The demand for compact ultraviolet laser devices is increasing, as they are essential in applications such as optical storage, photocatalysis, sterilization, ophthalmic surgery and nanosurgery. Many researchers are devoting considerable effort to finding materials with larger bandgaps than that of GaN. Here we show that hexagonal boron nitride (hBN) is a promising material for such laser devices because it has a direct bandgap in the ultraviolet region. We obtained a pure hBN single crystal under high-pressure and high-temperature conditions, which shows a dominant luminescence peak and a series of s-like exciton absorption bands around 215 nm, proving it to be a direct-bandgap material. Evidence for room-temperature ultraviolet lasing at 215 nm by accelerated electron excitation is provided by the enhancement and narrowing of the longitudinal mode, threshold behaviour of the excitation current dependence of the emission intensity, and a far-field pattern of the transverse mode.

Direct-bandgap properties and evidence for ultraviolet lasing of hexagonal boron nitride single crystal.

Quantum size effect in semiconductor microcrystals

Optical properties of highly excited direct gap semiconductors

The laser generation of ultrasound in solids is reviewed with particular emphasis on the application of this unique ultrasonic source. Three regimes for the generation of ultrasound in solids using lasers in the visible near infrared wavelength region exist: thermoelastic, plasma and constrained surface source regimes. The mechanism for ultrasonic generation in each of these regimes is given. Recent experimental investigations into laser-generated ultrasound are also described, including a description of a different mechanism for the generation of ultrasound in solids using a CO2 laser. Finally, the many applications of laser-generated ultrasound are reviewed. These applications range from nondestructive testing to the determination of candle flame parameters. Possible future applications are also outlined.

https://iopscience.iop.org/article/10.1088/0022-3727/26/3/001/pdf

Laser-generated ultrasound: its properties, mechanisms and multifarious applications

A refractive laser surgery process is disclosed for using compact, low-cost ophthalmic laser systems which have computer-controlled scanning with a non-contact delivery device for both photo-ablation and photo-coagulation in corneal reshaping. The basic laser systems may include flash-lamp and diode pumped UV solid state lasers (193-215 nm), compact excimer laser (193 nm), free-running Er:glass (1.54 microns), Ho:YAG (2.1 microns), Q-switched Er:YAG (2.94 microns), and tunable IR lasers, (750-1100) nm and (2.5-3.2) microns. The advantages of the non-contact, scanning device used in the process over other prior art lasers include being safer, reduced cost, more compact and more precise and with greater flexibility. The theory of beam overlap and of ablation rate and coagulation patterns is also disclosed for system parameters. Lasers are selected with energy of (0.01-10) mJ, repetition rate of (1-10,000), pulse duration of 0.01 nanoseconds to a few hundreds of microseconds, and with spot size of (0.05-2) mm for use with refractive laser surgery.

Ophthalmic surgery method using non-contact scanning laser

The group velocity of excitonic polariton in CuCl has been measured at 4.2 K using picosecond light pulses. It is derived by measuring both the time delay of light pulses and the thickness of samples. Experimental results are well explained by the dispersion relation of excitonic polariton including spatial dispersion. In addition, the translational effective mass of exciton in CuCl is directly derived, i.e. M =(2.0 ±0.1) m 0 .

Picosecond Time of Flight Measurements of Excitonic Polariton in CuCl

Absorption bands have been found at 655 nm, 830 nm and 1.28 µm in ZnS:Mn crystals under optical excitation. From the close correlation to the orange fluorescence in the time behavior and exciting power dependence, the initial state of these transitions is identified with the emitting state 4 T 1 ( 4 G ) of Mn 2+ ion. Comparison of the energies between the experiment and calculation leads to the assignment of the terminal levels as 4 A 2 ( 4 F ), 4 T 1 ( 4 P ) and 4 E ( 4 D ) states, which is consistent with the strengths and breadths of the observed trasitions. From the energy position of the 1.28 µm band, it is concluded that the excitationinduced absorption initiates from the relaxed excited state. It is also shown that both single Mn 2+ and exchange-coupled pair of Mn 2+ and exchange-coupled pair of MN 2+ controbute to the excited-state absorption.

Absorption Spectra of Optically Pumped ZnS:Mn

Excited-state absorption spectra of ZnS : Mn

Time-resolved luminescence spectra in the picosecond region of excitonic molecules in CuCl are studied at 4.2 K under band-to-band and exciton resonance excitations with time resolutions of ∼10 ps and ∼60 ps, respectively. The formation time and the lifetime of excitonic molecules are determined to be \({\lesssim}10\) ps and 300 ps, respectively, the latter of which agrees with the radiative lifetime calculated by using existing theories. The relaxation processes of the excitonic molecule system are discussed on the basis of the time variation of the luminescence line shape, and some analyses are made by considering both molecule-molecule collisions and molcule-acoustic phonon interactions. Transient behavior of the stimulated emission due to the excitonic molecule luminescence are also investigated.

Yuichi Tanaka

Papers

Picosecond Time of Flight Measurements of Excitonic Polariton in CuCl

Absorption Spectra of Optically Pumped ZnS:Mn

Excited-state absorption spectra of ZnS : Mn

Picosecond Time Analysis of Excitonic Molecule Luminescence Spectra in CuCl

Broadly tunable, repetitive, picosecond parametric oscillator