Advances in computational imaging over the last decade have sparked a revolution in metrologies aimed at understanding nano-material, magnetic, acoustic, and most recently—systems with non-repeatable and nonlinear dynamics. The most recent advances are being led by state-of-the-art single-shot, high-dimensional multiplexed imaging systems. In this talk, I will review progress on single-shot multiplexed coherent computational imaging and provide some perspective on how the technology is poised to usher in a new revolution in high-dimensional pulse-beam metrology. Uniquely, these new metrologies will enable optimizing the spatiotemporal profile of ultrashort pulses to reach the highest intensities. The talk will conclude with progress on a computational microscope capable of recording nonlinear dynamics with femtosecond frame-periods.
Ultrafast pulse-beams are four-dimensional, space–time phenomena that can exhibit complicated spatiotemporal coupling. Tailoring the spatiotemporal profile of an ultrafast pulse beam is necessary for a variety of applications ranging from basic science involving high-intensity light matter interactions to applied microscopy and advanced manufacturing and micromachining.
In this talk I will discuss the development of a novel single-pulse, reference-free spatiotemporal characterization technique based on two colocated synchronized measurements: (1) broadband single-shot ptychography and (2) single-shot frequency resolved optical gating. We apply the technique to measure the nonlinear propagation of an ultrafast pulse beam through a fused silica window. Our spatiotemporal characterization method represents a major contribution to the growing field of spatiotemporally engineered ultrafast laser pulse beams.
In this work, we exploit HHG in a noble gas to merge the azimuthally twisted wavefront of a vortex beam and the spatially varying polarization of a vector beam, yielding EUV vector-vortex beams (VVB) that are tailored simultaneously in their SAM and OAM. Employing a high-resolution EUV Hartmann wavefront sensor (EUV HASO, Imagine Optic), we perform the complete spatial intensity and wavefront characterization of the vertical polarization component of the 25th harmonic beam centered at a wavelength of 32.6 nm. By driving the HHG using IR VVB, we show that HHG enables the production of EUV VVB exhibiting radial, azimuthal, or even intermediate polarization distribution. Furthermore, the wavefront characterization allows for the unambiguous confirmation of the topological charge and OAM helicity of the upconverted harmonic VVB. Notably, our work reveals that HHG provides a means for the synchronous and controlled manipulation of SAM and OAM. The production of ultrafast EUV VVB with high OAM and adjustable polarization distributions opens up promising prospects for their applications at nanometric spatial and sub-femtosecond temporal resolutions using a table-top harmonic source.
High-order harmonic generation (HHG) is an instrumental process enabling the transfer of short infrared pulse coherence properties into the Extreme Ultraviolet (EUV) spectral range. This phenomenon has opened the way to ultrafast pump-probe experiments at the nanoscale level. Recently, HHG has provided a straightforward approach to frequency upconvert beams structured in their phase and/or polarization. An emblematic example is the optical vortex beam, which is characterized by an azimuthally twisting wavefront. From a fundamental point of view, such a beam exhibits a phase singularity on the propagation axis and is carrying orbital angular momentum (OAM). Vector beams denote another structured beam family, exhibiting a spatially varying polarization.
In this paper, we will present our recent results on the generation and characterization of EUV vortex beams exhibiting very high topological charges (up to 100). Besides, using a similar HHG up-conversion scheme, we will show the production of so-called EUV vector-vortex beams that present the combined characteristics of the vortex and vector beams. Finally, progress on plasma-based soft x-ray laser amplification of such structured beams will be outlined,
High frame rate (HFR) phase-and-amplitude contrast imaging is imperative for a better understanding of the underlying physics in dynamically evolving phenomena. Here we present, to our knowledge, the first experimental HFR ptychographic reconstructions using a novel Time-Resolved Imaging via Multiplexed Ptychography (TIMP) system. Our TIMP system is composed of a time-encoding device, which creates a train of temporally separated pulses of different phase profiles, and a novel beam-sampling single-shot ptychography microscope. HFR imaging is achieved by reconstructing the complex object from each pulse independently and time ordering the reconstructed objects based on the phase structure of the reconstructed probes.
Temporal shaping of picosecond duration pulses is challenging due to limitations of direct shaping with electro-optical technologies or spectral shaping because of limited spectral content. We present an experimental implementation of a non-collinear sum frequency generation scheme wherein picosecond duration pulses with tailored temporal profiles are derived from femtosecond pulses with modified spectral phase. We demonstrate temporally shaped pulses with >20 ps duration, flat-top profile, and near transform-limited spectral content while maintaining upwards of 40% conversion efficiency. Additionally, we provide a framework for extending this technique to arbitrary temporal profiles and wavelengths.
Ultrafast pulsed lasers are integral for a variety of industrial, medical, and scientific endeavors. Fundamentally, pulsed lasers are complicated four-dimensional space-time phenomena that can be described by their complex spatiotemporal electric field profiles. Here we present the first ever single-shot, full-field, reference-free spatiotemporal pulse-beam measurement system capable of single-pulse characterization. Our technique combines single-shot spatiospectral characterization from broadband single-shot ptychography (BBSSP) with a collocated single-shot spectral phase measurement from single-shot frequency resolved optical gating (SSFROG). Together, these measurements provide the full complex spatiospectral field at the plane, which can be computationally propagated and Fourier transformed to provide the spatiotemporal profile of the pulse at any plane.
Using multiplexed broadband ptychography (MBP), we characterize the EUV light in high-order harmonic generation (HHG). MBP can measure spectrally resolved complex beam profiles for different harmonic outputs without spectral dispersion. Through a simple change to the experimental setup, we also characterize the driving laser for the process. The experimentally measured driving laser profile is used in an SFA+ with dipole approximation simulation of high harmonic generation. The simulated results are compared to the experimentally measured high harmonics produced with the characterized driving laser. By characterizing the input and output in HHG we can further understand the generation mechanics to better control the process. We also can investigate how control of the fundamental laser can lead to control of the output harmonics.
The lasers and conventional optics used in charged-particle beam x-ray free electron laser photo-injectors are designed using fundamentally the same techniques for decades. We present a novel method for the generation and conditioning of the UV laser used for electron generation via photoemission that can enhance electron beam and X-ray performance. Additionally, we discuss laser-based spatio-temporal shaping and conditioning of electron beam phase space in order to selectively promote lasing operational modes.
In simultaneous spatial and temporal focusing (SSTF) a wide bandwidth pulse with transverse spatial chirp is focused, resulting in a pulse that is temporally compressed only near the focal plane. The pulse also has a pulse front tilt angle that depends on the amount of initial transverse chirp. In this work, we explore computationally and experimentally the properties of SSTF vortex and vector beams. To analyze the beam propagation, we build on the concept that a spatially chirped beam is a superposition of Gaussian beams with a position or angle that depends on frequency. We extend this to superpositions of Hermite-Gauss high-order modes to describe the singular beams. At focus, the beams of the ultrashort pulses are tilted versions of the familiar doughnut beams. Away from focus, however, where the spectral components do not fully overlap, we find that vortex and vector beams result in strikingly different mapping of the singularity mapping in the spatio-temporal domain. The use of higher-order modes increases the focal spot size without reducing the already short SSTF depth of focus. Experimentally, we use spiral phase plates to produce vortex beams and a linear to radial polarization converter for the vector beams. The vector beam is not distorted by the polarization-insensitive transmission gratings. The spatial chirp compressor is improved over our previous work to vary the chirp positive and negative. The sensitivity of the singular beam focus to grating misalignment can actually be used to optimize the compressor alignment.
In simultaneous spatial and temporal focusing (SSTF) a wide bandwidth pulse with transverse spatial chirp is focused, resulting in a pulse that is temporally compressed only near the focal plane. The pulse also has a pulse front tilt angle that depends on the amount of initial transverse chirp. Using an improved design of an asymmetric pulse compressor, we can easily vary the amount of output spatial chirp and thus the pulse front tilt at the focus. We direct this beam into a vacuum chamber and focus it onto an argon gas jet to achieve high harmonic generation (HHG). Since the harmonics are created with a tilted pulse, they emerge from the focus with an amount of angular chirp based on the input spatial chirp and harmonic number. We angularly disperse the harmonics in the direction perpendicular to the spatial chirp with a curved reflective grating, which focuses in the spectral direction onto an x-ray CCD camera. We observe that each of the harmonics possesses angular spatial chirp. To our knowledge, this is the first experimental verification of our earlier published theory of spatially chirped high harmonics. These harmonics are in a sense the Fourier complement to harmonics produced with the Lighthouse Effect. In that case, the attosecond pulse train is angularly dispersed while here each harmonic has angular spectral dispersion. This technique could be used for hyperspectral XUV spectroscopy and, when the beam is refocused, would allow for temporal focusing of the attosecond pulse train.
Simultaneous spatially and temporally focussing (SSTF) of ultrashort pulses allows for an unprecedented control of the intensity distribution of light. It has therefore a great potential for widespread applications ranging from nonlinear microscopy, ophthalmology to micro-machining. SSTF also allows to overcome many bottlenecks of ultrashort pulse micro-machining, especially non-linear effects like filamentation and self-focussing. Here, we describe and demonstrate in detail how SSTF offers an additional degree of freedom for shaping the focal volume. In order to obtain a SSTF beam, the output of an ultrafast laser is usually split by a grating into an array of copies of the original beam, which we refer to as beamlets. The ratio of the beamlet array width to the width of the invidual beamlet is the beam aspect ratio. The focal volume of the SSTF beam can now be tailored transversally by shaping the cross-section of the beamlets and axially by choosing the right beam aspect ratio. We will discuss the requirements of the setup for a successful implementation of this approach: Firstly, the group velocity dispersion and the third order dispersion have to be compensated in order to obtain a high axial confinement. Secondly, the beamlet size and their orientation should not vary too much spectrally. Thirdly, beamlet and SSTF focus should match. We will hence demonstrate how SSTF allows to inscribe tailored three-dimensional structures with fine control over their aspect ratio. We also show how the SSTF focus can be adapted for various glasses and crystals.
Transcleral photodisruption may provide a noninvasive method for creating partial thickness scleral channels to reduce elevated intraocular pressure associated with glaucoma. We achieved subsurface photodisruption in vitro without damaging overlying tissues with three techniques: (1) use of long laser wavelengths, (2) application of pressure, and (3) application of a dehydrating agent. Using 1 and 3, we were able to photodisrupt the internal surface of a full thickness block of sclera by focusing through the tissue.
To evaluate transscleral glaucoma surgery techniques using ultrashort pulsed lasers, we attempted to produce photodisruption on the inner surface of the sclera without damaging the overlying tissue. We identified two methods, using pulses centered at 1700 nm and a transparency inducing drug, to produce the spatial and temporal confinement of the pulse necessary to produce photodisruption in the highly scattering sclera. When fully developed these concepts may help address the longstanding limitations of current glaucoma surgical techniques.
A recently developed laser-produced plasma channel is shown to be a promising means to produce an efficient, compact soft x-ray laser. The channel provides a route for efficient high power laser pumping through optical waveguiding of the pump. The channel also acts as a waveguide for generated soft x-rays, since it has wavelength independent mode structure. Channel creation and guided laser pulses of moderate duration and energy can be highly effective in driving nonequilibrium behavior of these plasmas to generate substantial population inversions.
We present results from a 1-D plasma dynamics calculation, describing the evolution of strongly heated material in the vicinity of a solid-vacuum interface. We find that the radiation emitted by the hot material in the range hν > kTe, where Te is the initial peak plasma temperature, comes primarily from the region of the original step function interface. This emission is dominated by recombination radiation. The emitted radiation pulse is extremely short; the cooling at the interface is dominated by expansion. It is seen that thermal conduction minimally affects the radiation pulse intensity and duration.
In these proceedings, we report on a time-resolved investigation of the hydrodynamics of a laser-produced plasma. A sub-100ps pulse is focused into a chamber filled with xenon for various pulse energies and pressures. This pulse (the pump pulse) forms a plasma, which is probed by a second pulse (the probe pulse) with a variable delay of up to 2.5 ns. The gradients in the plasma density profile produce a lensing effect on the probe pulse. The beam transmitted through the plasma is viewed with a CCD camera. The diffraction pattern of the probe pulse can be seen by subtracting the image of the first pulse beam from the image produced by the two-pulse beam.
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