Alan R. Kost

Optical limiting with C60 in polymethyl methacrylate

We demonstrate optical limiting for the C(60) fullerene in polymethyl methacrylate (PMMA) as a solid polymer host. It is shown that the optical-limiting behavior is consistent with excited-state absorption (reverse saturable absorption) as a mechanism. We suggest that a higher threshold for optical limiting compared with that of C(60) in toluene is due to nonlinear scattering for the liquid. The performance of C(60) in PMMA is compared with that in chloroaluminum phthalocyanine, N-methylthioacridone, Kings complex, and ruthenium Kings complex in PMMA. Optical damage thresholds are reported.

Mid-wave infrared diode lasers based on GaInSb/InAs and InAs/AlSb superlattices

We report the characterization of a set of broad‐area semiconductor diode lasers with mid‐wave infrared (3–5 μm) emission wavelengths. The active region of each laser structure is a 5‐ or 6‐period multiple quantum well (MQW) with Ga0.75In0.25As0.22Sb0.78 barriers and type‐II (broken‐gap) Ga0.75In0.25Sb/InAs superlattice wells. The cladding layers of each laser structure are n‐ and p‐type InAs/AlSb (24 A /24 A) superlattices grown lattice‐matched to a GaSb substrate. By tailoring constituent layer thicknesses in the Ga0.75In0.25Sb/InAs superlattice wells, laser emission wavelengths ranging from 3.28 μm (maximum operating temperature=170 K) to 3.90 μm (maximum operating temperature=84 K) are obtained.

Screening effects in (111)B AlGaAs-InGaAs single quantum well heterostructures

A reduction in luminescence decay time and a shift toward higher optical transition energy is observed in response to an increase in photogenerated carrier density for a p‐i‐n (111)B Al0.15Ga0.85As‐In0.055Ga0.945As strained‐layer single quantum well heterostructure. These effects, which are attributed to free‐carrier screening of the strain‐induced electric field, are expected to be useful for designing novel optoelectronic devices that exploit the unique electro‐optic properties of (111) strained quantum wells.

Subpicosecond spin relaxation in GaAsSb multiple quantum wells

Spin relaxation times in GaAsxSb1−x quantum wells are measured at 295 K using time-resolved circular dichroism induced by 1.5 μm, 100 fs pulses. Values of 1.03 and 0.84 ps are obtained for samples with x=0 and 0.188, respectively. These times are >5 times shorter than those in InGaAs and InGaAsP wells with similar band gaps. The shorter relaxation times are attributed to the larger spin-orbit conduction-band splitting in the Ga(As)Sb system, consistent with the D’yakonov–Perel theory of spin relaxation [M. I. D’yakonov and V. I. Perel, Sov. Phys. JETP 38, 177 (1974)]. Our results indicate the feasibility of engineering an all-optical, polarization switch at 1.5 μm with response time <250 fs.

Picosecond investigations of optical limiting mechanisms in King's complex

We have investigated the nonlinear optical mechanisms responsible for optical limiting of both picosecond and nanosecond 532-nm optical pulses in the organometallic compound cyclopentadienyliron carbonyl tetramer (Kings complex). For fluences below ~200 mJ/cm 2 , picosecond pump-probe measurements in solutions of the Kings complex reveal a prompt reverse saturable absorption (RSA) that recovers with a time constant of 120 ps. We attribute this RSA to excited-state absorption within the singlet system of the Kings complex, and we demonstrate that the RSA is completely characterized by a simple three-level model. We find, however, that the material parameters extracted from these picosecond measurements cannot account for the strong optical limiting previously observed in identical solutions of this compound using nanosecond excitation at higher fluences. Picosecond measurements at fluences greater than 200 mJ/cm 2 reveal the onset of an additional loss mechanism that appears ~1 ns after excitation. The magnitude of this loss depends on both the laser repetition rate and the solvent, indicating that the loss is not directly related to the intrinsic properties of the Kings complex but is most likely thermal in origin. Using nanosecond excitation pulses, we have performed angularly resolved transmission and reflection measurements, which reveal strong forward- and backward-induced scattering at these fluences. Furthermore, when the Kings complex is incorporated in a solid host, we observe negligible induced scatter and the response is completely described by the singlet parameters extracted from the picosecond measurements. These observations indicate that the nanosecond optical limiter response of solutions of Kings complex is dominated by thermally induced scattering.

Optical nonlinearities and ultrafast charge transport in all‐binary InAs/GaAs strained hetero n‐i‐p‐i’s

We use picosecond differential spectroscopy to temporally and spectrally resolve the formation and decay of nonlinearities and space‐charge fields in a hetero n‐i‐p‐i that contains quantum wells in the intrinsic regions that are composed of all‐binary InAs/GaAs short‐period strained‐layer superlattices. The evolution of the optical response is determined by competition between excitonic bleaching and the excitonic shift caused by screening of the built‐in electric field of the n‐i‐p‐i. The relative contributions of the two resulting optical nonlinearities are complicated functions of fluence, time, and wavelength, with the detailed dynamics determined by thermionic emission from the wells, picosecond charge transport over nanometer dimensions, screening, and recombination. At low fluences, excitonic bleaching is the source of an ultrafast nonlinear response that can be turned on and off in <10 ps. This initial excitonic bleaching gives way to a blue shift of the exciton as the carriers escape the wells in...

Optical control of microwaves with semiconductor n‐i‐p‐i structures

We control the microwave transmission of a GaAs n‐i‐p‐i structure by illuminating it with a cw argon ion laser. Tests in a broadband microwave modulator wave spectrometer show that an optical intensity of 800 mW/cm2 produces a 50% change in transmission for microwaves between 10 and 50 GHz.

Optical limiting with C60 solutions

We examine optical limiting with C60 solutions for nanosecond optical pulses at 532 nm and 694 nm and microsecond pulses at 514 nm. When the input fluence is less than 50 J/cm2, optical limiting is due to a combination of reverse saturable absorption (excited state absorption) and self-defocusing. Nonlinear scattering was not observed. For a peak input fluence greater than 50 J/cm2, an acoustic report and broadband emission indicate that optical breakdown occurs. At 532 nm, optical limiting for C60-toluene is comparable to carbon black suspension (CBS). For use at 694 nm, C60-toluene has a larger excited state absorption than at 532 nm, which partially compensates a much smaller ground state absorption. For microsecond optical pulses, C60 appears to be a better optical limiter than CBS.

Suppression of intervalley scattering in Ga(As)Sb quantum wells

Femtosecond time-resolved reflectivity was measured near the 1.55 μm absorption edge of several GaAsxSb1−x/AlSb quantum well samples. On the basis of differences in the reflectivity recovery kinetics and plateau values, we deduce that Γ–L intervalley scattering can be effectively suppressed for x⩾0.19. This is consistent with calculations incorporating confinement and strain effects which give L–Γ energy separations of 29 (x=0) and 109 meV (x=0.19). Suppression of intervalley scattering can lead to increased internal quantum efficiency and higher carrier mobility in 1.55 μm based devices.

Fullerene-based large-area passive broadband laser filters

We examine optical limiting with C60-toluene for nanosecond optical pulses at 532 nm. When the input fluence is less than 50 J/cm2, optical limiting is due to a combination of reverse saturable absorption (excited state absorption) and self-defocusing. Nonlinear scattering was not observed. For a peak input fluence greater than 50 J/cm2, an acoustic report and broad-band emission indicate that optical breakdown occurs. We find C60- tetrahydronaphthalene to be a better optical limiter, at 532 nm, than tetrahydronaphthalene solutions of C76, C78, or C84. C84-tetrahydronaphthalene is shown to be an optical limiter at 1.064 micrometers .