Multi-watt, multi-octave, mid-infrared femtosecond source

In vivo efficacy investigation of long-wavelength infrared femtosecond lasers for skin rejuvenation

Ablative lasers such as erbium-doped laser and carbon dioxide laser are currently primary tools for skin rejuvenation and treating dermatological disorders. However, during treatment, as the thermal effect exerts on both target and normal tissues simultaneously, significant effectiveness is often accompanied by a high risk of adverse reactions. To attain an appropriate thermal diffusion and thus favorable therapeutic outcome and fewer side effects, collagen-resonant femtosecond (fs) lasers hold promise as innovative tools for laser cosmetic treatments. In this study, we report, for the first time to the best of our knowledge, an in vivo experiment of fs laser resurfacing with collagen-resonant wavelengths of 6.1 and 7.5 μm, via an optical parametric amplifier. Our results demonstrate that long-wavelength infrared (LWIR) lasers effectively enhance the components of the dermal matrix without causing dermal ablation. The structure of collagen fiber is significantly improved with a substantial amount of new collagen formation. The increased expression of various collagen types in immunofluorescence image further demonstrates the efficacy of the LWIR fs laser in skin rejuvenation. In addition, improvement in the epidermis is more pronounced at a wavelength of 6.1 μm, with a more suitable depth of action. We anticipate that LWIR fs laser could become widely applicable in clinical settings for skin regeneration and rejuvenation.

Spectral broadening of µJ-level pulses around 8 µm in a Germanium-based multi-pass scheme

A 2.6-fold spectral broadening of mid-infrared femtosecond µJ-level pulses has been achieved using an unfolded multi-pass configuration of germanium plates and zinc selenide lenses. This method maintains a throughput higher than 60% while preserving the spatial quality and the temporal duration of the input beam. Numerical simulations match the experimental results and show the potential to tailor the parameters of the cell to obtain different spectra.

Broadband spectral tuning and multi-molecular detection in a BaGa4Se7 optical parametric amplifier

Broadband spectral tuning of long-wavelength infrared (LWIR) femtosecond lasers without rotating nonlinear crystals has special usefulness in applications of nonlinear integrated photonics and microscopic ultrafast dynamics studies with stringent requirements on beam pointing. Here, we demonstrate, for the first time to the best of our knowledge, a temperature-tuning LWIR femtosecond optical parametric amplifier (OPA), based on a BaGa4Se7 (BGSe) crystal. Broadband spectral tunability from 8.4 to 17.1 µm over a crystal temperature range of 20-140°C at three fixed phase-matching (PM) angles is achieved with mini-watts output power. As a proof of concept, multiple trace gas detections are demonstrated on sulfur hexafluoride, ethane, and acetylene through only temperature variation. Our results validate the feasibility of achieving an ultra-broadband LWIR spectral tuning through temperature variation in a BGSe OPA, which is beneficial for unique applications such as on-chip spectroscopy and microscopic pump-and-probe experiments.

Sub-attosecond-precision optical-waveform stability measurements using electro-optic sampling

The generation of laser pulses with controlled optical waveforms, and their measurement, lie at the heart of both time-domain and frequency-domain precision metrology. Here, we obtain mid-infrared waves via intra-pulse difference-frequency generation (IPDFG) driven by 16-femtosecond near-infrared pulses, and characterise the jitter of sub-cycle fractions of these waves relative to the gate pulses using electro-optic sampling (EOS). We demonstrate sub-attosecond temporal jitter at individual zero-crossings and sub-0.1%-level relative amplitude fluctuations in the 10-kHz-0.625-MHz band. Chirping the nearly-octave-spanning mid-infrared pulses uncovers wavelength-dependent attosecond-scale waveform jitter. Our study validates EOS as a broadband (both in the radio-frequency and the optical domains), highly sensitive measurement technique for the jitter dynamics of optical waveforms. This sensitivity reveals outstanding stability of the waveforms obtained via IPDFG and EOS, directly benefiting precision measurements including linear and nonlinear (infrared) field-resolved spectroscopy. Furthermore, these results form the basis toward EOS-based active waveform stabilisation and sub-attosecond multi-oscillator synchronisation/delay tracking.

UV 30 fs laser pulse generation using a multi-pass cell

Ultrashort ultraviolet (UV) pulses are pivotal for resolving ultrafast electron dynamics. However, their efficient generation is strongly impeded by material dispersion and two-photon absorption, in particular, if pulse durations around a few tens of femtoseconds or below are targeted. Here, we present a new (to our knowledge) approach to ultrashort UV pulse generation: using the fourth-harmonic generation output of a commercial ytterbium laser system delivering 220 fs UV pulses, we implement a multi-pass cell (MPC) providing 5.6 µJ pulses at 256 nm, compressed to 30.5 fs. Our results set a short-wavelength record for MPC post-compression while offering attractive options to navigate the trade-off between upconversion efficiency and acceptance bandwidth for UV pulse production.

Extraordinary Enhancement of Nonlinear Optical Interaction in NbOBr2 Microcavities

2D materials are burgeoning as promising candidates for investigating nonlinear optical effects due to high nonlinear susceptibilities, broadband optical response, and tunable nonlinearity. However, most 2D materials suffer from poor nonlinear conversion efficiencies, resulting from reduced light-matter interactions and lack of phase matching at atomic thicknesses. Herein, a new 2D nonlinear material, niobium oxide dibromide (NbOBr2) is reported, featuring strong and anisotropic optical nonlinearities with scalable nonlinear intensity. Furthermore, Fabry-Pérot (F-P) microcavities are constructed by coupling NbOBr2 with air holes in silicon. Remarkable enhancement factors of ≈630 times in second harmonic generation (SHG) and 210 times in third harmonic generation (THG) are achieved on cavity at the resonance wavelength of 1500 nm. Notably, the cavity enhancement effect exhibits strong anisotropic feature tunable with pump wavelength, owing to the robust optical birefringence of NbOBr2. The ratio of the enhancement factor along the b- and c-axis of NbOBr2 reaches 2.43 and 5.27 for SHG and THG at 1500 nm pump, respectively, which leads to an extraordinarily high SHG anisotropic ratio of 17.82 and a 10° rotation of THG polarization. The research presents a feasible and practical strategy for developing high-efficiency and low-power-pumped on-chip nonlinear optical devices with tunable anisotropy.

High-efficiency, single-stage tunable optical parametric amplifier for visible photocathode applications

We present a single-stage optical parametric amplifier (OPA) with an average conversion efficiency up to 38%, tunable between 1.01 and 1.18 µm. The OPA seed is produced by a gain-managed nonlinear fiber amplifier. Numerical modeling of the seed pulse generation shows a linear chirp, a smoothly broadened redshifted spectrum, and a high spectral energy density. When up-converted to the visible through second-harmonic generation, the signal pulses are suitable for visible photocathode excitation.

Nonlinear compression of naturally down-chirped superradiance pulses from a free-electron laser oscillator by thick germanium plates

Naturally down-chirped superradiance pulses, with mirco-pulse energy, peak wavelength, and micropulse duration of 40 µJ, 8.7 μm, and 5.1 optical cycles, respectively, emitted from a free-electron laser (FEL) oscillator were nonlinearly compressed down to 3.7 optical cycles using a 30-mm-thick Ge plate. The peak power enhancement owing to nonlinear compression was found to be 40%. The achieved peak power and pulse duration were comparable to those of recently developed high-intensity and few-cycle long-wavelength infrared sources based on solid-state lasers. FEL oscillators operating in the superradiance regime can serve as unique tools for studying strong-field physics in long-wavelength infrared regions.

Highly efficient octave-spanning long-wavelength infrared generation with a 74% quantum efficiency in a χ(2) waveguide

The realization of compact and efficient broadband mid-infrared (MIR) lasers has enormous impacts in promoting MIR spectroscopy for various important applications. A number of well-designed waveguide platforms have been demonstrated for MIR supercontinuum and frequency comb generations based on cubic nonlinearities, but unfortunately third-order nonlinear response is inherently weak. Here, we propose and demonstrate for the first time a χ(2) micrometer waveguide platform based on birefringence phase matching for long-wavelength infrared (LWIR) laser generation with a high quantum efficiency. In a ZnGeP2-based waveguide platform, an octave-spanning spectrum covering 5-11 μm is generated through optical parametric generation (OPG). A quantum conversion efficiency of 74% as a new record in LWIR single-pass parametric processes is achieved. The threshold energy is measured as ~616 pJ, reduced by more than 1-order of magnitude as compared to those of MIR OPGs in bulk media. Our prototype micro-waveguide platform could be extended to other χ(2) birefringence crystals and trigger new frontiers of MIR integrated nonlinear photonics.

Sub-two-cycle gigawatt-peak-power LWIR OPA for ultrafast nonlinear spectroscopy of condensed state materials

The application of high-power, few-cycle, long-wave infrared (LWIR, 8-20 µm) pulses in strong-field physics is largely unexplored due to the lack of suitable sources. However, the generation of intense pulses with >6 µm wavelength range is becoming increasingly feasible with the recent advances in high-power ultrashort lasers in the middle-infrared range that can serve as a pump for optical parametric amplifiers (OPA). Here we experimentally demonstrate the feasibility of this approach by building an OPA pumped at 2.4 µm that generates 93 µJ pulses at 9.5 µm, 1 kHz repetition rate with sub-two-cycle pulse duration, 1.6 GW peak power, and excellent beam quality. The results open a wide range of applications in attosecond physics (especially for studies of condensed phase samples), remote sensing, and biophotonics.

Measurement of nonlinear refractive indices of bulk and liquid crystals by nonlinear chirped interferometry

The nonlinear refractive indices (n₂) of a selection of bulk (LiB₃O₅, KTiOAsO₄, MgO:LiNbO₃, LiGaS₂, ZnSe) and liquid (E7, MLC2132) crystals are measured at 1030 nm in the sub-picosecond regime (200 fs) by nonlinear chirped interferometry. The reported values provide key parameters for the design of near- to mid-infrared parametric sources, as well as all-optical delay lines.

Sub-Hz relative linewidths from an interferometrically stabilized mid-infrared frequency comb

Frequency combs present a unique tool for high-precision and rapid molecular spectroscopy. Difference frequency generation (DFG) of near-infrared sources is a common approach to generate passively stabilized mid-infrared combs. However, only little attention has been paid so far to precisely measure the coherence properties of such sources. Here, we investigate these using a Raman-soliton based DFG source driven by an Yb:fiber frequency comb. A heterodyne beat between the second harmonic of the phase-locked DFG comb near 4 µm and a 2 µm Tm:fiber frequency comb locked to the same optical reference is performed. Using this method, we measure the relative phase noise power spectral density of both combs. This results in a sub-Hz relative linewidth between the DFG comb and the Tm:fiber comb. We also introduce a new pump/seed delay locking mechanism based on interferometry for long-term stable intensity noise suppression.

Intensity noise in difference frequency generation-based tunable femtosecond MIR sources

We characterize the intensity noise of two mid-infrared (MIR) ultrafast tunable (3.5-11 μm) sources based on difference frequency generation (DFG). While both sources are pumped by a high repetition rate Yb-doped amplifier delivering 200 μJ 300 fs at a central wavelength of 1030 nm, the first is based on intrapulse DFG (intraDFG), and the second on DFG at the output of an optical parametric amplifier (OPA). The noise properties are assessed through measurement of the relative intensity noise (RIN) power spectral density and pulse-to-pulse stability. The noise transfer mechanisms from the pump to the MIR beam is empirically demonstrated. As an example, improving the pump laser noise performance allows reduction of the integrated RIN (IRIN) of one of the MIR source from 2.7% RMS down to 0.4% RMS. The intensity noise is also measured at various stages and in several wavelength ranges in both laser system architectures, allowing us to identify the physical origin of their variation. This study presents numerical values for the pulse to pulse stability, and analyze the frequency content of the RINs of particular importance for the design of low-noise high repetition rate tunable MIR sources and future high performance time-resolved molecular spectroscopy experiments.

Few-cycle pulse generation by double-stage hybrid multi-pass multi-plate nonlinear pulse compression

Few-cycle pulses present an essential tool to track ultrafast dynamics in matter and drive strong field effects. To address photon-hungry applications, high average power lasers are used which, however, cannot directly provide sub-100-fs pulse durations. Post-compression of laser pulses by spectral broadening and dispersion compensation is the most efficient method to overcome this limitation. We present a notably compact setup which turns a 0.1-GW peak power, picosecond burst-mode laser into a 2.9-GW peak power, 8.2-fs source. The 120-fold pulse duration shortening is accomplished in a two-stage hybrid multi-pass, multi-plate compression setup. To our knowledge, neither shorter pulses nor higher peak powers have been reported to-date from bulk multi-pass cells alone, manifesting the power of the hybrid approach. It puts, for instance, compact, cost-efficient, and high repetition rate attosecond sources within reach.

Ultrastrong Optical Harmonic Generations in Layered Platinum Disulfide in the Mid-Infrared

Nonlinear optical activities (e.g., harmonic generations) in two-dimensional (2D) layered materials have attracted much attention due to the great promise in diverse optoelectronic applications such as nonlinear optical modulators, nonreciprocal optical device, and nonlinear optical imaging. Exploration of nonlinear optical response (e.g., frequency conversion) in the infrared, especially the mid-infrared (MIR) region, is highly desirable for ultrafast MIR laser applications ranging from tunable MIR coherent sources, MIR supercontinuum generation, and MIR frequency-comb-based spectroscopy to high harmonic generation. However, nonlinear optical effects in 2D layered materials under MIR pump are rarely reported, mainly due to the lack of suitable 2D layered materials. Van der Waals layered platinum disulfide (PtS₂) with a sizable bandgap from the visible to the infrared region is a promising candidate for realizing MIR nonlinear optical devices. In this work, we investigate the nonlinear optical properties including third-and fifth-harmonic generation (THG and FHG) in thin layered PtS₂ under infrared pump (1550-2510 nm). Strikingly, the ultrastrong third-order nonlinear susceptibility χ⁽³⁾(-3ω;ω,ω,ω) of thin layered PtS₂ in the MIR region was estimated to be over 10^-18 m²/V², which is about one order of that in traditional transition metal chalcogenides. Such excellent performance makes air-stable PtS₂ a potential candidate for developing next-generation MIR nonlinear photonic devices.

FLASH free electron laser pump-probe laser concept based on spectral broadening of high-power ytterbium picosecond systems in multi-pass cells

Within the FLASH2020+ upgrade, the pump-probe laser capabilities of the extreme ultraviolet and soft x-ray free-electron laser (XFEL) FLASH in Hamburg will be extended. In particular, providing wavelength tunability, shorter pulse durations, and reduced arrival time jitter will increase the scientific opportunities and the time resolution for the XFEL-optical laser pump-probe experiments. We present here a novel concept for the pump-probe laser at FLASH that is based on the post-compression of picosecond pulses emitted from high-power Ytterbium:YAG slab amplifiers. Flexible reduction of the pulse duration is facilitated by spectral broadening in pressure-tunable multi-pass cells. As an application, we show the pumping of a commercial optical parametric amplifier with 150 fs post-compressed pulses. By means of an additional difference frequency generation stage, tunable spectral coverage from 1.3 to 16 μm is reached with multi-μJ, sub-150 fs pulses. Finally, a modular reconfiguration approach to the optical setups close to the free-electron laser instruments is implemented. This enables fast installation of the nonlinear frequency converters at the end stations for user operation and flexibility between different instruments in the two experimental halls.

Random quasi-phase-matching for pulse characterization from the near to the long wavelength infrared

Experiments requiring ultrafast laser pulses require a full characterization of the electric field to glean meaning from the experimental data. Such characterization typically requires a separate parametric optical process. As the central wavelength range of new sources continues to increase so too does the need for nonlinear crystals suited for characterizing these wavelengths. Here we report on the use of poly-crystalline zinc selenide as a universal nonlinear crystal in the frequency resolved optical gating characterization technique from the near to long-wavelength infrared. Due to its property of random quasi-phase-matching it's capable of phase matching second-harmonic and sum-frequency generation of ultra-broadband pulses in the near and long wavelength infrared, while being crystal orientation independent. With the majority of ultra-fast laser sources being in this span of wavelengths, this work demonstrates a greatly simplified approach towards ultra-fast pulse characterization spanning from the near to the long-wavelength infrared. To our knowledge there is no single optical technique capable of such flexible capabilities.

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.

Simultaneously Wavelength- and Temperature-Insensitive Mid-Infrared Optical Parametric Amplification with LiGaS2 Crystal

Ultrafast mid-infrared (mid-IR) lasers with a high pulse repetition rate are in great demand in various fields, including attosecond science and strong-field physics. Due to the lack of suitable mid-IR laser gain medium, optical parametric amplifiers (OPAs) are used to generate an ultrafast mid-IR laser. However, the efficiency of OPA is sensitive to phase mismatches induced by wavelength and temperature deviations from the preset points, which thus limits the pulse duration and the average power of the mid-IR laser. Here, we exploited a noncollinear phase-matching configuration to achieve simultaneously wavelength- and temperature-insensitive mid-IR OPA with a LiGaS2 crystal. The noncollinearity can cancel the first-order dependence of phase matching on both wavelength and temperature. Benefitting from the thermal property of the LiGaS2 crystal, some collinear phase-matching solutions derived from the first-order and even third-order wavelength insensitivity have comparatively large temperature bandwidths and can be regarded as approximate solutions with simultaneous wavelength and temperature insensitivity. These simultaneously wavelength- and temperature-insensitive phase-matching designs are verified through numerical simulations in order to generate few-cycle, high-power mid-IR pulses.

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.

Fourier transform spectrometer based on high-repetition-rate mid-infrared supercontinuum sources for trace gas detection

We present a fast-scanning Fourier transform spectrometer (FTS) in combination with high-repetition-rate mid-infrared supercontinuum sources, covering a wavelength range of 2-10.5 µm. We demonstrate the performance of the spectrometer for trace gas detection and compare various detection methods: baseband detection with a single photodetector, baseband balanced detection, and synchronous demodulation at the repetition rate of the supercontinuum source. The FTS uses off-the-shelf optical components and provides a minimum spectral resolution of 750 MHz. It achieves a noise equivalent absorption sensitivity of ∼10^-6 cm^-1 Hz^-1/2 per spectral element, by using a 31.2 m multipass absorption cell.

Broadband Infrared Spectroscopy of Molecules in Solutions with Two Intrapulse Difference-Frequency-Generated Mid-Infrared Frequency Combs

Mid-infrared (mid-IR) spectroscopy is an incisive tool for studying structures and dynamics of complicated molecules in condensed phases. Developing a compact and broadband mid-IR spectrometer has thus been a long-standing challenge. Here, we show that a highly coherent and broadband mid-IR frequency comb can be generated by using an intrapulse difference-frequency-generation with a train of pulses from a few-cycle pulse Ti:sapphire oscillator. By tightly focusing the oscillator output beam into a single-pass, fan-out-type periodically poled lithium niobate crystal and tilting the orientation of the crystal, we show that a mid-IR frequency comb with more than an octave spectral bandwidth from 1550 cm^-1 (46 THz) to 3650 cm^-1 (110 THz) and vanishing carrier-envelope-offset phase can be generated. Using two coherent mid-IR frequency combs with different repetition frequencies, we demonstrate that a broadband mid-IR dual-frequency comb spectroscopy of aromatic compounds or amino acids in solutions is feasible. We thus anticipate that researchers will find our mid-IR frequency combs useful for developing ultrafast and broadband linear and nonlinear IR spectroscopy of chemically reactive or biologically important molecules in condensed phases.

Infrared Comb Spectroscopy of Buffer-Gas-Cooled Molecules: Toward Absolute Frequency Metrology of Cold Acetylene

We review the recent developments in precision ro-vibrational spectroscopy of buffer-gas-cooled neutral molecules, obtained using infrared frequency combs either as direct probe sources or as ultra-accurate optical rulers. In particular, we show how coherent broadband spectroscopy of complex molecules especially benefits from drastic simplification of the spectra brought about by cooling of internal temperatures. Moreover, cooling the translational motion allows longer light-molecule interaction times and hence reduced transit-time broadening effects, crucial for high-precision spectroscopy on simple molecules. In this respect, we report on the progress of absolute frequency metrology experiments with buffer-gas-cooled molecules, focusing on the advanced technologies that led to record measurements with acetylene. Finally, we briefly discuss the prospects for further improving the ultimate accuracy of the spectroscopic frequency measurement.

Few-cycle pulses tunable from 3 to 7 µm via intrapulse difference-frequency generation in oxide LGN crystals

An ultrashort mid-infrared (IR) source beyond 5 µm is crucial for a plethora of existing and emerging applications in spectroscopy, medical diagnostics, and high-field physics. Nonlinear generation of such sources from well-developed near-IR lasers, however, remains a challenge due to the limitation of mid-IR crystals. Based on oxide La₃Ga_5.5Nb_0.5O₁₄ (LGN) crystals, here we report the generation of femtosecond pulses tunable from 3 to 7 µm by intrapulse difference-frequency generation of 7.5 fs, 800 nm pulses. The efficiency and bandwidth dependences on pump polarization and crystal length are studied for both Type-I and Type-II phase-matching configurations. Maximum pulse energy of ∼10nJ is generated at 5.2 µm with a conversion efficiency of ∼0.14%. Because of the few-cycle pump pulse duration, the generated mid-IR pulses are as short as about three cycles. These results, to the best of our knowledge, represent the first experimental demonstration of LGN in generating mid-IR ultrashort pulses.

Mid-infrared frequency combs at 10 GHz

We demonstrate mid-infrared (MIR) frequency combs at 10 GHz repetition rate via intra-pulse difference-frequency generation (DFG) in quasi-phase-matched nonlinear media. Few-cycle pump pulses (≲15fs, 100 pJ) from a near-infrared electro-optic frequency comb are provided via nonlinear soliton-like compression in photonic-chip silicon-nitride waveguides. Subsequent intra-pulse DFG in periodically poled lithium niobate waveguides yields MIR frequency combs in the 3.1-4.8 µm region, while orientation-patterned gallium phosphide provides coverage across 7-11 µm. Cascaded second-order nonlinearities simultaneously provide access to the carrier-envelope-offset frequency of the pump source via in-line f-2f nonlinear interferometry. The high-repetition rate MIR frequency combs introduced here can be used for condensed phase spectroscopy and applications such as laser heterodyne radiometry.

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.

Widely tunable cavity-enhanced frequency combs

We describe the cavity enhancement of frequency combs over a wide tuning range of 450-700 nm (${ \gt }7900\;{{\rm cm}^{ - 1}} $>7900cm^-1), covering nearly the entire visible spectrum. Tunable visible frequency combs from a synchronously pumped optical parametric oscillator are coupled into a four-mirror, dispersion-managed cavity with a finesse of 600-1400. An intracavity absorption path length enhancement greater than 190 is obtained over the entire tuning range, while preserving intracavity spectral bandwidths capable of supporting sub-200 fs pulse durations. These tunable cavity-enhanced frequency combs can find many applications in nonlinear optics and spectroscopy.

Mid-infrared frequency comb with 6.7 W average power based on difference frequency generation

We report on the development of a high-power mid-infrared frequency comb with 100 MHz repetition rate and 100 fs pulse duration. Difference frequency generation is realized between two branches derived from an Er:fiber comb, amplified separately in Yb:fiber and Er:fiber amplifiers. Average powers of 6.7 W and 14.9 W are generated in the 2.9 µm idler and 1.6 µm signal, respectively. With high average power, excellent beam quality, and passive carrier-envelope phase stabilization, this light source is a promising platform for generating broadband frequency combs in the far infrared, visible, and deep ultraviolet.

Characterization of e-beam evaporated Ge, YbF3, ZnS, and LaF3 thin films for laser-oriented coatings

Thin films of Ge, ZnS, YbF₃, and LaF₃ produced using e-beam evaporation on ZnSe and Ge substrates were characterized in the range of 0.4-12 µm. It was found that the Sellmeier model provides the best fit for refractive indices of ZnSe substrate, ZnS, and LaF₃ films; the Cauchy model provides the best fit for YbF₃ film. Optical constants of Ge substrate and Ge film as well as extinction coefficients of ZnS, YbF₃, LaF₃, and ZnSe substrate are presented in the frame of a non-parametric model. For the extinction coefficient of ZnS, the exponential model is applicable. Stresses in Ge, ZnS, YbF₃, and LaF₃ were estimated equal to (-50)MPa, (-400)MPa, 140 MPa, and 380 MPa, respectively. The surface roughness does not exceed 5 nm for all films and substrates.

Field-resolved infrared spectroscopy of biological systems

The proper functioning of living systems and physiological phenotypes depends on molecular composition. Yet simultaneous quantitative detection of a wide variety of molecules remains a challenge^1-8. Here we show how broadband optical coherence opens up opportunities for fingerprinting complex molecular ensembles in their natural environment. Vibrationally excited molecules emit a coherent electric field following few-cycle infrared laser excitation^9-12, and this field is specific to the sample's molecular composition. Employing electro-optic sampling^10,12-15, we directly measure this global molecular fingerprint down to field strengths 10⁷ times weaker than that of the excitation. This enables transillumination of intact living systems with thicknesses of the order of 0.1 millimetres, permitting broadband infrared spectroscopic probing of human cells and plant leaves. In a proof-of-concept analysis of human blood serum, temporal isolation of the infrared electric-field fingerprint from its excitation along with its sampling with attosecond timing precision results in detection sensitivity of submicrograms per millilitre of blood serum and a detectable dynamic range of molecular concentration exceeding 10⁵. This technique promises improved molecular sensitivity and molecular coverage for probing complex, real-world biological and medical settings.

Ultrafast nonlinear refraction measurements of infrared transmitting materials in the mid-wave infrared

We utilize the conventional Z-scan technique to provide absolute measurements of third-order nonlinear refraction coefficients (n₂) in the mid-wave infrared at 2 µm and 3.9 µm of common optical materials that have transparency windows spanning this regime. We study a variety of narrow band gap and wide band gap semiconductors, fluoride crystals (BaF₂, CaF₂, LiF, and MgF₂) and optical glasses, and a series of chalcogenide glasses. The n₂ is found to span on the order of ∼10^-15 to ∼10^-12 cm²/W for the semiconductors, ∼10^-16 cm²/W for the fluoride crystals and glasses, and ∼10^-14 to ∼10^-13 cm²/W for the chalcogenides. The experimental results are compared to previous measurements of n₂ conducted in the visible and near-infrared along with empirical and theoretical formulations.

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.

Unveiling coupled electronic and vibrational motions of chromophores in condensed phases

The quest for capturing molecular movies of functional systems has motivated scientists and engineers for decades. A fundamental understanding of electronic and nuclear motions, two principal components of the molecular Schrödinger equation, has the potential to enable the de novo rational design for targeted functionalities of molecular machines. We discuss the development and application of a relatively new structural dynamics technique, femtosecond stimulated Raman spectroscopy with broadly tunable laser pulses from the UV to near-IR region, in tracking the coupled electronic and vibrational motions of organic chromophores in solution and protein environments. Such light-sensitive moieties hold broad interest and significance in gaining fundamental knowledge about the intramolecular and intermolecular Hamiltonian and developing effective strategies to control macroscopic properties. Inspired by recent experimental and theoretical advances, we focus on the in situ characterization and spectroscopy-guided tuning of photoacidity, excited state proton transfer pathways, emission color, and internal conversion via a conical intersection.

Broadband dispersive Ge/YbF3 mirrors for mid-infrared spectral range

Broadband dispersive mirrors operating in the mid-infrared spectral range of 6.5-11.5 μm are developed for the first time, to the best of our knowledge. The mirrors comprise Ge and YbF₃ layers, which have not been used before for manufacturing of multilayer dispersive optics. The design and production processes are described; mechanical stresses of the coatings are estimated based on experimental data; and spectral and phase properties of the produced mirrors are measured. The mirrors compensate group delay dispersion of ultrashort laser pulses accumulated by propagation through 4 mm ZnSe windows and additional residual phase modulation of an ultrashort laser pulse.

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.

Directly diode-pumped, Kerr-lens mode-locked, few-cycle Cr:ZnSe oscillator

Lasers based on Cr²⁺-doped II-VI material, often known as the Ti:Sapphire of the mid-infrared, can directly provide few-cycle pulses with octave-spanning spectra, and serve as efficient drivers for generating broadband mid-infrared radiation. It is expected that the wider adoption of this technology benefits from more compact and cost-effective embodiments. Here, we report the first directly diode-pumped, Kerr-lens mode-locked Cr²⁺-doped II-VI oscillator pumped by a single InP diode, providing average powers over 500 mW and pulse durations of 45 fs - shorter than six optical cycles at 2.4 µm. These correspond to a sixty-fold increase in peak power compared to the previous diode-pumped record, and are at similar levels with respect to more mature fiber-pumped oscillators. The diode-pumped femtosecond oscillator presented here constitutes a key step toward a more accessible alternative to synchrotron-like infrared radiation and is expected to accelerate research in laser spectroscopy and ultrafast infrared optics.

Octave-spanning single-cycle middle-infrared generation through optical parametric amplification in LiGaS2

We report the generation of extremely broadband and inherently phase-locked mid-infrared pulses covering the 5 to 11 µm region. The concept is based on two stages of optical parametric amplification starting from a 270-fs Yb:KGW laser source. A continuum seeded, second harmonic pumped pre-amplifier in β-BaB₂O₄ (BBO) produces tailored broadband near-infrared pulses that are subsequently mixed with the fundamental pump pulses in LiGaS₂ (LGS) for mid-infrared generation and amplification. The pulse bandwidth and chirp is managed entirely by selected optical filters and bulk material. We find an overall quantum efficiency of 1% and a mid-infrared spectrum smoothly covering 5-11 µm with a pulse energy of 220 nJ at 50 kHz repetition rate. Electro-optic sampling with 12-fs long white-light pulses directly from self-compression in a YAG crystal reveals near-single-cycle mid-infrared pulses (32 fs) with passively stable carrier-envelope phase. Such pulses will be ideal for producing attosecond electron pulses or for advancing molecular fingerprint spectroscopy.

Mid-infrared long-pass filter for high-power applications based on grating diffraction

A gold-coated silicon grating has been developed, enabling efficient spatial separation of broadband mid-infrared (MIR) beams with wavelengths >5  μm from collinearly propagating, broadband, high-power light in the near-infrared (NIR) spectral range (centered at 2 μm). The optic provides spectral filtering at high powers in a thermally robust and chromatic-dispersion-free manner such as that necessary for coherent MIR radiation sources based on parametric frequency downconversion of femtosecond NIR lasers. The suppression of a >20  W average-power, 2 μm driving pulse train by three orders of magnitude, while retaining high reflectivity of the broadband MIR beam, is presented.

Broadband vibrational sum-frequency generation spectrometer at 100 kHz in the 950-1750 cm-1 spectral range utilizing a LiGaS2 optical parametric amplifier

We present a 100 kHz broadband vibrational sum-frequency generation (VSFG) spectrometer operating in the 5.7-10.5 µm (950-1750 cm^-1) wavelength range. The mid-infrared beam of the system is obtained from a collinear, type-I LiGaS₂-crystal-based optical parametric amplifier seeded by a supercontinuum and pumped directly by 180 fs, ~32 µJ, 1.03 µm pulses from an Yb:KGd(WO₄)₂ laser system. Up to 0.5 µJ mid-infrared pulses with durations below 100 fs were obtained after dispersion compensation utilizing bulk materials. We demonstrate the utility of the spectrometer by recording high-resolution, low-noise vibrational spectra of Langmuir-Blodgett supported lipid monolayers on CaF₂. The presented VSFG spectrometer scheme offers superior signal-to-noise ratios and constitutes a high-efficiency, low-cost, easy-to-use alternative to traditional schemes relying on optical parametric amplification followed by difference frequency generation.

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.

3.5-mJ 150-fs Fe:ZnSe hybrid mid-IR femtosecond laser at 4.4 μm for driving extreme nonlinear optics

We report on entering a new era of mid-IR femtosecond lasers based on amplification in a relatively new gain chalcogenide medium, Fe:ZnSe. Our hybrid all-solid-state laser system is based on direct pulse amplification of femtosecond seed from three-stage AGS-based-optical parametric amplification (OPA) in a Fe:ZnSe laser crystal optically pumped by a Cr:Yb:Ho:YSGG Q-switched nanosecond laser. The development of the pump source with output energy up to 90 mJ operating at a 10 Hz repetition rate regime and highly efficient grating compressor (80%) provides 3.5-mJ 150-fs femtosecond pulses centered at 4.4 μm. Diode-pumped Er:YAG/Er:YLF lasers make it possible to increase the beam quality and repetition rate of the proposed laser system up to 100 Hz. Focusing such a laser radiation into the ∼3λ beam diameter allows us to reach a focus laser intensity up to 10¹⁶  W/cm² which is only an order of magnitude lower than a relativistic intensity of 10¹⁷  W/cm² and enough to drive strong nonlinear optics in mid-IR. We show as a proof-of-principle experiment the generation of four-octave spanning (from 350 nm up to 5.5 μm) supercontinuum in xenon.

Mid-infrared feed-forward dual-comb spectroscopy

Mid-infrared high-resolution spectroscopy has proven an invaluable tool for the study of the structure and dynamics of molecules in the gas phase. The advent of frequency combs advances the frontiers of precise molecular spectroscopy. Here we demonstrate, in the important 3-µm spectral region of the fundamental CH stretch in molecules, dual-comb spectroscopy with experimental coherence times between the combs that exceed half an hour. Mid-infrared Fourier transform spectroscopy using two frequency combs with self-calibration of the frequency scale, negligible contribution of the instrumental line shape to the spectral profiles, high signal-to-noise ratio, and broad spectral bandwidth opens up opportunities for precision spectroscopy of small molecules. Highly multiplexed metrology of line shapes may be envisioned.

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.

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.