Lithography-Free Nanofabrication on Various Semiconductors by the Codeposition Etching Technique.

Nano-/microstructures can be formed with the aid of small amounts of impurities during deposition with noble gas plasma irradiation, which is referred to as codeposition etching. This can be a new method for lithography-free semiconductor nanofabrication. Here, the codeposition etching method was employed with argon plasma and molybdenum (Mo) impurities on various semiconductors. Structures can be formed only on substrates that have a lower sputtering yield than the seed impurity. The density, area, and height of structures are related to both the impurity deposition rate and the substrate material. Moreover, two mechanisms of impurity nucleation are proposed according to time dependence results for the formation of the structures.

Verdet constant dispersion of magnesium fluoride for deep-ultraviolet and vacuum-ultraviolet Faraday rotators.

The Verdet constant dispersion in magnesium fluoride (MgF2) crystals was evaluated over a wavelength range of 190-300 nm. The Verdet constant was found to be 38.7 rad/(T·m) at a wavelength of 193 nm. These results were fitted using the diamagnetic dispersion model and the classical Becquerel formula. The fitted results can be used for the designing of suitable Faraday rotators at various wavelengths. These results indicate the possibility of using MgF2 as Faraday rotators not only in deep-ultraviolet regions, but also in vacuum-ultraviolet regions owing to its large bandgap.

Tb,Y:SrF2 crystal for efficient laser operation in the visible spectral region.

A Tb,Y:SrF2 crystal with high optical quality is grown using the temperature gradient technique. The spectroscopic and laser properties of the crystal in the visible spectral region are studied. A fluorescence lifetime of 5.6 ms is measured from the crystal, which is beneficial for laser operation with a low threshold. A continuous-wave Tb,Y:SrF2 laser delivers an output power of 259 mW at 545 nm, with a slope efficiency of 35.2%. To the best of the authors' knowledge, this is the first report on a Tb-doped SrF2 laser and represents the highest output power for visible alkaline-earth fluoride lasers. The limitations for power scaling are discussed.

Efficient diode-pumped Er:YAP master-oscillator power-amplifier system for laser power improvement at 2920†nm.

We report on the first demonstration of laser-diode-pumped master-oscillator power-amplifier (MOPA) system based on Er-doped bulk material working at 2920 nm. The relaxation oscillation at the beginning of the laser pulse from the Er:YAlO3 (YAP) oscillator was suppressed effectively when the pump frequency was increased to 140 Hz, as a result of the establishment of a three-level system. In the amplifier, the small signal gain of the Er:YAP strongly depends on pump duration and repetition frequency, and can reach the upper limit of parasitic oscillation. Further, 25.5 mJ of output pulse energy has been achieved from the amplifier at 150 Hz frequency (2.2 ms pump duration), with over 32% of optical-to-optical efficiency. Further improvement of the amplification ability of the MOPA system was discussed.

Highly efficient Er:YAP laser with 6.9 W of output power at 2920†nm.

We report on the efficient high-power operation of a laser-diode-pumped Er3+-doped yttrium aluminum perovskite (Er:YAP) laser in the 3 µm spectral region at room temperature. 6.9 W of continuous-wave (CW) output power was obtained at 2920 nm. The slope efficiency was as high as 30.6% with respect to the absorbed pump power, which is close to the quantum defect limit (33.4%). To the best of our knowledge, this is the highest CW output power generated from 3 µm Er3+-doped solid state lasers at room temperature. Furthermore, our analysis has shown that more than 10 W of output power based on Er:YAP is possible by further mitigating the thermal lens effect.

Efficient Pr:YAlO3 lasers at 622†nm, 662†nm, and 747†nm pumped by semiconductor laser at 488†nm.

We report laser operation of Pr:YAlO3 pumped by a frequency-doubled optically pumped semiconductor laser. Continuous-wave laser oscillations at around 622 nm, 662 nm, and 747 nm were demonstrated in plano-concave or/and plano-plano cavities. The maximum slope efficiencies were found to be 37%, 35%, and 59%, respectively, which are record-high values for Pr:YAlO3 lasers. Furthermore, lasing at 622 nm was demonstrated at room temperature for the first time to the best of our knowledge.

Compact deep ultraviolet frequency-doubled Tb:LiYF4 lasers at 272 nm.

A new laser system has been developed to generate coherent deep ultraviolet (DUV) radiation at 272 nm. The DUV lasers were produced via intra-cavity frequency doubling of the Tb3+:LiYF4 lasers emitting fundamentally at 544 nm. Continuous-wave (cw) and Q-switched operations were performed with a type I phase-matched ß-BaB2O4 nonlinear crystal. The cw operation produces 127 mW of averaged DUV output power. Passive Q-switched operation was realized by using Co2+:MgAl2O4 saturable absorbers. At an initial transmittance (excluding Fresnel reflections) of 99% at 544 nm, stable pulsed output at 272 nm with maximum single-pulse energy of 7.6 µJ and peak power of 6.1 W was obtained. Furthermore, by employing a smaller initial transmittance of 94.7%, we achieved maximum averaged DUV output power of 277 mW. The statistically averaged single-pulse DUV energy and peak power were estimated to be around 100 µJ and 320 W, respectively, which indicates great potential for this DUV laser system toward high energy and peak power.

40 kHz, 20 ns acousto-optically Q-switched 4 µm Fe:ZnSe laser pumped by a fluoride fiber laser.

An actively Q-switched mid-infrared Fe:ZnSe laser pumped by a continuous wave fluoride fiber laser has been developed. Stable operation with a pulse duration of 20 ns and a repetition rate of 40 kHz at 4 µm was achieved. The maximum peak power was 1.1 kW. The high-repetition rate, high-peak power nanosecond pulsed laser, which has been created for the first time, to the best of our knowledge, in an actively Q-switched Fe:ZnSe laser, should prove a suitable light source for laser processing and molecular sensing.

Power scalable 30-W mid-infrared fluoride fiber amplifier.

A fluoride-fiber-based master oscillator power amplifier (MOPA) for 30-W class continuous-wave (cw) operation at 2.8-µm wavelength has been demonstrated. To overcome the low durability of ZBLAN fibers, various novel technologies for using fluoride glass with a ZBLAN-fiber-based side-pump combiner have been adopted in the system. A maximum cw output power of 33 W and stable operation under 23-W output have been demonstrated. We suggest that such fiber MOPA systems will open up advanced fluoride fiber technology for next-generation high-power mid-IR lasers.

Optimization of laser emission at 2.8 µm by Er:Lu2O3 ceramics.

We have demonstrated the continuous-wave operation of a highly efficient 2.8 µm Er-doped Lu2O3 ceramic laser at room temperature. An Er:Lu2O3 ceramic with a doping concentration of 11 at.% afforded a slope efficiency of 29% and an output power of 2.3 W with pumping at 10 W. To our knowledge, these are the highest slope efficiency and output power obtained to date for an Er:Lu2O3 ceramic laser at 2.8 µm. In addition, we prepared ceramics with various doping concentrations and determined their emission cross sections by fluorescence lifetime measurements and emission spectroscopy.

Plane-by-plane femtosecond laser inscription of first-order fiber Bragg gratings in fluoride glass fiber for in situ monitoring of lasing evolution.

We report the femtosecond laser inscription of fiber Bragg gratings (FBGs) in an Er-doped fluoride glass fiber used for lasing at a mid-infrared wavelength of 2.8 µm. The lasing evolution is discussed in terms of the FBG reflectivity, wavelength transition to the Bragg wavelength, and output power of the mid-infrared fiber laser. A first-order and short (2.5-mm-long) Bragg grating showed a reflectivity of 97%, because of a laser-induced index modulation of 1.1 × 10-3. This modulation was sufficient to saturate this system's output power. The laser oscillator is designed to lase in the atmospheric window of 2799-2800 nm slope. Further, this oscillator's efficiency is as high as 29.1% for the launched pump power over the range of 0.4-4.6 W and at a lasing wavelength of 2799.7 nm. This oscillator also exhibited a FWHM bandwidth of 0.12 nm.

Efficient continuous wave and quasi-continuous wave operation of a 2.8 µm Er:Lu2O3 ceramic laser.

We have demonstrated a highly efficient 2.8 µm Er-doped Lu2O3 ceramic laser and investigated the lasing dynamics by time-resolved spectroscopy. During room-temperature continuous wave operation, a slope efficiency of 22% was achieved with a high-quality transparent ceramic. To our knowledge, this is the highest slope efficiency obtained by an Er:Lu2O3 ceramic laser. In addition, an output peak power of 1.2 W was obtained during quasi-continuous wave operation. Time-resolved spectroscopy showed that the emission wavelengths exhibited a red shift from 2715 to 2845 nm, which indicated that continuous wave operation may be possible at 2740 and 2845 nm.

Fast nondiffusive response of heat and turbulence pulse propagation.

The experimental findings from the Large Helical Device have demonstrated a fast, nondiffusive behavior during the propagation of heat pulses, with an observed increase in speed with reduction in their temporal width. Concurrent propagation of the temperature gradient and turbulence, in a timeframe spanning from a few milliseconds to tens of milliseconds, aligned with the avalanche model. These results indicate that the more spatiotemporally localized the heat and turbulence pulses are, the greater the deviation of the plasma from its equilibrium state, coupled with faster propagation velocity. This insight is pivotal for future fusion reactors, which necessitate the maintenance of a steady-state, non-equilibrium condition.

Broadband mid-infrared amplified spontaneous emission from Er/Dy co-doped fluoride fiber with a simple diode-pumped configuration.

Er3+/Dy3+ co-doped double-clad ZBLAN optical fiber has been used to obtain amplified spontaneous emission (ASE) broadband light sources cladding-pumped by 980-nm multimode laser diode (LD) sources. It has been demonstrated that mid-infrared broadband emission extending from 2515 to 3735 nm was obtained by energy transfer between Er3+ and Dy3+. We experimentally investigated the optimum design of Er3+/Dy3+ co-doped ZBLAN fiber in terms of ion concentration, fiber length, pumping configuration, and pumping power. The ASE output power was more than 2.5 mW when the LD pump power was set at 5 W. To assess its potential for gas sensing applications, the fabricated ASE light source was used to successfully detect methane gas with concentrations at 1% and 5%. The simple and stable construction of our ASE light source is suitable for practical purposes.

Dual Strong Couplings Between TPPS J-Aggregates and Aluminum Plasmonic States.

We report on the spectral properties of strong coupling between the localized surface plasmon resonances (LSPRs) of aluminum (Al) nanostructures and tetraphenylporphyrin tetrasulfonic acid hydrate (TPPS) J-aggregates. Because of their wide spectral range of LSPR bands from ultraviolet to near-infrared wavelengths by controlling structural size, Al nanodisks can realize strong coupling with different excitons of TPPS J-aggregates. The Rabi splitting energies of the excitons based on Soret and Q bands are 300 and 180 meV, respectively. In addition to extinction spectrum, we have also measured an excitation spectrum to determine the essential absorption of the hybrid states and successfully confirmed a shoulder peak corresponding to a lower branch of hybrid states. In Al nanorod systems, strong coupling with two excitons can also be selectively induced by merely rotating the polarization of the incident light, which constituted a simple platform for the dynamic control of exciton/plasmon coupling states.