Publisher Summary This chapter describes the coherent population trapping in laser spectroscopy. Coherent population trapping may be also described as the pumping of the atomic system in a particular state, the coherent superposition of the atomic states, which is a nonabsorbing state. The exciting radiation creates an atomic coherence, such that the atom's evolution is prepared exactly out of phase with the incoming radiation and no absorption takes place. The chapter discusses the basic properties of an atomic system prepared with the coherent population-trapping superposition of states and outlines experimental observations concerned with the establishment of coherent trapping in different discrete systems. The chapter also discusses the theoretical and experimental aspects of trapping that involve states of the continuum and reviews the theoretical and experimental features associated with coherent population trapping in laser cooling, adiabatic transfer, lasing without inversion, pulse matching, and photon statistics. The theoretical aspect of coherent population trapping created by spontaneous emission is also discussed in the chapter.

Coherent Population Trapping in Laser Spectroscopy

For many years, laser-driven ion acceleration, mainly proton acceleration, has been proposed and a number of proof-of-principle experiments have been carried out with lasers whose pulse duration was in the nanosecond range. In the 1990s, ion acceleration in a relativistic plasma was demonstrated with ultra-short pulse lasers based on the chirped pulse amplification technique which can provide not only picosecond or femtosecond laser pulse duration, but simultaneously ultra-high peak power of terawatt to petawatt levels. Starting from the year 2000, several groups demonstrated low transverse emittance, tens of MeV proton beams with a conversion efficiency of up to several percent. The laser-accelerated particle beams have a duration of the order of a few picoseconds at the source, an ultra-high peak current and a broad energy spectrum, which make them suitable for many, including several unique, applications. This paper reviews, firstly, the historical background including the early laser-matter interaction studies on energetic ion acceleration relevant to inertial confinement fusion. Secondly, we describe several implemented and proposed mechanisms of proton and/or ion acceleration driven by ultra-short high-intensity lasers. We pay special attention to relatively simple models of several acceleration regimes. The models connect the laser, plasma and proton/ion beam parameters, predicting important features, such as energy spectral shape, optimum conditions and scalings under these conditions for maximum ion energy, conversion efficiency, etc. The models also suggest possible ways to manipulate the proton/ion beams by tailoring the target and irradiation conditions. Thirdly, we review experimental results on proton/ion acceleration, starting with the description of driving lasers. We list experimental results and show general trends of parameter dependences and compare them with the theoretical predictions and simulations. The fourth topic includes a review of scientific, industrial and medical applications of laser-driven proton or ion sources, some of which have already been established, while the others are yet to be demonstrated. In most applications, the laser-driven ion sources are complementary to the conventional accelerators, exhibiting significantly different properties. Finally, we summarize the paper.

https://iopscience.iop.org/article/10.1088/0034-4885/75/5/056401/pdf

Review of laser-driven ion sources and their applications.

Arrays of micrometer-scale needles could be used to deliver drugs, proteins, and particles across skin in a minimally invasive manner. We therefore developed microfabrication techniques for silicon, metal, and biodegradable polymer microneedle arrays having solid and hollow bores with tapered and beveled tips and feature sizes from 1 to 1,000 μm. When solid microneedles were used, skin permeability was increased in vitro by orders of magnitude for macromolecules and particles up to 50 nm in radius. Intracellular delivery of molecules into viable cells was also achieved with high efficiency. Hollow microneedles permitted flow of microliter quantities into skin in vivo, including microinjection of insulin to reduce blood glucose levels in diabetic rats.

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Microfabricated needles for transdermal delivery of macromolecules and nanoparticles: Fabrication methods and transport studies

As a form of matter intermediate between molecules and bulk solids, atomic clusters have been much studied1 Light-induced processes in clusters can lead to photo-fragmentation2,3 and Coulombic fission4, producing atom and ion fragments with a few electronvolts (eV) of energy However, recent studies of thephotoionization of atomic clusters with high intensity (>1016 W cm−2) femtosecond laser pulses have shown that these interactions can be far more energetic5,6,7,8,9,10,11,12,13—excitation of large atomic clusters can produce a superheated microplasma that ejects ions with kinetic energies up to 1 MeV (ref 10) This phenomenon suggests that through irradiation of deuterium clusters, it would be possible to create plasmas with sufficient average ion energy for substantial nuclear fusion Here we report the observation of nuclear fusion from the explosions of deuterium clusters heated with a compact, high-repetition-rate table-top laser We achieve an efficiency of about 105 fusion neutrons per joule of incident laser energy, which approaches the efficiency of large-scale laser-driven fusion experiments Our results should facilitate a range of fusion experiments using small-scale lasers, and may ultimately lead to the development of a table-top neutron source, which could potentially find wide application in materials studies

Nuclear fusion from explosions of femtosecond laser-heated deuterium clusters

Nonlinear Optical Crystals: A Complete Survey

A new nonlinear optical crystal CsLiB6O10 (CLBO), is described that can be grown from either stoichiometric melt or from solution. A large, high quality single crystal with dimensions of 14×11×11 cm3 was obtained by the top‐seeded Kyropoulos method. Fourth harmonic and fifth harmonic generations of the 1.064 μm Nd:YAG laser radiation with type‐I phase matching were realized in the CLBO crystal. Output pulse energies obtained were 110 mJ at 266 nm and 35 mJ at 213 nm.

New nonlinear optical crystal: Cesium lithium borate

On the basis of the anionic group theory, a new nonlinear optical crystal cesium triborate, (CsB3O5, CBO) was discovered. Single crystals of CBO in centimeter size were grown from a stoichiometric melt. The transparent range of CBO covers from 170 to 3000 nm. The preliminary measurement of its second harmonic generation coefficient shows that its d14 is ∼0.468×d11 (β−BaB2O4). Its measured damage threshold is as high as 26 GW/cm2 at 1.053 μm, 1.0 ns.

CsB3O5: A new nonlinear optical crystal

A new nonlinear optical (NLO) crystal, CsLiB6O10 (CLBO) has been discovered. CLBO can quadruple and quintuple the Nd:YAG laser output. Large CLBO crystal with dimensions 13×12×10 cm3 could be grown in twelve days.

New Nonlinear Optical Crystal Cesium Lithium Borate

L‐arginine phosphate monohydrate (LAP) and deuterated LAP (DLAP) are new organic nonlinear optical materials useful for higher harmonics of radiation from high‐power lasers. We measured the bulk laser damage threshold of these crystals using light from a 1.05 μm laser with 1 and 25 ns pulse widths and 0.53 μm laser light with 0.6 and 20 ns pulse widths. In every case, these crystals show much higher thresholds than potassium dihydrogen phosphate (KDP and fused silica. These crystals are very interesting not only as a frequency converter but also as other optical components of high‐power lasers, because of their extremely high damage threshold.

Extremely high damage threshold of a new nonlinear crystal L-arginine phosphate and its deuterium compound

Nonlinear optical (NLO) properties of CsLiB6O10 (CLBO) have been investigated. Frequency quadrupling of the 1.064 µ m Nd:YAG laser output for type-I phase matching was observed by using CLBO. The walk-off angle and the effective NLO coefficient deff were estimated to be 1.83° and 1.01 pm/V for fourth harmonics generation of Nd:YAG laser output, respectively. Phase matching for sum frequency generation of 4ω (fourth harmonics of Nd:YAG laser)+ω (fundamental of Nd:YAG laser), resulting in fifth harmonics of Nd:YAG laser output, has been observed.

https://iopscience.iop.org/article/10.1143/JJAP.34.L296/pdf

Sadao Nakai

Papers

New nonlinear optical crystal: Cesium lithium borate

CsB3O5: A new nonlinear optical crystal

New Nonlinear Optical Crystal Cesium Lithium Borate

Extremely high damage threshold of a new nonlinear crystal L-arginine phosphate and its deuterium compound

Nonlinear optical properties of cesium lithium borate