Mid-infrared Femtosecond Pulses Research Articles

165 MHz highly stable femtosecond Er:ZBLAN fiber laser operating at 2.8 µm.

Three-micrometer mid-infrared (MIR) femtosecond pulse sources with a high repetition rate (HRR) have potential applications in a number of fields such as biological imaging, optical frequency combs, and gas detection. In this paper, by optimizing the fiber length and the cavity structure, we demonstrated a highly stable, self-starting mode-locked fluoride fiber laser (MLFFL) with a fundamental repetition rate of ∼165 MHz and a signal-to-noise ratio (SNR) of 90 dB. As far as we know, this stands as the highest fundamental repetition rate ever acquired directly from an ultrafast MLFFL in the >2.5 µm MIR region. Stable 352-fs pulses at 2795 nm with an average output power of 392 mW and a low integrated relative intensity noise (RIN) of 0.018% [10 Hz, 10 MHz] were generated. The root mean square (RMS) power fluctuation is 0.17% over 2 h, which indicates excellent oscillator stability. This high-performance laser offers a practicable scheme both for scaling the repetition frequency in MIR MLFFLs and high-precision ultrafast applications at longer wavelengths.

Modeling generation of harmonics in the water window region in hollow core waveguides by mid-infrared femtosecond pulses

We numerically investigate generation of harmonics in the water window region (down to 2.8 nm) by 2 μm femtosecond pulses propagating in hollow core waveguides filled with high pressure He. Numerical calculations are based on a three dimensional macroscopic model, which solves the pulse propagation by a split-step method, uses the strong field approximation to evaluate the single atom response, and integrates it coherently to obtain the harmonic field. Two configurations for the waveguides are considered: the standard one with a constant diameter of 70 μm and a conical one with a decreasing diameter from 70 to 50 μm. We demonstrate that harmonic field enhancement can be obtained in spectral domains of great practical interest, from 2.8 to 20 nm, and identify quasi-phase matching induced by multimode beating as the mechanism responsible for this enhancement.

An optical fiber integrated device for nonlinear generation of femtosecond mid-infrared pulses

Compact and broadband mid-infrared (MIR) sources are in high demand because of a wide range of potential applications such as molecular sensing in the fingerprint region. The generation of coherent MIR radiation at arbitrary frequencies typically requires nonlinear mixing between at least two input waves, which is often cumbersome to implement. We present an integrated and, therefore, adjustment-free solution combining few-femtosecond pulse compression in a germanosilicate optical fiber and optical rectification. To this end, a 16-μm-thin GaSe crystal is directly mounted on the end facet of a highly nonlinear fiber assembly exploiting a focused ion beam. With input pulses of a minute energy of 5 nJ and a duration of 120 fs at the telecom wavelength of 1.55 μm, we directly obtain ultrabroadband and phase-stable output transients. Electro-optic sampling in free space reveals single-cycle pulses with spectral components covering the entire MIR from 10 to 120 THz.

Efficient single-cycle mid-infrared femtosecond laser pulse generation by spectrally temporally cascaded optical parametric amplification with pump energy recycling.

We proposed spectrally temporally cascaded optical parametric amplification (STOPA) using pump energy recycling to simultaneously increase spectral bandwidth and conversion efficiency in optical parametric amplification (OPA). Using BiB3O6 and KTiOAsO4 nonlinear crystals, near-single-cycle mid-infrared (MIR) pulses with maximum energy conversion efficiencies exceeding 25% were obtained in simulations. We successfully demonstrated sub-two-cycle, CEP-stable pulse generation at 1.8 µm using a four-step STOPA system in the experiment. This method provides a solution to solve the limitations of the gain bandwidth of nonlinear crystals and the low conversion efficiency in broadband OPA systems, which is helpful for intense attosecond pulse generation and strong laser field physics studies.

Intense mid-infrared laser pulse generated via flying-mirror red-shifting in near-critical-density plasmas

Abstract Relativistic femtosecond mid-infrared pulses can be generated efficiently by laser interaction with near-critical-density plasmas. It is found theoretically and numerically that the radiation pressure of a circularly polarized laser pulse first compresses the plasma electrons to form a dense flying mirror with a relativistic high speed. The pulse reflected by the mirror is red-shifted to the mid-infrared range. Full three-dimensional simulations demonstrate that the central wavelength of the mid-infrared pulse is tunable from 3 $\rm{\mu m}$ to 14 $\rm{\mu m}$ and the laser energy conversion efficiency can reach as high as 13%. With a 0.5-10 PW incident laser pulse, the generated mid-infrared pulse reaches a peak power of 10-180 TW, which could be interesting for various applications in ultrafast and high-field sciences.

Efficient and high-spatiotemporal-quality terawatt-class mid-infrared optical parametric amplifiers by spatially shaped pumping

We propose a method to efficiently generate terawatt (TW)-class mid-infrared (MIR) femtosecond laser pulses with high spatiotemporal quality through optical parametric chirped-pulse amplification (OPCPA). By transforming the pump-beam profile for the OPCPA from Gaussian to flat-top using a designed field mapping optics consisting of two aspherical lenses, we obtain a TW-class femtosecond laser pulse at 2 µm with a conversion efficiency of over 36% according to our simulations. Furthermore, the spatiotemporal coupling effects are greatly suppressed in our method compared to an OPCPA system that is pumped by a widely employed Gaussian profile beam. Our work provides a simple and robust method for developing OPCPA systems with high efficiency and high pulse quality.

Multiple conical odd harmonics from filament-inscribed nanogratings

We report on the observation of conical third, fifth, seventh, and ninth harmonics that gradually emerge during the supercontinuum generation by filamentation of femtosecond midinfrared pulses in lithium strontium hexafluoroaluminate crystal. We show that the generation of conical odd harmonics is an optical signature of light-driven material reorganization in the form of volume nanogratings at the site irradiated by repetitive femtosecond filaments. The angle-resolved spectral measurements demonstrate remarkably broad spectra of individual odd harmonics, benefiting from a spectrally broadened pump pulse (supercontinuum), and reveal that filament-inscribed nanogratings represent photonic structures that are able to provide ultrabroad phase-matching bandwidths covering the wavelength range from the ultraviolet to the near infrared. We propose a scenario that interprets the generation of conical fifth, seventh, and ninth harmonics as nanograting phase-matched cascaded noncollinear four-wave mixing processes.

Microjoule‐Level Mid‐Infrared Femtosecond Pulse Generation in Hollow‐Core Fibers (Laser Photonics Rev. 17(6)/2023)

Mid-Infrared Femtosecond Pulse Generation in Hollow-Core Fibers In article number 2200882, Ang Deng, Wonkeun Chang, and colleagues present a novel approach to generate high-energy femtosecond pulses in mid-infrared from a gas-filled hollow-core fiber. The efficient conversion of near-infrared pump to mid-infrared is facilitated by engineering the cross-sectional geometry of cladding elements in the hollow-core fiber. It promises new opportunities in molecular spectroscopy.

Ionization dynamics and damage conditions in fused silica irradiated with mid-infrared femtosecond pulses

The employment of ultrashort laser sources at the mid-infrared (mid-IR) spectral region for dielectrics is expected to open innovative routes for laser patterning and a wealth of exciting applications in optics and photonics. To elucidate the material response to irradiation with mid-IR laser sources, a consistent analysis of the interaction of long wavelength femtosecond pulses with dielectric materials is presented. The influence of the pulse duration is particularly emphasized in specifying the laser parameters for which photoionization and impact ionization are important. Simulation results using pulses at 2.2, 3.2, and 5 μm are conducted to illustrate optimum conditions for the onset of damage on the solid that is related to the occurrence of the optical breakdown. The results predict that the damage threshold scales as ∼τpa (0.31≤a≤0.37) at all laser wavelengths. Given the significant effect of the induced excitation level on excitation of surface plasmons (SPs), which account for the formation of laser-induced periodic structures oriented perpendicular to the laser polarization, a correlation of the produced electron densities with SPs and the threshold of SP excitation (∼τpβ, 0.33≤β≤0.39) are also discussed in this as yet unexplored spectral region. The results are expected to guide development of an innovative approach to surface patterning using strong mid-IR pulses for advanced applications.

Single-shot selective femtosecond and picosecond infrared laser crystallization of an amorphous Ge/Si multilayer stack

Pulsed laser crystallization is an efficient annealing technique to obtain polycrystalline silicon or germanium films on non-refractory substrates. This is important for creating “flexible electronics” and can also be used to fabricate thin-film solar cells. In this work, near- and mid-infrared femtosecond and picosecond laser pulses were used to crystallize a Ge/Si multilayer stack consisting of alternating amorphous thin films of silicon and germanium. The use of infrared radiation at wavelengths of 1030 and 1500 nm with photon energies lower than the optical absorption edge in amorphous silicon allowed obtaining selective crystallization of germanium layers with a single laser shot. The phase composition of the irradiated stack was investigated by the Raman scattering technique. Several non-ablative regimes of ultrashort-pulse laser crystallization were found, from partial crystallization of germanium without intermixing the Ge/Si layers to complete intermixing of the layers with formation of GexSi1-x solid alloys. The roles of single- and two-photon absorption, thermal and non-thermal (ultrafast) melting processes, and laser-induced stresses in selective pico- and femtosecond laser annealing are discussed. It is concluded that, due to a mismatch of the thermal expansion coefficients between the adjacent stack layers, efficient explosive solid-phase crystallization of the Ge layers is possible at relatively low temperatures, well below the melting point.

Generation of single solitons tunable from 3 to 3.8 μm in cascaded Er3+-doped and Dy3+-doped fluoride fiber amplifiers

High-power tunable femtosecond mid-infrared (MIR) pulses are of great interest for many scientific and industrial applications. Here we demonstrate a compact fluoride-fiber-based system that generates single solitons tunable from 3 to 3.8 μm. The system is composed of an Er:ZBLAN fiber oscillator and amplifier followed by a fusion-spliced Dy:ZBLAN fiber amplifier. The Er:ZBLAN fiber amplifier acts as a power booster as well as a frequency shifter to generate Raman solitons up to 3 μm. The Dy:ZBLAN fiber amplifier transfers the energy from the residual 2.8 μm radiation into the Raman solitons using an in-band pumping scheme, and further extends the wavelength up to 3.8 μm. Common residual pump radiation and secondary solitons accompanying the soliton self-frequency shift (SSFS) are recycled to amplify Raman solitons, consequently displaying a higher output power and pulse energy, a wider shifting range, and an excellent spectral purity. Stable 252 fs pulses at 3.8 μm with a record average power of 1.6 W and a pulse energy of 23 nJ are generated. This work provides an effective way to develop high-power widely tunable ultrafast single-soliton MIR laser sources, and this method can facilitate the design of other SSFS-based laser systems for single-soliton generation.

Time-resolved vibrational-pump visible-probe spectroscopy for thermal conductivity measurement of metal-halide perovskites.

Understanding thermal transport at the microscale to the nanoscale is crucially important for a wide range of technologies ranging from device thermal management and protection systems to thermal-energy regulation and harvesting. In the past decades, non-contact optical methods, such as time-domain and frequency-domain thermoreflectance, have emerged as extremely powerful and versatile thermal metrological techniques for the measurement of material thermal conductivities. Here, we report the measurement of thermal conductivity of thin films of CH3NH3PbI3 (MAPbI3), a prototypical metal-halide perovskite, by developing a time-resolved optical technique called vibrational-pump visible-probe (VPVP) spectroscopy. The VPVP technique relies on the direct thermal excitation of MAPbI3 by femtosecond mid-infrared optical pump pulses that are wavelength-tuned to a vibrational mode of the material, after which the time dependent optical transmittance across the visible range is probed in the ns to the μs time window using a broadband pulsed laser. Using the VPVP method, we determine the thermal conductivities of MAPbI3 thin films deposited on different substrates. The transducer-free VPVP method reported here is expected to permit spectrally resolving and spatiotemporally imaging of the dynamic lattice temperature variations in organic, polymeric, and hybrid organic-inorganic semiconductors.

High-Speed Mid-Wave Infrared Uni-Traveling Carrier Photodetector Based on InAs/InAsSb Type-II Superlattice

High-speed photodetectors operating at mid-wave infrared are crucial for free-space optical communication and frequency comb spectroscopy. In this letter, we report a high-speed mid-wave infrared uni-traveling carrier photodiode based on InAs/InAsSb type-II superlattice at room temperature for the first time. The device exhibits a cut-off wavelength of around 5.5 <inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> at room temperature. The responsivity of the device is about 0.6 A/W (at 4.5 <inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula>) under &#x2212;1 V at room temperature. The frequency response of the device is characterized by an optical parametric amplification system generating mid-infrared femtosecond pulses. A device with a 20 <inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> diameter has a 3-dB bandwidth of 12.8 GHz at &#x2212;4 V. These promising results suggest that the device could be potential candidates to be employed in the emerging high-speed MWIR applications.

Fifth-order nonlinear optical response of Alq3 thin films

The fifth-order optical nonlinearity of Tris-(8-hydroxyquinoline) aluminium (Alq3) thin films deposited on quartz substrates when exposed to an intense laser excitation of mid-infrared femtosecond pulses is investigated. This property was examined by generating the fifth-harmonic frequency of the driving field whose signal strength as a function of laser intensity is well described by a power-law scaling. The perturbative nature of the underlying mechanism enabled the linking of the process directly to the fifth-order nonlinear optical susceptibility χ(5). We also utilized the open-aperture z-scan for studying the nonlinear absorption aspect of the third-order nonlinear optical properties of the sample. It was found that Alq3 thin-films inherit the same saturable absorption character reported in Alq3 solutions under continuous laser excitation in the visible range, with a comparable value of the nonlinear absorption coefficient βeff (in the order of 10−2 cm/W).

Isolated attosecond pulse generation from helium atom irradiated by a three-color laser pulse

Isolated attosecond pulses enables the studying and controlling of ultrafast electron processes in atoms and molecules. High-order harmonic generation (HHG) is the most promising way to generate such pulses, benefiting from the broad plateau structure of the typical HHG spectrum. We theoretically investigate high-order harmonic and attosecond pulse generation from helium atom in a three-color laser field, which is synthesized by 16 fs/1600 nm, 15 fs/1100 nm and 5 fs/800 nm pulse laser. Compared with harmonic spectrum generated by a two-color laser field synthesized by 16 fs/1600 nm and 15 fs/1100 nm, the harmonic spectrum generated from the synthesized three-color field exhibits high conversion efficiency and broadband supercontinuum characteristics. The continuous spectrum range covers from 230th to 690th harmonics, and the generation of 128 attosecond isolated short pulses with higher intensity is realized after Fourier transform. This result is attributable to the fact that the synthesized three-color electric field exhibits high-intensity and few-cycle mid-infrared femtosecond pulse laser characteristics, which can effectively control atomic ionization and recombination occurring within an effective optical period of the mid-infrared femtosecond pulse. This scheme solves the problems faced by the current femtosecond pulse laser technology, i.e. the few-cycle mid-infrared femtosecond pulse laser cannot have both carrier envelope phase stability and high power output.

Selective ultrashort laser annealing of amorphous Ge/Si multilayer stacks

We report on single-short laser crystallization of Ge/Si multilayer stacks consisting of alternating amorphous nanosized films of silicon and germanium using near- and mid-infrared femtosecond and picosecond laser pulses. The phase composition of the irradiated stacks was investigated by the Raman scattering technique. Several non-ablative regimes of crystallization were found, from partial crystallization of germanium without intermixing the Ge/Si layers to complete intermixing of the layers with formation of GexSi1-x solid alloys. The roles of one- and two-photon absorption, thermal and non-thermal (ultrafast) melting processes, and laser-induced stresses in selective pico- and femtosecond laser annealing are analysed based on theoretical estimations and comparison with experimental data. It is concluded that, due to a mismatch of the thermal expansion coefficients between the adjacent stack layers, efficient explosive solid-phase crystallization of the Ge layers is possible at relatively low temperatures, well below the melting point. The possibility of ultrafast non-thermal phase transition in germanium in the studied regimes is also discussed.

Mono-cycle terahertz pulses from intersubband shift currents in asymmetric semiconductor quantum wells

Both second-order frequency mixing in nonlinear optical media and photoconductive antennas have provided terahertz (THz) transients in a wide parameter range. Here, we demonstrate a novel type of ultrafast nonlinear optical response in asymmetric semiconductor quantum wells, originating from electron shift currents. Resonant intersubband excitation by a femtosecond mid-infrared pulse induces a transient spatial shift of electronic charge, which leads to the emission of a mono-cycle THz pulse. This mechanism is characterized and separated from conventional difference frequency mixing by nonlinear two-dimensional THz spectroscopy. The amplitude of the THz electric field emitted by the shift current reaches several percent of the mid-infrared driving field, significantly higher than in difference frequency mixing. Changes in time structure of the THz pulse upon nonlinear saturation of intersubband absorption allow for generating tailored THz transients. The present concept can be implemented in highly compact devices driven by mid-infrared pulses at megahertz repetition rates to provide versatile THz pulses for spectroscopy and optoelectronics.

Broadly tunable mid-infrared femtosecond pulses directly generated by an optical parametric amplifier

We introduce an optical parametric amplifier, pumped by an amplified femtosecond Yb:KGW laser, which directly generates broadly tunable mid-infrared (MIR) pulses, covering the whole vibrational spectrum from 3 to 10 µm. The avoidance of the traditional difference-frequency generation stage to access the MIR range simplifies the setup while enabling high conversion efficiencies. The two-stage design employs in the second stage either periodically poled lithium niobate, optimized for the CH/OH stretching region (3-5 µm) or LiGaS2, which allows extending the tunability to the fingerprint region (up to 10 µm). We anticipate applications of this versatile source to ultrafast vibrational spectroscopy and infrared microscopy.

Characteristics of nonlinear terahertz-wave radiation generated by mid-infrared femtosecond pulse laser excitation

We report on efficient terahertz-wave generation in organic and inorganic crystals by nonlinear wavelength conversion approach using a 3.3 μm femtosecond pulse laser. Experimental results reveal the relation between pump power and terahertz-wave output power, which is proportional to the square of the pump power at the range of mega- to tera-watt cm−2 class even if the pump wavelength is different. Damage threshold of organic and inorganic crystals are recorded 0.6 and 18 tera-watt cm−2 by reducing several undesirable nonlinear optical effects using mid-infrared source.

Acousto-optic modulator based dispersion scan for phase characterization and shaping of femtosecond mid-infrared pulses.

Compression, shaping and characterization of broadband mid-infrared (MIR) pulses based on an acousto-optic modulator (AOM) pulse shaper is presented. Characterization of the spectral phase is achieved by an AOM-shaper based implementation of a dispersion scan (d-scan). The abilities of the setup are demonstrated by imprinting several test phases with increasing complexity on broadband MIR pulses centered at 3.2 µm and retrieval of the imprinted phases with the presented d-scan method. Phase characterization with d-scan in combination with an evolutionary algorithm allows us to compress the MIR pulses below 50 fs FWHM autocorrelation after the shaper.