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Abstract
We present a compact, few-cycle, short-wave infrared light source delivering 13 µJ, carrier-envelope phase (CEP) stable pulses around 2 µm, operating at 200 kHz repetition rate. Starting from an ytterbium fiber amplifier, the seed is produced via white-light generation followed by difference frequency generation, and later amplified in two BiBO nonlinear crystals. A pulse duration of 15.8 fs is measured with the dispersion scan technique, while the CEP stability is assessed via a monolithic spectral interferometry scheme. We demonstrate the potential of the system to drive strong-field experiments by performing high-order harmonic generation in argon gas.
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1. Introduction
Breakthroughs in ultrafast and strong-field physics have been closely connected to the advancements in laser technology. Short and powerful laser pulses have been instrumental in unlocking new regimes of light-matter interaction. For example, the discovery of high-harmonic generation (HHG) in gases was made possible by employing intense picosecond and femtosecond pulses [1,2]. The push towards pulse lengths comparable to the duration of the carrier cycle (i.e. few-cycle pulses) has led to the generation of isolated attosecond pulses [3] and observation of carrier-envelope-phase-(CEP)-sensitive effects, e.g., directional emission of electrons in above-threshold-ionization [4] or production of lightwave-driven electrical currents in dielectrics and semiconductors [5,6].
The generation of few-cycle pulses has been most commonly performed in the visible and near-infrared via well-established methods such as nonlinear compression of titanium-sapphire chirped pulse amplifiers (CPA) in gas-filled hollow-core fibers [7,8] or construction of broadband optical parametric chirped pulse amplification (OPCPA) systems [9,10]. However, a number of strong-field studies would greatly benefit from employing few-cycle pulses with central frequencies further in the infrared. Again, one notable example being the HHG process, where the maximum obtained photon energy, the so-called harmonic cut-off, scales favourably with increasing wavelength of the driving radiation (as $\lambda ^2$) [11,12], at the cost of significant reduction in conversion efficiency [13,14]. This cut-off extension not only gives access to coherent radiation in the highly sought-after "water window" [15], but makes it possible to synthesize shorter attosecond pulses by increasing the carrier frequency and bandwidth [16,17]. Few-cycle mid-infrared pulses are also desirable when exploring intense light-matter interaction in solids. The use of longer wavelengths reduces the constraint on the electric field strengths needed to observe strong-field effects [18,19] (e.g. CEP-dependent electron photoemission in metallic structures [20], HHG in solids [21,22]) while simultaneously avoiding detrimental effects of resonant and multiphoton carrier excitations and giving access to experimental studies in a wider range of materials and structures.
These advantages have become the driving force behind significant research and engineering efforts towards building light sources capable of providing such pulses. Over the last two decades, a large number of systems delivering intense few-cycle pulses with central wavelengths in 1500-3000 nm (the so-called short-wave infrared region, or SWIR for short) has been reported [23–31]. The majority of these setups are based on difference frequency generation (DFG) followed by optical parametric amplification (OPA) processes, which offer immediate benefits such as passive CEP stability [32], wavelength tunability and a broad amplification bandwidth. The implementation of additional post-compression stages can bring the pulse duration towards the single-cycle regime [33].
In this work, we report on the development of a few-cycle SWIR OPCPA. Pumped by an ytterbium-based (Yb) laser amplifier, the system delivers CEP-stable 13-µJ (2.6 W average power) pulses centered around a wavelength of 2 µm at 200 kHz repetition rate. Using the dispersion scan (d-scan) technique [34] as a pulse characterization, we estimate the pulse duration to be equal to 15.8 fs full-width-at-half-maximum (FWHM), corresponding to 2.3 oscillations of the carrier wave. The CEP stability is evaluated to be better than 300 mrad over 10 minutes by simply focusing a small portion of the beam into a thick nonlinear crystal and detecting CEP-sensitive spectral interference fringes with a fiber spectrometer, an approach confirmed by a separate measurement with a conventional f - 2f interferometer. We demonstrate the capabilities of the setup by generating high-order harmonics in argon, reaching photon energies exceeding 140 eV. The detection of higher photon energies is restricted by a reduced efficiency of the diffraction grating in the extreme ultraviolet (XUV) spectrometer.
2. Setup description
A schematic of the SWIR light source is presented in Fig. 1. The backbone of our architecture is a continuous-wave-pumped, Yb-doped rod-type fiber CPA. A fraction around 1030 nm from the spectrum of a few-cycle Titanium:Sapphire oscillator seeds an Yb fiber pre-amplifier. The pulses are stretched by a chirped fiber Bragg grating, while two acousto-optic modulators reduce the repetition rate of the system from 80 MHz to 200 kHz The output of the pre-amplifier is sent into a rod-type fiber amplifier which boosts the pulse energy to 200 µJ. The chirped and amplified pulses are compressed in a grating compressor with an efficiency of $\approx$90%. After the compressor, the fundamental pulses around 1030 nm have a pulse energy of $\approx$180 µJ (average power of 36 W) and a duration of 300 fs.
Fig. 1. Sketch of the set-up. WLG - white-light generation, CM - chirped mirrors. A piezo-driven delay stage in the 1030 nm arm of the DFG stage (CEP control) can be moved to adjust the CEP of the idler pulse
In order to reach the desired wavelength regime and sufficient bandwidth, we employ several nonlinear frequency conversion and amplification stages. The scheme to generate CEP-stable infrared pulses is based on the approach of Homann and colleagues [35], which has proven before to be successful in implementing a 2-µm 10-W few-cycle OPA [24]. First, a 20:80 beam splitter separates the majority of the fundamental pulse energy for later pump stages (see Fig. 1). The remaining 20% are further split with a ratio of 10:90. Roughly 4 µJ are used to generate a supercontinuum in a 4-mm-thick yttrium aluminum garnet (YAG) crystal. The majority of the fundamental (32 µJ) is focused into a 1-mm-thick beta barium borate (BBO) crystal cut at $\theta = 23.4^\circ$ for second harmonic generation (SHG). The conversion efficiency is $\approx$53%, yielding $\approx$17 µJ of pulse energy at 515 nm. The white-light and the second harmonic pulses are combined in a non-collinear optical parametric amplification (NOPA) stage to amplify a spectral region spanning approximately $630-760$ nm. Here, another BBO crystal ($\theta$ = 23.6° , thickness = 4 mm) is used as a nonlinear medium and the non-collinear angle $\alpha$ is kept at approximately 2.1°. This NOPA stage has a gain of roughly two orders of magnitude, with an output power of $\approx$0.5 W. The typical amplification spectrum, together with the supercontinuum seed spectrum, are shown in Fig. 2(a).
Fig. 2. Spectra of different stages of the few-cycle SWIR light source. The vertical axes are not to scale. (a) Spectrum of the generated supercontinumm in YAG (WLG) and the corresponding amplified spectrum (NOPA). (b) SWIR spectrum after DFG and two NOPA stages. The inset shows the near-field two-photon absorption beam profile, recorded 1 m away from the second amplification crystal.
In the next step, the output of the visible NOPA acts as a pump in a DFG stage where it is mixed with $\approx$15 µJ of the fundamental 1030 nm radiation in a 1-mm-thick BBO crystal ($\theta = 20.5^\circ$) in collinear configuration. After the crystal, the pump and seed pulses are separated from the idler by a long-pass filter. We routinely achieve up to $\approx$0.5 µJ in the idler. A typical spectrum is shown in Fig. 2(b). The idler spans the wavelength range from 1.6 to 2.5 µm and supports a pulse with a FWHM duration of 13.9 fs, corresponding to 2.2 cycles at 1900 nm central wavelength. Since pump (NOPA) and seed (1030 nm) pulses are both derived from the same source, the output of the DFG stage is passively CEP-stable [32]. The CEP of the resulting idler pulses can be controlled by a piezo-driven delay stage in the 1030 nm arm.
To reach sufficient intensity to drive certain strong-field processes, such as gas phase HHG, the output power of the SWIR pulses has to be increased. For this purpose, we implement two consecutive NOPA stages based on two bismuth borate (BiBO) crystals. The output of the DFG stage is first focused into a crystal with 4 mm thickness (XZ plane cut, $\theta = 7.3^\circ$). The first NOPA stage is pumped by the fundamental light (1030 nm) with a pulse energy of $\approx$27 µJ. The amplified SWIR light is refocused into the second BiBO crystal (XZ plane cut, $\theta = 7.8^\circ$, 3 mm thickness) pumped by the remaining $\approx$108 µJ of the 1030 nm radiation.
In both amplification stages, we use type-I phase matching configuration and pump intensities of $\approx$100 GW/cm2. As the seed and the idler have the same polarization and possess a significant spectral overlap, a small non-collinear angle is introduced to separate the amplified signal pulses from the co-generated idler. We use slightly different phase matching angles between the two stages in order to optimize the amplified spectral bandwidth albeit at the cost of decreased conversion efficiency. The first amplification stage mainly targets the short-wavelength side, while the second stage is phase matched to amplify the longer wavelengths. We obtain $\approx$13 µJ of energy in the SWIR pulse, corresponding to an average power of 2.6 W, with a clean beam profile, as indicated by a two-photon absorption image taken by a Silicon-based camera [see inset of Fig. 2(c)]. The Fourier-limited pulse duration of the amplified spectrum is 15.6 fs. This is slightly longer as compared to the DFG. However, the number of cycles under the intensity envelope has only increased to 2.3 cycles, which can be attributed to a shift of the central wavelength.
In order to compress pulses with a broad bandwidth, it is essential to estimate the overall dispersion in the setup and carefully design the pulse compressor. The Group Delay Dispersion (GDD) can simply be compensated by using a combination of different materials featuring negative or positive GDD values in the relevant spectral range. The Third-Order Dispersion (TOD), however, cannot be easily compensated by materials only. Thus, we use custom-made TOD-compensating chirped mirrors (Ultrafast Innovations TOD2102) together with a pair of antireflection-coated zinc sulfide (ZnS) glass wedges. The wedges are motorized to fine tune the dispersion, which also allows us to perform pulse characterization using the dispersion scan (d-scan) technique [34], discussed in the following section.
3. Results
3.1 Pulse compression and characterization
To evaluate the pulse duration, we use third-harmonic generation (THG) d-scan by focusing the pulses into a thin fused silica plate and recording the third harmonic spectrum while scanning the ZnS wedges. We limit the incoming pulse energy into the plate in order to minimize self-phase-modulation-based spectral broadening, as it can affect the shape of the measured d-scan traces and give misleading results.
Firstly, the chirp of the idler beam directly after the DFG is evaluated. This is done to estimate the amount of GDD that is needed to match the pulse duration in the NOPAs as well as to minimize the influence of the third-order phase on the amplification process. The results of the d-scan measurement of the seed pulse are presented in Fig. 3. We retrieve a compressed pulse duration of 14.1 fs FWHM, very close to the Fourier-limit of 13.9 fs. We use the Nelder-Mead optimization algorithm in order to retrieve the spectral phase, a proven and robust method for pulse reconstruction from d-scan traces [34,36]. By calculating the amount of material dispersion and adjusting the number of bounces on the chirped mirrors needed to compress these pulses, we estimate that after the DFG crystal the idler contains approximately 500 fs2 and 4100 fs3 (at 2000 nm wavelength) of GDD and TOD, respectively. Thus, we introduce roughly −1100 fs2 of GDD and −5000 fs3 of TOD into the beam path (a combination of 7 mm of Sapphire and 4 reflections from the chirped mirrors, which remove 100 fs2 of GDD and 2000 fs3 of TOD per reflection). The resulting seed pulse for the NOPA has negative net GDD and TOD, which helps to avoid pulse compression and thus large parasitic nonlinear effects inside the BiBO crystals.
Fig. 3. THG d-scan measurement of the seed (DFG output) pulse. (a) and (c) Measured and retrieved d-scan traces, respectively; (b) Retrieved pulse intensity profile, plotted with the Fourier-limited pulse (FL); (d) Measured spectrum and retrieved spectral phase
To compress the pulses after the two NOPAs, we add only two more bounces from the additional chirped mirror pair placed after amplification in order to compensate the third order phase acquired in the two NOPA stages, and the additional GDD is compensated by moving the ZnS wedges. The results of the d-scan measurement of the amplified output are presented in Fig. 4. The retrieved pulse duration is 15.8 fs FWHM, and the retrieved pulse shape is almost identical to that of the Fourier-limited pulse, with the main differences being in the shape of pre- and post-pulses. Good agreement between measured and retrieved traces for both DFG and NOPA measurements (RMS errors of $0.72$% and $1.9$%, with trace dimensions of 500 and 256 points in wavelength and insertion axes, respectively) signifies reliable retrieval results.
Fig. 4. THG d-scan measurement of the amplified pulse. (a) and (c) Measured and retrieved d-scan traces, respectively; (b) Retrieved pulse intensity profile, plotted with the Fourier-limited pulse (FL); (d) Measured spectrum and retrieved spectral phase
3.2 Carrier-envelope phase stability
Due to the way the seed is generated, the output of the SWIR OPCPA is passively CEP-stable. The CEP stability is evaluated in several ways. By tightly focusing a small fraction of the output into a thick nonlinear crystal [Fig. 5(a)], we generate supercontinuum light and observe CEP-sensitive spectral fringes in the near-infrared and visible regions of the spectrum, corresponding to the interference between various frequency-mixing components (primarily second and third harmonics of the SWIR pulse) and the broadened fundamental - the crystal functions as a spectral CEP interferometer. The CEP sensitivity of the supercontinuum generated by intense SWIR pulses has been reported before in air [37] and various solid targets (e.g. fused silica or CaF2 and YAG crystals [38,39]). In our case, we use a thick nonlinear crystal due to several reasons: (1) it reduces the requirement on the pulse energy needed to generate the supercontinuum, (2) even-order harmonics can be generated simultaneously with the odd harmonics and the supercontinuum, allowing various interference mechanisms with CEP sensitivity (e.g. f - 2f, 2f - 3f), (3) the thickness introduces delay between interfering frequency components which reduces the spacing between interference fringes.
Fig. 5. Monolithic setup for measuring CEP stability. (a) Focusing the SWIR light into a thick nonlinear crystal produces CEP-dependent fringes in various spectral regions, which can be filtered and detected by a spectrometer, OAP - off-axis parabola; (b) Piezo stage delay scan of spectral fringes, the fringe oscillation period is 3.4 fs
In our experiment, we focus $\approx$250 nJ of pulse energy into a 5-mm-thick BBO crystal. By carefully tuning the crystal phase matching condition we obtain spectral fringes in the 600-700 nm region. To confirm that the observed fringes are indeed CEP-sensitive, we perform a time-resolved scan by moving the piezo-driven delay stage located in the 1030 nm arm of the DFG [CEP control in Fig. 5(b)]. The spectral fringe pattern repeats itself every 3.4 fs, corresponding to one oscillation of the electric field of the 1030 nm signal beam. From these observations, we can speculate that the fringes originate either from 2f - 3f or f - 2f interference schemes. In both cases, the $2f$ component is the second harmonic of the broadened fundamental spectrum around 1300 nm. It then can interfere with either a third harmonic $3f$ or directly with supercontinuum $f$. The complicated interplay of spectral broadening and harmonic generation calls for additional future experiments and extensive simulations in order to better understand the origin of the observed signal.
Continuously recording the fringes over 10 minutes (Fig. 6(a), each spectrum averaged over 1.5 ms), we obtain the root-mean-square (RMS) value of the CEP noise to be equal to 255 mrad. We also perform a more conventional CEP measurement using an f - 2f interferometer and obtain 290 mrad RMS, comparable to the measurement with the BBO. Due to the integration time of the spectrometer, high-frequency noise (> 667 Hz) is washed out, and thus we expect the shot-to-shot noise to be higher. In both cases, we observe a slow drift of the interference fringes, which can be attributed to the changes in environmental conditions in the laboratory. On the hourly scale, the fringes tend to move and such long drifts can be straightforwardly compensated by implementing a feedback loop to the piezo stage.
Fig. 6. CEP stability comparison, each spectrum was recorded with 1.5 ms spectrometer integration time. (a) monolithic BBO-based measurement. (b) standard f - 2f interferometer.
3.3 Applications
We test the capabilities of the SWIR amplifier for driving strong-field effects by tightly focusing the output pulses into a high-pressure gas jet and generating high-order harmonics in argon gas. The details of the harmonic generation setup can be found elsewhere [40]. At a backing pressure of 15 bar, our estimates suggest that the gas density in the interaction region corresponds to a pressure of 5.9 bar at standard conditions, considering a gas jet of a 40 µm diameter. The typical spectrum is shown in Fig. 7(a) and extends up to 150 eV in photon energy with distinctive harmonic peaks appearing up to 90 eV. The apparent decrease in flux for high photon energies above 110 eV can be explained by a greatly reduced efficiency of the XUV diffraction grating, as the used XUV spectrometer is designed for photon energies <100 eV (i.e. for characterizing the HHG output from a near-infrared OPCPA). We also carry out an HHG dispersion scan measurement, where we record the high harmonic spectra as a function of dispersion applied to the fundamental pulse by moving the ZnS wedges, thus changing the pulse duration and the CEP of the driving pulse. We observe a steady shift of the harmonic structure due to variations in the CEP (Fig. 7(b)), a characteristic feature of few-cycle CEP-stable HHG sources, which further proves the phase stability of the setup.
Fig. 7. (a) HHG spectrum generated in Argon, the signal is accumulated over 20 seconds. (b) HHG dispersion scan, showing the shift of CEP-sensitive harmonic peaks.
4. Conclusions
In conclusion, we demonstrate a few-cycle OPCPA-based light source operating in the short-wave infrared spectral region at 200 kHz repetition rate. Derived from the output of an Yb fiber CPA system, we generate the SWIR beam via DFG between the visible NOPA and the fundamental 1030 nm radiation, with the idler beam further amplified in two consecutive NOPA stages to an energy of $\approx$13 µJ.
The compression is achieved using TOD-compensating chirped mirrors and a pair of ZnS wedges, resulting in 14.1 fs and 15.8 fs pulse duration for the seed and amplified output, respectively. The source is inherently passively CEP-stabilized, which is confirmed by two independent measurements. We demonstrate a monolithic CEP measurement scheme, straightforward to implement. This method is confirmed by a separate measurement based on f - 2f interferometry.
This SWIR OPCPA constitutes an attractive driving source for strong-field processes e.g. high-harmonic generation in gases and solids, and the high repetition rate of 200 kHz is desirable in many statistics-hungry experiments, e.g. photoionization coincidence studies. Additionally, compressed 2-cycle pulses directly from the DFG without additional amplification can be extremely useful in surface science and plasmonics studies, e.g. when investigating properties of electron photoemission process from metallic nanostructures [41]. Even shorter pulses, possibly approaching the single cycle, can potentially straight-forwardly be obtained by nonlinear post-compression schemes, e.g., nonlinear self-compression in a bulk solid [42]. We performed the first test of the system by generating high-order harmonics in argon and obtained photon energies exceeding 140 eV. With a future planned upgrade to the Yb CPA, the system will be further developed and optimized to produce coherent high-flux soft X-Ray radiation in the water window.
Funding
Vetenskapsrådet (2013-8185, 2016-04907, 2019-06275, 2021-04691); Crafoordska Stiftelsen (20210924); European Research Council (884900); Knut och Alice Wallenbergs Stiftelse; Wallenberg Center for Quantum Technology (WACQT).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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