Emission cross sections and spectroscopy of Ho3+ laserchannels in KGd(WO4)2 single crystal

The spectroscopic properties of Ho³⁺ laser channels in KGd(WO₄)₂ crystals have been investigated using optical absorption, photoluminescence, and lifetime measurements. The radiative lifetimes of Ho³⁺ have been calculated through a Judd-Ofelt (JO) formalism using 300-K optical absorption results. The JO parameters obtained were Ω₂=15.35×10^-20 cm², Ω ₄=3.79×10^-20 cm², Ω₆ =1.69×10^-20 cm². The 7-300-K lifetimes obtained in diluted (8·10¹⁸ cm^-3) KGW:0.1% Ho samples are: τ(⁵F₃)≈0.9 μs, τ( ⁵S₂)=19-3.6 μs, and τ(⁵F₅ )≈1.1 μs. For Ho concentrations below 1.5×10²⁰ cm^-3, multiphonon emission is the main source of non radiative losses, and the temperature independent multiphonon probability in KGW is found to follow the energy gap law τ_ph ^-1(0)=βexp(-αΔE), where β=1.4×10^-7 s^-1, and α=1.4×10³ cm. Above this holmium concentration, energy transfer between Ho impurities also contributes to the losses. The spectral distributions of the Ho³⁺ emission cross section σ_EM for several laser channels are calculated in σ- and π-polarized configurations. The peak a σ_EM values achieved for transitions to the ⁵I₈ level are ≈2×10^-20 cm² in the σ-polarized configuration, and three main lasing peaks at 2.02, 2.05, and 2.07 μm are envisaged inside the ⁵I₇→⁵I₈ channel     

Gain and saturation intensity of the green Ho:ZBLAN upconversionfiber amplifier and laser

The gain and saturation intensity of the green Ho-doped fluorozirconate (ZBLAN) glass fiber amplifier and laser, pumped in the red (643 ⩽ λ_p ⩽ 649 nm; ⁵F₅ ← ⁵I₈), have been measured. For a 2.4-μm core diameter fiber 45 cm in length, the single-pass gain at 543.4 nm exceeds 12 dB for 90 mW of pump power at 643.5 nm. The saturation power for the ⁵F₄, ⁵S₂ → ⁵I₈ lasing transition was determined from gain measurements to be 970 ± 175 μW, which corresponds to a saturation intensity of 19.8 ± 3.5 kW · cm^-2 , and a stimulated emission cross section approximately one order of magnitude larger than theoretical estimates     

Saturable absorbers based on impurity and defect centers incrystals

Saturation of near-infrared absorption and transmission dynamics are investigated in tetravalent-chromium-doped Gd₃Sc₂ Ga₃O₁₂, Gd₃Sc₂Al₃ O₁₂, and Mg₂SiO₄ crystals, as well as in reduced SrTiO₃ using 20 ps 1.08 μm laser pulses. An absorption cross section of (5±0.5)×10^-18 cm² in garnets and (2.3±0.3)×10^-18 cm² in forsterite is estimated for the ³A ₂-³T₂ transition of tetrahedral Cr⁴⁺. Q-switched and ultra-short pulses are realized in neodymium lasers using chromium-doped crystals as the saturable absorbers. Saturation of free-carrier absorption with ultra-short relaxation time is observed in SrTiO₃ at 10⁸-10 ¹⁰ W/cm² pump intensities, while at 10¹⁰-10¹¹ W/cm² three-photon interband transitions predominate. The free-carrier absorption cross section is estimated to be (2.7±0.3)×10^-18 cm²     

Analysis of competing mechanisms in transitions between excitedstates in Er-doped integrated waveguides

A new procedure for determining the prevalence of either excited-state absorption (ESA) or nonradiative cooperative energy transfer between erbium ions in transitions between excited states in Er-doped integrated waveguides is presented. These transitions are currently very attractive for the development of upconversion lasers. The procedure is based on the analysis of the dependence on the transition-originating mechanisms of the modulation transfer from the pump to the excited levels' population. The accuracy and validity range of the method are studied numerically using ordinary integrated structures. By using this procedure, the ratio of the contributions of the two competing mechanisms to the ⁴I_13/2 ⇒ ⁴I_9/2 transition is determined from fluorescence measurements on a Er,Ti:LiNbO₃ sample excited by a 1480-nm pump. Moreover, new values of the excited-state pump-absorption cross section from level ⁴I_13/2, σ₂₄ (≈1480 nm) = 0.8 × 10^-26 m², and of the parameter associated with nonradiative cooperative energy transfer between Er³⁺ ions, C₂₂ = 3 × 10^-24 m³ s·^-1, are reported     

A comparison of optoelectronic properties of lattice-matched andstrained quantum-well and quantum-wire structures

The k·p formalism is used to study the absorption spectra, material and differential gain in quantum wires as a function of orientation, built-in strain, and wire dimensions. The results for material and differential gain are compared with those for an optimized quantum-well structure. We find that for quantum wires at 300 K, the gain becomes positive at a carrier density of 1.27·10¹⁸ cm^-3, while in quantum wells this density is calculated to be 1.82·10¹⁸ cm^-3. Incorporating tensile strain in the wires reduces the transparency carrier concentration to 0.96·10¹⁸ cm^-3 while compressive strain allows one to obtain positive gain for densities greater than 1.08·10¹⁸ cm^-3. Orienting the wire along the [111] direction reduces the transparency carrier density to 0.60·10¹⁸ cm^-3. The differential gain in quantum-well structures for injections near the threshold is on the order of 10^-14 cm^-4, while for 50 Å·100-Å quantum wires the differential gain near the threshold is found to be on the order of 10^-13 cm^-4 . The differential gain in wires whose wire axis is parallel to the [111] direction has also been found to be on the order of 10^-13 cm^-4 for carrier injections close to the threshold     

Cr4+:YAG as passive Q-switch and Brewster plate in apulsed Nd:YAG laser

We demonstrate the performance of a Nd:YAG laser, passively Q-switched with a Cr⁴⁺:YAG plate, which plays the double role of a passive Q-switch and a Brewster plate. The Brewster plate configuration contributes an intracavity loss of approximately 3.2-10 ^-3 cm^-1 along the cavity length. Losses contributed by the active Cr⁴⁺ ions in the plate relate to their excited state absorption. A freshly measured transmission saturation curve of Cr⁴⁺:YAG suggests a ground state absorption cross section σ_gs=(8.7±0.8)-10^-19 cm², and an excited state absorption cross section σ_es=(2.2±0.2)-10^-19 cm² of the Cr⁴⁺ ions at λ=1064 nm     

Enhancing the Near-Infrared Spectral Response of Silicon Optoelectronic Devices via Up-Conversion

A silicon-based optoelectronic device that exhibits an enhanced response to subbandgap light is described. The device structure consists of a bifacial silicon solar cell with an up- converting (UC) layer attached to the rear. Erbium-doped sodium yttrium fluoride (NaY_0.8F₄ : Er_0.2 ³⁺) phosphors are the optically active centers responsible for the UC luminescence. The unoptimized device is demonstrated to respond effectively to wavelengths (lambda) in the range of 1480-1580 nm with an external quantum efficiency (EQE) of 3.4% occurring at 1523 nm at an illumination intensity of 2.4 W/cm² (EQE = 1.4 times 10^-2 cm²/W). An analysis of the optical losses reveals that the luminescence quantum efficiency (LQE) of the device is 16.7% at 2.4 W/cm² of 1523-nm excitation (LQE = 7.0 times 10^-2 cm²/W), while further potential device improvements indicate that an EQE of 14.0% (5.8 times 10^-2 cm²/W) could be realistically achieved.     

Generation-recombination noise in GaN/AlGaN heterostructure fieldeffect transistors

Local levels with a large activation energy E_a~0.8-1.0 eV have been observed in low-frequency noise measurements of GaN/AlGaN heterostructure field effect transistors (HFETs and MOS-HFETs) grown on 4N-SiC substrates. The noise might come from the thin (30 nm) AlGaN barrier layer. The estimates of the level parameters based on this assumption resulted in reasonable values of capture cross section σ_n≈(10^-12-10^-13) cm² and trap concentration N_t≈5-10¹⁶ cm^-3     

New-user identification in a CDMA system

We present three detector/estimators (DEs) which allow multiuser detection and parameter estimation without a side channel in a dynamic asynchronous code-division multiple-access (CDMA) system in which users are entering and leaving the system. These DEs optimally detect a new user given only the chip rate and the spreading factor of the new user. Two of these DEs, the maximum-likelihood detector/estimator (MLDE) and the generalized maximum-likelihood detector/estimator (GMLDE), produce maximum-likelihood estimates of the new user's signature sequence, delay, and amplitude, which are then incorporated into a multiuser detector. The third DE, the cyclic detector/estimator (CDE), is the most computationally efficient of the three processors. This DE detects the new user by testing for cyclostationarity and then uses suboptimal schemes to estimate the new user's signature sequence, delay, and amplitude. Simulations indicate that all three DEs reliably detect a new user for an E_s/σ² (symbol-energy-to-noise ratio) of 5 dB. The MLDE and GMLDE produce signature sequence and delay estimates with probability of error less than 0.07 for an E_s/σ² of 10 dB, and the CDE produces signature sequence and delay estimates with probability of error less than 0.13 for an E_s/σ² of 15 dB     

Optical absorption and stimulated emission of neodymium in yttriumorthosilicate

A spectroscopic investigation of the biaxial crystal yttrium orthosilicate doped with Nd³⁺(Nd³⁺:Y₂SiO₅) has been performed. Spectrally and orientationally resolved emission cross sections necessary for the evaluation of laser performance on the Nd^{3+ 4}F_3/2-⁴I _9/2 and ⁴F_3/2-⁴I_11/2 transitions have been determined. The Judd-Ofelt theory has been applied to measured values of optical absorption line strengths to obtain the orientation averaged intensity parameters: Ω₂-3.34×10^-20 cm², Ω ₄=4.35×10^-20 cm², and Ω₆=5.60×10^-20 cm². These Judd-Ofelt intensity parameter values are significantly different from those previously reported by A.M. Tkachuk et al. Using these intensity parameters, the Nd^{3+ 4}F₂ metastable state lifetime is predicted to be 225 μs. Measured low Nd concentration ⁴F_3/2 lifetimes of 214 μs indicate a high radiative quantum efficiency. Because of the Stark level splitting of the Nd^{3+ 4}F_3/2 and ⁴I_9/2 manifolds, laser operation at twice one of the Cs atomic resonance filter acceptance wavelengths is possible     

Evaluation of absorption and emission cross sections of Er-dopedLiNbO3 for application to integrated optic amplifiers

The polarization-dependent absorption and emission spectra of the ⁴I_13/2-⁴I_15/2 transition (λ~1.5 μm) in single crystal bulk Er:LiNbO₃ have been measured. Low-temperature (10 K) measurements of the Stark split energy levels of these two manifolds indicate at least two Er³⁺ sites. McCumber theory is applied to determine the Er:LiNbO₃ absorption and emission cross sections. These values are used to calculate the gain characteristics of Er:LiNbO₃ channel waveguides. Calculations indicate that a gain of 10 dB is achievable in a waveguide of several centimeters using ~20-mW pump power     

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 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     

Carrier-dependent nonlinearities and modulation in an InGaAs SQWwaveguide

The authors report measurements of optically induced carrier-dependent refractive index changes and their saturation in an InGaAs single quantum well centered within a linear multiple quantum well guided-wave Fabry-Perot resonator using diode laser sources. A low-excitation nonlinear refractive cross-section, σ_n=-1×10^-19 cm³, was deduced for probe wavelengths near the TM (transverse magnetic) absorption edge, falling only to σ_n=-3.1×10^-20 cm³, at over 0.16 μm from the band edge. For an incident irradiance of 18 kW/cm ², refractive index changes in the InGaAs quantum well as large as -0.16 were deduced near the absorption edge, while the index change at a wavelength 0.16 μm from the absorption edge was -0.055. This large off-resonant index change is attributed to an enhanced free-carrier contribution within a 2D system     

The generation and characterization of electron and hole trapscreated by hole injection during low gate voltage hot-carrier stressingof n-MOS transistors

Hot-carrier stressing carried out on conventional and MDD n-MOS transistors under low gate voltage conditions (V_g⩽V_d/4) is discussed. Following the stress, the devices were subjected to short alternate phases of electron and hole injection into the oxide in order to identify the damage species generated. It is shown that the damage created consists principally of hole and electron oxide traps. This is confirmed using the charge pumping technique. Maximum damage is obtained for conditions of maximum hole injection, indicating that hot holes are responsible for both types of defects. Comparison with maximum interface state damage shows that degradation due to electron traps can be significantly greater than interface state creation in the stressing of n-MOS devices at high drain voltages. The damage is shown to be localized. Two-dimensional simulation of localized charge placed close to the drain junction suggests that equal quantities of positive and negative charge might be created by this stressing. Measurements of capture cross sections for electron trapping reveal two cross sections, σ(1)≈3×10^-15 and σ(2)≈3×10^-16 cm²     

Low-resistance bandgap-engineeredW/Si1-xGex/Si contacts

Bandgap-engineered W/Si_1-xGe_x/Si junctions (p⁺ and n⁺) with ultra-low contact resistivity and low leakage have been fabricated and characterized. The junctions are formed via outdiffusion from a selectively deposited Si_0.7Ge _0.3 layer which is implanted and annealed using RTA. The Si _1-xGe_x layer can then be selectively thinned using NH₄OH/H₂O₂/H₂O at 75°C with little change in characteristics or left as-deposited. Leakage currents were better than 1.6×10^-9 A/cm² (areal), 7.45×10^-12 A/cm (peripheral) for p⁺/n and 3.5×10^-10 A/cm² (peripheral) for n⁺/p. W contacts were formed using selective LPCVD on Si_1-xGe_x. A specific contact resistivity of better than 3.2×10^-8 Ω cm² for p ⁺/n and 2.2×10^-8 Ω cm² for n⁺/p is demonstrated-an order of magnitude n⁺ better than current TiSi₂ technology. W/Si_1-xGe _x/Si junctions show great potential for ULSI applications     

A semi-empirical representation of antenna diversity gain atcellular and PCS base stations

Predictions of uplink space-diversity gain in the cellular and personal communications systems (PCS) bands (near 850 MHz and 1.9 GHz, respectively), suffer from incomplete modeling of multipath angular spread σ. Using previously published measurements to reduce the gap, we show that σ~[distance]^-1/2 and that diversity gains are about 2 dB higher for PCS than for cellular     

Electrical Characterization of ZrO2/Si Interface Properties in MOSFETs With ZrO2 Gate Dielectrics

MOSFETs incorporating ZrO₂ gate dielectrics were fabricated. The I_DS-V_DS, I_DS-V_GS , and gated diode characteristics were analyzed to investigate the ZrO₂/Si interface properties. The interface trap density (D _it) was determined to be about 7.4times10¹² cm ^-2middoteV^-1 using subthreshold swing measurement. The surface-recombination velocity (s₀) and the minority carrier lifetime in the field-induced depletion region (tau _0,FIJ) measured from the gated diodes were about 3.5times10 ³ cm/s and 2.6times10^-6 s, respectively. The effective capture cross section of surface state (sigma_s) was determined to be about 5.8times10^-16 cm² using the gated diode technique and the subthreshold swing measurement. A comparison with conventional MOSFETs using SiO₂ gate oxides was also made     

A New InP/InGaAs Double Heterojunction Bipolar Transistor With a Step-Graded InAlGaAs Collector Structure

An interesting InP/InGaAs double heterojunction bipolar transistor with a step-graded InAlGaAs layer at the base-collector (B-C) heterojunction is fabricated and studied. Simulated results reveal that the potential spike at the B-C heterointerface is completely eliminated. Experimentally, the operation regime is wider than 11 decades in magnitude of the collector current (I_c = 10^-12 A to I_c = 10^-1 A). Furthermore, the studied device exhibits a relatively high common-emitter breakdown voltage and low output conductance even at high temperature. In the microwave characteristics, the unity current gain cutoff frequency f_T = 72.7 GHz and the maximum oscillation frequency f _max = 50 GHz are achieved for a nonoptimized device (A_E = 6 times 6 mum²).     

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