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Nanostructured optical surfaces allow exquisite control over linear and nonlinear light interactions, where the surface actively creates new frequencies up to high-order harmonics of an intense infrared driving laser field. The function and performance of these surfaces depend sensitively on the distribution of the high-harmonic fields in and between the nanostructured elements, as the high-harmonic wavelength becomes comparable to the nanoscale features of the surface. Imaging the nonlinear response at the active surface with nanometer resolution would greatly benefit the optimization of the metasurface's function. Here we demonstrate an approach to lensless imaging of extreme ultraviolet high harmonics that resolves the amplitude and phase of nonlinear polarization at the active nanostructured surface of an MgO crystal. Imaging the near-field distribution of high harmonics is the gateway to optimized functional high-harmonic metasurfaces and the integration of high harmonics on a chip.

In this work we use helium ion beam nanolithography to structure the surface of an MgO crystal and control the emission of extreme-ultraviolet high harmonics. We demonstrate their focusing to below 300 nm in waist diameter.

Plane-by-plane fabrication of fiber Bragg gratings in optical fibers using short-pulse femtosecond IR laser is proposed and demonstrated. By incorporating a cylindrical lens in the fabrication setup, a plane of index modification can be directly inscribed in fiber core by a single laser pulse through the proposed method. This plane-by-plane method simplifies the grating inscription process and allows for the fabrication of complicated grating structures., 25th International Conference on Optical Fiber Sensors 2017, 24-28 April 2017, Jeju, South Korea, Series: Proceedings of SPIE; no. 10323

Hitting the highs in solid state The ability to generate high harmonics of optical frequencies through the nonlinear interaction between intense light pulses and gas atoms has opened up the area of ultrafast optics and spectroscopy. Sivis et al. now show that high harmonics can also be generated with a solid-state sample. They used nanofabricated structured targets of ZnO and varied the chemical composition of the sample to demonstrate that (modest) high harmonics can be generated as the light interacts with the target materials. The results present the possibility of developing solid-state ultrafast optical devices. Science , this issue p. 303

It has been known for many years that during filamentation of femtosecond light pulses in air, gain is observed on the B to X transition in N_{2}^{+}. While the gain mechanism remains unclear, it has been proposed that recollision, a process that is fundamental to much of strong field science, is critical for establishing gain. We probe this hypothesis by directly comparing the influence of the ellipticity of the pump light on gain in air filaments. Then, we decouple filamentation from gain by measuring the gain in a thin gas jet that we also use for high harmonic generation. The latter allows us to compare the dependence of the gain on the ellipticity of the pump with the dependence of the high harmonic signal on the ellipticity of the fundamental. We find that gain and harmonic generation have very different behavior in both filaments and in the jet. In fact, in a jet we even measure gain with circular polarization. Thus, we establish that recollision does not play a significant role in creating the inversion.

We demonstrate new possibilities to tailor high harmonic emission in semiconductors using nanofabrication. Morphology and local composition can each localize and enhance emission, which can be used to realize new high harmonic devices.

Attosecond extreme-ultraviolet pulses have a complex space–time structure. However, at present, there is no method to observe this intricate detail; all measurements of the duration of attosecond pulses are, to some extent, spatially averaged. A technique for determining the full space–time structure would enable a detailed study of the highly nonlinear processes that generate these pulses as a function of intensity without averaging. Here, we introduce and demonstrate an all-optical method to measure the space–time characteristics of an isolated attosecond pulse. Our measurements show that intensity-dependent phase and quantum-path interference both play a key role in determining the pulse structure. In the generating medium, the attosecond pulse is strongly modulated in space and time. Propagation modifies but does not erase this modulation. Quantum-path interference of the single-atom response, previously obscured by spatial and temporal averaging, may enable measuring the laser-field-driven ion dynamics with sub-cycle resolution.

The nanoscale localization of light aided by the plasmonic response of materials goes hand in hand with enhancing its oscillating electric field. In the vicinity of the antenna, the electric field of a nano-Joule laser pulse can become strong enough to convert the laser light into high order harmonics through an extremely nonlinear interaction with gas atoms that occupy the nanoscopic volume. However, previously reported plasmon-assisted high harmonics [1,2] are suspected to be fluorescence [3,4]. The problem seems to be (i) the small number of gas atoms that can occupy the nanoscopic volume and (ii) the low damage threshold of the plasmonic antennas [5]. Here we use monopole nano-plasmonic antennas to demonstrate plasmon-assisted high harmonic generation directly from the high-density crystalline substrate that hosts an array of antennas. Built on a silicon substrate, the low bandgap of silicon relative to the ionization potential of rare gas atoms allows harmonic emission at much lower intensities. Imaging the high harmonic radiation will allow nanometer and attosecond measurement of the plasmonic field, unveiling collective relaxation dynamics and field reshaping while opening a new path to XUV frequency combs.

Here, we investigate the enhancement and localization of high harmonic generation in a nanostructured ZnO-crystal by spectral analysis and microscopic imaging. We show a robust method of distinguishing site-selective harmonic emission from the bulk emission.

We produce oriented rotational wave packets in CO and measure their characteristics via high harmonic generation. The wave packet is created using an intense, femtosecond laser pulse and its second harmonic. A delayed 800 nm pulse probes the wave packet, generating even-order high harmonics that arise from the broken symmetry induced by the orientation dynamics. The even-order harmonic radiation that we measure appears on a zero background, enabling us to accurately follow the temporal evolution of the wave packet. Our measurements reveal that, for the conditions optimum for harmonic generation, the orientation is produced by preferential ionization which depletes the sample of molecules of one orientation.

We bring the methodology of orienting polar molecules together with the phase sensitivity of high harmonic spectroscopy to experimentally compare the phase difference of attosecond bursts of radiation emitted upon electron recollision from different ends of a polar molecule. This phase difference has an impact on harmonics from aligned polar molecules, suppressing emission from the molecules parallel to the driving laser field while favoring the perpendicular ones. For oriented molecules, we measure the amplitude ratio of even to odd harmonics produced when intense light irradiates CO molecules and determine the degree of orientation and the phase difference of attosecond bursts using molecular frame ionization and recombination amplitudes. The sensitivity of the high harmonic spectrum to subtle phase differences in the emitted radiation makes it a detailed probe of polar molecules and will drive major advances in the theory of high harmonic generation.

We introduce generic approach for complete space-time characterization of the light pulses. Measured spectrally resolved wavefront across one plane and knowledge of temporal profile at one point in space allow complete space-time reconstruction of the pulse.

We introduce new approach for complete space-time reconstruction of the attosecond pulses. Measured spectrally resolved wavefront across one plane and knowledge of temporal profile at one point in space enables complete space-time characterization of the pulse.

We demonstrate an all-optical spatio-temporal characterization method for the attosecond pulses produced through high harmonic generation. A spatio-temporal profile is retrieved from the spatial modulation of the harmonic spectra.

A focused ultrashort pulse can reach high enough intensity that non-linear ionization dominates its interaction with transparent media while still having relatively low fluence. In this case, the energy extracted from the beam can counter self-focusing by energy depletion and plasma formation, providing controlled energy deposition that can modify the material in a highly local manner. We demonstrate that non-linear absorption limits the intensity that can be reached and that the energy is deposited prior to the focus. We model the energy distribution, and predict and measure the energy transmitted through the focus. We establish the threshold intensity for non-linear ionization in dielectrics at ~ 10(1)(3) W cm-(2). We use the refractive index modification that the non-linear ionization causes in glass to image the spatial distribution of the energy deposition.

Summary form only given. Modification of the refractive index of transparent materials by UV laser radiation is widely used for writing waveguides and Bragg gratings in different type of glasses. Extending controlled writing to the inside of transparent materials is important since it opens a wide range of applications for integrated optical elements. In this work we report on direct writing of diffraction gratings and other optical elements inside different silica glasses with a train of femtosecond laser pulses at high repetition rate and study the influence of laser repetition rate and the number of pulses per focused area.

High harmonic generation (HHG) [1, 2] in gas-phase atomic or molecular targets has been extensively studied and developed over the past few decades, enabling attosecond spectroscopy [3] and tomographic imaging of molecular orbitals [4]. Recently, HHG could also been demonstrated in solid-state systems [5-8], which allows for applying attosecond spectroscopy techniques to condensed matter in an all-optical fashion as well as utilizing a solid's electrons as a probe to investigate structural anisotropies [9]. Here, we extend these novel approaches by nanoscale engineering of the solid targets and demonstrate that HHG in semiconductors can be tailored and controlled by modification of the local chemical composition or the microstructure.

The solid state opens new possibilities for the generation of high harmonics extending to the extreme ultraviolet. Additionally, an array of tools and techniques exists for straightforward fabrication of solid-state devices, with nanoscale precision. In this work, we explore new ways to control high-harmonic emission in semiconductors using nanofabrication. We measure high-harmonic emission from tailored silicon and zinc oxide targets, up to the ninth order of the fundamental driving frequency (corresponding to a wavelength of ~225 nm). Morphology and local composition can each localize and enhance emission. We utilize this control to demonstrate prototype devices which simultaneously emit and focus harmonic radiation., CLEO: QELS_Fundamental Science, 13–18 May 2018, San Jose, California

We present a novel spatio-temporal characterization method for the attosecond pulses produced through high harmonic generation. A spatio-temporal profile is reconstructed from the spatial modulation of the harmonic spectra in the far-field.

We demonstrate an all-optical technique that measures instantaneous field of unknown optical pulses. We show that the field can be imprinted onto the deflection of the attosecond XUV pulse.