Tailored Laser Pulse Research Articles

Tailored laser pulse method to manipulate filler wire melt metallurgy from thermal cycles

A robust method is introduced to simulate and study the filler wire metallurgy for controlled cooling conditions after melting, enabling efficient mapping with prompt analysis of trends. Proposed is a reduced, though representative, process with more controllable conditions. Short lengths of filler wires are preplaced in a cavity, drilled into a base metal sheet. Irradiation by a pulsed laser beam melts the wire to generate a sample nugget. Pulse shaping influences the cooling rate, granting the ability to tailor weldament microstructures. The method is demonstrated for S1100QL steel and undermatched filler wire, to obtain high toughness for processes like laser-arc hybrid welding, where a representative thermal cycle is needed. For high toughness, a controlled amount of acicular ferrite and, in turn, nonmetallic inclusions is desirable. This “snapshot” method has revealed a characteristic histogram of inclusion sizes, for different pulse shapes. Additional information on the thermal cycle can be acquired by employing thermocouples, a pyrometer, or advanced methods like high speed imaging or modeling. The method offers a wide spectrum of variants and applications.

Tailored laser pulse chirp to maintain optimum radiation pressure acceleration of ions

Ion beams generated with ultra-intense laser-plasma accelerators hold promises to provide compact and affordable beams of relativistic ions. One of the most efficient acceleration setups was demonstrated to be direct acceleration by the laser's radiation pressure. Due to plasma instabilities developing in the ultra-thin foils required for radiation pressure acceleration, however, it is challenging to maintain stable acceleration over long distances. Recent studies demonstrated, on the other hand, that specially tailored laser pulses can shorten the required acceleration distance suppressing the onset of plasma instabilities. Here, we extend the concept of specific laser pulse shapes to the experimentally accessible parameter of a frequency chirp. We present a novel analysis of how a laser pulse chirp may be used to drive a foil target constantly maintaining optimal radiation pressure acceleration conditions for in dependence on the target's areal density and the laser's local field strength. Our results indicate that an appropriately frequency chirped laser pulse yields a significantly enhanced acceleration to higher energies and over longer distances suppressing the onset of plasma instabilities.

Weak coherent pulses for single-photon quantum memories

Attenuated laser pulses are often employed in place for single photons in order to test the efficiency of the elements of a quantum network. In this work we analyze theoretically the dynamics of storage of an attenuated light pulse (where the pulse intensity is at the single photon level) propagating along a transmission line and impinging on the mirror of a high finesse cavity. Storage is realized by the controlled transfer of the photonic excitations into a metastable state of an atom confined inside the cavity and occurs via a Raman transition with a suitably tailored laser pulse, which drives the atom and minimizes reflection at the cavity mirror. We determine the storage efficiency of the weak coherent pulse which is reached by protocols optimized for single-photon storage. We determine the figures of merit and we identify the conditions on an arbitrary pulse for which the storage dynamics approaches the one of a single photon. Our formalism can be extended to arbitrary types of input pulses and to quantum memories composed by spin ensembles, and serves as a basis for identifying the optimal protocols for storage and readout.

Optimal storage of a single photon by a single intra-cavity atom

We theoretically analyze the efficiency of a quantum memory for single photons. The photons propagate along a transmission line and impinge on one of the mirrors of a high-finesse cavity. The quantum memory is constituted by a single atom within the optical resonator. Photon storage is realized by the controlled transfer of the photonic excitation into a metastable state of the atom and occurs via a Raman transition with a suitably tailored laser pulse, which drives the atom. Our study is supported by numerical simulations, in which we include the modes of the transmission line and we use the experimental parameters of existing experimental setups. It reproduces the results derived using input–output theory in the corresponding regimes and can be extended to compute dynamics where the input–output formalism cannot be straightforwardly applied. Our analysis determines the maximal storage efficiency, namely, the maximal probability to store the photon in a stable atomic excitation, in the presence of spontaneous decay and cavity parasitic losses. It further delivers the form of the laser pulse that achieves the maximal efficiency by partially compensating parasitic losses. We numerically assess the conditions under which storage based on adiabatic dynamics is preferable to non-adiabatic pulses. Moreover, we systematically determine the shortest photon pulse that can be efficiently stored as a function of the system parameters.

Technique for Forming Solid D2 and D-T Layers for Shock Timing Experiments at the National Ignition Facility

Capsule implosion experiments on the National Ignition Facility (NIF) are driven with a carefully tailored laser pulse that delivers a sequence of shocks to the ablator and fuel. To ensure the shocks converge at the desired position, the shock strength and velocity are measured in experimental platforms referred to as keyhole targets. Shock measurements have been made on capsules completely filled with liquid deuterium for the solid deuterium tritide (D-T) layer campaigns. Modeling has been used to extend these results to form an estimate of the shock properties in solid D-T layers.To verify and improve the surrogacy of the liquid-filled keyhole measurements, we have developed a technique to form a solid layer inside the keyhole capsule. The layer is typically uniform over a 400-μm-diameter area. This is sufficient to allow direct measurement of the shock velocity. This layering technique has been successfully applied to 13 experiments on the NIF. The technique may also be applicable to fast-igniter experiments since some proposed designs resemble keyhole targets. We discuss our method in detail and give representative results.

Selective photo-dissociative ionization of methane molecule with TDDFT study

Three dimensional calculation of control dynamics for finding the optimized laser filed is formulated using an iterative method and time-dependent density functional approach. An appropriate laser pulse is designed to control the desired products in the dissociation of methane molecule. The tailored laser pulse profile, eigenstate distributions and evolution of the efficient occupation numbers are predicted and exact energy levels of this five-atomic molecule is obtained. Dissociation rates of up to 78%, 80%, 90%, and 82% for CH2+, CH+, C+ and C++ are achieved. Based on the present approach one can reduce the controlling costs.

Quasi-monoenergetic ion generation by hole-boring radiation pressure acceleration in inhomogeneous plasmas using tailored laser pulses

It is proposed that laser hole-boring at a steady speed in inhomogeneous overdense plasma can be realized by the use of temporally tailored intense laser pulses, producing high-fluence quasi-monoenergetic ion beams. A general temporal profile of such laser pulses is formulated for arbitrary plasma density distribution. As an example, for a precompressed deuterium-tritium fusion target with an exponentially increasing density profile, its matched laser profile for steady hole-boring is given theoretically and verified numerically by particle-in-cell simulations. Furthermore, we propose to achieve fast ignition by the in-situ hole-boring accelerated ions using a tailored laser pulse. Simulations show that the effective energy fluence, conversion efficiency, energy spread, and collimation of the resulting ion beam can be significantly improved as compared to those found with un-tailored laser profiles. For the fusion fuel with an areal density of 1.5 g cm–2, simulation indicates that it is promising to realize fast ion ignition by using a tailored driver pulse with energy about 65 kJ.

Directivity and frequency control of an intense underwater laser acoustic source for navy applications.

We develop an intense laser acoustic source, in which a tailored laser pulse can compress underwater at a predetermined remote location. Optical compression results in laser-induced breakdown (LIB), localized heating, and acoustic shock generation. Recent experiments include near-field acoustic source characterization using lens-focused 400-, 800-, 532-, and 1064-nm pulses of Ti:sapphire and Nd:YAG lasers. Sound pressure levels over 215-dB were achieved using a compact laser. We have demonstrated control of the shape of the LIB plasma volume, and thereby control of the acoustic frequency spectrum and acoustic source directivity. By superposition of volume elements within the LIB, the acoustic pulse duration in a given propagation direction is determined by the parallel dimension of the LIB, divided by the parallel acoustic transit time across the LIB. Thus, the shape of the LIB strongly affects the acoustic pulse duration and directivity, and aspherical LIB volumes result in strongly anisotropic acoustic sources. In our experiments, the LIB volume shape was varied by laser pulse length, energy, optical bandwidth, and focusing angle. We also studied acoustic propagation in a 30 000-gal bubbly salt water tank. [This work is supported by the U.S. Office of Naval Research.]

Electrical pulse compression using photoconductive zinc selenide opening switches

Electrical pulse compression and power gain were observed in an inductive energy storage pulsed power circuit with a photoconductive polycrystalline ZnSe opening switch. The ZnSe opening switch was optically activated by a specially tailored laser pulse operated at the blue-green region. A maximum voltage gain of 8 and a power gain of 64 was obtained with a 509 V DC bias voltage. We observed in the ZnSe opening switch a recoverable breakdown behavior similar to lock-on in GaAs. A peak output voltage of 8.9 kV was obtained from a 1 mm thick switch with 5 kV DC bias. >

Compression of matter to superhigh densities by an accelerating heat wave

A one-dimensional analytical model is presented for the compression of matter (e.g., thermonuclear fusion fuel to densities of more than 103 times the solid density) by an accelerating subsonic heat wave. The heat wave, driven possibly by the absorption of a temporally tailored laser pulse, launches a large number of successive weak shock waves which compress and heat the target quasiadiabatically in order to keep the energy expenditure at a minimum. The net power to drive this heat wave must have a stepwise increase in time approximated by W = 98ρ0c03K2/τ2, where K = Dfρf/D0ρ0 is the fraction of the initial target mass (of density ρ0 and thickness Df) which reaches the final state (of ρf and Df. The adiabaticondition sets the requirement that K should be much smaller than one. The time t is scaled as τ = 1 − t/t8, where t8 = D0/c0 and c0 is the initial sound speed. The sound speed in the compressed matter, from which pressure and density can be obtained using the adiabatic equations of state, will increase approximately as c0(K/τ)1/4. Half of the total driving energy is expended by the time tf[τf = (Df/D0) (ρ0/ρf)1/3], from which time the power has to be held constant at Wf = 98(ρf/ρ0)8/3ρ0c03 until time t8.