Diode-pumped ytterbium-doped Sr5(PO4)3F laser performance

The performance of the first diode-pumped Yb³⁺-doped Sr ₅(PO₄)₃F (Yb:S-FAP) solid-state laser is discussed. An InGaAs diode array has been fabricated that has suitable specifications for pumping a 3×3×30 mm Yb:S-FAP rod. The saturation fluence for diode pumping was deduced to be 5.5 J/cm ² for the particular 2.8 kW peak power diode array utilized in our studies. This is 2.5× higher than the intrinsic 2.2 J/cm ² saturation fluence as is attributed to the 6.5 nm bandwidth of our diode pump array. The small signal gain is consistent with the previously measured emission cross section of 6.0×10^-20 cm², obtained from a narrowband-laser pumped gain experiment. Up to 1.7 J/cm³ of stored energy density was achieved in a 6×6×44 mm Yb:S-FAP amplifier rod. In a free running configuration, diode-pumped slope efficiencies up to 43% (laser output energy/absorbed pump energy) were observed with output energies up to ~0.5 J per 1 ms pulse. When the rod was mounted in a copper block for cooling, 13 W of average power was produced with power supply limited operation at 70 Hz with 500 μs pulses     

Energy storage and heating measurements in flashlamp-pumpedCr:Nd:GSGG and Nd:YAG

The authors have experimentally measured the energy stored and the heat generated in flashlamp-pumped Cr:Nd:GSGG for three Cr³⁺ concentrations in the range of 1-2×10²⁰ ions/cm³ . It has been found that the energy storage efficiency in these samples is 1.7 times greater than that of the Nd:YAG sample, and the normalized heating parameter χ, defined as the heat deposited per unit of stored energy, is 2.5 under the specified pumping conditions, with no evident dependence on the Cr³⁺ concentration. It has been been found that the measured χ value for the sample of Nd:YAG is 2.9 for the same pumping conditions. These observed χ values exceed expected values by factors of ~1.1 and 2 for the Cr:Nd:GSGG and Nd:YAG samples, respectively. The thermal focal length in the two materials was measured showing that the lensing is shorter in GSGG by a factor of 2.6 for the same available output power, or a factor of 4.5 for the same input pump power. The expected thermal lensing was determined using measured heat loads with no adjustable parameters, achieving satisfactory agreement with measured lensing values     

Output beam focusability of a Nd:glass zig-zag slab laser

The focusability of a zig-zag optical path laser with a 230×65×5 mm Nd:glass for application to soft X-ray generation is described. A Q-switched laser output energy of 18.3 J with a pulse width of 28 ns was obtained in a phosphate glass LHG-5 (Nd:8 wt.%), with an extraction efficiency of 71% from a stored energy of 26 J at a pumping energy of 4 kJ. The focusability was achieved up to 6.2×10¹² W/cm² in a spot size of 100 μm, which was sufficient intensity to generate soft X-rays for X-ray lithography     

Stimulated Raman shifting of the Nd:YAG fourth harmonic (266 nm) inH2, HD, and D2

The dependence of the first Stokes stimulated Raman conversion efficiency of fourth-harmonic radiation from a Nd:YAG laser at 266 nm has been studied for the isotopic species H₂, HD, and D₂ as a function of gas pressure and laser energy using a low numerical aperture (~4.5×10^-3) pumping geometry. While the laser energy threshold for first Stokes conversion is seen to vary significantly between the species it has been found that photon conversion efficiencies of at least 50% can be achieved for all of them for laser pump energies at 266 nm ⩽50 mJ/pulse. This study provides a new measurement of the differential cross section for stimulated Raman scattering in HD of 8.1±2.4×10^-29 cm²/sr at 266 nm and at high pressures, and agreement is found with previous measurements of the cross sections for H₂ and D₂. The results have been used to optimize the laser transmitter system for a differential absorption lidar (DIAL) system to measure tropospheric ozone concentration profiles     

Arrays of microdischarge devices having 50-100 μm squarepyramidal Si anodes and screen cathodes

Microdischarge devices having (50-100 μm)² pyramidal Si anodes and metal screen cathodes have been operated continuously at Ne gas pressures up to 1350 Torr and voltages below 95 V. More than 34 μW of output power is produced by a single device in a solid angle of ~5×10^-2 sr for a Ne pressure of 500 Torr. 3×3 arrays of these devices have been fabricated     

N掺杂MgZnO薄膜的p型导电稳定性研究

利用磁控溅射技术,以Mg0.06Zn0.94O为陶瓷靶材,制备了N掺杂p型Mg0.1 3Zn0.8 7O薄膜,薄膜的电阻率为42.45Ω·cm,载流子浓度为3.70×1017/cm3,迁移率为0.40cm2·V-1·s-1。研究了该薄膜p型导电性质在室温空气下随时间的变化情况。实验结果表明,薄膜的电阻率逐渐升高,载流子浓度降低,五个月以后,薄膜转变为n型导电,电阻率为85.58Ω·cm,载流子浓度为4.53×1016/cm3,迁移率为1.61cm2·V-1·s-1。真空热退火后重新转变为p型。结果显示,其p型导电类型的转变与在空气中吸附H2O或H2等形成浅施主有关。     

32×32 ultraviolet Al0.1Ga0.9N/GaN p-i-nphotodetector array

We report the fabrication and performance of a 32×32 Al_0.1Ga_0.9N-GaN ultraviolet p-i-n photodetector array. The devices exhibit very low dark current, the mean dark current density is ~4 nA/cm² at 5-V reverse bias, and the dark current distribution is very uniform (~98% of the devices exhibit dark current density <90 nA/cm²). Owing to the design of the p-Al_0.13Ga_0.87N window layer, the external quantum efficiency is as high as 72% at 357 nm. The photocurrent distribution is also presented. The detectivity is estimated to be as high as 8×10 ¹⁴ cm·Hz^1/2·W^-1     

Dynamic characterisation of Si/SiGe power HBTs

Si/SiGe power heterojunction bipolar transistors (HBTs) grown by MBE were dynamically characterised in the common-base configuration. At an emitter current density of 1.1×10⁵ A/cm², a maximum frequency of oscillation of 49 GHz was observed. At 10 GHz a maximum unilateral gain of 14 dB is available, and a CW output power of 1.3 W/mm for a device with 10 parallel emitter-fingers of 1×10 μm² each was predicted, from CW measurements     

Degradation of InGaAs/InP double heterojunction bipolar transistorsunder electron irradiation

The dc characteristics of InGaAs/InP double heterojunction bipolar transistors (DHBTs) are studied under high-energy (~1 MeV) electron irradiation up to a fluence of 14.8×10¹⁵ electrons/cm ². The devices show an increase in common-emitter current gain (h_fe) at low levels of dose (<10¹⁵ electrons/cm²) and a gradual decrease in h_fe and an increase in output conductance for higher doses. The decrease in h _fe is as much as ~80% at low base currents (~10 μA) after a cumulative dose of 14.8×10¹⁵ electrons/cm². The observed degradation effects in collector current-voltage (I-V) characteristics are studied quantitatively using a simple SPICE-like device model. The overall decrease in h_fe is attributed to increased recombination in the emitter-base junction region caused by radiation-induced defects. The defects introduced in the collector-base junction region are believed to be responsible for the observed increase in the output conductance     

Low-noise back-illuminated AlxGa1-xN-basedp-i-n solar-blind ultraviolet photodetectors

We report the growth, fabrication and characterization of Al_0.4Ga_0.6N-Al_0.6Ga_0.4N back-illuminated, solar-blind p-i-n photodiodes. The peak responsivity of the photodiodes is 27 and 79 mA/W at λ≈280 nm for bias voltages of 0 V and -60 V, respectively, with a UV-to-visible rejection ratio of more than three decades (at 400 nm). These devices exhibit very low dark current densities (~5 nA/cm² at -10 V). At low frequencies, the noise exhibits a 1/f-type behavior. The noise power density is S₀≈5×10^-25 A²/Hz at -12.7 V and the detectivity (D*) at 0 V is estimated to be in the range of 4×10¹¹-5×10¹³ cm·Hz^1/2 /W. Time-domain pulse response measurements in a front-illumination configuration indicate that the devices are RC-time limited and show a strong spatial dependence with respect to the position of the incident excitation, which is mainly due to the high resistivity of the p-type Al_0.4Ga_0.6 N layer     

Carrier transport related analysis of high-power AlGaN/GaN HEMTstructures

Shubnikov-de Haas (SdH) oscillation and Hall measurement results were compared with HEMT DC and RF characteristics for two different MOCVD grown AlGaN-GaN HEMT structures on semiinsulating 4H-SiC substrates. A HEMT with a 40-nm, highly doped AlGaN cap layer exhibited an electron mobility of 1500 cm²/V/s and a sheet concentration of 9×10¹² cm at 300 K (7900 cm²/V/s and 8×10¹² cm^-2 at 80 K), but showed a high threshold voltage and high DC output conductance. A 27-nm AlGaN cap with a thinner, lightly doped donor layer yielded similar Hall values, but lower threshold voltage and output conductance and demonstrated a high CW power density of 6.9 W/mm at 10 GHz. The 2DEG of this improved structure had a sheet concentration of n_SdH=7.8×10¹² cm^-2 and a high quantum scattering lifetime of τ_q=1.5×10^-13 s at 4.2 K compared to n_SdH=8.24×10¹² cm^-2 and τ_q=1.72×10^-13 s for the thick AlGaN cap layer structure, Despite the excellent characteristics of the films, the SdH oscillations still indicate a slight parallel conduction and a weak localization of electrons. These results indicate that good channel quality and high sheet carrier density are not the only HEMT attributes required for good transistor performance     

Pumping with diodes

The development of semiconductor lasers for pumping solid-state laser crystals is discussed. The basic operating principles of solid-state devices are reviewed to show why this new development is so important. Among the first of the new diode-laser technologies to be applied to pumping was the phase-locked array, which produced much higher power outputs and higher brightness from room-temperature CW diode-laser systems. With diode-laser arrays capable of power outputs between 0.2 and 2 W, and relatively good beam quality, it became possible to pump Nd:YAG and other crystals longitudinally, with the pump and laser output beams either parallel or antiparallel in the laser crystal. Diode pumping allows the flowing cooling liquids and fluctuating arc lamps used in conventional lamp-pumped lasers to be replaced by solid heat sinks and stabilized pump lasers. As a result, designers can build low-power solid-state lasers having high mechanical and thermal stability. Short-term linewidths in the kilohertz range can be obtained from diode-pumped devices, and the long-term drift of monolithic devices can be kept within 50 MHz for over an hour (for a center frequency of 3×10⁸ MHz) by controlling the laser-crystal temperature     

High peak tunnel current densityGa0.47In0.53As Esaki diodes

High peak current density Ga_0.47In_0.53As interband tunnel diodes were fabricated by metal organic molecular beam epitaxy. A room temperature peak-to-valley current ratio of 16 and a peak tunnel current density of 9.2 kA/cm² were obtained in diodes doped to ~3×10¹⁹cm³ on both n-type and p-type sides. A peak-to-valley current ratio of 3.8, and a peak tunnel current density of 93.2 kA/cm² were obtained in diodes doped to ~10²⁰ cm^-3 on both n-type and p-type sides     

Experimental study of gain and output coupling characteristics of aCW chemical oxygen-iodine laser

Gain and output coupling characteristics of the CW chemical oxygen-iodine laser (COIL) are determined experimentally by means of varying the output coupling method. Under the conditions that the Cl₂ flow rate is 11.8 mmol/s, the I₂ molar flow rate is from 20 to 50 μmol/s, and the duct pressure is 200 Pa, the following were obtained from the experimental data: maximum values of output power of 58 W, and optimal output coupling factor of 1.50%, a resonator efficiency of 4.8%, an unsaturated small-signal gain of 1.55×10^-3 cm^-1, a threshold small-signal gain of 1.31×10^-3 cm^-1, a saturation intensity of 1150 W/cm², intraresonator losses of 9%, and an atomic iodine concentration of 2.85×10¹⁴ cm^-3. A comparison of these results to the published data of other COIL systems is presented     

Superconducting thin films of Bi-Sr-Ca-Cu-O deposited using a `freelasing' infrared Nd:YAG laser

The authors have used a free lasing Nd:YAG laser operating at 1·064 μm for depositing thin films of the new ceramic superconductor Bi-Sr-Ca-Cu-O. The stringent baking cycles required to produce good-quality films exhibiting zero resistance at 80 K is discussed. Current densities exceeding 15×10³ A/cm² at 4·2 K have been achieved     

Noncontact high-speed waveform measurements with the picosecondphotoelectron scanning electron microscope

The authors have replaced the electron gun and beam blanking system of a conventional voltage contrast scanning electron microscope by a pulse-laser/photocathode combination, resulting in a source producing electron pulses of order 1 ps in duration at a 100 MHz repetition rate and with a peak brightness of 3 10⁸ A/cm² sr at 1.8 keV. This novel instrument has demonstrated stroboscopic noncontact waveform measurements on metal interconnect lines in different environments with a temporal resolution between than 5 ps, a voltage resolution of 3 mV/(Hz)^1/2, and a spatial resolution of 0.1 μm. These measurements are achieved with extraction fields above the sample of about 1 kV/mm     

A reasonably practical XUV laser for applications

We describe a laboratory-scale 109 nm Xe Auger laser pumped by an all-commercial high-repetition-rate Nd:YAG laser system. The Xe laser provides pulse energies up to 1 μJ, and an average flux of 3×10 ¹² photons/s at 11.5 eV. Measurements of the gain, output energy, and output beam shape are reported, along with an investigation of the time dependence of the gain     

Simultaneous Q-switching and mode-locking in a diode-pumped Nd:YVO4-Cr4+:YAG laser

Parametric studies of passive Q-switching and mode-locking in a Nd ³⁺:YVO₄-Cr⁴⁺:YAG laser were theoretically carried out. Simultaneous mode-locking and Q-switching was also experimentally studied. It was found that over 90% of the output power could be mode-locked in a diode-pumped passively Q-switched Nd³⁺:YVO₄-Cr⁴⁺:YAG laser. The average pulse duration of the mode-locked pulse train was estimated to he around 110~150 ps. The highest peak power of a single pulse near the maximum of the Q-switched envelope was greater than 100 kW     

Experimental determination of electron drift velocity in 4H-SiC p+-n-n+ avalanche diodes

4H-SiC p⁺-n-n⁺ diodes of low series resistivity (<1×10^-4 Ω·cm²) were fabricated and packaged. The diodes exhibited homogeneous avalanche breakdown at voltages U_b=250-270 V according to the doping level of the n layer. The temperature coefficient of the breakdown voltage was measured to be 2.6×10^-4 k^-1 in the temperature range 300 to 573 K. These diodes were capable of dissipating a pulsed power density of 3.7 MW/cm² under avalanche current conditions. The transient thermal resistance of the diode was measured to be 0.6 K/W for a 100-ns pulse width, An experimental determination of the electron saturated drift velocity along the c-axis in 4H-SIC was performed for the first time, It was estimated to be 0.8×10⁷ cm/s at room temperature and 0.75×10⁷ cm/s at approximately 360 K     

Experimental studies of proton-implanted GaAs-AlGaAsmultiple-quantum-well modulators for low-photocurrent applications

We describe the first attempts to control photocurrent, and thus power dissipation, in surface-normal multiple-quantum-well (MQW) modulators. We have made detailed experimental studies of proton-implanted p-i-n GaAs-Al_xGa_1-xAs MQW modulators having barrier layers of x=0.3, 0.45, and 1.0. Structures were implanted to levels of 1×10¹² cm^-2, 1×10¹³ cm^-2, and 1×10¹⁴ cm ^-2. Photocurrent progressively decreased with increasing implant-dose and barrier mole fraction (x). Exciton linewidths showed a strong voltage and implant dose dependence, demonstrating a tradeoff between photocurrent and modulation performance. We obtained our best results with x=1.0 barriers. For example, 1×10¹³ cm^-2-implanted asymmetric Fabry-Perot modulators were realized in which the optical performance was similar to that of unimplanted devices. The photocurrent responsivity was, however, only 0.007 A/W at 12.5 V bias. We report measurements of carrier lifetime in these materials that show the reduction in photocurrent arises from a reduction in lifetime due to implant-induced damage. In addition, the reduced lifetime decreases the optically-excited quantum-well carrier population, leading to an increase in cw saturation intensity. Specifically, 1×10¹³ cm^-2-implanted devices with x=1.0 have a saturation intensity of roughly 45 kW/cm², while unimplanted devices have 3.5 kW/cm². Asymmetric self electro-optic effect devices (A-SEED's) are demonstrated, and power dissipation issues associated with the use of low-photocurrent modulators in integrated systems are discussed