Ariel Paul

Quasi-phase-matched generation of coherent extreme-ultraviolet light

High-harmonic generation is a well-known method of producing coherent extreme-ultraviolet (EUV) light, with photon energies up to about 0.5 keV (refs 1, 2). This is achieved by focusing a femtosecond laser into a gas, and high harmonics of the fundamental laser frequency are radiated in the forward direction. However, although this process can generate high-energy photons, efficient high-harmonic generation has been demonstrated only for photon energies of the order 50–100 eV (ref. 5). Ionization of the gas prevents the laser and the EUV light from propagating at the same speed, which severely limits the conversion efficiency. Here we report a technique to overcome this problem, and demonstrate quasi-phase-matched frequency conversion of laser light into EUV. Using a modulated hollow-core waveguide to periodically vary the intensity of the laser light driving the conversion, we efficiently generate EUV light even in the presence of substantial ionization. The use of a modulated fibre shifts the energy spectrum of the high-harmonic light to significantly higher photon energies than would otherwise be possible. We expect that this technique could form the basis of coherent EUV sources for advanced lithography and high-resolution imaging applications. In future work, it might also be possible to generate isolated attosecond pulses.

High numerical aperture tabletop soft x-ray diffraction microscopy with 70-nm resolution.

Light microscopy has greatly advanced our understanding of nature. The achievable resolution, however, is limited by optical wavelengths to ≈200 nm. By using imaging and labeling technologies, resolutions beyond the diffraction limit can be achieved for specialized specimens with techniques such as near-field scanning optical microscopy, stimulated emission depletion microscopy, and photoactivated localization microscopy. Here, we report a versatile soft x-ray diffraction microscope with 70- to 90-nm resolution by using two different tabletop coherent soft x-ray sources—a soft x-ray laser and a high-harmonic source. We also use field curvature correction that allows high numerical aperture imaging and near-diffraction-limited resolution of 1.5λ. A tabletop soft x-ray diffraction microscope should find broad applications in biology, nanoscience, and materials science because of its simple optical design, high resolution, large depth of field, 3D imaging capability, scalability to shorter wavelengths, and ultrafast temporal resolution.

Tabletop soft-x-ray Fourier transform holography with 50 nm resolution

We present what we believe to be the first implementation of Fourier transform (FT) holography using a tabletop coherent x-ray source. By applying curvature correction to compensate for the large angles inherent in high-NA coherent imaging, we achieve image resolution of 89 nm using high-harmonic beams at a wavelength of 29 nm. Moreover, by combining holography with iterative phase retrieval, we improve the image resolution to <53 nm. We also demonstrate that FT holography can be used effectively with short exposure times of 30 s. This technique will enable biological and materials microscopy with simultaneously high spatial and temporal resolution on a tabletop soft-x-ray source.

Strain relaxation in patterned strained silicon directly on insulator structures

Strain relaxation is studied in strained silicon directly on insulator (SSDOI) substrates patterned with nanoscale features. Using interference lithography, biaxially strained SSDOI substrates with 30nm thick strained Si on insulator films were patterned into grating structures with 90nm wide stripes, and arrays of 80nm×170nm pillars. The strain profiles of these patterned structures were examined by ultraviolet Raman spectroscopy. Raman analysis of the SSDOI gratings indicates strain relaxation in the 90nm wide stripes, compared to the strain measured in unpatterned portions of the SSDOI wafer. Three-dimensional finite-element modeling of the stress distributions in the grading structures predicts that 95% of the strain is maintained in the direction along the length of the stripes. These simulations are used to decouple the strain components along the width and length of the SSDOI grating structure, inferred from Raman measurements. The results are consistent with substantial stress relaxation across th...

Phase-matching techniques for coherent soft X-ray generation

Coherent beams at soft X-ray (SXR) wavelengths can be generated using extreme nonlinear optics by focusing an intense laser into a gas. In this paper, we discuss phase-matching and quasi-phase-matching techniques that use gas-filled modulated waveguides to enhance the frequency conversion process. This leads to the generation of SXR beams that are both spatially and temporally coherent.

Highly coherent light at 13 nm generated by use of quasi-phase-matched high-harmonic generation

By measuring the fringe visibility in a Youngs double pinhole experiment, we demonstrate that quasi-phase-matched high-harmonic generation produces beams with very high spatial coherence at wavelengths around 13 nm. To our knowledge these are the highest spatial coherence values ever measured at such short wavelengths from any source without spatial filtering. This results in a practical, small-scale, coherent, extreme-ultraviolet source that is useful for applications in metrology, imaging, and microscopy.

Time-resolved momentum imaging system for molecular dynamics studies using a tabletop ultrafast extreme-ultraviolet light source

We describe a momentum imaging setup for direct time-resolved studies of ionization-induced molecular dynamics. This system uses a tabletop ultrafast extreme-ultraviolet (EUV) light source based on high harmonic upconversion of a femtosecond laser. The high photon energy (around 42 eV) allows access to inner-valence states of a variety of small molecules via single photon excitation, while the sub--10-fs pulse duration makes it possible to follow the resulting dynamics in real time. To obtain a complete picture of molecular dynamics following EUV induced photofragmentation, we apply the versatile cold target recoil ion momentum spectroscopy reaction microscope technique, which makes use of coincident three-dimensional momentum imaging of fragments resulting from photoexcitation. This system is capable of pump-probe spectroscopy by using a combination of EUV and IR laser pulses with either beam as a pump or probe pulse. We report several experiments performed using this system.

Long-term carrier-envelope phase stability from a grating-based, chirped pulse amplifier

We demonstrate a carrier-envelope phase (CEP) stabilized, chirped pulse laser amplifier that exhibits greatly improved intrinsic long-term CEP stability compared with that of other amplifiers. This system employs a grating-based stretcher and compressor and a cryogenically cooled laser amplifier. Single-shot carrier envelope phase noise measurements are also presented that avoid underestimation of this parameter caused by fringe averaging and represent a rigorously accurate upper limit on CEP noise.

Absolute determination of the wavelength and spectrum of an extreme-ultraviolet beam by a Young’s double-slit measurement

The interference pattern produced by irradiation of a pair of pinholes with a beam contains information on both the spatial and the temporal coherence properties of the beam, as well as its power spectrum. We demonstrate experimentally for what is believed to be the first time that the spectrum of an extreme-ultraviolet (EUV) beam can be obtained from a measurement of the interference pattern produced by a pinhole pair. This approach offers a convenient method of making absolute wavelength and relative spectral intensity calibrations in the EUV.

Phase matching, quasi-phase matching, and pulse compression in a single waveguide for enhanced high-harmonic generation

We demonstrate, for the first time to our knowledge, that the efficient region of high-harmonic generation can be shifted from lower to higher photon energies by combining phase matching, quasi-phase matching, and pulse compression in a single gas-filled waveguide. An intrawaveguide pulse compression process that works through a combination of ionization-induced refraction and guiding shortens the laser pulse as it propagates through an Ar-filled waveguide. This leads to enhanced harmonic emission at high photon energies near 95 eV while it reduces emission at low photon energies near 45 eV. The waveguide geometry also mitigates ionization-induced refraction, allowing Ar gas with high effective nonlinear susceptibility to be used.