Mid-infrared Pulses Research Articles

Two-octave mid-infrared pulses via chirp-optimized cascaded intra-pulse difference frequency generation

Two-octave mid-infrared pulses via chirp-optimized cascaded intra-pulse difference frequency generation

Flexible delivery of broadband, 100 fs mid-infrared pulses in the water-absorption band using hollow-core photonic crystal fiber

High-quality free-space and over-fiber transmission of mid-infrared light is limited by factors such as material-related absorption, diffraction, light leakage, and nonlinearity. Conventional vacuum apparatus can be utilized for high-quality laser-beam delivery to address these issues; however, the deployment of such apparatus would increase system complexity, which is detrimental to their practical applications. Here, we report the successful use of evacuated hollow-core photonic crystal fiber (PCF) to flexibly transmit ultrafast mid-infrared pulses over several meters, while preserving exceptional spatial, spectral, and temporal fidelities. The PCF was engineered to feature a low-loss transmission band within the water absorption range, and an evacuated 5 m length was used to transmit Watt-level, 100 fs pulses centered at ∼2.8µm. A comparison between free-space transmission and air-filled PCF highlights the superior performance of the evacuated hollow-core PCF, indicating its strong suitability for the flexible delivery of sub-ps laser pulses in the mid-infrared.

Probing amplified Josephson plasmons in YBa2Cu3O6+x by multidimensional spectroscopy

The nonlinear driving of collective modes in quantum materials can lead to a number of striking non-equilibrium functional responses, which merit a comprehensive exploration of underlying dynamics. However, the coherent coupling between nonlinearly-driven modes frequently involves multiple mode coordinates at once, and is often difficult to capture by one-dimensional pump probe spectroscopy. One example is phonon-mediated amplification of Josephson plasmons in YBa2Cu3O6+x, a phenomenon likely associated with the mysterious superconducting-like optical response observed in this material. Here, we report two-dimensional nonlinear spectroscopy measurements in driven YBa2Cu3O6+x. We excite apical oxygen phonons with pairs of mutually-delayed carrier envelope phase stable mid-infrared pump pulses, and detect time-modulated second-order nonlinear optical susceptibility. We find that the driven phonons parametrically amplify coherent pairs of fluctuating opposite-momentum Josephson plasma polaritons, corresponding to a squeezed state of the Josephson plasma.

Electro-optical switching in a nonlinear metasurface

Nonlinear metasurfaces are multifunctional photonic elements that generate and control light, enabling multiple proof-of-principle applications, such as in nonlinear holography, beam shaping, and nanoscale sources of entangled photon pairs. Active tuning of nonlinear metasurfaces will considerably expand their prospects for integrated photonics. Here, we demonstrate a highly-tunable metasurface device that combines electrical and optical switching in a broadband frequency conversion process. Our device employs electrically tunable hybrid graphene-gold plasmons to convert mid-infrared pulses into visible light by optical gating. A strong electrical modulation of the generated visible light is attained for a broad range of mid-infrared input frequencies, and this modulation is up to 3.5× stronger for the metasurface than for plain graphene. All-optical switching measurements indicate that the device can be optically switched at Terahertz rates. Our results may lead to new applications of metasurfaces such as ultrafast signal processing, nonlinear sensing, and optical transduction.

Designing Ho:ZBLAN photonic crystal fiber for high power mid-infrared wavelength conversion at 2.86 µm

We present a theoretical framework for an optical parametric amplifier influencing four-wave mixing (FWM) using a ZBLAN photonic crystal fiber (PCF) to achieve high-power mid-infrared pulse generation at 2.86 µm. In this prolific wavelength conversion scheme, the Stokes wavelength is precisely tuned via a degenerate FWM process by varying the pump wavelength of a continuous wave source and the signal frequency of picosecond pulses within a specific range. The proposed fiber design, optimized with a lattice pitch of 8.95 µm and an air hole diameter of 3 µm, enables efficient Stokes photon generation at 2.86 µm when pumped with 1551 nm light using a 4 cm ZBLAN PCF. The signal source consists of 3 ps pulses at 6 GHz with a wavelength of 1.064 µm. Additionally, the Stokes output power is significantly enhanced using a holmium-doped PCF. Numerical simulations of this frequency conversion approach demonstrate the generation of periodic picosecond pulses at 2.86 µm with a duration of approximately 5 ps and an output power of 9.15 W.

Theoretical research on the effective generation of an isolated attosecond pulse from the synthesized pulse laser with optimized waveform

Abstract A mid-infrared femtosecond pulse laser with a single cycle and high intensity is an ideal driving light source for generating isolated attosecond pulses. Due to current experimental limitations, it is difficult to directly achieve this type of laser light source in the laboratory. In this paper, we obtain such an ideal light source by adding a Ti sapphire pulse to the combined pulse laser consisting of two mid-infrared pulses. Specifically, by combining the synthesized pulse consisting of 8 fs/1200 nm/1.62 × 1014 W cm−2 and 12 fs/1800 nm/2.71 × 1014 W cm−2 with an additional 8 fs/800 nm/1.26 × 1014 W cm−2 Ti sapphire pulse, the resulting electric field waveform is very close to that of a 1170 nm femtosecond pulse with an intensity of 1.4 × 1015 W cm−2, a single-cycle pulse width, and a carrier-envelope phase of 0.25π. Numerical simulations show that both cases produce high-order harmonic emission spectra with broadband supercontinuum spectra, however, the bandwidth of the supercontinuum spectra and the harmonic intensities in the synthesized pulses are significantly better than those in the single 1170 nm pulse. After inverse Fourier transform, we obtain 66 as a high-intensity isolated attosecond pulse, whose intensity is five orders of magnitude higher than that of a monochromatic field. Here, the phase differences between three combined pulse lasers have little effect on the numerical simulation results when they vary in the range of 0.3π.

Wavelength-tunable, mid-infrared ultrafast pulse generation through Raman self-frequency shift in an all-solid fluorotellurite fiber.

We report a mid-infrared (mid-IR) fiber laser system that can deliver ultrafast soliton-like pulses with a wavelength tunable from 2.8 to 4.0 μm. The pump light source of the system is a mid-IR fiber laser mode-locked at 2.8 μm, which has an average output power of ∼500 mW, a repetition rate of ∼30 MHz, a pulse energy of ∼15 nJ, and a pulse duration of ∼150 fs. The pump light was then launched into a short (∼70 cm) sample of fluorotellurite (TeO2-BaF2-Y2O3) glass fiber as the high-nonlinearity waveguide with simultaneously good features of large Raman gain, tailored dispersion, and high-power capability. High-efficiency Raman soliton self-frequency-shift phenomenon can be obtained in this short fiber sample, leading to the generation of broadband tunable (3-4 μm), mid-IR pulses with hundreds-of-fs pulse duration and tens-of-mW average power, corresponding to a pulse energy level of ∼1 nJ. The frequency-conversion efficiency inside the nonlinear fiber was measured to be as high as ∼16%. The present system, combining the advanced techniques of fluorotellurite fiber fabrication and mid-IR ultrafast fiber laser, highlights its application potentials for generating low-noise, high-beam-quality, mid-IR ultrafast pulses with a compact fiber configuration.

Above-mJ optical parametric chirped pulse amplifier at 3 μm for laser-driven coherent soft x-ray generation beyond the water window

Optical parametric chirped pulse amplification (OPCPA) provides an excellent platform to generate ultrashort mid-infrared pulses in the spectral window beyond the scope of traditional mode-locked lasers. This technology has paved the path toward tabletop coherent soft x-ray (SXR) sources in recent years. Commercial availability of high-power Yb:YAG lasers as the pump lasers has enabled OPCPA to generate high-energy femtosecond mid-IR pulses at a high repetition rate. However, it is still difficult to achieve above mJ, high repetition rate OPCPA at 3 μm with less than 100 fs pulsewidth. Here, we present a 10 kHz, few-cycle OPCPA at 3.1 μm generating compressed pulses of 1.1 mJ energy with a record temporal width of 58 fs and an excellent rms stability of 0.8%. Our experimental results are further compared with two different simulation codes for optimization. To increase the amplification efficiency, we utilize a pulse-front tilt matching configuration resulting in 80% more energy in the first power OPCPA stage and expect up to 3 mJ of pulse energy in total with all three power OPCPA stages. These pulses open up the opportunity to access, in particular, the magnetically dichroic L-absorption edges of the 3d metals through the generation of ultrashort SXRs via high harmonic generation beyond the water window (500–900 eV) in a laboratory setup. This provides the prospect of availing femtosecond pump-probe spectroscopy with SXR pulses for studying the electronic structure dynamics of numerous condensed phase systems via resonant transitions from core levels of functionally relevant metals without having to resort to large-scale facilities.

Generation of relativistic few-cycle radially polarized mid-infrared pulse in plasma channel

Generation of relativistic few-cycle radially polarized mid-infrared pulse in plasma channel

Full Brillouin zone, multi-band reconstruction of the electronic structure from high-harmonic spectra

While the all-optical characterization of electronic band structure has been touted as an exciting application of the solid-state high harmonic generation, the practical realization of the idea proves difficult, and neither the full potential nor the limitation of the approach are properly understood. This work demonstrates that a few suitably chosen high-harmonic spectra excited by a single quasi-monochromatic mid-infrared pulse provide sufficient information for a three-dimensional reconstruction of multiple electronic bands extending over the entire Brillouin zone. As a by-product of the surrogate-Hamiltonian approach introduced in this work, individual band-structure components such as transition dipole moments and Berry curvatures can also be obtained.

Megawatt peak-power, single-mode, mid-infrared femtosecond pulse delivery at 5-6 μm via a silica-based anti-resonant hollow core fiber.

We demonstrate the first, to our knowledge, delivery of megawatt peak power, single-mode mid-infrared (MIR) femtosecond pulses at 5-6 μm using a silica-based anti-resonant hollow core fiber (AR-HCF). Benefiting from the light confinement inside the hollow core, the AR-HCF exhibits high damage thresholds, reliable power stability, efficient spatial beam self-cleaning, and pulse shape preservation. Pumped by a homemade LGS-based two-stage optical parametric amplifier generating high-power ∼200 fs pulses, the fiber achieves a maximum delivered peak power of 4 MW at 5.1 μm and 5 MW at 6.1 μm, with peak intensities reaching 100 GW/cm2, despite fiber losses exceeding 2 dB/m. This flexible, meter-scale delivery system demonstrates exceptional potential for addressing the challenges of high peak power MIR laser delivery in precise, minimally invasive interventional ablation, particularly at resonant peaks such as amide-I (6.1 μm) and cholesterol esters (5.75 μm).

Mid-infrared frequency combs and pulse generation based on single section interband cascade lasers

Interband cascade lasers (ICLs) are semiconductor lasers emitting in the mid-wave infrared (MWIR 3–6 μm) and can operate as frequency combs (FCs). These demonstrations are based on double section cavities that can reduce dispersion and/or are adapted for radio frequency operation. Here, we show that ICL FCs at long wavelengths, where the refractive index dispersion reduces, can be realized in a single long section cavity. We show FC generation for ICLs operating at λ ∼ 4.2 μm, demonstrating narrow electrical beatnotes over a large current range. We also reconstruct the ultrafast temporal response through a modified shifted wave interference Fourier transform spectroscopy setup with two fast MWIR detectors, which shows a frequency modulated response in free running operation. Further, we show that, through active mode-locking, the ICL can be forced to generate short pulses on the order of 3 ps. This temporal response is in agreement with Maxwell–Bloch simulations, highlighting that these devices possess long dynamics (∼100 ps) and potentially makes them appropriate for the generation of large peak powers in the MWIR.

Attosecond metrology of vacuum-ultraviolet high-order harmonics generated in semiconductors via laser-dressed photoionization of alkali metals

Semiconductor crystals driven by strong mid-infrared pulses offer advantages for studying many-body physics and ultrafast optoelectronics via high-harmonic generation. While the process has been used to study solids in the presence strong mid-infrared fields, its potential as an attosecond light source is largely underexplored. We demonstrate that high-harmonics emitted from zinc-oxide crystals produce attosecond pulses, measured through spectroscopy of alkali metals. Using a cross-correlation approach, we photoionize Cesium atoms with vacuum-ultraviolet high-harmonics in the presence of a mid-infrared laser field. We observe oscillations in the photoelectron yield, originating from the instantaneous polarization of atoms by the laser field. The phase of these oscillations encodes the attosecond synchronization of the high-harmonics and is used for attosecond pulse metrology. This source opens new spectral windows for attosecond spectroscopy, enabling studies of bound-state dynamics in natural systems with low ionization energies, while facilitating the generation of non-classical entangled light states in the visible-VUV.

BaGa4Se7-based 7-15 µm tunable broadband optical parametric amplifier pumped around 2 µm.

The non-oxide BaGa4Se7 (BGSe) crystal with broad transparency range, large nonlinearity, and high damage threshold has been widely utilized to build optical parametric oscillators/amplifiers that convert the well-developed near-infrared pump laser around 1 µm into the developing mid-infrared radiation. However, the inherent narrow phase-matching bandwidth of BGSe with a pump around 1 µm hampers the generation of ultrashort mid-infrared pulses. Here, we demonstrate that by pumping the BGSe crystal around 2 µm, it is possible to achieve a sufficient phase-matching bandwidth to support ultrashort pulses across a broad mid-infrared spectral range. In the experiments, two synchronized 1 kHz optical parametric chirped-pulse amplification sources centered at 2.35 µm and 3.1 µm are used to pump and seed a BGSe-based optical parametric amplifier, generating 52 µJ sub-four-cycle pulses at 9.7 µm. The central wavelength of the generated mid-infrared pulse can be tuned from 7 to 15 µm by finely adjusting the pump and seed wavelengths as well as the crystal orientation. These results reveal the enormous potential and bright prospects of BGSe for generating ultrashort intense pulses in the long-wave infrared region.

Dynamics of mid-infrared semiconductor switching controlled by femtosecond laser pulses

Semiconductor switching of sub-picosecond mid-infrared laser pulses between 10 and 14 µm is characterized in GaAs, n-Ge, and ZnSe controlled by 30 fs pulses with photon energy above the band gap of the material. The reflectivity and lifetime are studied for multiple wavelengths. Time domain dynamics of semiconductor plasma reflectivity observed in experiments correlate with that derived in diffusion-recombination theory. Potential application of ultrafast semiconductor switching as a photonic device for use in high-power mid-infrared laser systems is discussed.

Carrier-envelope-phase characterization of ultrafast mid-infrared laser pulses through harmonic generation and interference in argon

The propagation of an intense, femtosecond, mid-infrared laser pulse in a gaseous medium results in the efficient generation of spectrally overlapping low-order harmonics, whose optical carrier phases are linked to the carrier-envelope phase (CEP) of the mid-infrared driver pulse. Random peak-power fluctuations of the driver pulses, converted to the fluctuations of the nonlinear phases, acquired by the pulses on propagation, cause this phase correlation to smear out. We show that this seemingly irreversible loss of phase can be recovered, and that the complete information needed for the phase correction is contained in the harmonic spectra itself. The optical phases of the intense driver pulse and its harmonics, as fragile as they appear to be against even weak disturbances, evolve deterministically during highly nonlinear propagation through the extended ionization region.

Visible/Multi-terahertz 2-Dimensional Spectroscopy of Ultrafast Carrier Dynamics in Quantum Materials

Ultrafast dynamics observed at low energies carry insightful information about the complex many-body interactions in solid-state materials. Here, we present a highly sensitive and robust setup for asymmetric 2-dimensional spectroscopy performing 2-pulse visible excitation combined with probing in the 15- to 35-THz frequency range. This experimental setup is ideal for targeting the interplay of high- and low-energy correlations in functional materials with femtosecond temporal and millielectronvolt energy resolution. In addition, the sub-cycle field resolution of mid-infrared pulses enables tracking nonthermal interactions in the complex dielectric function. Prototypical measurements benchmark ultrafast carrier dynamics in thin-film graphite, showing in detail the interplay of direct and indirect optical transitions in the transient excited state. We further investigate the photo-induced collapse of the superconducting condensate in the high-temperature superconductor Bi 2 Sr 2 CaCu 2 O 8+ x at energies resonant to the optical bandgap, revealing a nontrivial instantaneous nonlinearity related to the excited quasiparticles in the material. Optical pump–terahertz probe experiments build the foundation for this evolutionary step in 2-dimensional spectroscopy as well as for terahertz 4-wave mixing with resonant driving and readout of the superconducting state. Our results offer exciting perspectives in the study of strong correlations and enable precise investigations of nontrivial many-body interactions in few-layer samples and nanostructures.

Isolated Attosecond Free-Electron Laser Based on a Subcycle Driver from Hollow Capillary Fibers

An attosecond light source provides an advanced tool for investigating electron motion using time-resolved-spectroscopy techniques. Isolated attosecond pulses, especially, will significantly advance the study of electron dynamics. However, achieving high-intensity isolated attosecond pulses is still challenging at the present stage. In this paper, we propose a novel scheme for generating high-intensity, isolated attosecond soft x-ray free-electron lasers (FELs) using a mid-infrared (MIR) subcycle modulation laser from gas-filled hollow capillary fibers. The multi-cycle MIR pulses are first compressed to subcycles using a krypton-filled hollow capillary fiber with a decreasing pressure gradient due to the soliton self-compression effect. By utilizing such subcycle MIR laser pulses to modulate an electron beam, we can obtain a quasi-isolated current peak, which can then produce an isolated FEL pulse with a high signal-to-noise ratio, naturally synchronizing with the subcycle MIR laser pulse. Numerical simulations have been carried out, including subcycle pulse generation, electron beam modulation, and FEL radiation processes. The simulation results indicate that an isolated attosecond pulse with a wavelength of 1 nm, a peak power of ~28 GW, a pulse duration of ~580 as, and a signal-to-noise ratio of ~96.2% can be generated by our proposed method. The numerical results demonstrated here pave a new way for generating a high-intensity isolated attosecond soft x-ray pulse, which may have many applications in nonlinear spectroscopy and atomic-site electronic processes.

Ultrafast mid-infrared interferometric photocurrents in graphene-based two-terminal devices

We demonstrate that graphene-based two-terminal devices allow autocorrelating femtosecond mid-infrared pulses with a pulse duration of about 100 fs in the wavelength regime of 5.5–14 μm. The results suggest that the underlying ultrafast detection principle relies on an electric field dominated autocorrelation in combination with the optoelectronic dynamics at the metal–graphene interfaces. The demonstrated scheme excels because of the ease in nanofabrication of two-terminal graphene-based optoelectronic devices and their robustness.

Spatial filtering and optimal generation of high-flux soft x-ray high harmonics using a Bessel–Gauss beam

In recent years, significant advancements in high-repetition-rate, high-average-power mid-infrared laser pulses have enabled the generation of tabletop high-flux coherent soft x-ray harmonics for photon-hungry experiments. However, for practical applications, it is crucial to effectively filter out the driving beam from the high harmonics. In this study, we leverage the distinctive properties of a Bessel–Gauss (BG) beam to introduce a novel approach for spatial filtering, specifically targeting soft x-ray harmonics, releasing with a high-photon flux simultaneously. Our simulations reveal that by finely adjusting the focus geometry and gas pressure, the BG beam naturally adopts an annular shape, emitting high harmonics with minimal divergence in the far field. To achieve complete spatial separation of the driving beam and harmonic emissions, we pinpoint the optimal gas pressure and focusing geometry, particularly under overdriven laser intensities, for achieving good phase matching of harmonic emissions from short-trajectory electrons within the gas medium when the exact ionization level is higher than the “critical” value. Additionally, we establish scaling relations for sustaining optimal phase-matching conditions crucial for spatially separating the driving laser and the high-harmonic field, especially as the wavelength of the driving laser increases. Furthermore, our analysis demonstrates a substantial enhancement of harmonic yields by at least one order of magnitude compared to a truncated Gaussian annular beam. We also show that under accessible experimental conditions, soft x-ray photon flux up to 1010 photons/s at 250 eV can be achieved. The utilization of the BG beam opens up a promising pathway for the development of high-flux attosecond soft x-ray light sources, poised to serve a wide range of applications.