Various capabilities of LP3 ASUR platform are presented with particular emphasis on the possibility to combine moderate and high-intensity laser beamlines for developing pump-probe approach with modular (optical/x-ray) time-resolved diagnostics. This unique and highly versatile combination in terms of laser excitation, laser – material arrangement and conditions, and in-operando diagnostics makes the ASUR platform an ideal tool for nurturing fundamental knowledge in laser – material interaction and for innovative laser-driven engineering of materials and devices.
We demonstrate laser-fabrication of length-controlled channels in the bulk of dielectrics with a spatial periodicity down to 0.7 µm and moderate aspect ratio (1:10), by single-shot ablation using different (fs/ps) pulse durations. We take advantage of beam shaping technique using an axicon and annular aperture to generate a Bessel beam with an extra control of the so-called “non-diffracting” length. The dimensions and pitch attained are suitable to envision the writing of NIR nanophotonic components. As a proof-of-principle demonstration, we fabricate patterns whose arrangements mimic photonic-crystal devices like waveguide, Y-coupler, and structures with square and triangular lattices as their basic units.
Periodic arrangements of micro- and nano-holes with controllable period, diameter and depth at the surface of materials are of high interest for a large range of applications. Here we present a laser-based approach using micro-Bessel beams with adjustable length to machine arrays of holes with subwavelength diameters and depths reaching several micrometers at the surface of fused silica. Suitability and limitations of the technique are investigated, including the challenge of avoiding crosstalk effects. The performance level shows the potential of the direct-laser-processing method towards the realization of integrated devices, as a flexible and cost-effective alternative technique to current multistep nanofabrication methods.
Arrays of nanoholes or nanochannels constitute the building block of integrated devices that open attractive applications like 2D photonic crystals, 2D metamaterials or nanostructured surfaces. Here we present a laser-based technique that enables to generate short-length micro-Bessel beams (irrespective of their core diameter) that we further use to machine depth-controlled holes with a cylindrical depth profile. We illustrate the potential of this method by fabricating square arrays of subwavelength-diameter holes with several-micrometers depth by direct laser ablation at the surface of fused silica.
High intensity femtosecond laser are now routinely used to produce energetic particles and photons via interaction with solid targets. However, the relatively low conversion efficiency of such processes requires the use of high repetition rate laser to increase the average power of the laser-induced secondary source. Furthermore, for high intensity laser-matter interaction, a high temporal contrast is of primary importance as the presence of a ns ASE pedestal (Amplified Spontaneous Emission) and/or various prepulses may significantly affect the governing interaction processes by creating a pre-plasma on the target surface. We present the characterization of a laser chain based on Ti:Sa technology and CPA technique, which presents unique laser characteristics : a high repetition rate (100 Hz), a high peak power (>5 TW) and a high contrast ratio (ASE<10-10) obtained thanks to a specific design with 3 saturable absorbers inserted in the amplification chain. A deformable mirror placed before the focusing parabolic mirror should allow us to focus the beam almost at the limit of diffraction. In these conditions, peak intensity above 1019W.cm-2 on target could be achieved at 100 Hz, allowing the study of relativistic optics at a high repetition rate.
We present the development of a laser damage and ablation test-bench able to accommodate ultrashort pulses down to 10
fs pulse duration. The laser test-bench is operated in air and we demonstrate its capability to accurately measure the
damage and ablation characteristics of optical materials, like fused silica, irradiated by single ultrashort pulses of < 15 fs
pulse duration. The careful characterization of beam propagation allows us to ascertain the precise retrieval of laserinduced
damage and ablation threshold fluences as well as to identify the energetic regime yielding to beam
filamentation.
Dielectrics as single layers and broadband high-reflective stacks were deposited by electron beam deposition
processes compatible with 1-meter class optics. After being physically and optically characterized, samples were
irradiated with several ultrafast lasers (KYW:Yb 500fs, Ti:Sa 40fs and Ti:Sa 11fs) with single and multi-pulses.
The setups of the test platforms, laser-induced damage threshold investigations of intrinsic materials, dielectric
multilayers and hybrid metal/dielectric multilayers and electric field intensity distributions are described.
The metrology of laser damage is essential for the development of intense laser chains and their applications, especially
in micromachining. We first present a test bench able to accurately measure the damage and ablation behaviour of
optical materials and components irradiated by femtosecond lasers (< 15 fs - 5 ps). We further illustrate the interest of
our measurements through examples related to laser technology and engineering, and also fundamental knowledge of
laser-matter interaction.
Results of laser induced damage threshold (LIDT) of fused silica, sapphire and Ti:Sa crystals in single shot mode in the femtosecond regime down to few optical cycles (< 10 fs) are presented. Different approaches to determine LIDT are described and compared. Our methodology yields accurate determination of damage/ablation threshold of any material (or component) irradiated by pulsed laser, as well as complementary physical results characterizing laser–matter interaction and/or concerning the deterministic character of femtosecond damage. It is shown that the abrupt decrease of both damage and ablation thresholds observed with ultra-short pulses (< 30 fs) is related to the significance of tunnel ionization in the ultrashort regime. Moreover, the results indicate that the laser damage occurrence is more deterministic below 30 fs.
Yuen Yap Cheng, Burkhard Fückel, Tim Schulze, Rowan MacQueen, Murad Tayebjee, Andrew Danos, Tony Khoury, Raphaël G. Clady, N. Ekins-Daukes, Maxwell Crossley, Bernd Stannowski, Klaus Lips, Timothy Schmidt
KEYWORDS: Solar cells, Solar energy, Upconversion, Photovoltaics, Amorphous silicon, External quantum efficiency, Absorption, Molecules, Energy efficiency, Silicon films
Photovoltaics (PV) offer a solution for the development of sustainable energy sources, relying on the sheer
abundance of sunlight: More sunlight falls on the Earth’s surface in one hour than is required by its inhabitants in a
year. However, it is imperative to manage the wide distribution of photon energies available in order to generate
more cost efficient PV devices because single threshold PV devices are fundamentally limited to a maximum
conversion efficiency, the Shockley-Queisser (SQ) limit. Recent progress has enabled the production of c-Si cells
with efficiencies as high as 25%,1 close to the limiting efficiency of ∼30%. But these cells are rather expensive, and ultimately the cost of energy is determined by the ratio of system cost and efficiency of the PV device. A strategy to radically decrease this ratio is to circumvent the SQ limit in cheaper, second generation PV devices. One promising approach is the use of hydrogenated amorphous silicon (a-Si:H), where film thicknesses on the order of several 100nm are sufficient. Unfortunately, the optical threshold of a-Si:H is rather high (1.7-1.8 eV) and the material
suffers from light-induced degradation. Thinner absorber layers in a-Si:H devices are generally more stable than
thicker films due to the better charge carrier extraction, but at the expense of reduced conversion efficiencies,
especially in the red part of the solar spectrum (absorption losses). Hence for higher bandgap materials, which
includes a-Si as well as organic and dye-sensitized cells, the major loss mechanism is the inability to harvest low
energy photons.
Hot carrier solar cells have a fundamental efficiency limit well in excess of single junction devices. Developing a
hot carrier absorber material, which exhibits sufficiently slow carrier cooling to maintain a hot carrier population
under realistic levels of solar concentration is a key challenge in developing real-world hot carrier devices.
We propose strain-balanced In0.25GaAs/GaAsP0.33 quantum wells as a suitable absorber material and present
continuous-wave photoluminescence spectroscopy of this structure. Samples were optimised with deep wells and
the GaAs surface buffer layer was reduced in thickness to maximise photon absorption in the well region. The
effect of well thickness on carrier distribution temperature was also investigated. An enhanced hot carrier effect
was observed in the optimised structures and a hot carrier distribution temperature was measured in the thick
well (14 nm) sample under photon flux density equivalent to 1000 Suns concentration.
We have investigated a photochemical up-conversion system comprising a molecular mixture of a palladium
porphyrin to harvest light, and a polycyclic aromatic hydrocarbon to emit light. The energy of harvested
photons is stored as molecular triplet states which then annihilate to bring about up-converted fluorescence.
The limiting efficiency of such triplet-triplet annihilation up-conversion has been believed to be 11% for some
time. However, by rigorously investigating the kinetics of delayed fluorescence following pulsed excitation, we
demonstrate instantaneous annihilation efficiencies exceeding 40%, and limiting efficiencies for the current system
of ≈60%. We attribute the high efficiencies obtained to the electronic structure of the emitting molecule, which
exhibits an exceptionally high T2 molecular state. We utilize the kinetic data obtained to model an up-converting
layer irradiated with broadband sunlight, finding that ≈3% efficiencies can be obtained with the current system,
with this improving dramatically upon optimization of various parameters.
We present the design of an OPCPA (Optical Parametric Chirped Pulse Amplification) pre-amplifier incorporated in a
visible (475 nm) high-contrast multiterawatt femtosecond laser chain based on hybrid (solid/gas) technology.
In this paper we describe the experimental validation of the technique of correction of wavefront aberration in the middle of the laser amplifying chain. This technique allows the correction of the aberrations from the first part of the laser system, and the pre-compensation of the aberrations built in the second part. This approach will allow an effective aberration management in the laser chain, to protect the optical surfaces and optimize performances, and is the only possible approach for multi-Petawatt laser system from the technical and economical point of view. This approach is now possible after the introduction of new deformable mirrors with lower static aberrations and higher dynamic than the standard devices.
A low density medium like a gas is attractive for laser amplification due to its high breakdown threshold and scalability to very large volumes. Moreover, the non-linear index of refraction of a gas is of three orders of magnitude lower than for a solid medium that is particularly suitable for direct amplification, without pulse stretching, of high-power ultrashort pulses. Among all gas laser media, application of the photolytical XeF(C-A) laser for high energy amplification is very attractive for the development of ultra-high power laser systems up to the petawatt power level due to the XeF(C-A) broad amplification bandwidth (80 nm FWHM centered near 475 nm) and a rather high saturation fluence (~0.05 J.cm-2), as well as a very low level of Amplified Spontaneous Emission. The paper presents the strategy of the LP3 laboratory to develop a high-contrast multiterawatt femtosecond laser chain based on a hybrid (solid/gas) technology, including a Ti:Sapphire oscillator generating 50 fs pulses at 950 nm, an Optical Parametric Chirped Pulse Amplification stage, a frequency converter, and a final high-energy amplification in the photolytical XeF(C-A) amplifier. Our approach is supported by first pilot experiments of femtosecond pulse amplification in a compact photolytical XeF(C-A) amplifier.
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