At the Texas Petawatt laser facility we developed a novel ultra-short pulsed laser-driven neutron source generating an unprecedented output peak flux. Our results show a dramatic onset of high-energy electron generation from petawatt laser-irradiated plastic targets for targets thinner than a few microns. In this regime, the copious amounts of multi-MeV electrons emitted from the target are utilized to generate photo-neutrons from a metal converter. The neutrons are generated with a <50 ps pulse duration and a flux of 1018 n/cm2/s, exceeding any other pulsed or CW neutron source. In this paper, we will report on our measurement of the neutron yields produced from high atomic number converters.
A steady increase of on-target laser intensity with also increasing pulse contrast is leading to light-matter interactions of extreme laser fields with matter in new physics regimes. At the Texas Petawatt laser we have realized interactions in the transparent-overdense regime, which is reached by interacting a highly relativistic, ultra-high contrast laser pulse with a solid density ultrathin target. The extreme fields in the laser focus are turning the overdense, opaque target transparent to the laser by the relativistic mass increase of the electrons. Thus, the interaction becomes volumetric, increasing the energy coupling from laser to plasma. Using plasma mirrors to increase the on-target contrast ratio, we demonstrated generation of over 60 MeV proton beams with pulse energies not exceeding 40 J (on target).
Organic luminescent solar concentrators (LSCs) have been widely investigated due to their potential in dramatically
decreasing the cost of collecting solar energy. We designed, fabricated organic LSCs at different sizes and characterized
their optical and electrical properties. The output efficiency enhancement methods for LSCs photovoltaics (PVs) are
explored including attaching white diffusers on the bottom surfaces of LSCs, and adding a refractive index matched
optical gel between the LSC edge surfaces and the attached PV cells. To further improve the output power conversion
efficiency, multi-layered LSCs are studied and compared with single layered LSCs. The distribution of the output current
from the LSC edges varies slightly, which is beneficial to collection of the concentrated light by attached PV cells. Also,
in comparison with applying a wavelength selective film, the alignment of dye molecules using polymerized liquid
crystal is discussed as a promising optical design and efficiency improvement method.
Size- and structure-dependent efficiency enhancement methods are studied for luminescent solar concentrators (LSCs) fabricated by casting organic laser dyes into PMMA matrixes. The enhancement are achieved mainly by attaching a white diffuser with an airgap at the bottom of the LSC and adding refractive index matched optical gel between the LSC's edges and the attached photovoltaic cells. The size-dependent efficiency enhancement is studied for a single layer by changing the size up to 120 cm. The results show that the enhancement from the white diffuser drops and then tends to plateau at a certain size of LSC. This also applies to multilayer LSCs. Together with optical gel, the efficiency enhancement is higher for multilayer structures than that for single layers. We also demonstrate the optimal length for the design of LSCs due to reabsorption of dyes. These results could be applied to optimize the design of other LSCs.
A luminescent solar concentrator (LSC) generally is a sheet of highly transparent materials embedded with luminescent
materials. Incident sunlight is absorbed by the luminescent materials, and then emitted through down conversion process
at longer wavelengths. A large portion of the emitted light is trapped in the sheet and travels to the edges where
photovoltaic solar cells are attached. In this study, we investigate the optical enhancement methods for LSCs with
different sizes mainly by using optical gel and white diffuser. The largest tested LSC is up to 1.2m in length and with
geometrical gain 64. This is, as we know, the largest reported size. It yields electrical gain 3.9 by optical enhancements.
And the optical efficiency is still as large as 10%. The study shows that the enhancement by white diffuser is more
sensitive to the size of the LSCs than that of the optical gel. Such enhancement drops with the increase of the sizes of
LSC, but tends to plateau at certain size.
Quantum dot (QD) luminescent solar concentrator (LSC) uses a sheet of highly transparent materials doped with
luminescent QDs materials. Sunlight is absorbed by these quantum dots and emitted through down conversion process.
The emitted light is trapped in the sheet and travels to the edges where it can be collected by photovoltaic solar cells. In
this study, we investigate the performance of LSCs fabricated with near infrared QDs (lead sulfide) and compared with
the performance of LSCs containing normal visible QDs (CdSe/ZnS), and LSCs containing organic dye (Rhodamine B).
Effects of materials concentrations (related to re-absorption) on the power conversion efficiency are also analyzed. The
results show that near infrared QDs LSCs can generate nearly twice as much as the output current from normal QDs and
organic dye LSCs. This is due to their broad absorption spectra. If stability of QDs is further improved, the near infrared
QDs will dramatically improve the efficiency of LSCs for solar energy conversion with lower cost per Wp.
It is well-known that conservation of phase-space volume or optical etendue leads to strict limits to concentration. Less
well- known is the connection between entropy and etendue. Entropy has a logarithmic dependence on etendue in
addition to the familiar linear dependence on heat. This trade-off permits in principle an exponential boost in
concentration. Optical systems that make use of this possibility will be discussed.
A new technique for ultrashort pulsed beams (UPBs) shaping that uses volume holographic gratings (VHGs) recorded in
anisotropic crystals by modulating the polarization states of the input UPBs is proposed and analyzed. Our approach is
based on the investigation that when a VHG recorded in anisotropic crystals is illuminated by a UPB, the spectral and
temporal widths and the shapes of the diffracted and transmitted pulsed beams vary with the polarization state of the
input UPB, based on the extended coupled wave theory of Kogelnik which combines with the dispersion effect of the
crystal materials. By using this approach, the modulation ranges of the spectral and temporal widths of the output UPB
can be controlled by the parameters of the grating. Moreover, the diffraction efficiency of the volume holographic grating
is analyzed, which also varies with the polarization state of the input UPB. An example of using this approach is given
that a linearly polarized Gaussian-shaped UPB with duration of 30fs is effectively shaped with a transmission volume
holographic grating recorded in photorefractive LiNbO3 crystals.
The diffraction behavior of an ultrashort optical pulse with Gaussian-profile both in time and space diffracted by a volume grating is numerically investigated using two-wave first-order coupled wave theory, which elegantly expressed the theory of volume holography by Kogelnik. Simple analytical expressions for the spectrum and spatial profiles of the transmitted and diffracted beams in the near field are obtained. The total diffraction efficiency is also presented. Detailed diffraction characteristics for the ultrashort pulsed Gaussian beams are investigated. The analysis and diffraction characteristics presented in this paper are important for the ultrashort pulse shaping and volume grating applications.
Using the two-dimensional coupled-wave theory, the diffraction characteristics of volume holographic gratings (VHGs)
with finite size planar are studied for the ultrashort pulsed beam (UPB) readout. Numerical simulations are show for the
special case of the overlapping VHGs reconstructed by a Gaussian-shaped UPB in temporal domain. The effects of the
material dispersion and the finite size of the grating on the intensity distributions of the diffracted and transmitted pulsed
beams, and the total diffraction efficiency are given. Our study also shows the differences between the diffraction
characteristics of the finite size planar VHG for the UPB readout and those for the CW readout. And, comparison of the
diffraction characteristics between the finite size VHGs and the one dimensional VHGs under the UPB readout is given.
With digital holographic interferometry, we have investigated the phase differences of a plane wave transmitting through
a partly-poled RuO2:LiNbO3 crystal sample. The holograms recorded by a CCD array are reconstructed by both the
Fresnel transform method and the angular spectrum backward propagation algorithm. Comparison of the reconstructed
results shows a higher resolution can be achieved by the angular spectrum backward propagation algorithm.
In order to obtain the two dimensional distribution of the refractive index changes due to the domain inversion in the RuO2:LiNbO3 crystal, we make use of the Mach-Zehnder interferometer system to record holograms, with the object light transmitting through the poling RuO2:LiNbO3 crystal, and then filter the spectrum in the frequency domain to get the object light spectrum for numerically reconstructing the object wavefront. The results confirm that the electro-chromic effect region well agrees with the domain inversion region in RuO2:LiNbO3 crystal.
For an ultrashort pulsed beam (UPB) with different polarization states diffracted by a volume holographic grating (VHG), the effects of the bandwidth to its diffraction properties are studied. By developing the coupled wave theory of Kogelnik, the formulas of the spectral and temporal distributions of the transmission and diffraction pulse beams are analytically given, with considering the effect of the dispersion of the grating media further. Then the bandwidths of the VHG for the UPB with different polarization states are investigated. We show the changes of the spectral and temporal distributions of the transmission and diffraction pulsed beams with different ratios of the bandwidth of the UPB to the bandwidth of the VHG. Finally, the varieties of the diffraction efficiency of the VHG for the UPB with different polarization states are studied.
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