Search for: All records

Vigorous efforts to harness the topological properties of light have enabled a multitude of novel applications. Translating the applications of structured light to higher spatial and temporal resolutions mandates their controlled generation, manipulation, and thorough characterization in the short-wavelength regime. Here, we resort to high-order harmonic generation (HHG) in a noble gas to upconvert near-infrared (IR) vector, vortex, and vector-vortex driving beams that are tailored, respectively, in their spin angular momentum (SAM), orbital angular momentum (OAM), and simultaneously in their SAM and OAM. We show that HHG enables the controlled generation of extreme-ultraviolet (EUV) vector beams exhibiting various spatially dependent polarization distributions, or EUV vortex beams with a highly twisted phase. Moreover, we demonstrate the generation of EUV vector-vortex beams (VVB) bearing combined characteristics of vector and vortex beams. We rely on EUV wavefront sensing to unambiguously affirm the topological charge scaling of the HHG beams with the harmonic order. Interestingly, our work shows that HHG allows for a synchronous controlled manipulation of SAM and OAM. These EUV structured beams bring in the promising scenario of their applications at nanometric spatial and sub-femtosecond temporal resolutions using a table-top harmonic source. 

While there has been success in Wakefield acceleration of electrons, there are a number of applications that could benefit from acceleration to modest energy (~MeV) by the laser field, for example, ultrafast electron diffraction and injection into higher-energy laser-driven accelerators. Here we outline our scheme for ponderomotive acceleration of electrons (and in principle, positrons) in which we control the group velocity of ultrafast pulses through pulse front tilt. Provided the intensity is above the threshold for capture of electrons, the leading part of the pulse front effectively acts like a moving mirror whose shape is controlled by the spatio-temporal topology of the intensity profile. Our analytic models of the propagation of spatially-chirped beams, simple relativistic single-particle models of the laser-electron interaction and our implementation of these beams in particle-in-cell simulations help to predict the output electron energy and direction. We are preparing experiments on the ALEPH laser system at Colorado State University in which we will use the diagnostic techniques that we have developed to align our scaled-up design of a high-energy pulse compressor that will deliver spatially chirped pulses. 

We􀀁explore􀀁an􀀁electron􀀁acceleration􀀁scheme􀀁which􀀁uses􀀁the􀀁ponderomotive􀀁force􀀁of􀀁a􀀁tilted􀀁ultrafast􀀁laser􀀁as􀀁the􀀁drive􀀁mechanism􀀁for􀀁acceleration. The􀀁effect􀀁of􀀁pulse􀀁front􀀁curvature􀀁on􀀁the􀀁acceleration􀀁process􀀁is􀀁also􀀁discussed. 

The study of the physics of naturally occurring electrostatic discharges (ESDs) at early times is challenged by the difficulty in overcoming pre-trigger requirements of laser probes. In this work, ultraviolet (UV) pulses from a diode-pumped solid-state, Q-switched laser system are used to trigger ESDs. We use an open-air spark gap with a gap voltage held near threshold. The laser intensity is in the microjoule range so that seed electrons are produced through the photoelectric effect on the cathode. In contrast to laser-triggered spark gaps, the resulting discharges are anticipated to be very similar to those produced by random seed electrons. The triggering produces ESDs with a yield of >65%. While there is ~10ns jitter, co-recording of the current pulse will allow for time-resolved experimental diagnostics with ns timing resolution. Early results show a relatively short delay between triggering and the arc discharge (~100ns), indicating that collisional UV generation is a more likely source of secondary electrons than ion return current. Our experiment will be compared to our numerical models for plasma temperature and species evolution measurements in ESDs. Future experiments will be completed in a discharge chamber which allows for control of the gas composition and pressure. 

Using a novel pulse compressor for the CSU ALEPH facility, we demonstrate direct ponderomotive acceleration of electrons with 1.5J, tilted ultrafast pulses. The < 500keV electrons are directed normal to the tilted pulse front as predicted. 

We introduce a self-referenced system that retrieves the full spatio-temporal profile of an ultrashort pulse using a Shack-Hartmann and second harmonic generation FROG. The key feature is the precise co-location of a spectral phase measurement at one spatial position with the spectrally resolved spatial measurements. 

We generalize our method for propagating spatially chirped Gaussian beams to properly calculate the evolution of geometric spectral phase through a lens. By expanding the spectral phase around the local central frequency, we analytically calculate the spatio-temporal field. Applications to intentionally detuned pulse compressors are discussed. 

We present a ptychographic phase retrieval algorithm which solves the square root problem in second order pulse measurement techniques and reconstructs the fields of multiple incoherent pulses simultaneously from a single dispersion scan trace. 

We demonstrate a novel dispersion scan algorithm using grating dispersion. We also propose using the intrinsic dispersion of temporally focused laser pulses to characterize the pulse structure by scanning a nonlinear crystal through focus.