Efficient middle-infrared generation in LiGaS2 by simultaneous spectral broadening and difference-frequency generation

In-line synthesis of multi-octave phase-stable infrared light

Parametric downconversion driven by modern, high-power sources of 10-fs-scale near-infrared pulses, in particular intrapulse difference-frequency generation (IPDFG), affords combinations of properties desirable for molecular vibrational spectroscopy in the mid-infrared range: broad spectral coverage, high brilliance, and spatial and temporal coherence. Yet, unifying these in a robust and compact radiation source has remained a key challenge. Here, we address this need by employing IPDFG in a multi-crystal in-line geometry, driven by the 100-W-level, 10.6-fs pulses of a 10.6-MHz-repetition-rate, nonlinearly post-compressed Yb:YAG thin-disk oscillator. Polarization tailoring of the driving pulses using a bichromatic waveplate is followed by a sequence of two crystals, LiIO₃ and LiGaS₂, resulting in the simultaneous coverage of the 800-cm^-1-to-3000-cm^-1 spectral range (at -30-dB intensity) with 130 mW of average power. We demonstrate that optical-phase coherence is maintained in this in-line geometry, in theory and experiment, the latter employing ultra-broadband electro-optic sampling. These results pave the way toward coherent spectroscopy schemes like field-resolved and frequency-comb spectroscopy, as well as nonlinear, ultrafast spectroscopy and optical-waveform synthesis across the entire infrared molecular fingerprint region.

Below-threshold harmonic generation in gas-jets for Th-229 nuclear spectroscopy

The generation of below-threshold harmonics in gas-jets constitutes a promising path towards optical frequency combs in the vacuum ultra-violet (VUV) spectral range. Of particular interest is the 150 nm range, which can be exploited to probe the nuclear isomeric transition of the Thorium-229 isotope. Using widely available high-power, high-repetition-rate Ytterbium-based laser sources, VUV frequency combs can be generated through the process of below-threshold harmonic generation, in particular 7^th harmonic generation of 1030 nm. Knowledge about the achievable efficiencies of the harmonic generation process is crucial for the development of suitable VUV sources. In this work, we measure the total output pulse energies and conversion efficiencies of below-threshold harmonics in gas-jets in a phase-mismatched generation scheme using Argon and Krypton as nonlinear media. Using a 220 fs, 1030 nm source, we reach a maximum conversion efficiency of 1.1 × 10^-5 for the 7^th harmonic (147 nm) and 0.78 × 10^-4 for the 5^th harmonic (206 nm). In addition, we characterize the 3^rd harmonic of a 178 fs, 515 nm source with a maximum efficiency of 0.3%.

Inline amplification of mid-infrared intrapulse difference frequency generation

We demonstrate an ultrafast mid-infrared source architecture that implements both intrapulse difference frequency generation (iDFG) and further optical parametric amplification (OPA), in an all-inline configuration. The source is driven by a nonlinearly compressed high-energy Yb-doped-fiber amplifier delivering 7.4 fs pulses at a central wavelength of 1030 nm, at a repetition rate of 250 kHz. It delivers 1 µJ, 73 fs pulses at a central wavelength of 8 µm, tunable over more than one octave. By enrolling all the pump photons in the iDFG process and recycling the long wavelength pump photons amplified in the iDFG in the subsequent OPA, we obtain an unprecedented overall optical efficiency of 2%. These performances, combining high energy and repetition rate in a very simple all-inline setup, make this technique ideally suited for a growing number of applications, such as high harmonic generation in solids or two-dimensional infrared spectroscopy experiments.

Generation of 8–20 μm Mid-Infrared Ultrashort Femtosecond Laser Pulses via Difference Frequency Generation

Mid-infrared (MIR) ultrashort laser pulses have a wide range of applications in the fields of environmental monitoring, laser medicine, food quality control, strong-field physics, attosecond science, and some other aspects. Recent years have seen great developments in MIR laser technologies. Traditional solid-state and fiber lasers focus on the research of the short-wavelength MIR region. However, due to the limitation of the gain medium, they still cannot cover the long-wavelength region from 8 to 20 µm. This paper summarizes the developments of 8–20 μm MIR ultrafast laser generation via difference frequency generation (DFG) and reviews related theoretical models. Finally, the feasibility of MIR power scaling by nonlinear-amplification DFG and methods for measuring the power of DFG-based MIR are analyzed from the author’s perspective.

Efficient generation of few-cycle pulses beyond 10 μm from an optical parametric amplifier pumped by a 1-µm laser system

Nonlinear vibrational spectroscopy profits from broadband sources emitting in the molecular fingerprint region. Yet, broadband lasers operating at wavelengths above 7 μm have been lacking, while traditional cascaded parametric frequency down-conversion schemes suffer from exceedingly low conversion efficiencies. Here we present efficient, direct frequency down-conversion of femtosecond 100-kHz, 1.03-μm pulses to the mid-infrared from 7.5 to 13.3 μm in a supercontinuum-seeded, tunable, single-stage optical parametric amplifier based on the wide-bandgap material Cd_0.65Hg_0.35Ga₂S₄. The amplifier delivers near transform-limited, few-cycle pulses with an average power > 30 mW at center wavelengths between 8.8 and 10.6 μm, at conversion efficiencies far surpassing that of optical parametric amplification followed by difference-frequency generation or intrapulse difference-frequency generation. The pulse duration at 10.6 μm is 101 fs corresponding to 2.9 optical cycles with a spectral coverage of 760–1160 cm⁻¹. Cd_xHg_1−xGa₂S₄ is an attractive alternative to LiGaS₂ and BaGa₄S₇ in small-scale, Yb-laser-pumped, few-cycle mid-infrared optical parametric amplifiers and offers a much higher nonlinear figure of merit compared to those materials. Leveraging the inherent spatial variation of composition in Cd_xHg_1−xGa₂S₄, an approach is proposed to give access to a significant fraction of the molecular fingerprint region using a single crystal at a fixed phase matching angle.

Mid-Infrared Few-Cycle Pulse Generation and Amplification

In the past decade, mid-infrared (MIR) few-cycle lasers have attracted remarkable research efforts for their applications in strong-field physics, MIR spectroscopy, and bio-medical research. Here we present a review of MIR few-cycle pulse generation and amplification in the wavelength range spanning from 2 to ~20 μm. In the first section, a brief introduction on the importance of MIR ultrafast lasers and the corresponding methods of MIR few-cycle pulse generation is provided. In the second section, different nonlinear crystals including emerging non-oxide crystals, such as CdSiP2, ZnGeP2, GaSe, LiGaS2, and BaGa4Se7, as well as new periodically poled crystals such as OP-GaAs and OP-GaP are reviewed. Subsequently, in the third section, the various techniques for MIR few-cycle pulse generation and amplification including optical parametric amplification, optical parametric chirped-pulse amplification, and intra-pulse difference-frequency generation with all sorts of designs, pumped by miscellaneous lasers, and with various MIR output specifications in terms of pulse energy, average power, and pulse width are reviewed. In addition, high-energy MIR single-cycle pulses are ideal tools for isolated attosecond pulse generation, electron dynamic investigation, and tunneling ionization harness. Thus, in the fourth section, examples of state-of-the-art work in the field of MIR single-cycle pulse generation are reviewed and discussed. In the last section, prospects for MIR few-cycle lasers in strong-field physics, high-fidelity molecule detection, and cold tissue ablation applications are provided.

Broadband (8.5-13.5 µm) intra-pulse difference frequency generation in a LiGaS2 crystal of a 90 fs 744 nm laser pulse after its continuous redshift in air

Broadband frequency downconversion of a 90 fs 744 nm Ti:sapphire laser pulse into the mid-infrared (IR) was demonstrated via its filamentation-induced self-frequency shift in air and subsequent intra-pulse difference frequency generation in a LiGaS₂ crystal. The filamentation of the laser pulse in air provided its continuous spectral broadening to the Stokes wing with spectral humps separated by ∼1000cm^-1 that was appropriate for the laser pulse difference frequency conversion into the mid-IR. The difference frequency emission spectrum spanned from 8.5 to 13.5 µm at the e^-2 level. The transform limited pulse duration of the mid-IR pulse was 47 fs, which corresponded to a 1.3-cycle laser pulse. Energy conversion efficiency was up to 10^-4 and 5⋅10^-4 without and with chirp compensation, respectively.

Overcoming gas ionization limitations with divided-pulse nonlinear compression

We simulate Kerr and plasma nonlinearities in a hollow-core fiber to show how plasma effects degrade the output pulse. Our simulations predict the plasma effects can be avoided entirely by implementing divided-pulse nonlinear compression. In divided-pulse nonlinear compression, a high-energy pulse is divided into multiple low-energy pulses, which are spectrally broadened in the hollow-core fiber and then recombined into a high-energy, spectrally broadened pulse. With the plasma effects overcome, spectral broadening can be scaled to larger broadening factors and higher pulse energies. We anticipate this method will also be useful to scale spectral broadening in gas-filled multipass cells.

High-energy mid-infrared intrapulse difference-frequency generation with 5.3% conversion efficiency driven at 3 µm

Intrapulse difference-frequency generation (IPDFG) is a relatively simple technique to produce few-cycle mid-infrared (MIR) radiations. The conversion efficiency of IPDFG could be potentially improved by using the long driving wavelength to reduce the quantum defect. In this paper, we report a high-energy MIR IPDFG source with a record-high conversion efficiency of up to 5.3%, driven by 3 µm, 35 fs, 10 kHz pulses. The IPDFG output has a 5 µJ pulse energy and 50 mW average power. It spans over a spectral range from 6 to 13.2 µm. A 68 fs of IPDFG pulse width is measured, corresponding to 2.1 cycles, centered at 9.7 µm. The high-energy, two-cycle IPDFG pulses are used to produce a 3-octave supercontinuum in a KRS-5 crystal, spanning from 2 to 16 µm, with a 2.4 µJ pulse energy and a 24 mW average power.

On the effect of third-order dispersion on phase-matched terahertz generation via interfering chirped pulses

High-energy narrowband terahertz (THz) pulses, relevant for a plethora of applications, can be created from the interference of two chirped-pulse drive lasers. The presence of third order dispersion, an intrinsic feature of many high-energy drive lasers, however, can significantly reduce the optical-to-THz conversion efficiency and have other undesired effects. Here, we present a detailed description of the effect of third-order dispersion (TOD) in the pump pulse on the generation of THz radiation via phase-matching of broadband highly chirped pulse trains. Although the analysis is general, we focus specifically on parameters typical to a Ti:Sapphire chirped-pulse amplification laser system for quasi-phase-matching in periodically-poled lithium niobate (PPLN) in the range of THz frequencies around 0.5 THz. Our analysis provides the tools to optimize the THz generation process for applications requiring high energy and to control it to produce desired THz waveforms in a variety of scenarios.

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.

Efficient generation of broadband MIR radiation by difference–frequency generation in LiGaS2

We report a surprisingly infrared (MIR) pulse generation in simultaneous interplay of intra-pulse self-phase-modulation and dispersion. broadband and efficient middle-LiGaS2 (LGS) by invoking a difference frequency generation,

Efficient middle-infrared generation in LiGaS2by simultaneous spectral broadening and difference-frequency generation: erratum

In this erratum, we correct the abstract and introduction of Opt. Lett.43, 1742 (2018)OPLEDP0146-959210.1364/OL.43.001742 to not include a wrong name for the crystal.