Propagation of intense laser pulses in plasma with a prepared phase-space distribution.

Optimizing the laser wakefield accelerator (LWFA) requires control of the intense driving laser pulse and its stable propagation. This is usually challenging because of mode mismatching arising from relativistic self-focusing, which invariably alters the velocity and shape of the laser pulse. Here we show how an intense pre-pulse can prepare the momentum/density phase-space distribution of plasma electrons encountered by a trailing laser pulse to control its propagation. This can also be used to minimize the evolution of the wakefield thus enhancing the stability of the LWFA, which is important for applications.

Using a passively stable attosecond beamline for relative photoemission time delays at high XUV photon energies.

We present and demonstrate an experimental scheme that enables overlap-free reconstruction of attosecond beating by interference of two-photon transitions (RABBITT) measurements at high extreme-ultraviolet (XUV) photon energies. A compact passively-stabilized attosecond beamline employing a multilayer (ML) mirror allows us to obtain XUV pulses consisting of only two odd high-harmonic orders from an attosecond pulse train (APT). We compare our new technique to existing schemes that are used to perform RABBITT measurements and discuss how our scheme resolves the limitations imposed by spectral complexity of the harmonic comb at high photon energies. We further demonstrate first applications of our scheme for rare gases and gas mixtures, and show that this scheme can be extended to gas-molecule mixtures.

Extreme-ultraviolet high-order harmonic generation from few-cycle annular beams.

An annular infrared (IR) laser beam has been used for high-order harmonic generation reaching a cut-off energy of 90 eV for extreme-ultraviolet-infrared (XUV-IR) pump-probe experiments in an intrinsically stable attosecond beamline. The generation of harmonics along the laser axis in the missing portion of the laser beam decreases the IR power load on thin metallic foils that are used for removing the residual IR and shaping the XUV pulses from high-harmonic generation. This finds applications in high-average-power few-cycle laser systems, where high-average IR power destroys the foils. The spatial separation of IR and XUV will, moreover, simplify the realization of attosecond time-resolved measurements.

Photoionization delays in xenon using single-shot referencing in the collinear back-focusing geometry.

Attosecond photoemission delays for all the valence (5p3/2, 5p1/2, 5s, 4d5/2, 4d3/2) subshells of xenon have been accessed using the interferometric RABBITT technique. The 4d subshell delays in Xe have been accessed for the first time, to the best of our knowledge, due to the high photon energy used. A novel technique of single-shot referencing in the collinear back-focusing geometry has been introduced. This enables us to distinguish the signal from principal photoelectron peaks due to ionization by extreme ultraviolet radiation only and infrared-induced sideband contributions, especially in the regions of spectral overlap.

Extreme-ultraviolet high-harmonic generation in liquids.

High-harmonic generation (HHG) in gases has been the main enabling technology of attosecond science since its discovery. Recently, HHG from solids has been demonstrated, opening a lively area of research. In contrast, harmonic generation from liquids has so far remained restricted to low harmonics in the visible regime. Here, we report the observation and detailed characterization of extreme ultraviolet HHG from liquid water and several alcohols extending beyond 20 eV. This advance was enabled by the implementation of the recent liquid flat-microjet technology, which we show to facilitate the spatial separation of HHG from the bulk liquid and the surrounding gas phase. We observe striking differences between the HHG spectra of water and several alcohols. A comparison with a strongly-driven few-band model establishes the sensitivity of HHG to the electronic structure of liquids. Our results suggest liquid-phase high-harmonic spectroscopy as a new method for studying the electronic structure and ultrafast scattering processes in liquids.

Complete reconstruction of ultra-broadband isolated attosecond pulses including partial averaging over the angular distribution.

Attosecond streaking is a powerful tool to investigate ultrafast electron dynamics on the attosecond time scale. To obtain the highest temporal resolution in a pump-probe experiment, soft-X-ray (SXR) and infrared (IR) pulses have to be carefully characterized. Here, we present a detailed description of our recent generalization of the Volkov-transform generalized projection algorithm (VTGPA) and its application to multiple overlapping photoelectron bands. This method allows for the complete temporal reconstruction of both IR and SXR pulses under the inclusion of accurate complex photoionization matrix elements (PMEs). In this article, we compare the performance of our new method with traditional algorithms. We particularly focus on the important role played by the photoelectron angular distribution (PAD) which needs to be taken into account for the highest fidelity of attosecond pulse reconstruction. For this purpose, we investigate numerically the influence of the finite collection angle of the electron spectrometer on the retrieval and the obtained pulse parameters. We further theoretically demonstrate the reliability of the reconstruction for pulse durations even shorter than the atomic unit of time.

Photoelectron spectrometer for liquid and gas-phase attosecond spectroscopy with field-free and magnetic bottle operation modes.

A compact time-of-flight spectrometer for applications in attosecond spectroscopy in the liquid and gas phases is presented. It allows for altering the collection efficiency by transitioning between field-free and magnetic-bottle operation modes. High energy resolution (ΔE/E = 0.03 for kinetic energies >20 eV) is achieved despite the short flight-tube length through a homogeneous deceleration potential at the beginning of the flight tube. A closing mechanism allows isolating the vacuum system of the flight tube from the interaction region in order to efficiently perform liquid-microjet experiments. The capabilities of the instrument are demonstrated through photoelectron spectra from multiphoton ionization of argon and xenon, as well as photoelectron spectra of liquid and gaseous water generated by an attosecond pulse train.

Streaking of 43-attosecond soft-X-ray pulses generated by a passively CEP-stable mid-infrared driver.

Attosecond metrology has so far largely remained limited to titanium:sapphire lasers combined with an active stabilization of the carrier-envelope phase (CEP). These sources limit the achievable photon energy to ∼100 eV which is too low to access X-ray absorption edges of most second- and third-row elements which are central to chemistry, biology and material science. Therefore, intense efforts are underway to extend attosecond metrology to the soft-X-ray (SXR) domain using mid-infrared (mid-IR) drivers. Here, we introduce and experimentally demonstrate a method that solves the long-standing problem of the complete temporal characterization of ultra-broadband (≫10 eV) attosecond pulses. We generalize the recently proposed Volkov-transform generalized projection algorithm (VTGPA) to the case of multiple overlapping photoelectron spectra and demonstrate its application to isolated attosecond pulses. This new approach overcomes all key limitations of previous attosecond-pulse reconstruction methods, in particular the central-momentum approximation (CMA), and it incorporates the physical, complex-valued and energy-dependent photoionization matrix elements. These properties make our approach general and particularly suitable for attosecond supercontinua of arbitrary bandwidth. We apply this method to attosecond SXR pulses generated from a two-cycle mid-IR driver, covering a bandwidth of ∼100 eV and reaching photon energies up to 180 eV. We extract an SXR pulse duration of (43±1) as from our streaking measurements, defining a new world record. Our results prove that the popular and broadly available scheme of post-compressing the output of white-light-seeded optical parametric amplifiers is adequate to produce high-contrast isolated attosecond pulses covering the L-edges of silicon, phosphorous and sulfur. Our new reconstruction method and experimental results open the path to the production and characterization of attosecond pulses lasting less than one atomic unit of time (24 as) and covering X-ray absorption edges of most light elements.

Autonomic Reactivity Differs in Young Adults Classified using Revised Indian and WHO Guidelines for Obesity.

INTRODUCTION: Many studies have reported alteration in autonomic activity in obesity. However, there is paucity of literature comparing autonomic reactivity using different guidelines of obesity. As Indian guidelines were revised recently and WHO states that countries should use all categories of BMI for reporting purposes, it is prudent to compare physiological state in different categories of BMI. AIM: The aim of the present study was to compare the autonomic alteration in young adults using revised Indian and WHO guidelines for obesity. MATERIALS AND METHODS: A battery of autonomic tests (Valsalva Manoeuvre (VM), Deep Breathing Test (DBT), Lying to Standing Test (LST) and Hand Grip isometric exercise Test (HGT) was conducted on 34 overweight and obese and 30 normal weight volunteers categorised using revised Indian guidelines of body mass index. Same participants were regrouped and analysed using WHO guidelines of BMI and waist hip ratio (WHR). RESULTS: For analysis, participants were grouped into 3 categories of normal, overweight and obese using revised Indian guidelines for obesity. Same participants were regrouped according to WHO guidelines. E:I ratio during DBT, 30:15 ratio during LST, Valsalva ratio during VM and increase in DBP during HGT were compared in different subgroups. There was no difference in sympathetic and parasympathetic activities in participants classified according to revised Indian guidelines. In participants classified using WHO criteria, sympathetic reactivity in overweight subjects was significantly less as compared to normal subjects (p<0.05). CONCLUSION: Autonomic alterations might be more related to body fat percent rather than BMI. Indian guidelines are based on the observation that Asian population has more adipose tissue in WHO range of BMI. As the guidelines of BMI are applicable to all age groups and do not consider physical activity profile, they might still not be a good predictor of body fat.