A major concern of High Harmonic Generation (HHG) is the small conversion efficiency at the single emitter level. Thus, ensuring that the emission at different locations are emitted in phase is crucial. At high pump intensities, it is impossible to phase match the radiation without reverting to ordered modulations of either the medium or the pump field itself, a technique known as Quasi-Phase-Matching (QPM). To date, demonstrated QPM techniques of HHG were usually complicated and/or lacked tunability. Here we demonstrate experimentally a relatively simple, highly and easily tunable all-optical QPM technique in which different spatial modes are superposed together to create a pump beam in which either the intensity or the polarization is periodically modulated. With these we demonstrate on-the-fly, tunable QPM of harmonic orders 25 to 41 with up to 40 fold enhancement of the emission at pressures ranging from 15 to 100 Torr.
We experimentally broke the temporal Fourier focusing limit of an ultra-short optical pulse and used it to demonstrate temporal super-resolution detection of a temporal event . The envelope function of the pulse is synthesized in the form of a Super-Oscillating Beat (SOB) signal, made of pairs of optical modes (i.e. beat modes) centered around a common carrier frequency. The mathematical form for the synthesis of the SOB signal is based on a known super-oscillatory function . Suited with the right amplitude and phases these beat modes interfere to create a lobe in the temporal waveform of the field’s envelope which can be arbitrarily narrow, at the cost of reduced amplitude at the fast oscillation. In our case, we achieved a temporal feature that is approximately three times shorter than the duration of a transform-limited Gaussian pulse having a comparable bandwidth while maintaining 30% visibility of the super-oscillating feature.
We then used this SOB signal to demonstrate experimentally temporal super-resolution. Specifically, the SOB signal was used to resolve the existence of a temporal double-slit, a pair of adjacent pulses which are detected as a single temporal event by a transform-limited Gaussian pulse having the same bandwidth. Formally, this experiment constitutes a temporal analogue to super resolution imaging by using a super oscillating point-spread-function [3,4]. Numerical simulations analyse in which cases the SOB signal outperform transform-limited signals for detection of short temporal events.
 Y. Eliezer, L. Hareli, L. Lobachinsky, S. Froim and A. Bahabad, “Breaking the Temporal Resolution Limit by Superoscillating Optical Beats”, Physical Review Letters, 119, 043903 (2017).
 M. V Berry and S. Popescu, "Evolution of Quantum Superoscillations and Optical Supperresolution without Evanescent Waves," J. Phys. A. Math. Gen. 39, 6965 (2006).
 A. M. H. Wong and G. V Eleftheriades, "An Optical Super-microscope for Far-field, Real-time Imaging Beyond the Diffraction Limit" Sci. Rep. 3, (2013).
 E. T. F. Rogers, J. Lindberg, T. Roy, S. Savo, J. E. Chad, M. R. Dennis, and N. I. Zheludev, "A Super-oscillatory Lens Optical Microscope for Subwavelength Imaging," Nat. Mater. 11, 432 (2012).
Single-mode optical fibers for the mid-IR (λ=3-30μm) are needed for many applications such as IR fiber lasers
and spatial filtering for nulling interferometry. In the past, we have already reported the design and fabrication of stepindex
single mode fibers for the mid-IR. Index guiding photonic crystal fibers (IG-PCF) offer many advantages over
step-index fibers, such as a wide spectral range, large mode area and low bending losses. So far, only limited success has
been achieved in the development of such fibers, due to the lack of suitable materials that are transparent in this spectral
range. We report here the design, fabrication and optical characterization of single-mode IG-PCFs for the mid-IR.
Triangular and octagonal IG-PCFs were fabricated from silver halide polycrystalline materials which transmit well in the
spectral range 2-20μm. The photonic crystal fibers were characterized by near-field and far-field measurements and they
demonstrated a single-mode behavior with relatively low losses and a large mode area, in agreement with our simulations. As predicted from the simulations, the octagonal arrangement of the rods in the fiber resulted in a single mode fiber with lower losses, a better mode shape and a higher rejection of high order modes, in comparison to the