Probe of high-order harmonic generation in a hollow waveguide geometry using counterpropagating light

Enhanced XUV harmonics generated in mixed noble gases using three-color laser fields

High repetition coherent extreme ultraviolet (XUV) harmonics offer a powerful tool for investigating electron dynamics and understanding the underlying physics in a wide range of systems. We demonstrate the utilization of combined three-color (ω+2ω+3ω) laser fields in the generation of coherent extreme ultraviolet radiation in mixed noble gases. The three-color field results from the combination of fundamental, second-, and third-order harmonics of the near-infrared laser pulses in the nonlinear crystals. Different noble gases were selected as gas targets based on their ionization potentials, which are important parameters for generating higher cut-offs and intensities for the XUV harmonics. Enhanced XUV harmonic intensities were observed in the mixture of He + Kr gases when using three-color laser fields, compared to harmonics generated in the He + Kr mixture under a single-color pump. On the other hand, suppression of XUV harmonic intensity was observed in the mixture of He + Xe under the three-color pump due to the highest ionization level for these two mixed gases at similar laser conditions. Strong harmonic yields in the range of 25 to 80 eV of photon energy were observed.

Broadband second harmonic generation of counter-propagating ultrashort pulses

Counter-propagating ultrafast pulses can disrupt the phase of harmonic generation, and offer a means to achieve quasi-phase matching in processes like high-order harmonic generation. Optimizing this process requires accurate modeling. Using second harmonic generation (SHG) as a simpler and more accessible proxy, we compare the results of two numerical simulations to experimental measurements of SHG with counter-propagating pulses. The first follows previous theoretical work in assuming a quasi-CW pulse and solving the nonlinear wave equation in the time-domain. However, we find that adapting a frequency-domain model to account for the broadband nature of ultrafast pulses better reproduces the salient features we observe in our experimental results.

On-the-Fly Control of High-Harmonic Generation Using a Structured Pump Beam

We demonstrate experimentally a relatively simple yet powerful all-optical enhancement and control technique for high harmonic generation. This is achieved by using as a pump beam two different spatial optical modes interfering together to realize tunable periodic quasi-phase matching of the interaction. With this technique, we demonstrate on-the-fly quasi-phase matching of harmonic orders 29-41 at ambient gas pressure levels of 50 and 100 Torr, where an up to 100-fold enhancement of the emission is observed. The technique is scalable to different harmonic orders and ambient pressure conditions.

Controlled growth and optical response of a semi-hollow plasmonic nanocavity and ultrathin sulfide nanosheets on Au/Ag platelets

Herein, we report a strategy to construct a semi-hollow plasmonic nanocavity and grow ultrathin sulfide nanosheets inside. The competition and cooperation of Au deposition with Ag etching based on flat Ag nanoplates are proposed. For the establishment of the semi-hollow nanocavity, Au shells are grown on Ag nanoplates, which serve as a stable frame, followed by partial etching of the Ag nanoplates. By controlling the thickness of the initial Ag nanoplates or the injected amount of etchant, the nanocavity size is fine-tuned. Significantly, the remaining unetched Ag layers provide a flat platform for the growth of 2D ultrathin sulfides of Ag₂S and CdS inside the semi-hollow plasmonic nanocavity. Strong plasmon resonance and large local field enhancement are exhibited inside the plasmonic cavity where the ultrathin semiconductor sulfides are grown, indicating strong plasmon-exciton interactions in the hybrids. Furthermore, this synthetic approach is extended to grow other metal sulfides such as Bi₂S₃ and PbS. The combination of a flat plasmonic cavity with ultrathin semiconductor nanosheets in this study provides a new strategy for the development of unique plasmon-based hybrids with excellent optical properties.

Quasi-phase-matched high-order harmonic generation using tunable pulse trains

A simple technique for generating trains of ultrafast pulses is demonstrated in which the linear separation between pulses can be varied continuously over a wide range. These pulse trains are used to achieve tunable quasi-phase-matching of high harmonic generation over a range of harmonic orders up to the harmonic cut-off, resulting in enhancements of the harmonic intensity in excess of an order of magnitude. The peak enhancement of the harmonics is clearly shown to depend on the separation between pulses, as well as the number of pulses in the train, representing an easily tunable source of quasi-phase-matched high harmonic generation.

Quasi-phase-matching high harmonic generation using trains of pulses produced using an array of birefringent plates

Quasi-phase-matched high harmonic generation using trains of up to 8 counter-propagating pulses is explored. For trains of up to 4 pulses the measured enhancement of the harmonic signal scales with the number of pulses N as (N + 1)², as expected. However, for trains with N > 4, no further enhancement of the harmonic signal is observed. This effect is ascribed to changes of the coherence length Lc within the generating medium. Techniques for overcoming the variation of Lc are discussed. The pressure dependence of quasi-phase-matching is investigated and the switch from true-phase-matching to quasi-phase-matching is observed.

Phase matching of high harmonic generation in the soft and hard X-ray regions of the spectrum

We show how bright, tabletop, fully coherent hard X-ray beams can be generated through nonlinear upconversion of femtosecond laser light. By driving the high-order harmonic generation process using longer-wavelength midinfrared light, we show that, in theory, fully phase-matched frequency upconversion can extend into the hard X-ray region of the spectrum. We verify our scaling predictions experimentally by demonstrating phase matching in the soft X-ray region of the spectrum around 330 eV, using ultrafast driving laser pulses at 1.3-microm wavelength, in an extended, high-pressure, weakly ionized gas medium. We also show through calculations that scaling of the overall conversion efficiency is surprisingly favorable as the wavelength of the driving laser is increased, making tabletop, fully coherent, multi-keV X-ray sources feasible. The rapidly decreasing microscopic single-atom yield, predicted for harmonics driven by longer-wavelength lasers, is compensated macroscopically by an increased optimal pressure for phase matching and a rapidly decreasing reabsorption of the generated X-rays.

Quasi-phase-matching and dispersion characterization of harmonic generation in the perturbative regime using counterpropagating beams

It is shown theoretically that second harmonic generation can be quasi-phase-matched by using a pump beam consisting of a forward propagating field and a counterpropagating pulse train. The counterpropagating setup can also be used for direct measurement of the coherence length of the nonlinear process which can determine the dispersion properties of the medium.

Quasi-periodic and random quasi-phase matching of high harmonic generation

Quasi-phase matching schemes employing quasi-periodic or random spatial modulations, previously applied to perturbative nonlinear optics, are demonstrated theoretically for the extreme nonlinear optical process of high harmonic generation. We show that quasi-periodic quasi-phase matching of high harmonic generation can be used for simultaneous enhancement of arbitrarily chosen spectral regions. We also demonstrate enhancement of a single extremely wide bandwidth using random quasi-phase matching.

Lensless diffractive imaging using tabletop coherent high-harmonic soft-X-ray beams

We present the first experimental demonstration of lensless diffractive imaging using coherent soft x rays generated by a tabletop soft-x-ray source. A 29 nm high harmonic beam illuminates an object, and the subsequent diffraction is collected on an x-ray CCD camera. High dynamic range diffraction patterns are obtained by taking multiple exposures while blocking small-angle diffraction using beam blocks of varying size. These patterns reconstruct to images with 214 nm resolution. This work demonstrates a practical tabletop lensless microscope that promises to find applications in materials science, nanoscience, and biology.

Grating-assisted phase matching in extreme nonlinear optics

We propose a new technique for phase matching high harmonic generation that can be used for generating bright, tabletop, tunable, and coherent x-ray sources at keV photon energies. A weak quasi-cw counterpropagating field induces a sinusoidal modulation in the phase of the emitted harmonics that can be used for correcting the large plasma-induced phase mismatch. We develop an analytical model that describes this grating-assisted x-ray phase matching and predicts that very modest intensities (<10(10) W/cm2) of quasi-cw counterpropagating fields are required for implementation.