Pulsed laser damage thresholds in vitro for intraocular lenses and membranes

Energy and power density damage thresholds were determined in air, for plastic IOL's and membranes at the focal point of several solid-state laser systems: 1) 694 nm,Q-switched single pulse (30 ns), multimode, 2) 1064 nm,Q-switched single pulse (20 ns), TEM00, 3) 1060 nm, mode-locked single pulse, 15 ps, TEM11, 4) 530 nm, mode-locked single pulse, 15 ps, TEM11, and 5) 1064 nm, mode-locked pulse train (9-11 pulses, 30 ps), TEM00. Pulse energies bracketing damage thresholds as well as focal diameter and pulse duration for each system were determined. Energy density thresholds are lower, and power density thresholds higher, for shorter duration pulses-e.g., 23 J/cm²(1.15 GW/cm²) versus 6 J/cm²(400 GW/cm²) at the same wavelength as in systems 2) and 3) (p = 0.005). Damage thresholds for glass IOL's are 37 J/cm²(1.9 GW/cm²) and 37 J/cm²(1235 GW/cm²) as in systems 2) and 5). Damage threshold values for plastic membranes (Saran Wrap^®) exposed to nanosecond and picosecond pulse trains of Nd:YAG at 1064 nm are about half that of plastic IOL's. When laser pulses with a cone angle of 14° from systems 2) and 5) are focused on plastic membrane next to the IOL, damage thresholds are 30 J/cm²(1.5 GW/cm²) for 20 nsQ-switched pulses and 20 J/cm²(670 GW/cm²) for trains of 30 ps mode-locked pulses. Damage thresholds of IOL's immersed in 0.9 percent saline are approximately the same as those obtained in air.     

Optical power limiting and stabilization based on a novel polymercompound

Optical power limiting and stabilization based on the two-photon absorption (TPA) mechanism is performed in a polymer solution excited by ~810 nm and ~7-ns laser pulses. The solute is a novel polymer, a poly(2,5-dialkoxy-p-phenylene ethynylene) derivative (EBO-OPPE). Using 1-cm path-length EHO-OPPE solution in chloroform of d₀=0.03 mol of repeat unit/liter as the nonlinear absorptive medium, the dynamic transmission changes from T=0.92 to 0.28 when the input intensity of the ~810-nm laser beam is increased from I₀=15 to 600 MW/cm² . The measured nonlinear absorption coefficient is 14.5 cm/GW. Optical power stabilization is demonstrated at an average input intensity level of I₀≈400 MW/cm² with a Δ≈±25% peak-power fluctuation of the laser pulse. After passing through the nonlinear medium, the output peak-power fluctuation is reduced to Δ≈±8%. The spectral-width effect of the input laser beam on the nonlinear absorption of the EHO-OPPE solution is investigated. For three different spectral structures of the input laser beam (single narrow spectral line, multiple spectral lines, and broad spectral band), measured values of TPA cross section for EHO-OPPE are σ₂=66, 80, and 101×10^-20 cm⁴/GW, respectively. This means that EHO-OPPE is one of the best known nonlinear absorptive materials for power limiting purposes     

Compensation of nonlinear self-focusing in high-power lasers

We report on experimental results and the numerical modeling of the compensation of nonlinear self-focusing effects occurring in a high-power Nd:glass laser system at intensities above 1 GW/cm². A GaAs wafer with a negative nonlinear refractive index was used for the cancellation of the accumulated phase difference along the beam direction in a nonlinear medium (B integral). A numerical model is developed to calculate the behavior of the experimental setup     

Nonlinear pulse switching using long-period fiber gratings

We show that the intensity required to achieve all-optical switching in long-period fiber gratings can be reduced by orders of magnitude through the use of uniform phase-shifting regions between gratings. Predicted switching intensities are of the order 100 MW/cm² compared to 10 GW/cm² for normal device configuration     

Study of the Nd: Glass laser radiation

The Nd:glass laser has become one of the most useful sources of light pulses a few picoseconds in duration. In this paper, we review the results of an extensive study of the time and spectral structure of the Nd:glass laser radiation. The time structure was studied by means of two-photon fluorescence (TPF) patterns: these were scanned by a very thin (28-μ) cell containing the fluorescent dye, the fluorescence being monitored by a photomultiplier. When the Nd:glass laser isQswitched by a rotating mirror or when it is free running, we find TPF patterns fully consistent with a model where the laser emission has the character of Gaussian noise (thermal light), i.e., a model where the modes are randomly phased. When the laser was simultaneouslyQswitched and mode locked we made two observations : 1) the TPF patterns show that the ultra-short pulses observed previously have an overall duration of ∼8 ps, but also possess an internal substructure containing peaks 0.4-0.8 ps in duration; 2) the spectral width of these pulses is <20 cm^-1at the beginning of theQ-switched train and expands to ∼80 cm^-1in the middle of the train. This rapid spectral broadening during pulse buildup is attributed to self-phase modulation in the laser glass matrix due to a nonlinear index n2which we evaluate asn{2}= (2 pm 1) times 10^{-22}m²/V²(or1.8 times 10^{-13}esu). Gain limiting due to this effect and self-focusing become very important at power densities above 1 GW/cm², presenting a serious limitation on the energy density (J/cm²), which one may hope to extract from Nd: glass laser systems.     

Steady-state and transient forward current-voltage characteristicsof 4H-silicon carbide 5.5 kV diodes at high and superhigh currentdensities

Steady-state and transient forward current-voltage I-V characteristics have been measured in 5.5 kV p⁺-n-n⁺ 4H-SiC rectifier diodes up to a current density j≈5.5×10 ⁴ A/cm². The steady-state data are compared with calculations in the framework of a model, in which the emitter injection coefficient decreases with increasing current density. To compare correctly the experimental and theoretical results, the lifetime of minority carriers for high injection level, τ_ph, has been estimated from transient characteristics. At low injection level, the hole diffusion length L_pl has been measured by photoresponse technique. For a low-doped n-base, the hole diffusion lengths are L_pl≈2 μm and L_ph≈6-10 μm at low and high injection levels respectively. Hole lifetimes for low and high injection levels are τ_pl≈15 ns and τ_ph≈140-400 ns. The calculated and experimental results agree well within the wide range of current densities 10 A/cm ²3 A/cm². At j>5 kA/cm², the experimental values of residual voltage drop V is lower than the calculated ones. In the range of current densities 5×10³ A/cm²4 A/cm², the minimal value of differential resistance R_d =dV/dj is 1.5×10^-4 Ω cm². At j>25 kA/cm², R_d increases with increasing current density manifesting the contribution of other nonlinear mechanisms to the formation steady-state current-voltage characteristic. The possible role of Auger recombination is also discussed     

10 A, 2.4 kV Power DiMOSFETs in 4H-SiC

We report the characteristics of large area (3.3 × 3.3 mm ²) high-voltage 4H-SiC DiMOSFETs. The MOSFETs show a peak MOS channel mobility of 22 cm²/V·s and a threshold voltage of 8.5 V at room temperature. The DiMOSFETs exhibit an on-resistance of 4.2 mΩ·cm² at room temperature and 85 mΩ·cm² at 200°C. Stable avalanche characteristics at approximately 2.4 kV are observed. An on-current of 10 A is measured on a 0.103 cm² device. High switching speed is also demonstrated. This suggests that the devices are capable of high-voltage, high-frequency, low-loss switching applications     

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     

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     

Efficient third-harmonic generation of a picosecond laser pulsewith time delay

A new configuration based on the polarization-mismatching scheme with time delay for efficient frequency tripling conversion is proposed in this paper. The calculated results showed that the requirement for the efficient frequency tripling conversion of a 1-ps laser pulse is not only the optimization of peak intensity of the second-harmonic pulse, but also the optimization of the pulse duration ratio and temporal difference between the o-polarization second-harmonic pulse and the e-polarization first-harmonic pulse due to group-velocity mismatch among the interacting pulses. With the proposed scheme the group velocity mismatch can be compensated. Overall energy conversion efficiency increases from 55% to 75% under the optimized conditions at the intensity of 6 GW/cm². The temporal shape of the third-harmonic pulse with 1 ps pulse duration has no subpulses. The optimization of the efficient frequency tripling conversion for intensities of over 75 GW/cm² is also described. The results showed that the maximum tripling energy conversion efficiency is close to 80% with the optimized doubler and tripler     

Dye-doped photorefractive liquid crystals for dynamic and storageholographic grating formation and spatial light modulation

The basic mechanisms of photo-induced space charge field formation, director axis re-orientation, and refractive index changes in fullerene C60- and dye-doped nematic liquid crystals films are presented. In particular, in aligned methyl-red-doped nematic liquid crystal film, we observe a nonlinear index change coefficient as high as 10 cm²/W, associated with purely optically induced liquid crystal director axis re-orientation. Experimental observations of dynamic and high-resolution storage holographic grating formation, two beam coupling with gain of nearly 3000 cm^-1, optical limiting action at nanowatt cw laser power, and incoherent-coherent image conversion at μW/cm² light intensity level are discussed     

Crystal growth, frequency doubling, and infrared laser performanceof Yb3+:BaCaBO3F

Yb:BaCaBO₃F(Yb:BCBF) has been investigated as a new laser crystal with potential for self-frequency doubling, Yb³⁺ in BCBF exhibits a maximum absorption cross section at 912 nm of 1.1×10^-20 cm² with a bandwidth (FWHM) of 19 nm. The maximum emission cross section at 1034 nm is 1.3×10^-20 cm² with a transition bandwidth of 24 nm. The measured emission lifetime of Yb³⁺ is 1.17 ms. An Yb:BCBF laser has been demonstrated with a Ti:sapphire pump source, and a measured slope efficiency of 38% has been obtained for the fundamental laser output. Single crystal powders of BCBF have been compared with KD ⁺P for a relative measure of the second-harmonic generating potential, yielding d_eff(BCBF)~0.26 pm/V. The phasematching angle has been estimated from the refractive index data for type I second-harmonic generation of 0.517 μm light; the predicted angle is 37° from the c-axis. The growth, spectroscopy, laser performance, and linear and nonlinear optical properties of Yb:BCBF are reported     

High-quality two-dimensional electron gas inAlxGa1-xAs/GaAs heterostructures by LP-OMVPE

A combination of high mobility and high sheet carrier density in Al_xGa_1-xAs/GaAs two-dimensional electron gas (2DEG) elements was obtained by low-pressure organometallic vapor phase epitaxy (OMVPE). The sheet charge densities (n_s) and mobilities (μ) at 77 K are 1.2×10¹²/cm² and 90000 cm²/V-s for single-channel, and 2.0× 10¹²/cm² and 64500 cm²/V-s for double-channel elements, respectively. Strong correlations between the photoluminescence spectrum of the Al_xGa_1-xAs layers and the 2DEG mobility were found. The 2DEG elements were used as mixers and detectors at millimeter wavelengths. Mixing at 94 GHz with a 1.7-GHz IF bandwidth and detection of signals as high as 238 GHz under a magnetic field were achieved with these devices     

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.     

Investigation of inductively coupled plasma gate oxide on lowtemperature polycrystalline-silicon TFTs

By optimizing the inductively coupled plasma (ICP) oxidation condition, a thin oxide of 10 nm has been grown at 350°C to achieve excellent gate oxide integrity of low leakage current<5×10^-8 A/cm² (at 8 MV/cm), high breakdown field of 9.3 MV/cm and low interface trap density of 1.5×10¹¹ /eV cm². The superior performance poly-Si TFTs using such a thin ICP oxide were attained to achieve a high ON current of 110 μA/μm at V_D=1 V and V_G=5 V and the high electron field effect mobility of 231 cm²/V·S     

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     

Solid-phase epitaxial Pd/Ge ohmic contacts to In1-xGaxAsyP1-y/InP

A Pd/Ge metallization to InGaAsP/InP semiconductors, formed with solid-phase epitaxy (SPE) technique, has been investigated in this study. With this method, ohmic contacts with low specific contact resistance (rho_{c} approx 2.3 times 10^{-6}Ω.cm²) have been achieved on p-type In0.53Ga0.47As(p approx 1.8 times 10^{19}/cm³). The same contact scheme also gives low specific contact resistance (rho_{c} approx 6 times 10^{-7}Ω.cm²) on n-type In0.53Ga0.47As (n approx 1.0 times 10^{19}/cm³). Excellent surface morphology is observed in all the samples, and the contacts do not deteriorate for at least 4 h at temperatures between 300 and 500°C.     

Dispersive subband-resonant nonlinearity in amorphous Si/SiO2 quantum well structures

Dispersive nonlinearity in amorphous Si/SiO₂ quantum well structures (QW's) has been investigated. The refractive index changes obtained from the intensity-dependent reflection spectra are nonlinearly dependent on the excitation intensity and can be described by the model of the saturating nonlinearity at low pump intensities. The nonlinear refractive index reveals resonant behavior associated with the subband structure of the QW's. The saturated nonlinear index and the saturation intensity have been obtained as Δn_s=-0.11 and I_s=1.9 MW/cm² at the transitions between the lowest subbands, and Δn_s~0.3 and I_s~0.5 MW/cm² at the transitions between the second subbands of the valence and conduction bands. The nonlinearity for the second subband transitions has been found high enough to provide potentially bistable operation, but the bistability is not expected at the transitions between the ground subbands. Carrier lifetime less than 1 ps restricting the switching time of the nonlinearity has been estimated from the saturation intensity     

Lasing mode behavior of a single-mode MQW laser injected with TMpolarized light

The lasing mode behavior of a multiple quantum well (MQW) distributed feedback (DFB) laser was measured when intensity-modulated orthogonally polarized transverse magnetic (TM) mode light was injected. The 3-dB bandwidth of the frequency response shows a trend different from that observed with conventional bias current modulation: at high bias currents, it decreases with increasing bias current. The maximum bandwidth of 3 dB was observed when the normalized bias current was 4, and it reached 16 GHz at this bias current. The gain saturation coefficients for the transverse electric (TE) and TM modes estimated from these results were ∈_p^E; 2.5×10^-17 cm³ and ∈_q^E 5.7×10^-18 cm³ for the TE mode, and ∈_p^M: 6.0×10^-17 cm³ and ∈_q^M: 2.0×10^-14 cm³ for the TM mode     

InGaAs/InAlAs/InP collector-up microwave heterojunction bipolartransistors

Collector-up InGaAs/InAlAs/InP heterojunction bipolar transistors (HBTs) were successfully fabricated, and their DC and microwave characteristics measured. High collector current density operation (J_c>30 kA/cm²) and high base-emitter junction saturation current density (J₀>10^-7 A/cm²) were achieved. A cutoff frequency of f _t=24 GHz and a maximum frequency of oscillation f _max=20 GHz at a collector current density of J₀ =23 kA/cm² were achieved on a nominal 5-μm×10-μm device