Alon Bahabad

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-μm 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.

Photonic quasicrystals for nonlinear optical frequency conversion.

We present a general method for the design of 2-dimensional nonlinear photonic quasicrystals that can be utilized for the simultaneous phase matching of arbitrary optical frequency-conversion processes. The proposed scheme--based on the generalized dual-grid method that is used for constructing tiling models of quasicrystals--gives complete design flexibility, removing any constraints imposed by previous approaches. As an example we demonstrate the design of a color fan--a nonlinear photonic quasicrystal whose input is a single wave at frequency omega and whose output consists of the second, third, and fourth harmonics of omega, each in a different spatial direction.

Characterizing isolated attosecond pulses from hollow-core waveguides using multi-cycle driving pulses

The generation of attosecond-duration light pulses using the high-order harmonic generation process is a rapidly evolving area of research. In this work, we combine experimental measurements with careful numerical analysis, to demonstrate that even relatively long-duration, 15 fs, carrier-envelope-phase (CEP) unstabilized near-infrared (NIR) pulses can generate isolated attosecond extreme-ultraviolet (EUV) pulses by the dynamically-changing phase matching conditions in a hollow-core waveguide geometry. The measurements are made using the laser-assisted photoelectric effect to cross-correlate the EUV pulse with the NIR pulse. A FROG CRAB analysis of the resulting traces (photoelectron signal versus photoelectron energy and EUV-NIR delay) is performed using a generalized projections (GP) algorithm, adapted for a wide-angle photoelectron detection geometry and non-CEP stabilized driving laser pulses. In addition, we performed direct FROG CRAB simulations under the same conditions. Such direct simulations allow more freedom to explore the effect of specific pulse parameters on FROG CRAB traces than is possible using the automated GP retrieval algorithm. Our analysis shows that an isolated pulse with duration of ≈ 200 attoseconds can result from CEP unstabilized, high intensity ≈ 15 fs multi-cycle driving pulses coupled into a hollow-core waveguide filled with low-pressure Argon gas. These are significantly longer driving pulses than used in other experimental implementations of isolated attosecond pulses.

Generation of Optical Vortex Beams by NonlinearWave Mixing

It is shown that optical vortex beams can be generated from a non-vortex fundamental beam by an optical frequency conversion process taking place within a twisted nonlinear photonic crystal. This is done without any first-order (linear) refractive optics. Through such a proposed structure, all-optical switching of vortices with different helicities is made possible, as well as the simultaneous application of counter-rotating vortex beams of different frequencies.

Engineering two-dimensional nonlinear photonic quasi-crystals

A known algorithm for modeling quasi-periodic lattices is used to generate two-dimensional quadratic nonlinear photonic quasi-crystals containing a set of desired discrete spectral components. This allows us to fabricate optical devices in which an arbitrary set of nonlinear optical processes can be efficient. We demonstrate this capability by fabricating two devices: a multidirectional single-frequency doubler and a multidirectional, multifrequency doubler that is capable of nearly collinear doubling of cw radiation in the optical communication C band (1530-1570 nm) through angle tuning.

All-optical polarization switch in a quadratic nonlinear photonic quasicrystal

We present an all-optical intensity-dependent polarization switch based on cascaded three-wave-mixing interactions in a quasiperiodic quadratic nonlinear photonic crystal. The polarization switching is realized by simultaneous quasiphase matching of upconversion and downconversion processes in LiNbO3 and achieves three orders of magnitude better efficiency than previous devices based on cascaded cubic nonlinearities. The switch allows extending mode-cleaning and mode-locking techniques to considerably lower input power. We demonstrate experimentally that a single linearly polarized 1550 nm fundamental wave generates a new fundamental wave of orthogonal polarization.

Two-center interferences in photoionization of a dissociating H2+ molecule

numerical simulations for the ionization yield as a function of the time delay between the two pulses exhibit characteristic oscillations due to interferences between the partial electron waves emerging from the two protons in the dissociating hydrogen molecular ion. We show that the photon energy of the pump pulse should be in resonance with the σg–σu transition and the pump pulse duration should not exceed 5 fs in order to generate a well-confined nuclear wave packet. The spreading of the nuclear wave packet during the dissociation is found to cause a decrease of the amplitudes of the oscillations as the time delay increases. We develop an analytical model to fit the oscillations and show how dynamic information about the nuclear wave packet, namely, velocity, mean internuclear distance, and spreading, can be retrieved from the oscillations. The predictions of the analytical model are tested well against the results of our numerical simulations.

Nonlinear Photonic Quasicrystals for Novel Optical Devices

Two well-known methods for the design of quasicrystal models are used to create novel nonlinear optical devices. These devices are useful for efficient three-wave mixing of several different processes, and therefore offer greater flexibility with respect to the more common periodic nonlinear photonic crystals. We demonstrate applications for polarization switching as well as multi-wavelength and multi-directional frequency doubling. The generalized dual grid method is proven to be efficient for designing photonic quasicrystals for one-dimensional collinear devices as well as elaborate two-dimensional multi-directional devices. The cut-and-project method is physically realized by sending finite-width optical beams at an irrational angle through a periodic two-dimensional nonlinear photonic crystal. This enables the creation of two simultaneous collinear optical processes that can be varied by changing the angle of the beams.

Quasi-phase-matched concurrent nonlinearities in periodically poled KTiOPO 4 for quantum computing over the optical frequency comb

We report the successful design and experimental implementation of three coincident nonlinear interactions, namely ZZZ (type 0), ZYY (type I), and YYZ/YZY (type II) second-harmonic generation of 780 nm light from a 1560 nm pump beam in a single, multigrating, periodically poled KTiOPO(4) crystal. The resulting nonlinear medium is the key component for making a scalable quantum computer over the optical frequency comb of a single optical parametric oscillator.

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.