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20 Nov 2006

Volume 89, Issue 21, Articles (21xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 89, 211101 (2006); http://dx.doi.org/10.1063/1.2392673 (3 pages)

H. W. Choi, K. N. Hui, P. T. Lai, P. Chen, X. H. Zhang, S. Tripathy, J. H. Teng, and S. J. Chua
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Lasing in GaN microdisks pivoted on Si

H. W. Choi, K. N. Hui, P. T. Lai, P. Chen, X. H. Zhang, S. Tripathy, J. H. Teng, and S. J. Chua

Appl. Phys. Lett. 89, 211101 (2006); http://dx.doi.org/10.1063/1.2392673 (3 pages) | Cited 25 times

Online Publication Date: 20 November 2006

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Arrays of pivoted GaN microdisks have been fabricated on a GaN/Si material by a combination of dry and wet etching. The Si material beneath the GaN microdisks is removed by wet etching, leaving behind a fine pillar to support the disks. Raman spectroscopy reveals substantial strain relaxation in these structures. Resonant modes, corresponding to whispering gallery modes, are observed in the photoluminescence spectra. Stimulated emission is achieved at higher optical pumping intensities.
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42.55.Sa Microcavity and microdisk lasers
42.60.By Design of specific laser systems
42.60.Da Resonators, cavities, amplifiers, arrays, and rings
81.65.Cf Surface cleaning, etching, patterning
78.66.Fd III-V semiconductors
78.45.+h Stimulated emission
78.30.Fs III-V and II-VI semiconductors

High filling fraction gallium phosphide inverse opals by atomic layer deposition

E. Graugnard, V. Chawla, D. Lorang, and C. J. Summers

Appl. Phys. Lett. 89, 211102 (2006); http://dx.doi.org/10.1063/1.2387874 (3 pages) | Cited 8 times

Online Publication Date: 20 November 2006

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High filling fraction gallium phosphide (GaP) inverse opals were fabricated by atomic layer deposition within the void spaces of silica colloidal crystal templates. Depositions were performed from 400 to 500 °C using trimethylgallium and tris(dimethylamino)phosphine precursors. The resulting films were characterized by optical reflectance, which indicated infiltration as high as 100% of the conformal film growth maximum, corresponding to a volume filling fraction of 0.224. X-ray diffraction measurements confirmed the crystallinity of the film. These results indicate the fabrication of three-dimensional photonic crystals using a III-V optoelectronic material with sufficient dielectric contrast to form a full photonic band gap in the visible.
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81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.55.A- Nucleation and growth
78.66.Fd III-V semiconductors
42.70.Qs Photonic bandgap materials

Effect of different laser polarization direction on high order harmonic generation of N2 and H2

Lei Cui, J. Zhao, Y. J. Hu, Y. Y. Teng, X. H. Zeng, and B. Gu

Appl. Phys. Lett. 89, 211103 (2006); http://dx.doi.org/10.1063/1.2387977 (3 pages) | Cited 2 times

Online Publication Date: 20 November 2006

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The high order harmonic generations of N2 and H2 irradiated by ultrastrong femtosecond pulsed laser are simulated by the time dependent density functional theory method, and effect of different laser polarization directions on high order harmonic generation (HHG) is discussed. The results indicate that for N2, with its outermost σg electron, the HHG yield is maximal when the molecules are aligned with the laser polarization direction. Good agreements are found with recent experiments, and for H2, with its outermost σg electron, the HHG yield also peaks when the molecules are aligned at about 0° form polarization axis.
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42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
33.80.-b Photon interactions with molecules
31.15.E- Density-functional theory

Magnetoencephalography with an atomic magnetometer

H. Xia, A. Ben-Amar Baranga, D. Hoffman, and M. V. Romalis

Appl. Phys. Lett. 89, 211104 (2006); http://dx.doi.org/10.1063/1.2392722 (3 pages) | Cited 49 times

Online Publication Date: 21 November 2006

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The authors demonstrate detection and mapping of brain magnetic fields evoked by auditory stimulation with a noncryogenic magnetometer based on spin precession of potassium atoms in spin-exchange-relaxation-free regime. Optical readout using a photodiode array allows flexibility in detector placement while using common elements for most components of the multichannel system. Absence of a cryogenic dewar eliminates magnetic Johnson noise from radiation shields and allows the use of a compact magnetic shield with a high shielding factor. The magnetometer sensitivity is currently equal to 3.5 fT/Hz1/2 at 10 Hz, similar to superconducting quantum interference device magnetoencephalography systems.
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87.80.-y Biophysical techniques (research methods)
07.55.Ge Magnetometers for magnetic field measurements
85.75.Ss Magnetic field sensors using spin polarized transport

Strong optical activity in chiral metamaterials of metal screw hole arrays

Fumiaki Miyamaru and Masanori Hangyo

Appl. Phys. Lett. 89, 211105 (2006); http://dx.doi.org/10.1063/1.2392787 (3 pages)

Online Publication Date: 21 November 2006

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The authors demonstrate the change of the polarization of the electromagnetic wave induced by chiral metamaterials in the terahertz region. The polarization of the incident terahertz wave changes dramatically after transmitting through the chiral metamaterials made of the array of screw holes in metal slabs. A strong resonant polarization change (ellipticity and polarization azimuth) is observed at frequencies corresponding to the standing cavity modes of the metal waveguide with a finite length. Such experimental results indicate that the metal screw hole arrays work as a strong optically active material.
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42.70.-a Optical materials
84.40.-x Radiowave and microwave (including millimeter wave) technology

Magnetic-field-induced birefringence and particle agglomeration in magnetic fluids

Ziyun Di, Xianfeng Chen, Shengli Pu, Xiao Hu, and Yuxing Xia

Appl. Phys. Lett. 89, 211106 (2006); http://dx.doi.org/10.1063/1.2392824 (3 pages) | Cited 16 times

Online Publication Date: 21 November 2006

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Birefringent effect dependent on magnetic intensity and wavelength in a water-based Fe3O4 magnetic fluid over the wavelength range of 400–700 nm is studied in this letter. The results are compared with the theory proposed by Llewellyn [J. Phys. D 16, 95 (1983) ], in which the optical anistropic properties of the magnetic fluid due to the aggregation of the particles at a relatively low density is suggested. It is shown that the degree of aggregation corresponds to the value of the axial ratio of the aggregated particles. So does the magnitude of the birefringence.
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78.20.Fm Birefringence
78.20.Ls Magneto-optical effects
75.50.Mm Magnetic liquids
75.50.Dd Nonmetallic ferromagnetic materials
42.70.-a Optical materials

Plasmon-enhanced erbium luminescence

H. Mertens and A. Polman

Appl. Phys. Lett. 89, 211107 (2006); http://dx.doi.org/10.1063/1.2392827 (3 pages) | Cited 50 times

Online Publication Date: 21 November 2006

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It is demonstrated that the photoluminescence intensity of optically active erbium ions positioned in close proximity of anisotropic Ag nanoparticles is significantly enhanced if the nanoparticles support plasmon modes that are resonant with the erbium emission. In addition, the photoluminescence intensity enhancement is found to be polarized corresponding to polarization of these plasmon modes. Both observations demonstrate that the photoluminescence enhancement is due to coupling of the Er3+ math−math transition dipoles with plasmon modes in the Ag nanoparticles. As this coupling mechanism is known to affect the emission rate, metal nanoparticles provide an opportunity to reduce the effect of temperature or concentration quench processes that are known to occur in a wide range of erbium-doped materials.
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78.55.Hx Other solid inorganic materials
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
42.70.-a Optical materials
78.68.+m Optical properties of surfaces

Cascaded active regions in 2.4 μm GaInAsSb light-emitting diodes for improved current efficiency

J. P. Prineas, J. T. Olesberg, J. R. Yager, C. Cao, C. Coretsopoulos, and M. H. M. Reddy

Appl. Phys. Lett. 89, 211108 (2006); http://dx.doi.org/10.1063/1.2392993 (3 pages) | Cited 4 times

Online Publication Date: 21 November 2006

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By cascading multiple GaInAsSb active regions, the authors have fabricated 2.4 μm light-emitting diodes that, for a given light output, operate at reduced current and higher voltage, which can be advantageous for battery-powered sensor applications. Tunnel heterojunctions separating emission regions add no measurable series resistance. Devices are demonstrated at room temperature with continuous wave output.
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85.60.Jb Light-emitting devices

Efficient surface plasmon field confinement in one-dimensional crystal line-defect waveguides

J.-C. Weeber, A.-L. Baudrion, A. Bouhelier, A. Bruyant, G. Colas des Francs, R. Zia, and A. Dereux

Appl. Phys. Lett. 89, 211109 (2006); http://dx.doi.org/10.1063/1.2392998 (3 pages) | Cited 2 times

Online Publication Date: 21 November 2006

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The authors operate a near-field optical microscope to investigate surface plasmon polariton (SPP) propagation along linear waveguides opened into one-dimensional (1D) plasmonic crystals, i.e., crystals featuring a single lattice plane orientation. They show that efficient SPP field confinement can be achieved by this type of waveguide although no band gap exists in the direction perpendicular to the waveguide axis. From computed wave-vector diagrams, they show that 1D plasmonic crystals can open a wide range of prohibited propagation directions preventing from a significant coupling of the waveguide SPP modes with the crystal Bloch modes. Finally, the authors demonstrate that the field distributions of the guided SPP modes can be qualitatively understood from simple Ewald constructions.
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42.79.Gn Optical waveguides and couplers
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
78.68.+m Optical properties of surfaces
71.36.+c Polaritons (including photon-phonon and photon-magnon interactions)

GaSb quantum-well-based “buffer-free” vertical light emitting diode monolithically embedded within a GaAs cavity incorporating interfacial misfit arrays

M. Mehta, G. Balakrishnan, S. Huang, A. Khoshakhlagh, A. Jallipalli, P. Patel, M. N. Kutty, L. R. Dawson, and D. L. Huffaker

Appl. Phys. Lett. 89, 211110 (2006); http://dx.doi.org/10.1063/1.2396897 (3 pages) | Cited 9 times

Online Publication Date: 21 November 2006

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The authors demonstrate a monolithic, electrically injected, vertically emitting GaSb/AlGaSb light emitting diode (LED) emitting at 1.6 μm comprised of a hybrid GaAs/GaSb-based structure. The LED is comprised of a GaSb/AlGaSb quantum well/barrier active region embedded within high index contrast GaAs/AlGaAs distributed Bragg reflectors (DBRs) using two interfacial misfit (IMF) arrays to relieve the strain induced from the high 8% lattice mismatch between the material systems. The first IMF is formed under compressive strain conditions to enable strain-free, defect-free deposition of GaSb active region directly on the lower GaAs/AlAs DBRs without need for thick buffer. The second IMF is formed under tensile conditions to enable the upper GaAs/AlAs DBRs on the GaSb active region. The device demonstrates a maximum output power of 3.5 μW. Initial diode optical and electrical characteristics along with IMF band structure are discussed.
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85.60.Jb Light-emitting devices
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)

Direct monitoring of the excited state population in biased SiGe valence band quantum wells by femtosecond resolved photocurrent experiments

P. Rauter, T. Fromherz, G. Bauer, N. Q. Vinh, B. N. Murdin, J. P. Phillips, C. R. Pidgeon, L. Diehl, G. Dehlinger, and D. Grützmacher

Appl. Phys. Lett. 89, 211111 (2006); http://dx.doi.org/10.1063/1.2397004 (3 pages) | Cited 5 times

Online Publication Date: 21 November 2006

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The authors report a direct measurement of the optical phonon intersubband hole relaxation time in a SiGe heterostructure and a quantitative determination of hole relaxation under electrically active conditions. The results were obtained by femtosecond resolved pump-pump photocurrent experiments using a free electron laser (wavelength 7.9 μm). Additionally, the intensity dependence of the nonlinear photocurrent response was measured. Both types of experiments were simulated using a density matrix description. With one parameter set, a consistent modeling was achieved confirming the significance of the extracted heavy hole relaxation times. For an intersublevel spacing of 160 meV, a value of 550 fs was obtained.
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78.67.De Quantum wells
73.63.Hs Quantum wells
72.40.+w Photoconduction and photovoltaic effects
73.21.Fg Quantum wells
63.22.-m Phonons or vibrational states in low-dimensional structures and nanoscale materials
78.47.-p Spectroscopy of solid state dynamics

Tunable pseudogaps in electrochromic WO3 inverted opal photonic crystals

Julia Khalack and P. V. Ashrit

Appl. Phys. Lett. 89, 211112 (2006); http://dx.doi.org/10.1063/1.2397012 (3 pages) | Cited 3 times

Online Publication Date: 21 November 2006

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The numerical investigation of the optical properties of electrochromic WO3 inverted opal structure has been carried out with the help of plane wave expansion and finite difference time domain simulation methods. Electrochromic effect is incorporated through the tunable parameters of the frequency-dependent Drude dielectric function. Dependence of photonic bands on free electron density and radius of air spheres is investigated. Five kinds of partial band gaps in (111) direction are found to appear with the change of the sphere radius. The photonic band gaps tend to be upshifted in frequency with insertion of free electrons or with increase of the sphere radius.
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42.70.Qs Photonic bandgap materials
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.20.Jq Electro-optical effects

Theory of three-dimensional subdiffraction imaging

Ricardo Marques, Manuel J. Freire, and Juan D. Baena

Appl. Phys. Lett. 89, 211113 (2006); http://dx.doi.org/10.1063/1.2397022 (3 pages) | Cited 9 times

Online Publication Date: 21 November 2006

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Subdiffraction image formation and measurement are analyzed. It is shown that subdiffraction imaging devices cannot produce focusing of energy into three-dimensional spots of subdiffraction size. However, three-dimensional subdiffraction imaging is possible as a consequence of a matching/tunneling process.
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42.30.-d Imaging and optical processing

Site selective studies of Er3+ emission centers in Er-implanted 6H-SiC

V. Glukhanyuk and A. Kozanecki

Appl. Phys. Lett. 89, 211114 (2006); http://dx.doi.org/10.1063/1.2397025 (3 pages) | Cited 6 times

Online Publication Date: 21 November 2006

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In this work the high resolution site selective photoluminescence (PL) using Fourier transform spectrometer and PL excitation spectra near 1.54 μm in Er-implanted 6H-SiC were investigated. Direct evidence for the existence of three different Er3+ emitting centers which dominate luminescence at 1.54 μm has been presented. The Stark splitting of the math excited states and math ground states for all three centers were determined. All centers reveal low local symmetry, probably due to complexes with impurities and defects.
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78.55.-m Photoluminescence, properties and materials
78.20.Jq Electro-optical effects
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
61.72.up Other materials

Coulomb scattering in the Monte Carlo simulation of terahertz quantum-cascade lasers

J. T. Lü and J. C. Cao

Appl. Phys. Lett. 89, 211115 (2006); http://dx.doi.org/10.1063/1.2397028 (3 pages) | Cited 20 times

Online Publication Date: 21 November 2006

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The authors compare different Coulomb screening models in the Monte Carlo simulation of terahertz quantum-cascade lasers: the static multisubband screening model and two single subband models. In quantum-cascade structures, electrons are located in many different subbands. Coulomb screening from all these subbands influences the intra- and intersubband scattering processes. The simulation results show that one of the two single subband models overestimates the screening effect, while the other underestimates it. The authors show the reasons for this and propose simple modification to the single subband models.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
02.50.Ng Distribution theory and Monte Carlo studies

Phase contrast imaging using photothermally induced phase transitions in liquid crystals

Chandra S. Yelleswarapu, Sri-Rajasekhar Kothapalli, Francisco J. Aranda, D. V. G. L. N. Rao, Yvonne R. Vaillancourt, and Brian R. Kimball

Appl. Phys. Lett. 89, 211116 (2006); http://dx.doi.org/10.1063/1.2397030 (3 pages) | Cited 7 times

Online Publication Date: 21 November 2006

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Phase contrast imaging is performed for live biological species using photothermal induced birefringence in dye doped liquid crystals. Using typical 4-f configuration, when liquid crystal cell is at back focal plane of Fourier lens, low spatial frequencies at center of Fourier spectrum are intense enough to induce local liquid crystal molecules into isotropic phase, whereas high spatial frequencies on the edges are not intense enough and remain in anisotropic phase. This results in π/2 phase difference between high and low spatial frequencies. This simple, inexpensive, all-optical, user-friendly, self-adaptive phase contrast imaging technique using low-power laser offers several distinct advantages.
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87.63.-d Non-ionizing radiation equipment and techniques
64.70.M- Transitions in liquid crystals
78.20.Fm Birefringence

Rewritable photonic circuits

Francesca Intonti, Silvia Vignolini, Volker Türck, Marcello Colocci, Paolo Bettotti, Lorenzo Pavesi, Stefan L. Schweizer, Ralf Wehrspohn, and Diederik Wiersma

Appl. Phys. Lett. 89, 211117 (2006); http://dx.doi.org/10.1063/1.2392720 (3 pages) | Cited 55 times

Online Publication Date: 22 November 2006

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The authors present a technique that allows to modify the local characteristics of two-dimensional photonic crystals by controlled microinfiltration of liquids. They demonstrate experimentally that by addressing and infiltrating each pore with a simple liquid, e.g., water, it is possible to write pixel by pixel optical devices of any geometry and shape. Calculations confirm that the obtained structures indeed constitute the desired resonators and waveguide structures.
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42.82.Gw Other integrated-optical elements and systems
42.70.Qs Photonic bandgap materials

Generation of arbitrary terahertz wave forms in fanned-out periodically poled lithium niobate

J. R. Danielson, N. Amer, and Yun-Shik Lee

Appl. Phys. Lett. 89, 211118 (2006); http://dx.doi.org/10.1063/1.2392819 (3 pages) | Cited 5 times

Online Publication Date: 22 November 2006

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The authors demonstrate a flexible terahertz pulse-shaping technique, manipulating spatially dispersed multifrequency components generated by optical rectification in a fanned-out periodically poled lithium niobate crystal. Spatial masks of low pass, high pass, and double slit in front of the crystal manipulate the spatial pattern of the optical excitation beam on the crystal, which is mapped onto the intensity profile of the terahertz spectrum. The spatial dispersion of the terahertz spectrum is removed by the line-to-point imaging of a spherical mirror.
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42.65.Re Ultrafast processes; optical pulse generation and pulse compression

Low-loss single-mode terahertz waveguiding using Cytop

Glenda de los Reyes, Alex Quema, Carlito Ponseca, Romeric Pobre, Reuben Quiroga, Shingo Ono, Hidetoshi Murakami, Elmer Estacio, Nobuhiko Sarukura, Ko Aosaki, Yoshihiko Sakane, and Hideki Sato

Appl. Phys. Lett. 89, 211119 (2006); http://dx.doi.org/10.1063/1.2392990 (3 pages) | Cited 14 times

Online Publication Date: 22 November 2006

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A polymer-based, Cytop planar photonic crystal waveguide (PPCW) was designed for guiding terahertz radiation. Results indicate that the propagation and coupling losses in the 0.2–1.1 THz range are relatively small for a sheetlike thickness design. Spectral analysis of the transmission data reveals frequency selectivity of the PPCW. Calculations of the spatial distribution of the terahertz electric field through the waveguide show evidence of single-mode propagation at a 0.45 THz central frequency. The highly transparent nature of Cytop from deep ultraviolet to the far infrared region indicates its potential use as an integral component in hybrid optics.
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42.79.Gn Optical waveguides and couplers
42.70.Qs Photonic bandgap materials
42.70.Jk Polymers and organics
42.15.Eq Optical system design

pH-adaptive microlenses using pinned liquid-liquid interfaces actuated by pH-responsive hydrogel

Liang Dong and Hongrui Jiang

Appl. Phys. Lett. 89, 211120 (2006); http://dx.doi.org/10.1063/1.2393038 (3 pages) | Cited 6 times

Online Publication Date: 22 November 2006

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The authors report on variable-focus liquid microlenses self-adaptive to environmental pH. The microlens is formed via a water-oil interface stably pinned at a hydrophobic-hydrophilic contact line along an aperture in a flexible slip. A set of hydrogel microposts is photopatterned in a microfluidic chamber around the aperture; the microposts autonomously respond to the environmental pH variation by expanding or contracting, thus deforming the aperture slip and tuning the curvature of the water-oil interface and the focal length of the microlens. A single microlens has a tunable focal length from −7.6 mm to −∞ (divergent) and from 8.5 mm to +∞ (convergent).
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42.79.Bh Lenses, prisms and mirrors
42.15.Eq Optical system design

Observation of nanosecond laser-induced phase explosion in aluminum

Cristian Porneala and David A. Willis

Appl. Phys. Lett. 89, 211121 (2006); http://dx.doi.org/10.1063/1.2393158 (3 pages) | Cited 29 times

Online Publication Date: 22 November 2006

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The transition from normal vaporization to phase explosion during laser ablation of aluminum was investigated using a nanosecond Nd:YAG laser. The threshold nature of phase explosion was observed by a discontinuous jump in the ablation depth at approximately 5.2 J/cm2. Ablation was imaged using a shadowgraph technique that was capable of probing ablation with nanosecond exposure time and nanosecond time delay resolution with respect to laser heating. Images above the threshold captured a mixture of vapor and droplets generated by phase explosion, which began near the end of the laser pulse without a significant time lag.
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42.62.-b Laser applications
61.82.Bg Metals and alloys
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
82.33.Vx Reactions in flames, combustion, and explosions
64.70.Hz Solid-vapor transitions

Room temperature electron spin relaxation in GaInNAs multiple quantum wells at 1.3 μm

C. Reith, S. J. White, M. Mazilu, A. Miller, J. Konttinen, M. Guina, and M. Pessa

Appl. Phys. Lett. 89, 211122 (2006); http://dx.doi.org/10.1063/1.2396901 (3 pages) | Cited 5 times

Online Publication Date: 22 November 2006

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The authors report a direct measurement of electron spin relaxation in GaInNAs semiconductor multiple quantum wells at room temperature. Multiple quantum wells of widths 5.8, 7, and 8 nm exhibiting excitonic absorption around 1.3 μm have been studied. Spin relaxation times were found to increase with well width in the range of 77–133 ps. The spin relaxation time dependence on first electron confinement energy suggests the Elliot-Yafet mechanism [ A. Tackeuchi et al., Physica B 272, 318 (1999) ] as the dominant relaxation process.
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73.63.Hs Quantum wells
73.21.Fg Quantum wells
72.25.Rb Spin relaxation and scattering
72.25.Dc Spin polarized transport in semiconductors
78.67.De Quantum wells
71.35.-y Excitons and related phenomena

Gradient refractive index planar microlens in Si using porous silicon

S. Ilyas and M. Gal

Appl. Phys. Lett. 89, 211123 (2006); http://dx.doi.org/10.1063/1.2396929 (3 pages) | Cited 5 times

Online Publication Date: 22 November 2006

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Significant effort is being expended on the integration of silicon electronics and optoelectronics. Here the authors describe a method to create planar gradient refractive index (GRIN) lenses in Si using porous silicon (PSi) technology. The authors’ approach allows the fabrication of a single planar lens or an array of such lenses with focal length that can be adjusted to match existing device(s) on the chip. The lenses are transparent in the near IR, including the optical communication window (1.3 μm<λ<1.6 μm). In addition to being potential components in future Si based integrated optical circuits, PSi GRIN lenses can also be used to improve the light coupling efficiency of existing Si based devices, such as sensors, detectors, and waveguides.
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42.82.Gw Other integrated-optical elements and systems
42.79.Ry Gradient-index (GRIN) devices
42.79.Bh Lenses, prisms and mirrors
42.82.Cr Fabrication techniques; lithography, pattern transfer
85.60.-q Optoelectronic devices

Short-wavelength (λ ≈ 3.3 μm) InP-based strain-compensated quantum-cascade laser

M. P. Semtsiv, M. Wienold, S. Dressler, and W. T. Masselink

Appl. Phys. Lett. 89, 211124 (2006); http://dx.doi.org/10.1063/1.2397020 (3 pages) | Cited 10 times

Online Publication Date: 22 November 2006

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The authors describe the design and implementation of a short-wavelength quantum-cascade laser emitting at approximately 3.3 μm at 80 K. The active region is based on the strain-compensated In0.73Ga0.27As–In0.55Al0.45As–AlAs heterosystem on InP. The band structure and the strain are controlled through the use of both composite barriers as well as composite wells. The structure is designed so the transition resulting in laser emission is very spatially diagonal; the upper laser state is primarily located in a thick In0.55Al0.45As layer in the injector while the lower laser state is in an In0.73Ga0.27As well. This design allows the lasing transition to bypass (in energy–growth-coordinate space) the lowest indirect X and L valleys of In0.73Ga0.27As, and population inversion is achieved in spite of the upper laser state reaching the energy of the indirect X- and L-valley edges of the adjacent In0.73Ga0.27As well.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems

Enhanced coupling to whispering gallery modes by two-photon absorption induced by a highly focused field

Smitha Kuriakose, Dru Morrish, Xiaosong Gan, James W. M. Chon, and Min Gu

Appl. Phys. Lett. 89, 211125 (2006); http://dx.doi.org/10.1063/1.2397036 (3 pages) | Cited 2 times

Online Publication Date: 22 November 2006

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The authors report on enhanced coupling to whispering gallery modes in fluorescent polystyrene microspheres using two-photon absorption induced by a highly focused field. Due to the highly confined excitation nature under focused evanescent illumination achieved by a circularly obstructed beam, the whispering gallery modes can be excited within a small volume near the perimeter of the microsphere. As a result, the visibility, the Q factor, and the degree of polarization of the fluorescence spectra induced in the microsphere are enhanced by 60%, 37%, and five times, respectively.
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78.55.Kz Solid organic materials
42.70.Jk Polymers and organics
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