Quasi-cw Excitation Research Articles

Ultralow-threshold quasi-CW lasing from FAPbBr3 perovskite first-order DFB laser

Metal halide perovskites are emerging materials for integrated photonics. Here we report a quasi-CW pumped ultra-low ASE/lasing threshold formamidinium lead bromide (FAPbBr3) laser. The laser achieved stable lasing at 555 nm with a full width at half maximum (FWHM) of 0.6 nm, showing a low lasing threshold of 22.6 μJ cm−2 under 3.5 nanosecond quasi-CW excitation at room temperature. The material also showed an ultra-low ASE threshold of 46 μJ cm−2 under the same pumping condition. Through polymer doping, we showed that the material’s performance can be improved by increasing bimolecular recombination rate with reduced grain size.

Measurement of the temperature-dependent output of lead zirconate titanate transducers

The effect of temperature and electrical drive conditions on the output of lead zirconate titanate (PZT) transducers is of particular interest in ultrasound metrology and medical ultrasound applications. In this work, the temperature-dependent output of two single-element PZT transducers was measured between 22 °C and 46 °C. Two independent measurement methods were used, namely radiation force balance measurements and laser vibrometry. When driven at constant voltage using a 50 Ω matched signal generator and amplifier using continuous wave (CW) or quasi-CW excitation, the output of the two transducers increased on average by 0.6 % per degree, largely due to an increase in transducer efficiency with temperature. The two measurement methods showed close agreement. Similar trends were observed when using single cycle excitation with the same signal chain. However, when driven using a pulser (which is not electrically matched), the two transducers exhibited different behaviour depending on their electrical impedance. Accounting for the temperature-dependent output of PZT transducers could have implications for many areas of ultrasound metrology, for example, in therapeutic ultrasound where a coupling fluid at an increased or decreased temperature is often used.

Energy-Transfer-Upconversion Models, Their Applicability and Breakdown in the Presence of Spectroscopically Distinct Ion Classes: A Case Study in Amorphous Al2O3:Er3+

The influence of energy migration and energy-transfer upconversion (ETU) among neighboring Er3+ ions on luminescence decay and steady-state population densities in Al2O3:Er3+ thin films is investigated by means of photoluminescence decay measurements under quasi-CW excitation. The experimental results are analyzed by several models. As expected from the basic physical assumptions made by these models, only Zubenko’s microscopic model provides good agreement with the experimental data, while other donor−acceptor treatments found in the literature are unsuccessful and the macroscopic rateequation approach provides meaningful results only when misinterpreting the intrinsic lifetime as a free fit parameter. Furthermore, a fast quenching process induced by, e.g., active ion pairs and clusters, undesired impurities, or host material defects such as voids, that is not revealed by any particular signature in the luminescence decay curves because of negligible emission by the quenched ions under quasi-CW excitation, is verified by pump-absorption experiments. This quenching process strongly affects device performance as an amplifier. Since Zubenko’s microscopic model treats all ions equally, it is unable to describe a second, spectroscopically distinct class of ions involving a fast quenching process. The model is extended to take into account the fraction of quenched ions. This approach finally leads to excellent agreement between the luminescence-decay, pump-absorption, and small-signal-gain experiments within the frame of a single theoretical description.

Diode-pumped continuous-wave, quasi-continuous-wave, and Q-switched laser operation of Yb3+,Tm3+: YLiF4 at 1.5 and 2.3 μm

We report continuous-wave (cw), quasi-cw and Q-switched laser oscillation of Yb3+,Tm3+-doped YLiF4 at 1.5 and 2.3 μm. Maximum cw output powers of 300 mW at 1.5 μm and 450 mW at 2.3 μm have been obtained. Quasi-cw excitation revealed 20 mJ pulses at 1.5 μm. The cw pumped 1.5 μm laser was passively Q-switched using a U:CaF2 crystal as saturable absorber with a repetition rate of 65 kHz and a pulsewidth of 800 ns. An average output power of 19 mW was achieved. The dynamics of the system was simulated using a rate equation model.

Space-charge effects in type-II strained layer superlattices

The low-temperature photoluminescence (PL) spectra of a range of CdS-CdSe and CdS-ZnSe type-II superlattices was measured under both cw and quasi-cw excitation over the excitation range from mW cm −2 to MW cm −2. In all cases, a single PL band due to the recombination of separated electrons and holes in the type-II bandstructure was observed. As the excitation density increases from a low value, the PL peak shifts logarithmically towards the blue. Total shifts of up to 200 meV for a CdS-CdSe sample of 10 nm period are observed, with a corresponding shift-factor of 30 meV per decade of excitation density. The shift factor increases in a regular fashion with the superlattice period. A comparison with the literature shows that a logarithmic dependance of the blue shift with increasing excitation density appears to be a universal feature of type-II SL, even in the absence of strain.

Stimulated emission of argon dimers excited by a pulsed plasma-cathode discharge

Results are reported from a theoretical and experimental investigation aimed at obtaining lasing on transitions of an Ar2 excimer laser pumped by a pulsed electric discharge. It is shown that by using a recently developed plasma-cathode discharge, quasi-cw excitation of the laser active medium can be achieved with a gain of around 0.1 cm−1. The results of preliminary experiments performed using a model system indicate an effect wherein the duration of the spontaneous emission at the gl=126 nm transition of the Ar2 dimer decreases monotonically as its peak intensity increases. This behavior, which is observed with increasing pump intensity, is evidence of a stimulated emission effect in the system. However, the measurements indicate that no amplification takes place in the active medium because of accidental impurities in the working gas.

Different carrier temperatures in the wells and in the barriers of InGaAs/GaAs single quantum well structures

Optical carrier heating by quasi-CW excitation has been studied in an InGaAs/GaAs single quantum well structure. At lower excitation intensities, until the quantum well is filled with the photoexcited carriers, the carrier temperature in the well is much higher than in the barriers. At higher excitation intensities, additional carrier heating in the barriers by the hot carriers located in the well is observed. The difference between carrier temperatures in the well and in the barriers is discussed comparing the carrier-LO phonon and carrier-carrier scattering times.

Controlled dissociation of I 2 via optical transitions between the X and B electronic states

Recent progress in both theoretical and experimental methods has led to the widespread belief that control of chemical reactions with ultrashort laser pulses is in principle achievable. In this article, the I 2 molecule, which has been the subject of a great deal of femtosecond work, is studied theoretically as a prospect for experimental control. An unconstrained optimization of the electric field is performed, with the objective of dissociation on the B state. Krotov's optimization theory is used for this unconstrained optimization, which reduces the CPU time usage by a factor of four. The field which results from the optimization yields a probability of 99% for B state dissociation. The Husimi transform of the optimal field indicates a simple underlying structure, in contrast with many previous fields obtained by optimal control theory: the optimal field closely resembles a sequence of three Gaussian pulses with the same central frequency. Analysis of the optimal field indicates it operates by a quasi-cw excitation directly into the B continuum. A constrained optimization was then performed: the electric field was restricted onto the frequency interval from 0 to 19752 cm −1 to avoid the quasi-continuous-wave excitation mechanism. The optimal solution of this problem uses a three-photon pump—dump—pump (PDP) mechanism. The optimal field was approximated by sequences of three and four Gaussian-shaped laser pulses; the optimal and the two approximating fields gave 34%, 6% and 11% for the dissociation probability, respectively. The fields from the constrained optimization have fwhm on the order of 30–50 fs and intensities in the range of 10 10 W/cm 2 and should be experimentally producible with available technology.

Kinetics of inversion loss in a laser utilizing the first negative nitrogen system

The luminescence due to the 0–0 transition in the first negative system of nitrogen under conditions of quasi-cw excitation of He–N2 and He–N2–H2 mixtures by a 1.5 μs electron beam was used to study the main processes resulting in loss of the inversion in a N2+ (B–X) laser. Binary collisions, mainly with helium atoms, were found to play a decisive role. It was established that deactivation of the upper and lower active states by hydrogen was highly selective.

Emission characteristics of an XeCl laser with x-ray preionization and quasi-cw excitation

A determination was made of the characteristics of an electric-discharge XeCl laser with x-ray preionization and quasi-cw excitation. An electrical line with ceramic capacitors was used for the first time as a storage device. A specific laser output energy of 3.25 J/liter and an efficiency of approx.1.6% was achieved. Matching of the line and discharge gap impedances was found to be possible at high charging voltages by adding helium to the Ne--Xe--HCl active medium.

Characteristics of Argon Ion Laser in Quasi-cw Excitation

Characteristics of Argon Ion Laser in Quasi-cw Excitation