High-spatial-resolution ultrafast framing imaging at 15 trillion frames per second by optical parametric amplification

Summary form only given. The coherent synthesis of custom-tailored, intense, sub-cycle optical waveforms is promising for attosecond science and strong-field physics, e.g., for precision control of strong-field interactions in atoms, molecules and solids, for the generation of intense isolated attosecond pulses, and for attosecond pump-probe spectroscopy.Recently, coherent pulse synthesis based on supercontinuum generation in a hollow-core fiber compressor allowed the generation of sub-cycle -300-μJ optical pulses. However, in this case ionization losses in the gas medium prevent further scaling to the mJ level. In contrast, parametric synthesizers do not face an energy scaling limit, and allow for spectral extension into the particularly appealing MIR region. In previous works we demonstrated coherent pulse synthesis of two optical parametric chirped-pulse amplifiers and of two optical parametric amplifiers (OPAs) on the few-μJ level [3]. Here we present the ongoing development of a novel 3-channel parametric synthesizer for generating a 2-octave-wide spectrum, which can easily be upscaled to the mJ-level. We start from a cryogenically cooled Ti:sapphire chirped-pulse amplifier (150 fs, 22 mJ, 0.8 μm, 1 kHz) and generate a CEP-stable continuum (0.5-2.3 μm) [5], by white-light generation in a YAG crystal pumped by the second harmonic (1.06 μm) of the CEP-stable idler of a NIR OPA. The continuum is split with custom-designed dichroic beam splitters (which will also be used for the final beam recombination) and seeds three OPAs, a VIS noncollinear OPA (NOPA), a NIR and an IR degenerate OPA (DOPA), each composed of 2 (later 3) amplification stages, and pumped by the pulses at 0.8 μm (IR DOPA), and by its second harmonic at 0.4 μm (VIS NOPA, NIR DOPA). To synthesize a coherent ultrashort pulse from these three OPAs, the relative timing of the pulses will be tightly locked using feedback loops with balanced optical cross-correlators, that can achieve sub-cycle synchronization with s0-as RMS timing jitter [2,3]. Figure 1(a) shows the measured output spectra and the energies from the second amplification stages operating in parallel. The transform-limited (TL) pulse duration from the synthesis of these spectra is 1.9 fs FWHM, corresponding to 0.7 optical cycles at 785 nm center wavelength. Our current work aims to scale up the energies (by a third amplification stage) to -0.5 mJ for the VIS NOPA/NIR DOPA and -2 mJ for the IR DOPA using the 18.5 mJ of pump remaining after the first two amplification stages. State-of-the-art double-chirped mirror pairs (at present in fabrication) will allow for the final pulse compression with ultralow residual ripple in the resulting total group-delay dispersion over the full bandwidth (0.52-2.3μm). Temporal characterization of the synthesized two-octave-spanning optical waveforms will be performed by two-dimensional spectral shearing interferometry (2DSI). We foresee that our 3-channel synthesizer, when further scaled in energy to the mJ level, will become a versatile tool for controlling strong-field interactions in atoms, molecules and solids and for attosecond pumpprobe spectroscopy employing ultrashort pulses in the VIS/IR and XUV/soft-X-ray regions.

Tilting the pump pulse front has been proposed for efficient phase-matched THz generation by optical rectification of femtosecond laser pulses in LiNbO3 (Hebling et al., 2002). By using amplified Ti:sapphire laser systems for pumping, this technique has recently resulted in generation of near-single-cycle THz pulses with energies on the 10-μJ scale (Yeh et al., 2007, Stepanov et al., 2008). Such high-energy THz pulses have opened up the field of subpicosecond THz nonlinear optics and spectroscopy (Gaal et al., 2006, Hebling et al., 2008a). The method of tilted-pulse-front pumping (TPFP) was introduced as a synchronization technique between the optical pump pulse and the generated THz radiation. Synchronization was accomplished by matching the group velocity of the optical pump pulse to the phase velocity of the THz wave in a noncollinear propagation geometry. Originally, TPFP was introduced for synchronization of amplified and excitation pulses in so called traveling-wave laser amplifiers (Bor et al., 1983). By using such traveling-wave excitation (TWE) of laser materials, especially dye solutions, extremely high gain (109) and reduced amplified spontaneous emission could be obtained (Hebling et al., 1991). Contrary to the case of TWE, when TPFP is used for THz generation by optical rectification, a wave-vector (momentum) conservation condition or, equivalently, a phase-matching condition has to be fulfilled. It was shown (Hebling et al., 2002), that such condition is automatically fulfilled if the synchronization (velocity matching) is accomplished. The reason is that in any tilted pulse front there is present an angular dispersion of the spectral components of the ultrashort light pulse and there is a unique connection between the tilt angle of the pulse front and the angular dispersion (Bor & Racz, 1985, Martinez 1986, Hebling 1996). Angular dispersion was introduced into the excitation beam of so called achromatic frequency doubler (Szabo & Bor, 1990, Martinez, 1989) and sum-frequency mixing (Hofmann et al., 1992) setups in order to achieve broadband frequency conversion and keeping the ultrashort pulse duration. It was pointed out that in non-collinear phasematched optical parametric generators (OPG) and optical parametric amplifiers (OPA) tilted pulse fronts are expected (Di Trapani et al., 1995). TPFP was used in the non-collinear OPA (NOPA) producing sub-5-fs pulses (Kobayashi & Shirakawa, 2000). The different aspect of tilted pulse front and angular dispersion is usually not mentioned in these papers dealing with broadband frequency conversion. It is well known that the bandwidth of parametric processes is connected to the relative group velocities of the interacting pulses (Harris, 1969). Phase matching to first order in frequency

We have developed a tunable, high-repetition-rate, broadband noncollinear optical parametric amplifier (NOPA) system pumped by a 1030 nm Yb:KGW laser. The system employs a prism-based dispersion compensation scheme that effectively corrects the wavefront tilt between the pump and seed pulses. Experimental results show that the NOPA provides single-shot output covering a spectral range of approximately 680–980 nm and enables tunable signal generation within 620–980 nm. The proposed design demonstrates excellent robustness and broad spectral coverage, offering a practical and efficient solution for realizing high-repetition-rate broadband light sources with great potential for applications in ultrafast spectroscopy and imaging.

Femtosecond pulses are primarily generated at 800 or 1030 nm in Tiisapphire or Yb based systems. To generate fully tunable pulses over the full visible and NIR range, optical parametric amplifiers and in particular white light seeded noncollinear optical parametric amplifiers (NOPA) are now widely used. For highest energy pulses this scheme is expanded to OPCPAs. Energy and beam pointing stability is a crucial topic in the application of all these sources. We investigated — to the best of our knowledge for the first time — how the beam pointing instability of the pump laser source translates to the NOPA output.

The spontaneous fluorescence background in optical parametric amplifiers is generally attributed to the zero-point fluctuations of the electromagnetic field. These are amplified in parallel to the seed light and lead to an uncompressible superfluorescence background that deteriorates the contrast in optical parametric chirped pulse amplifiers (OPCPA). The absolute level of the underlying parametric fluorescence has not been reported so far. Comparing the fluorescence to low level cw seed light and quantitatively monitoring the output of a noncollinear optical parametric amplifier for both sources, the level is now determined. In a situation of 50 nm visible output bandwidth and low Gaussian spatial modes about 58 photons are found in the signal direction within the femtosecond time window of the amplifier. The superfluorescence level is observed to be proportional to the pump area for constant signal amplification. The implications for the background in high power OPCPA are discussed.

Angular dispersion of pump frequencies is shown to be an efficient mechanism for bandwidth enhancement in a noncollinear optical parametric amplifier. We demonstrate the generation of a continuous, simultaneously phase-matched 250-THz parametrically amplified spectrum. The resultant visible-near-IR signal-wave pulses were compressed to a 4-fs duration by a micromachined flexible mirror. Feedback for an iterative computer-controlled dispersion compensation algorithm is based on pulse characterization by second-harmonic generation frequency-resolved optical gating.

Noncollinear optical parametric amplification (NOPA) is now established as a unique technology to generate ultrashort tunable pulses with high flexibility for spectroscopic applications. With the availability of commercial ultrafast fiber lasers delivering energies of several tens of microjoules, such devices can be designed to operate at unprecedented repetition rates of several MHz, enabling shorter acquisition time as well as better signal-to-noise ratio. Fiber laser-pumped tunable sources are also valuable tools for X-ray time resolved studies, to match the MHz repetition rate of synchrotrons or free electrons laser facilities. We report here on the development of a new tunable ultrashort NOPA pumped by a fiber laser at 500 kHz repetition rate with improved performances over previously published similar designs.

Ultrafast imaging using plane or diverging waves has recently enabled new ultrasound imaging modes with improved sensitivity and very high frame rates. Some of these new imaging modalities include shear wave elastography, ultrafast Doppler, ultrafast contrast-enhanced imaging and functional ultrasound imaging. Even though ultrafast imaging already encounters clinical success, increasing even more its penetration depth and signal-to-noise ratio for dedicated applications would be valuable.Ultrafast imaging relies on the coherent compounding of backscattered echoes resulting from successive tilted plane waves emissions; this produces high-resolution ultrasound images with a trade-off between final frame rate, contrast and resolution. In this work, we introduce multiplane wave imaging, a new method that strongly improves ultrafast images signal-to-noise ratio by virtually increasing the emission signal amplitude without compromising the frame rate. This method relies on the successive transmissions of multiple plane waves with differently coded amplitudes and emission angles in a single transmit event. Data from each single plane wave of increased amplitude can then be obtained, by recombining the received data of successive events with the proper coefficients.The benefits of multiplane wave for B-mode, shear wave elastography and ultrafast Doppler imaging are experimentally demonstrated. Multiplane wave with 4 plane waves emissions yields a 5.8 ± 0.5 dB increase in signal-to-noise ratio and approximately 10 mm in penetration in a calibrated ultrasound phantom (0.7 d MHz−1 cm−1). In shear wave elastography, the same multiplane wave configuration yields a 2.07 ± 0.05 fold reduction of the particle velocity standard deviation and a two-fold reduction of the shear wave velocity maps standard deviation. In functional ultrasound imaging, the mapping of cerebral blood volume results in a 3 to 6 dB increase of the contrast-to-noise ratio in deep structures of the rodent brain.

A novel superstructure fiber Bragg grating (SFBG) based comb gain equalizer for fiber optical parametric amplifier (OPA) is proposed. This gain equalizer is characterized by its comb reflectivity spectrum. It can have two functions at least, one is flattening the gain in multi ITU-T regulated channels, the other is it can be used as multi-channel isolation filters taking full advantage of its comb reflectivity spectrum. In addition it can be dispersionless or have dispersion compensation function. A one-bump fiber OPA, 30dB bandwidth >15nm, is designed with one of its peak gains (41dB) located at the center of work wavelengths in a 16-channel 100GHz spaced WDM optical transmission system according to corresponding Chinese criterion (1548.51nm~ 1560.61nm). The object reflectivity spectra of gain equalizer are calculated in two parts independently. Inner channel part for every channel is designed according to the gain spectrum of fiber OPA. As a comb filter, its inter-channel part reflectivities are the smaller the better. But inter-channel part reflectivities can not be simply zeroed, for which would result in the rising and descending edges of reflectivity spectrum being too sharp to be performed by any filter. To split the difference, inter-channel part reflectivities are calculated with Gaussian hypergeometric function (-0.5dB bandwidth 0.1nm, -45dB bandwidth 0.3nm) to obtain slower rising and descending edges than the simply zeroed case, which is very necessary and important to be sure the object reflectivity spectrum not only has sharp enough edges to take the shape of comb but also is physically realizable. Adopting channel-by-channel windowing method to apodize the object reflectivity spectrum, a superstructure fiber Bragg grating is designed with lay-peeling inverse scattering technique. At last, according to the coupling coefficient calculated, reflection spectrum and group time delay are analyzed based on the method of transformation matrix. Calculation results show that this SFBG satisfies all requirements. Further calculation results show that, using this SFBG as comb gain equalizer, the gain of fiber OPA is flattened in every channel to within ±0.4dB among 16 channels, with channel isolation high up to -35dB. It needs to point out that this method can also be used to design gain equalizer for other optical amplifiers.

INTRODUCTION The phenomenon of superfluorescence using photoexcitation followed by induced synchronization of the dipoles is important not only from the fundamental viewpoint of understanding the interaction between light and matter but also from the applied viewpoint of realizing the miniaturization of light sources. Organic-inorganic lead halide perovskite materials have various attractive properties not only for solar cells but also for LED and nanoscale lasers because of their wavelength tunability and low lasing threshold. Such an efficient lasing is indispensable for their applications, and the essential needs are miniaturization and low threshold, especially for quasi-2D systems[1]. In this study, we aim to fabricate quasi-two-dimensional perovskite crystals and observe superfluorescence phenomenon by transient absorption microscopy. EXPERIMENTAL SECTION 110 mg of lead bromide, 70 mg of cesium bromide, and 24 mg of phenethyl ammonium bromide were dissolved in 3 mL of dimethyl sulfoxide and the solution was sealed in a spray vial under a nitrogen atmosphere. Then spray was applied onto a cover glass under a nitrogen atmosphere, and crystals were allowed to grow under isopropanol vapor for 8 min at 50°C.Transient absorption microscopy setup was the system previously reported[2]. A regenerative amplified Ti: sapphire laser (Solstice, Spectra-Physics, 1 kHz, 795 nm, output power about 3.5 W) was a light source. The 795 nm fundamental beam output from the regenerative amplifier was then split into two equal intensities by a beam splitter, and output light was obtained from an optical parametric amplifier (OPA, TOPAS, light-conversion). The signal light (1.3 μm) guided from OPA was directed onto an amorphous CaF2 plate to generate white light. This white light was divided into two parts by a half-mirror. A portion of the white light was transmitted to the sample on the microscope as the monitoring light. And the other part was used as the reference light. Both the monitoring and reference lights were guided into a multichannel spectrometer (Orca quest) for spectral analysis. The reference light is to correct the intensity fluctuation of the white light. Excitation light was generated from a home-built non-collinear OPA (NOPA) at 550 nm and introduced to the microscope. The temporal resolution was ca. 150 fs. RESULTS AND DISCUSSION The excitation light intensity dependence of the emission spectrum of a quasi-two-dimensional perovskite crystal was measured with a femtosecond 400 nm excitation. A new emission maximum is observed above 173 mJ/cm2. To clarify the origin of this emission, the femtosecond transient absorption spectrum was measured (Exc. 400 nm). The time profiles of transient absorbance at 526 nm show a positive signal in the early stage and a negative signal over 1 ps in a single quasi-two-dimensional perovskite crystal. The positive signal suggests the photoinduced absorption of the carrier in a few layers of two-dimensional perovskite. The negative signal suggests the bleaching and stimulated emission signal of perovskite in bulk. Hence, these changes from positive to negative signals indicate the energy transfer from a few layers to bulk. Fast decay components of stimulated emission were changed by changing the intensity of excitation pulses. To analyze the decay components of stimulated emission, global analysis was done for each time profile. Fig. shows the excitation intensity dependence of the lifetime of the stimulated emission. The initial lifetime component was related to the excitation light intensity by 1/N. These behaviors show the same dependence on excitation light intensity as the superfluorescence phenomena observed in time-resolved luminescence measurements[1]. We will discuss the spectrum evolution and relationship between energy transfer and superfluorescence at the conference site.REFERENCES[1] M. Biliroglu, K. Gundogdu et al., Nature photonics, 2022, 16, 324-329.[2] T. Katayama, A. Furube et al., Jpn. J. Appl. Phys., 2023, 62(SG), SG1030. Figure 1

Theoretical analysis of noncollinear phase-matched optical parametric amplifier seeded by a white-light continuum

Generation of multi-color carrier-envelope phase locked pulse with continuous color tunability

We have investigated the possibility of using optical parametric amplifiers (OPA) as optical parametric chirped pulse amplifiers (OPCPA), in which energy from a narrow bandwidth pump source is efficiently transferred to a chirped signal pulse prior to its recompression (1).

Recently some of us have shown that the use of a correct ab initio approach to nonlinear pulse propagation simulations during nonlinear optical device designing can result in threefold efficiency increase with respect to the efficiency of existing solutions [1]. In that work we have focused on small beam size, and thus, high divergence regime where the effects of diffraction, spatial and temporal walk-off are difficult to separate and thus require numerical approach. Our collinear model of pulse propagation enabled modelling and optimization of a cascade third harmonic generation in a single element tripler. Herewith, we present the results of expanding our model to treatment of non-collinear optical configurations and the model application to our OPCPA design [2,3]. To the best of our knowledge this is a first propagation model that enables non-collinear configurations while in minimal assumptions regime: unidirectionality and paraxial approximation. By definition the non-collinear propagation is required when interaction of two or more beams is considered. In this case the use of not necessarily slowly varying envelope for each interacting beam is justified from both physical and computational reasons. In case of 3D non-collinear propagation the above conclusion leads to the concept of reference wavevector. Our model is based on unidirectional pulse propagation equation (UPPE [4]) in a rotated frame of reference. Rotation by a different angle has to be, however, performed separately for each of the interacting beams. Apparently even for quite high mutual beam angles it is enough to solve scalar, rather than vectorial, version of UPPE and sustain accuracy. Finally the initial conditions (rotated optical pulses) can be prepared through arbitrary 3D rotation through Fourier Transform shear operations. The idea, realization and advantages of the above mentioned, novel concepts: reference wavevector, rotated UPPE and arbitrary Fourier rotation will be discussed in the presentation. The real life examples of model results for the usage will be presented: an LBO based chirped pulse non-collinear optical parametric amplifier working in "exotic" (off major plane) phase matching conditions and the redesigned multiterawatt BiBO based OPCPA system [2]. 1. T. M. Kardaś, M. Nejbauer, P. Wnuk, B. Resan, C. Radzewicz, and P. Wasylczyk, "Full 3D modelling of pulse propagation enables efficient nonlinear frequency conversion with low energy laser pulses in a single-element tripler," Scientific Reports 7, 42889 (2017). 2. P Wnuk, Y Stepanenko, C Radzewicz "Multi-terawatt chirped pulse optical parametric amplifier with a time-shear power amplification stage", Optics Express 17(17), 15264 (2009) 3. Y. Stepanenko, "On the efficiency of a multiterawatt optical parametric amplifier: numerical model and optimization," JOSA B 28, 2337–2346 (2011). 4 M. Kolesik and J. V. Moloney, "Nonlinear optical pulse propagation simulation: From Maxwell's to unidirectional equations," Phys. Rev. E 70, (2004).

The noncollinear optical parametric amplification in BBO crystal is theoretically investigated. The phase matching angle, gain bandwidth, optimal noncollinear angle and conversion efficiency for both type-1 and type-II BBO are simulated. The numerical simulation results are important to the practical optical parametric amplification experiments with BBO crystal.