from the nonlinear compression and self-focusing measurements. Furthermore, we explore to which extent multiphoton absorption affects the nonlinear compression regime.

Pulse shaping in a midwave-IR OPCPA for multi-µJ few-cycle pulse generation at 12 µm via DFG

We report on dispersion management in mid-IR optical parametric chirped pulse amplifiers (OPCPA) aiming for high-energy few-cycle pulses beyond 4 µm. The available pulse shapers in this spectral region limit the feasibility of sufficient higher-order phase control. Intending the generation of high energy pulses at 12 µm via DFG driven by the signal and idler pulses of a midwave-IR OPCPA, we introduce alternative approaches for mid-IR pulse shaping, namely a germanium-prism pair and a sapphire-prism Martinez compressor. Furthermore, we explore the limits of bulk compression in Si and Ge for multi-mJ pulse energies.

Mid-infrared frequency domain optical parametric amplifier

We report on an optical architecture delivering sub-120 femtosecond laser pulses of 20 µJ tunable from 5.5 µm to 13 µm in the mid-infrared range (mid-IR). The system is based on a dual-band frequency domain optical parametric amplifier (FOPA) optically pumped by a Ti:Sapphire laser and amplifying 2 synchronized femtosecond pulses each with a widely tunable wavelength around 1.6 and 1.9 µm respectively. These amplified pulses are then combined in a GaSe crystal to produce the mid-IR few-cycle pulses by means of difference frequency generation (DFG). The architecture provides a passively stabilized carrier-envelope phase (CEP) whose fluctuations has been characterized to 370 mrad RMS.

Mid-infrared quasi-parametric chirped-pulse amplification based on Sm:LGN crystals

We numerically demonstrate highly efficient mid-infrared quasi-parametric chirped-pulse amplification (QPCPA) based on a recently developed Sm³⁺-doped La₃Ga_5.5Nb_0.5O₁₄ (Sm:LGN) crystal. At pump wavelength around 1 µm, the broadband absorption of Sm³⁺ on idler pulses can enable QPCPA for femtosecond signal pulses centered at 3.5 or 5 µm, with a conversion efficiency approaching the quantum limit. Due to suppression of back conversion, such mid-infrared QPCPA exhibits robustness against phase-mismatch and pump-intensity variation. The Sm:LGN-based QPCPA will provide an efficient approach for converting currently well-developed intense laser pulses at 1 µm to mid-infrared ultrashort pulses.

Amplification of mid-IR continuum for broadband 2D IR spectroscopy

We report the generation and characterization of microjoule level, broad bandwidth femtosecond pulses in the mid-infrared (MIR) using optical parametric amplification of continuum MIR seed pulses in GaSe. The signal (3 μm) and idler (6 μm) pulses have energies of 6 μJ and 3 μJ with bandwidths of ∼950 cm^-1 and 650 cm^-1 FWHM and pulse lengths of 34 fs and 80 fs. Broadband 2D IR spectra of O-H and N-H transitions are acquired with the signal beam demonstrating the capabilities of this source for cross peak and line shape measurements.

Nonlinear post-compression in multi-pass cells in the mid-IR region using bulk materials

We numerically investigate the regime of nonlinear pulse compression at mid-IR wavelengths in a multi-pass cell (MPC) containing a dielectric plate. This post-compression setup allows for ionization-free spectral broadening and self-compression while mitigating self-focusing effects. We find that self-compression occurs for a wide range of MPC and pulse parameters and derive scaling rules that enable its optimization. We also reveal the solitonic dynamics of the pulse propagation in the MPC and its limitations and show that spatiotemporal/spectral couplings can be mitigated for appropriately chosen parameters. In addition, we reveal the formation of spectral features akin to quasi-phase matched degenerate four-wave mixing. Finally, we present two case studies of self-compression at 3-μm and 6-μm wavelengths using pulse parameters compatible with driving high-field physics experiments. The simulations presented in this paper set a framework for future experimental work using few-cycle pulses at mid-IR wavelengths.

Interferometric measurements of nonlinear refractive index in the infrared spectral range

This study presents the development and application of interferometric technique for the measurement of nonlinear refractive index of optical materials, while directly accounting for experimentally determined laser pulse shape and beam profile. The method was employed in a systematic study of nonlinear refractive index on a series of common optical materials used in near and mid-IR spectral range, where experimental data on nonlinear material properties is still scarce. The values of nonlinear refractive index were determined at 1.03 µm, 2.2 µm, and 3.2 µm. The measurement results are compared to the values determined by previous studies (where available), and the influence of cascaded second-order nonlinearities is discussed.

Ultra-broadband long-wave-infrared pulse production using a chirped-pulse difference-frequency generation

We present a broadband light source based on near-infrared chirped-pulse difference-frequency mixing that is suitable for seeding long-wave-infrared (LWIR) optical parametric chirped-pulse amplification (OPCPA). A nitrocellulose pellicle is used in a Ti:sapphire regenerative amplifier to generate dual-frequency output pulses, which are subsequently mixed in a 0.4-mm thick AgGaS₂ crystal. LWIR pulses with ∼1 µm full width at half maximum (FWHM) bandwidth centered at 10.5 µm are generated by mixing transform-limited pulses. Assisted by genetic algorithm optimization, the bandwidth is broadened to ∼3 µm FWHM within the 8-12 µm atmospheric transmission window. The seed source paves the path towards tabletop ultrafast terawatt-class passively carrier-envelope-phase stabilized OPCPA in the LWIR region.

Long seed, short pump: converting Yb-doped laser radiation to multi-µJ few-cycle pulses tunable through 2.5-15 µm

We present a setup for generating broadband (up to 1050 cm^-1) and broadly tunable (2.5-15 µm) mid-infrared pulses using an Yb-doped femtosecond laser as the pump source. Our scheme, comprising two parametric amplifiers and a mixing stage, exploits favorable group velocity matching conditions in GaSe pumped at 2 µm to directly produce sub-70 fs pulses throughout the tuning range without any additional dispersion compensation, while 30-50 fs pulse durations are achieved with simple dispersion compensation by propagation through thin bulk media. The generated pulses have sub-1% short- and long-term energy noise, as well as stable spectral parameters, while delivering 0.5-2 W average mid-IR power. We expect the source to be useful for various spectroscopic applications in the mid-IR.

Efficient second harmonic generation of a high-power picosecond CO2 laser

A comparative analysis of AgGaSe₂, GaSe, CdGeAs₂, and Te for second harmonic generation (SHG) of a picosecond CO₂ laser at intensities up to 50 GW/cm² is presented. We demonstrate external energy conversion efficiency of >20% in AgGaSe₂. Conversion efficiency >5% is measured in GaSe and CdGeAs₂. Self-focusing and multifilamentation are found to severely limit the SHG process in CdGeAs₂ and Te at such high fields. Demonstration of ≥150 MW SH pulses for a 10 μm picosecond pump, in combination with femtosecond CO₂ laser development, will open new strong-field applications in the 4.5-5.5 μm range.

Millijoule 265 fs Tm:YAP regenerative amplifier for driving ultrabroad band collinear mid-infrared optical parametric amplifiers

Generation of 265-fs millijoule pulses at 1940 nm from a solid-state regenerative amplifier has been demonstrated. The amplification chain consists of a thulium-doped fluoride (Tm:ZBLAN) fiber oscillator, a two stage Tm:ZBLAN fiber preamplifier, and a regenerative amplifier with a thermoelectrically cooled thulium-doped yttrium aluminium perovskite crystal. The newly developed light source is used for pumping an ultra broadband mid-infrared optical parametric amplifier based on a gallium selenide crystal. The 2.5-4 µm range of a multioctave supercontinuum, generated in a polarization-maintaining ZBALN fiber, is used as the MIR seed. The amplified signal in combination with the corresponding idler pulses spread from 2.5 to 10 µm in a collinear geometry.

SWCNT-SA mode-locked Tm,Ho:LCLNGG laser

Sub-100 fs pulse generation from a passively mode-locked Tm,Ho-codoped cubic multicomponent disordered garnet laser at ∼2 µm is demonstrated. A single-walled carbon nanotube saturable absorber is implemented to initiate and stabilize the soliton mode-locking. The Tm,Ho:LCLNGG (lanthanum calcium lithium niobium gallium garnet) laser generated pulses as short as 63 fs at a central wavelength of 2072.7 nm with an average output power of 63 mW at a pulse repetition rate of ∼102.5 MHz. Higher average output power of 121 mW was obtained at the expense of longer pulse duration (96 fs) at 2067.6 nm using higher output coupling. To the best of our knowledge, this is the first report on mode-locked operation of the Tm,Ho:LCLNGG crystal.

Time-domain spectroscopy of methane excited by resonant high-energy mid-IR pulses

We describe the implementation of nonlinear time-domain spectroscopy of rotovibrational IR-active modes in methane through broadband Four-Wave Mixing driven by resonant high-energy mid infrared laser pulses. At high driving pulse intensities we observe an efficient vibrational ladder climbing triggered in the molecules. This study opens the possibility to impulsively and selectively excite molecules of biological interest to high-lying vibrational states and to characterize their dynamics.

Multi-millijoule, few-cycle 5 µm OPCPA at 1 kHz repetition rate

A table-top midwave-infrared optical parametric chirped pulse amplification (OPCPA) system generates few-cycle pulses with multi-10 GW peak power at a 1 kHz repetition rate. The all-optically synchronized system utilizes ZnGeP₂ nonlinear crystals and a highly stable 2 µm picosecond pump laser based on Ho:YLiF₄. An excellent energy extraction is achieved by reusing the pump pulse after the third parametric power amplification stage, resulting in 3.4 mJ idler pulses at a center wavelength of 4.9 µm. Pulses as short as 89.4 fs are achieved, close to only five optical cycles. Taking into account the pulse energy, a record high peak power of 33 GW for high-energy mid-IR OPCPAs beyond 4 µm wavelength is demonstrated.

Role of wavelength in photocarrier absorption and plasma formation threshold under excitation of dielectrics by high-intensity laser field tunable from visible to mid-IR

The development of high power mid-IR laser applications requires a study on laser induced damage threshold (LIDT) in the mid-IR. In this paper we have measured the wavelength dependence of the plasma formation threshold (PFT) that is a LIDT precursor. In order to interpret the observed trends numerically, a model describing the laser induced electron dynamics, based on multiple rate equations, has been developed. We show both theoretically and experimentally that PFT at mid-IR wavelengths is controlled by a transition from weak- to strong-field regime of free carrier absorption. In the case of MgF[Formula: see text] this transition occurs around 3-4 [Formula: see text]m corresponding to the region of the lowermost PFT. The region of the uppermost PFT is reached around 1 [Formula: see text]m and is governed by an interplay of photoionization and weak-field free carrier absorption which manifests itself in both MgF[Formula: see text] and SiO[Formula: see text]. The PFT observed in considered materials exhibits a universal dependence on the excitation wavelength in dielectrics. Thus, the presented results pave the route towards efficient and controllable laser-induced material modifications and should be of direct interest to laser researchers and application engineers for prevention of laser-induced damage of optical components in high-intensity mid-IR laser systems.

2.05 µm chirped pulse amplification system at a 1 kHz repetition rate-2.4 ps pulses with 17 GW peak power

Ho-doped yttrium lithium fluoride chirped pulse amplification (CPA) is implemented with a high-gain regenerative amplifier (RA) and a two-stage booster amplifier. We demonstrate the generation of 52.5 mJ pulses with a duration of 2.4 ps at a 1 kHz repetition rate. A peak power of 17 GW is achieved for the 2050 nm pulses. The CPA displays a remarkably high stability with a pulse-to-pulse rms as low as 0.23%. The RA operates without any signs of bifurcation and delivers 12 mJ pulses. Seeding the booster amplifier with the RA output scales the pulse energy linearly up into the 50-60 mJ range. The amplifier system is operated at room temperature and shows a high optical-to-optical efficiency of 20.3% with respect to the optical pump power.

Compact Ho:YLF-pumped ZnGeP2-based optical parametric amplifiers tunable in the molecular fingerprint regime

We report on a compact mid-infrared laser architecture, comprising a chain of $ {\rm ZnGeP}_2 $ZnGeP₂-based optical parametric amplifiers (OPAs), which afford a higher energy yield ($ \mathbin{\lower.3ex\hbox{$\buildrel \lt \over{\smash{\scriptstyle\sim}\vphantom{_x}}$}} 60\;\unicode{x00B5} {\rm J} $∼_x<60µJ at 1 kHz) compared to most conventional OPA gain media transparent in the 2-8-µm wavelength range. Specifically, our OPA scheme allows ready tunability in the molecular fingerprint regime and is tailored for strong-field excitation and coherent control of both stretch and bend (or torsional) vibrational modes in molecules. The OPAs are pumped and directly seeded (via supercontinuum generation) by a 2-µm, 3-ps Ho:YLF regenerative amplifier. The compressibility of the OPA output is demonstrated by a representative measurement of the near-Gaussian temporal profile of a dispersion-compensated 105-fs idler pulse at a central wavelength of 5.1 µm, corresponding to ${\sim}6 $∼6 optical cycles. Detailed numerical simulations closely corroborate the experimental measurements, providing a benchmark and a platform to further explore the parameter space for future design, optimization, and implementation of high-energy, ultrafast, mid-infrared laser schemes.

Long-wavelength-infrared laser filamentation in solids in the near-single-cycle regime

We experimentally demonstrate long-wavelength-infrared (LWIR) femtosecond filamentation in solids. Systematic investigations of supercontinuum (SC) generation and self-compression of the LWIR pulses assisted by laser filamentation are performed in bulk KrS-5 and ZnSe, pumped by ${\sim}{145}\;{\rm fs}$∼145fs, 9 µm, 10 µJ pulses from an optical parametric chirped-pulse amplifier operating at 10 kHz of repetition rate. Multi-octave SC spectra are demonstrated in both materials. While forming stable single filament, 1.5 cycle LWIR pulses with 4.5 µJ output pulse energy are produced via soliton-like self-compression in a 5 mm thick KrS-5. The experimental results quantitatively agree well with the numerical simulation based on the unidirectional pulse propagation equation. This work shows the experimental feasibility of high-energy, near-single-cycle LWIR light bullet generation in solids.

Phase-matching-free pulse retrieval based on transient absorption in solids

In this paper, we introduce a pulse characterization technique that is free of phase-matching constraints, exploiting transient absorption in solids as an ultrafast optical switch. Based on a pump-probe setup, this technique uses pump pulses of sufficient intensity to induce the switch, while the pulses to characterize are probing the transmissivity drop of the photoexcited material. This enables the characterization of low-intensity ultra-broadband pulses at the detection limit of the spectrometer and within the transparency range of the solid. For example, by using zinc selenide (ZnSe), pulses with wavelengths from 0.5 to 20 μm can be characterized, denoting five octaves of spectral range. Using ptychography, we retrieve the temporal profiles of both the probe pulse and the switch. To demonstrate this approach, we measure ultrashort pulses from a titanium-sapphire (Ti-Sa) amplifier, which are compressed using a hollow core fiber setup, as well as infrared to mid-infrared pulses generated from an optical parametric amplifier (OPA). The characterized pulses are centered at wavelengths of 0.77, 1.53, 1.75, 4, and 10 μm, down to sub-two optical cycles duration, exceeding an octave of bandwidth, and with energy as low as a few nanojoules.

Table-top high-energy 7 μm OPCPA and 260 mJ Ho:YLF pump laser

We present a state-of-the-art compact high-energy mid-infrared (mid-IR) laser system for TW-level eight-cycle pulses at 7 μm. This system consists of an Er:Tm:Ho:fiber MOPA which serves as the seeder for a ZGP-based optical parametric chirped pulse amplification (OPCPA) chain, in addition to a Ho:YLF amplifier which is Tm:fiber pumped. Featuring all-optical synchronization, the system delivers 260 mJ pump energy at 2052 nm and 16 ps duration at 100 Hz with a stability of 0.8% rms over 20 min. We show that chirp inversion in the OPCPA chain leads to excellent energy extraction and aids in compression of the 7 μm pulses to eight optical cycles (188 fs) in bulk BaF₂ with 93.5% efficiency. Using 21.7 mJ of the available pump energy, we generate 0.75 mJ energy pulses at 7 μm due to increased efficiency with a chirp inversion scheme. The pulse quality of the system's output is shown by generating high harmonics in ZnSe which span up to harmonic order 13 with excellent contrast. The combination of the passive carrier-envelope phase stable mid-IR seed pulses and the high-energy 2052 nm picosecond pulses makes this compact system a key enabling tool for the next generation of studies on extreme photonics, strong field physics, and table-top coherent X-ray science.

Recent advances in ultrafast X-ray sources

Over more than a century, X-rays have transformed our understanding of the fundamental structure of matter and have been an indispensable tool for chemistry, physics, biology, materials science and related fields. Recent advances in ultrafast X-ray sources operating in the femtosecond to attosecond regimes have opened an important new frontier in X-ray science. These advances now enable: (i) sensitive probing of structural dynamics in matter on the fundamental timescales of atomic motion, (ii) element-specific probing of electronic structure and charge dynamics on fundamental timescales of electronic motion, and (iii) powerful new approaches for unravelling the coupling between electronic and atomic structural dynamics that underpin the properties and function of matter. Most notable is the recent realization of X-ray free-electron lasers (XFELs) with numerous new XFEL facilities in operation or under development worldwide. Advances in XFELs are complemented by advances in synchrotron-based and table-top laser-plasma X-ray sources now operating in the femtosecond regime, and laser-based high-order harmonic XUV sources operating in the attosecond regime. This article is part of the theme issue 'Measurement of ultrafast electronic and structural dynamics with X-rays'.

9 μm few-cycle optical parametric chirped-pulse amplifier based on LiGaS2

We report a long-wavelength mid-infrared (mid-IR), few-cycle optical parametric chirped-pulse amplifier (OPCPA) based on LiGaS₂ crystals, pumped by a 1 μm Yb:YAG laser, at a 10 kHz repetition rate. The mid-IR OPCPA system generates pulses centered at 9 μm, with 1 4 μJ pulse energy and 140 mW average power. A 142 fs pulse width, which corresponds to less than 5 optical cycles at 9 μm, is measured by an interferometric autocorrelator. This is, to the best of our knowledge, the first long-wavelength mid-IR OPCPA pumped at 1 μm wavelength. It paves the way for the energy and power scaling of the ultrafast long-wavelength mid-IR lasers by utilizing advanced high-energy, high-power 1 μm pump lasers.

Optical parametric amplification of carrier-envelope phase-stabilized mid-infrared pulses generated by intra-pulse difference frequency generation

We report on a wavelength-tunable optical parametric amplifier (OPA) from 2.7 to 3.8 μm seeded with carrier-envelope phase (CEP) stabilized pulses generated by intra-pulse difference frequency generation (DFG) using a commercial Yb:KGW chirped-pulse amplifier. The Yb:KGW laser's output pulses are spectrally broadened in two-stage multi-plate pulse compression from 0.8 to 1.25 μm, which are compressed down to a sub-two-cycle duration of 6.5 fs using chirp mirrors. CEP-stabilized mid-infrared pulses are produced in intra-pulse DFG of the spectrally broaden pulses around 1.03 μm and parametrically amplified in KTiOAsO ₄ crystals. The output energy and temporal duration of the OPA output pulses range from 31 to 56 μJ and from 102 to 203 fs, respectively. The root mean square value of their CEP errors is measured to be 101 mrad.

Multimicrojoule GaSe-based midinfrared optical parametric amplifier with an ultrabroad idler spectrum covering 4.2-16 μm

We report a multimicrojoule, ultrabroadband midinfrared optical parametric amplifier based on a GaSe nonlinear crystal pumped at ∼2  μm. The generated idler pulse has a flat spectrum spanning from 4.5 to 13.3 μm at -3  dB and 4.2 to 16 μm in the full spectral range, with a central wavelength of 8.8 μm. The proposed scheme supports a subcycle Fourier-transform-limited pulse width. A (2+1)-dimensional numerical simulation is employed to reproduce the obtained idler spectrum. To our best knowledge, this is the broadest -3  dB spectrum ever obtained by optical parametric amplifiers in this spectral region. The idler pulse energy is ∼3.4  μJ with a conversion efficiency of ∼2% from the ∼2  μm pump to the idler pulse.

Millijoule few-cycle 5 μm source at 1 kHz repetition rate for generating broadband pulses from the mid- to far-infrared

We present a novel few-cycle 5 um source delivering 75 fs pulses with 1.2 mJ energy at a 1 kHz repetition rate and its first applications for broadband pulse generation from the mid- to far-infrared.

Watt-scale super-octave mid-infrared intrapulse difference frequency generation

The development of high-power, broadband sources of coherent mid-infrared radiation is currently the subject of intense research that is driven by a substantial number of existing and continuously emerging applications in medical diagnostics, spectroscopy, microscopy, and fundamental science. One of the major, long-standing challenges in improving the performance of these applications has been the construction of compact, broadband mid-infrared radiation sources, which unify the properties of high brightness and spatial and temporal coherence. Due to the lack of such radiation sources, several emerging applications can be addressed only with infrared (IR)-beamlines in large-scale synchrotron facilities, which are limited regarding user access and only partially fulfill these properties. Here, we present a table-top, broadband, coherent mid-infrared light source that provides brightness at an unprecedented level that supersedes that of synchrotrons in the wavelength range between 3.7 and 18 µm by several orders of magnitude. This result is enabled by a high-power, few-cycle Tm-doped fiber laser system, which is employed as a pump at 1.9 µm wavelength for intrapulse difference frequency generation (IPDFG). IPDFG intrinsically ensures the formation of carrier-envelope-phase stable pulses, which provide ideal prerequisites for state-of-the-art spectroscopy and microscopy.

Multi-octave supercontinuum generation in YAG pumped by mid-infrared, multi-picosecond pulses

High-energy, multi-octave supercontinuum (SC) generation in bulk media pumped with picosecond pulses in the mid-infrared, though pivotal in a myriad of applications, poses severe constraints due to wavelength scaling of the critical power criterion and the propensity to induce avalanche-ionization-seeded breakdown mechanisms. Here, we demonstrate a simple experimental geometry, relying on a very low numerical aperture for the pump pulse, and a crystal length commensurate with the Rayleigh length of the focusing geometry, generating a multi-octave, stable SC in yttrium aluminum garnet (YAG). The SC ranges from 500 nm to 3.5 μm (measured at -30  dB with spectral components at wavelengths up to 4.5 μm) when pumped by a 3 ps pulse centered at 2.05 μm in the anomalous dispersion regime. We also investigate the dynamics of filament formation in this interaction regime by monitoring the spectral and temporal evolution of the pulse during its propagation through the length of the crystal.

Generation of 1 kHz, 2.3 mJ, 88 fs, 2.5 μm pulses from a Cr2+:ZnSe chirped pulse amplifier

We demonstrate the generation of 2.3 mJ, 88 fs, 2.5 μm laser pulses at 1 kHz repetition rate from a three-stage chirped pulse amplifier employing Cr²⁺:ZnSe crystals as the active gain media. 5 μJ seed of the amplifier is obtained via intrapulse difference frequency generation in a bismuth triborate (BIBO) crystal from spectrally broadened Ti:Sapphire amplifier output. A multi-pass amplifier followed by two single-pass amplifiers pumped by Q-switched Ho:YAG lasers boost the pulse energy to 6.5 mJ, yielding 2.3 mJ, 88 fs pulses upon pulse compression. Our results show the highest peak power at 2.5 μm with 1 kHz repetition rate. Such a laser will be a powerful source for studying strong-field physics and extending high-harmonic generation towards the keV region.

Intense broadband mid-infrared pulses of 280 MV/cm for supercontinuum generation in gaseous medium

We produce extremely bright mid-infrared (mid-IR) pulses with a tunable wavelength of 7 μm to 15 μm through difference frequency generation. Optimization of beam quality and beam focusing results in an intense mid-IR field spatiotemporally confined in the lambda-cubic volume. A near planar wavefront is achieved through manipulating the wavefront curvature of the pumping pulse in the frequency downconversion process. Coherent mid-IR pulses are produced with the peak field of 280 MV/cm at 10 μm, and its intensity exceeds 100  TW/cm², estimated from measured pulse energy, and spatial and temporal pulse profiles. Interaction of such an intense mid-IR field with Xe and Kr gas forms plasma and generates a supercontinuum in the visible range.

Generation of 70-fs pulses at 2.86 μm from a mid-infrared fiber laser

We propose and demonstrate a simple route to few-optical-cycle pulse generation from a mid-infrared fiber laser through nonlinear compression of pulses from a holmium-doped fiber oscillator using a short length of chalcogenide fiber and a grating pair. Pulses from the oscillator with 265-fs duration at 2.86 μm are spectrally broadened through self-phase modulation in step-index As₂S₃ fiber to 141-nm bandwidth and then re-compressed to 70 fs (7.3 optical cycles). These are the shortest pulses from a mid-infrared fiber system to date, and we note that our system is compact, robust, and uses only commercially available components. The scalability of this approach is also discussed, supported by numerical modeling.