David T. Lloyd

General analytic solution for far-field phase and amplitude control, with a phase-only spatial light modulator

We present an analytical solution for the phase introduced by a phase-only spatial light modulator to generate far-field phase and amplitude distributions within a domain of interest. The solution is demonstrated experimentally and shown to enable excellent control of the far-field amplitude and phase.

Complete spatial characterization of an optical wavefront using a variable-separation pinhole pair.

Here we demonstrate a new technique for completely characterizing the spatial properties of high harmonic generation (HHG), capable of recovering the frequency-resolved intensity and phase profiles, as well as the spatial coherence of the source. In keeping with the nomenclature established by previous work, we dub our technique SCIMITAR (Scanning Interference Measurement for Integrated Transverse Analysis of Radiation).

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.

Improving the resolution obtained in lensless imaging with spatially shaped high-order harmonics

The resolution obtained with coherent diffractive imaging (CDI) is limited by a number of factors, one of which is the transverse coherence of the illuminating beam. For a successful reconstruction, it is accepted that the illuminating beam should have a lateral coherence length of at least twice the largest linear dimension of the sample [1].

Multimode quasi-phase-matching of high-order harmonic generation in gas-filled photonic crystal fibers

Driving bright high-order harmonic generation (HHG) with few-μJ pulses is a crucial step towards compact, high average power sources of coherent extreme ultraviolet (XUV) radiation for time-integrated applications including imaging. Unfortunately, reaching a sufficiently strong E-field to perform HHG with these pulses requires tight focusing, greatly reducing the interaction volume. An elegant solution to this problem is to restrict HHG to a hollow waveguide [1] and in particular a photonic crystal fiber [2]. Strong reabsorption in the XUV prohibits the use of multi-atmosphere pressures to achieve phase-matching [3], and instead quasi-phase-matching (QPM) is preferred. Here we demonstrate QPM of HHG for the first time within a gas-filled PCF.

Combined visible and near-infrared OPA for wavelength scaling experiments in strong-field physics

We report the operation of an optical parametric amplifier (OPA) capable of producing gigawatt peak-power laser pulses with tunable wavelength in either the visible or near-infrared spectrum. The OPA has two distinct operation modes (i) generation of > 350 μJ, sub 100 fs pulses, tunable between 1250 - 1550 nm; (ii) generation of > 170 μJ, sub 150 fs pulses tunable between 490 - 530 nm. We have recorded high-order harmonic spectra over a wide range of driving wavelengths. This flexible source of femtosecond pulses presents a useful tool for exploring the wavelength-dependence of strong-field phenomena, in both the multi-photon and tunnel ionization regimes.

Blind digital holographic microscopy

A blind variant of digital holographic microscopy is presented that removes the requirement for a well-characterized, highly divergent reference beam. This is achieved by adopting an off-axis recording geometry where a sequence of holograms is recorded as the reference is tilted, and an iterative algorithm that estimates the amplitudes and phases of both beams while simultaneously enhancing the numerical aperture. Numerical simulations have demonstrated the accuracy and robustness of this approach when applied to the coherent imaging problem.

Gaussian-Schell analysis of the transverse spatial properties of high-harmonic beams.

High harmonic generation (HHG) is an established means of producing coherent, short wavelength, ultrafast pulses from a compact set-up. Table-top high-harmonic sources are increasingly being used to image physical and biological systems using emerging techniques such as coherent diffraction imaging and ptychography. These novel imaging methods require coherent illumination, and it is therefore important to both characterize the spatial coherence of high-harmonic beams and understand the processes which limit this property. Here we investigate the near- and far-field spatial properties of high-harmonic radiation generated in a gas cell. The variation with harmonic order of the intensity profile, wavefront curvature, and complex coherence factor is measured in the far-field by the SCIMITAR technique. Using the Gaussian-Schell model, the properties of the harmonic beam in the plane of generation are deduced. Our results show that the order-dependence of the harmonic spatial coherence is consistent with partial coherence induced by both variation of the intensity-dependent dipole phase as well as finite spatial coherence of the driving radiation. These findings are used to suggest ways in which the coherence of harmonic beams could be increased further, which would have direct benefits to imaging with high-harmonic radiation.

Order Dependent Spatial Properties of High Harmonic Generation

We report on the dependence of the spatial properties of high harmonic generation on the harmonic order. We present interesting features in the transverse spatial coherence which may be important for coherent imaging experiments.

Complete spatial characterization of an optical wavefront using a variable-separation pinhole Pair

Here we demonstrate a new technique for completely characterizing the spatial properties of high harmonic generation (HHG), capable of recovering the frequency-resolved intensity and phase profiles, as well as the spatial coherence of the source. In keeping with the nomenclature established by previous work, we dub our technique SCIMITAR (Scanning Interference Measurement for Integrated Transverse Analysis of Radiation).