Femtosecond Optical Parametric Amplifier Research Articles

Water-window soft x-ray high-harmonic generation up to the nitrogen K-edge driven by a kHz, 2.1 μm OPCPA source

We report the generation of coherent water-window soft x-ray harmonics in a neon-filled semi-infinite gas cell driven by a femtosecond multi-mJ mid-infrared optical parametric chirped-pulse amplification (OPCPA) system at a 1 kHz repetition rate. The cutoff energy was extended to ∼450 eV with a 2.1 μm driver wavelength and a photon flux of photons/s/1% bandwidth was obtained at 350 eV. A comparable photon flux of photons/s/1% bandwidth was observed at the nitrogen K-edge of 410 eV. This is the first demonstration of water-window harmonic generation up to the nitrogen K-edge from a kHz OPCPA system. Finally, this system is suitable for time-resolved soft x-ray near-edge absorption spectroscopy. Further scaling of the driving pulse's energy and repetition rate is feasible due to the availability of high-power picosecond Yb-doped pump laser technologies, thereby enabling ultrafast, tabletop water-window x-ray imaging.

Lasing dynamics study by femtosecond time-resolved fluorescence non-collinear optical parametric amplification spectroscopy**Project supported by the National Natural Science Foundation of China (Grant Nos. 20925313 and 21503066), the Innovation Program of Chinese Academy of Sciences (Grant No. KJCX2-YW-W25), the Postdoctoral Project of Hebei University, China, and the Project of Science and Technology Bureau of Baoding City, China (Grant No. 15ZG029).

Femtosecond time-resolved fluorescence non-collinear optical parametric amplification spectroscopy (FNOPAS) is a versatile technique with advantages of high sensitivity, broad detection bandwidth, and intrinsic spectrum correction function. These advantages should benefit the study of coherent emission, such as measurement of lasing dynamics. In this letter, the FNOPAS was used to trace the lasing process in Rhodamine 6G (R6G) solution and organic semiconductor nano-wires. High-quality transient emission spectra and lasing dynamic traces were acquired, which demonstrates the applicability of FNOPAS in the study of lasing dynamics. Our work extends the application scope of the FNOPAS technique.

Experimental demonstration of broadband femtosecond optical parametric amplification based on YCOB crystal at near critical wavelength degeneracy

Broadband optical parametric amplification (OPA) in the near-infrared region (1.3–1.8μm) is demonstrated in YCOB crystal pumped by 1kHz Ti:sapphire based femtosecond laser at near critical wavelength degeneracy phase matching condition at first time. The gain bandwidth is closed to BIBO or BBO crystal OPA gain bandwidth. The energy obtained with 3.5mm-thick YCOB reached 20μJ with RMS 1.9%. After second harmonic generation, 17.6fs pulse is obtained, and the pulse compressibility is demonstrated.

Broadband high-power mid-IR femtosecond pulse generation from an ytterbium-doped fiber laser pumped optical parametric amplifier.

We report on a high-power periodically poled MgO-doped lithium niobate (MgO:PPLN)-based femtosecond optical parametric amplifier (OPA), featuring a spectral seamless broadband mid-infrared (MIR) output. By modifying the initial chirp and spectrum of the mode-locked seed laser, the Yb fiber pump laser exhibits a final output power of 14 W with sub-200-fs pulse duration after power amplification and compression. When the OPA was seeded with a broadband amplified spontaneous emission (ASE) source, a damage-limited 0.6 W broadband MIR radiation was experimentally obtained under the pump power of 10.15 W at 82 MHz repetition rate, corresponding to an overall OPA conversion efficiency of 32.7%. The 3 dB bandwidth of the mid-IR idler was 291.9 nm, centering at 3.34 μm.

Generation of high beam quality, high-energy and broadband tunable mid-infrared pulse from a KTA optical parametric amplifier

We have demonstrated efficient generation of high beam quality, high-energy and broadband tunable femtosecond mid-infrared pulses using a three-stage collinear optical parametric amplifier (OPA). The white-light continuum (WLC) seeded OPA setup, based on KTA crystal in three stages and pumped by a femtosecond laser pulse at 800nm, is capable of producing idler wavelength ranging from 2.4μm to 4.0μm with energy up to 82μJ at 3.27μm, which corresponds to signal energy of 350μJ at 1060nm. The output pulse has excellent intensity distribution with measured beam quality factor M2~1.1 for signal and M2~1.7 for idler. To our knowledge, this is the best beam quality reported in 3–5μm femtosecond OPA until now. The achieved mid-infrared pulse also has a good energy stability with a fluctuation of 1.01% rms over half an hour.

Multi-channel lock-in amplifier assisted femtosecond time-resolved fluorescence non-collinear optical parametric amplification spectroscopy with efficient rejection of superfluorescence background.

Superfluorescence appears as an intense background in femtosecond time-resolved fluorescence noncollinear optical parametric amplification spectroscopy, which severely interferes the reliable acquisition of the time-resolved fluorescence spectra especially for an optically dilute sample. Superfluorescence originates from the optical amplification of the vacuum quantum noise, which would be inevitably concomitant with the amplified fluorescence photons during the optical parametric amplification process. Here, we report the development of a femtosecond time-resolved fluorescence non-collinear optical parametric amplification spectrometer assisted with a 32-channel lock-in amplifier for efficient rejection of the superfluorescence background. With this spectrometer, the superfluorescence background signal can be significantly reduced to 1/300-1/100 when the seeding fluorescence is modulated. An integrated 32-bundle optical fiber is used as a linear array light receiver connected to 32 photodiodes in one-to-one mode, and the photodiodes are further coupled to a home-built 32-channel synchronous digital lock-in amplifier. As an implementation, time-resolved fluorescence spectra for rhodamine 6G dye in ethanol solution at an optically dilute concentration of 10(-5)M excited at 510 nm with an excitation intensity of 70 nJ/pulse have been successfully recorded, and the detection limit at a pump intensity of 60 μJ/pulse was determined as about 13 photons/pulse. Concentration dependent redshift starting at 30 ps after the excitation in time-resolved fluorescence spectra of this dye has also been observed, which can be attributed to the formation of the excimer at a higher concentration, while the blueshift in the earlier time within 10 ps is attributed to the solvation process.

Generation of broadband 17-μJ mid-infrared femtosecond pulses at 3.75 μm by silicon carbide crystal.

In this contribution, we report the generation of 17-μJ mid-infrared (MIR) pulses with duration of 70 fs and bandwidth of 550 nm centered at 3.75 μm at 1-kHz repetition rate, by a two-stage femtosecond optical parametric amplifier utilizing 4H-silicon carbide crystal as the nonlinear medium. The crystal is selected as it processes orders of magnitude higher damage threshold than traditional MIR nonlinear crystals, and it supports extreme broad parametric bandwidth. With its distinguished features such as MIR central wavelength, ultra-broad bandwidth, self-stable carrier-envelope phase, and potential for energy scaling, this kind of MIR source holds promise for new approaches to extreme short isolated attosecond pulse generation as well as MIR spectroscopy applications.

High power burst-mode optical parametric amplifier with arbitrary pulse selection

We present results from a unique burst-mode femtosecond non-collinear optical parametric amplifier (NOPA) under development for the optical - x-ray pump-probe experiments at the European X-Ray Free-Electron Laser Facility. The NOPA operates at a burst rate of 10 Hz, a duty cycle of 2.5% and an intra-burst repetition rate of up to 4.5 MHz, producing high fidelity 15 fs pulses at a center wavelength of 810 nm. Using dispersive amplification filtering of the super-continuum seed pulses allows for selectable pulse duration up to 75 fs, combined with a tuning range in excess of 100 nm whilst remaining nearly transform limited. At an intra-burst rate of 188 kHz the single pulse energy from two sequential NOPA stages reached 180 µJ, corresponding to an average power of 34W during the burst. Acousto- and electro-optic switching techniques enable the generation of transient free bursts of required length and the selection of arbitrary pulse sequences inside the burst.

Combining cw-seeding with highly nonlinear fibers in a broadly tunable femtosecond optical parametric amplifier at 42 MHz

We report on a precisely tunable and highly stable femtosecond oscillator-pumped optical parametric amplifier at a 41.7 MHz repetition rate for spectroscopic applications. A novel concept based on cw-seeding of a first amplification stage with subsequent spectral broadening and shaping, followed by two further amplification stages, allows for precise sub-nanometer and gap-free tuning from 1.35 to 1.75 μm and 2.55 to 4.5 μm. Excellent spectral stability is demonstrated with deviations of less than 0.008% rms central wavelength and 1.6% rms bandwidth over 1 h. Spectral shaping of the seed pulse allows precise adjustment of both the bandwidth and the pulse duration over a broad range at a given central wavelength. Transform-limited pulses nearly as short as 107 fs are achieved. More than half a Watt of average power in the near- and more than 200 mW in the mid-infrared with power fluctuations less than 0.6% rms over 1 h provide an excellent basis for spectroscopic experiments. The pulse-to-pulse power fluctuations are as small as 1.8%. Further, we demonstrate for the first time, to the best of our knowledge, that by using hollow-core capillaries with highly nonlinear liquids as a host medium for self-phase modulation, the signal tuning range can be extended and covers the region from 1.4 μm up to the point of degeneracy at 2.07 μm. Hence, the idler covers 2.07 to 4.0 μm.

Coherent photon interference elimination and spectral correction in femtosecond time-resolved fluorescence non-collinear optical parametric amplification spectroscopy

We report an improved setup of femtosecond time-resolved fluorescence non-collinear optical parametric amplification spectroscopy (FNOPAS) with a 210 fs temporal response. The system employs a Cassegrain objective to collect and focus fluorescence photons, which eliminates the interference from the coherent photons in the fluorescence amplification by temporal separation of the coherent photons and the fluorescence photons. The gain factor of the Cassegrain objective-assisted FNOPAS is characterized as 1.24 × 10(5) for Rhodamine 6G. Spectral corrections have been performed on the transient fluorescence spectra of Rhodamine 6G and Rhodamine 640 in ethanol by using an intrinsic calibration curve derived from the spectrum of superfluorescence, which is generated from the amplification of the vacuum quantum noise. The validity of spectral correction is illustrated by comparisons of spectral shape and peak wavelength between the corrected transient fluorescence spectra of these two dyes acquired by FNOPAS and their corresponding standard reference spectra collected by the commercial streak camera. The transient fluorescence spectra of the Rhodamine 6G were acquired in an optimized phase match condition, which gives a deviation in the peak wavelength between the retrieved spectrum and the reference spectrum of 1.0 nm, while those of Rhodamine 640 were collected in a non-optimized phase match condition, leading to a deviation in a range of 1.0-3.0 nm. Our results indicate that the improved FNOPAS can be a reliable tool in the measurement of transient fluorescence spectrum for its high temporal resolution and faithfully corrected spectrum.

Nonlinear temporal pulse cleaning techniques and application

AbstractTwo different pulse cleaning techniques for ultra-high contrast laser systems are comparably analysed in this work. The first pulse cleaning technique is based on noncollinear femtosecond optical-parametric amplification (NOPA) and second-harmonic generation (SHG) processes. The other is based on cross-polarized wave (XPW) generation. With a double chirped pulse amplifier (double-CPA) scheme, although temporal contrast enhancement in a high-intensity femtosecond Ti:sapphire chirped pulse amplification (CPA) laser system can be achieved based on both of the techniques, the two different pulse cleaning techniques still have their own advantages and are suitable for different contrast enhancement requirements of different laser systems.

Generation of 120 GW mid-infrared pulses from a widely tunable noncollinear optical parametric amplifier

We demonstrate a noncollinear optical parametric chirped-pulse amplification scheme for generating high-peak-power tunable mid-infrared (IR) pulses. The high-gain LiNbO(3)-based noncollinear parametric amplifier, seeded by a tunable femtosecond optical parametric amplifier, provides a wide wavelength tuning range from 3.3 to 3.95 μm and a large saturated gain of over 4000 in a single-stage amplifier. The compressed mid-IR pulse has a pulse energy of 13.3 mJ and pulse duration of 111 fs, with a peak power as high as 120 GW. To the best of our knowledge, this is the highest peak power ever reported for 3-5 μm tunable mid-IR lasers.

High conversion efficiency femtosecond optical parametric amplifier based on silicon waveguide in mid-infrared

The mid-infrared femtosecond optical parametric amplifier (OPA) in silicon waveguide is proposed and investigated numerically. Through appropriately engineering the waveguide dimensions, the zero-dispersion wavelength of the silicon waveguide can be tailored to 2.1μm and the broadband phase-matching can be realized. We achieve a conversion efficiency of −40dB and a conversion bandwidth of over 300nm for a low pump peak power in 8-mm-long silicon waveguide. We also analyze the evolution of the pump, signal and idler pulses in the process of optical parametric amplification. The proposed OPA exhibits compact configuration and can find important applications in integrated broadband optical source and ultrafast all-optical signal processing in mid-infrared.

Seeding of picosecond and femtosecond optical parametric amplifiers by weak single mode continuous lasers

Optical parametric amplifiers are typically seeded with either parametric superfluorescence or broadband continuum pulses. We show both with picosecond and femtosecond pump pulses, that single longitudinal mode cw lasers with mW power can be well used to generate nearly Fourier-transform-limited output pulses. The 532 nm pumped picosecond system is seeded in the near infrared and fully tunable from 1260 to 1630 nm. The femtosecond system operates stable with just hundreds of seed photons. The output spectral width matches closely to the width of individual spectral features found in single shot spectra of parametric superfluorescence. Both systems provide interesting radiation sources for nonlinear optics experiments that need highly controlled and clean excitation.

Application of Time-Resolved Coherent Anti-Stokes Raman Scattering Technique on the Study of Photocatalytic Hydrogen Production Kinetics

Based on the laser pulse output from a femtosecond regenerative amplifier and optical parametric amplifier (OPA), a broadband time-resolved coherent anti-Stokes Raman scattering (CARS) setup was assembled. Using this setup, the relationship of hydrogen CARS spectra to its amount in a mixture with air and the relevant detection time-delay were studied. Hydrogen CARS spectra without nonresonant background interference were obtained by adjusting the detection time-delay. The observed CARS intensity exhibited a linear relationship with the square of hydrogen concentration, which is consistent with the theoretical prediction. The signal-to-noise ratio showed that when the pressure of hydrogen-air mixed gas was 0.1 MPa, the detection limit of our setup was less than 0.2%. Using this setup, the hydrogen production kinetics of a platinum(II) terpyridyl acetylide molecular-cobalt catalysttriethanolamine (TEOA) system was studied. The kinetic mechanism of hydrogen production was discussed by considering the effect of changing pH. The results indicate that a high proton concentration will reduce the hydrogen production efficiency. This can be attributed to the inhibition of hydrolysis of TEOA under acidic conditions, because it is the electron and proton donor in this hydrogen production system. [Article] doi: 10.3866/PKU.WHXB201304281 www.whxb.pku.edu.cn 物理化学学报(Wuli Huaxue Xuebao) Acta Phys. -Chim. Sin. 2013, 29 (8), 1632-1638 August Received: January 31, 2013; Revised: April 28, 2013; Published on Web: April 28, 2013. ∗Corresponding author. Email: lmfu@chem.ruc.edu.cn; Tel: +86-10-62516604. The project was supported by the National Key Basic Research Program of China (973) (2009CB220008). 国家重点基础研究发展规划项目(973) (2009CB220008)资助 C Editorial office of Acta Physico-Chimica Sinica 1632 吕永钢等:利用时间分辨相干反斯托克斯拉曼散射技术研究光催化产氢反应动力学 No.8

Two-beam pumped infrared femtosecond nondegenerate optical parametric amplifier

We demonstrate a two-beam pumped infrared femtosecond (fs, 10−15s) broadband nondegenerate optical parametric amplifier (OPA). Compared with one-beam pumped femtosecond OPA, the output pulse energy is promoted relatively, and the broadband amplified pulse spectrum and pulse duration from the two-beam pumped infrared femtosecond OPA are similar. Though the two femtosecond pump beams are coherent and interferential with each other, the near field of the output beam is smooth and close to the Gaussian figuration. The results demonstrated that two pump sources can be used in the femtosecond OPA instead of one pump source for a combined higher pump energy and relative higher OPA amplified pulse energy with broadband femtosecond pulse duration.

Temporal and spectral characterizations of tunable noncollinear optical parametric amplifier

We experimentally investigate the temporal and spectral information from a broadband tunable femtosecond noncollinear optical parametric amplifier (NOPA), and present the spectral characterizations through the time–bandwidth products (τΔv). The time–bandwidth products of the signal are from 0.26 to 0.36, which are very similar to that of the pump (about 0.34) due to the time-correlated radiation. However, the time–bandwidth products of the idler are from 0.13 to 0.63, which present a larger variable range due to the more spatial freedom. We also experimentally demonstrate that the spectral bandwidth of the output signal is gradually widened when it is tuned to the longer central wavelength, which is approximatively following a λ2 scaling law.

High energy femtosecond OPA pumped by 1030 nm Yb:KGW laser.

We present a traveling-wave-type optical parametric amplifier (OPA) pumped at 1.03μm by a Yb:KGW laser that produces tunable high-energy pulses of 6.5–4μJ in the mid-infrared (mid-IR) region from 3.6 to 7μm. Pumping with negatively chirped pulses generates nearly transform-limited (TL) mid-IR pulses of 300–330fs length. Pumping with TL pulses of 200fs not only decreases the output energy by a factor of 1.5, but also decreases the mid-IR pulse-length to 160fs after additional compression. The compact and simple OPA setup is ideal for femtosecond infrared experiments in the fingerprint region.

Photoemission band mapping with a tunable femtosecond source using nonequilibrium absorption resonances

The authors show that the tunability of a femtosecond optical parametric amplifier combined with its high-repetition rate and short pulses provide a powerful tool for an alternate approach to conventional nonresonant band mapping by two-photon photoemission (2PPE). The authors demonstrate this 2PPE mapping via use of two model systems, i.e., the pair of sp surface and image states on flat Cu(111) and vicinal Cu(775) surfaces, over a photon energy range of 3.9–4.6 eV by making use of direct resonant band-to-band electronic transitions. Since the experimental excitation of the Cu image state from the surface state is comparable in time to the electron-electron equilibration time, the authors measure sharp resonant features in the electron energy distributions. In this approach, the authors track these resonant electronic transitions using 2PPE by varying the photon energy so as to achieve resonant excitation at each value of photoelectron emission angle over a large wavelength range on both types of surfaces. In addition, the authors explore the range of photon energies and optical intensities which may be used for this approach. The repetition rate of this laser was sufficient to yield a good signal-to-noise ratio while maintaining pump pulse intensities at levels that were low enough to prevent significant photon-induced space-charge broadening and electron-kinetic-energy shifting, even for photon energies close to the work function of the sample.

Scaling submillimeter single-cycle transients toward megavolts per centimeter field strength via optical rectification in the organic crystal OH1

We present the generation of high-power single-cycle terahertz (THz) pulses in the organic salt crystal 2-[3-(4-hydroxystyryl)-5.5-dimethylcyclohex-2-enylidene]malononitrile or OH1. Broadband THz radiation with a central frequency of 1.5 THz (λ(c)=200 μm) and high electric field strength of 440 kV/cm is produced by optical rectification driven by the signal of a powerful femtosecond optical parametric amplifier. A 1.5% pump to THz energy conversion efficiency is reported, and pulse energy stability better than 1% RMS is achieved. An approach toward the realization of higher field strength is discussed.