Experimental investigation of size broadening of a Kα x-ray source produced by high intensity laser pulses

The size of a hard Kα x-ray source ($${\mathrm{E}}_{{\rm{K}}_{\rm{\alpha }}}$$ E K α = 17.48 keV) produced by a high intensity femtosecond laser interacting with a solid molybdenum target is experimentally investigated for a wide range of laser intensity (I ~ 1017–2.8 × 1019 W/cm2) and for four values of the temporal contrast ratio (6.7 × 107 < CR < 3.3 × 1010). Results point out the size enlargement of the x-ray source with the increase of laser intensity and with the deterioration of temporal contrast. It amounts up to sixteen times the laser spot size at the highest laser intensity and for the lowest temporal contrast ratio. Using hydrodynamic simulations, we evaluate the density scale length of the pre-plasma L/λ just before the main pulse peak. This allows us to show that a direct correlation with the laser absorption mechanisms is not relevant to explain the large size broadening. By varying the thickness of the molybdenum target down to 4 µm, the impact of hot electron scattering inside the solid is also proved irrelevant to explain the evolution of both the x-ray source size and the Kα photon number. We deduce that the most probable mechanism yielding to the broadening of the source size is linked to the creation of surface electromagnetic fields which confine the hot electrons at the solid surface. This assumption is supported by dedicated experiments where the evolution of the size enlargement of the x-ray source is carefully studied as a function of the laser focal spot size for the highest contrast ratio.

Simultaneous Enhancement of Contrast and Power of Femtosecond Laser Pulses by Nonlinear Interferometer

We showed that the nonlinear Mach–Zehnder interferometer may be used not only for enhancing temporal contrast, as proposed earlier, but also for increasing pulse power due to efficient pulse compression. The interferometer introduces into the output pulse a nonlinear phase equal to π/2. This allows increasing laser power by a factor of 1.5 only by means of a chirped mirror. Use of an additional nonlinear plate leads to a multi-fold power increase retaining the contrast enhancement.

Single-Shot Temporal Contrast Enhancement Measurement of a Plasma Mirror by a Chirped Pulse

We focused on a single-shot method for directly measuring the temporal contrast enhancement of a single plasma mirror by analyzing the spectrum of a chirped pulse spatiotemporally overlapped with the igniting laser used for generating a plasma mirror. Experimentally, temporal contrast enhancement of 102 by one plasma mirror was successfully measured in a hundred picosecond timescale and was consistent with the theory. This single-shot measurement method caused no degradation on the performance of the plasma mirror, which was proved by monitoring the efficiency and far-field pattern of the igniting laser after the plasma mirror. Combined with calorimeters and CCD cameras, this method is expected to realize the single-shot online diagnosis of plasma mirrors. This method is expected to be an efficient approach for measuring the temporal contrast enhancement of the plasma mirrors.

Temporal properties of painful contrast enhancement using repetitive stimulation

Offset analgesia is characterized by a disproportionately large reduction in pain following a small decrease in a heat stimulus and is based on the phenomenon of temporal pain contrast enhancement (TPCE). The aim of this study is to investigate whether this phenomenon can also be induced by repetitive stimulation, i.e., by stimuli that are clearly separated in time. With this aim, the repetitive TPCE paradigm was induced in healthy, pain-free subjects (n=33) at the volar non-dominant forearm using heat stimuli. This paradigm was performed applying three different interstimulus intervals (ISIs): 5, 15, and 25 seconds. All paradigms were contrasted with a control paradigm without temperature change. Participants continuously rated the perceived pain intensity. In addition, electrodermal activity was recorded as a surrogate measure of autonomic arousal. Temporal pain contrast enhancement was confirmed for both ISI 5 seconds (p < 0.001) and ISI 15 seconds (p = 0.005), but not for ISI 25 seconds (p = 0.07), however the magnitude of TPCE did not differ between ISIs (p = 0.11). Electrodermal activity was consistent previous pain ratings, but showing significantly higher autonomic activity being measured. Thus, the phenomenon of temporal contrast enhancement of pain can also be induced by repetitive stimulation. Both the involvement of the autonomic nervous system and the involvement of habituation processes are conceivable, which consequently points to both central and peripheral mechanisms of TPCE.

Past and Present Subjective Wellbeing: The Role of Contrast and Memory

This study is an attempt to analyse the interrelationships between current, past and expected future subjective wellbeing (SWB) through the intermediating role of memory using the National Income Dynamics Study panel dataset for South Africa. The contribution of this study is in exploring the role of contrast (inter-temporal and social) in determining; a) the current levels of SWB and, b) the drivers of recall bias; within a causal framework in the context of a country with low average SWB and high SWB inequality. The results indicate significant presence of hedonic adaptation and reveals past and future contrast as important correlates of current subjective wellbeing. In addition, a perceived improvement in current happiness from the past, is associated with higher levels of current SWB, highlighting the intermediating role of memory. Memory, however, is observed to be biased with only 42% accuracy. Higher levels of current subjective wellbeing are found to enhance the probability of correct recall of past. An overall improvement bias is evident among those in the lower segment of the SWB distribution. The results reinforce the hypothesized simultaneous relationships between current SWB, inter-temporal contrast, and recall behaviour. Further the differences in our findings from European studies emphasise the relevance of context in driving these relationships.