Volume 105, Issue 11, 01 June 2009
- lasers, optics, and optoelectronics
- plasmas and electrical discharges
- structural, mechanical, thermodynamic, and optical properties of condensed matter
- electronic structure and transport
- magnetism and superconductivity
- dielectrics and ferroelectricity
- nanoscale science and design
- device physics
- applied biophysics
- interdisciplinary and general physics
Index of content:
- LASERS, OPTICS, AND OPTOELECTRONICS
105(2009); http://dx.doi.org/10.1063/1.3129319View Description Hide Description
Voltage-tunable plasmon resonances in the two-dimensional electron gas(2DEG) of a high electron mobility transistor(HEMT) fabricated from the InGaAs/InP materials system are reported. The device was fabricated from a commercial HEMT wafer by depositing source and drain contacts using standard photolithography and a semitransparent gate contact that consisted of a period transmission grating formed by electron-beam lithography. Narrow-band resonant absorption of terahertz radiation was observed in transmission in the frequency range of . The resonance frequency depends on the gate-tuned sheet charge density of the 2DEG. The observed separation of resonance fundamental from its harmonics and their shift with gate bias are compared with theory.
105(2009); http://dx.doi.org/10.1063/1.3137198View Description Hide Description
We studied both theoretically and experimentally the influence of the lattice constant on the resonance frequencies of cut-wire-pair structures, which are essential components in assembling left-handed materials. These structures were designed and fabricated, and the transmission spectra were measured in the microwave-frequency regime. Numerical simulation was performed using the transfer-matrix method. All the numerical results were in good agreement with the experimental data. It was found that the lattice parameters of the cut-wire-pair structure provide sensitive effects on the electric resonance, especially, in the propagation and the electric field directions, while the magnetic resonance frequency is nearly unchanged, except for its bandwidth and depth. In addition, we also studied the effect of dielectric layer thickness on the resonance frequencies of the cut-wire-pair structure. These sensitive changes in the electric resonance might have remarkable effects on the effective plasma frequency and therefore on the left-handed behavior of combined structures consisting of cut-wire pairs and continuous wires.
Monte Carlo simulation of terahertz quantum cascade laser structures based on wide-bandgap semiconductors105(2009); http://dx.doi.org/10.1063/1.3137203View Description Hide Description
Wide-bandgap semiconductors such as and quantum wells are promising for improving the spectral reach and high-temperature performance of terahertz quantum cascade lasers, due to their characteristically large optical phonon energies. Here, a particle-based Monte Carlo model is developed and used to quantify the potential of terahertz sources based on these materials relative to existing devices based on quantum wells. Specifically, three otherwise identical quantum cascade structures based on , , and quantum wells are designed, and their steady-state carrier distributions are then computed as a function of temperature. The simulation results show that the larger the optical phonon energies (as in going from the AlGaAs to the MgZnO to the AlGaNmaterials system), the weaker the temperature dependence of the population inversion. In particular, as the temperature is increased from , the population inversions are found to decrease by factors of 4.48, 1.50, and 1.25 for the AlGaAs, MgZnO, and AlGaN structure, respectively. Based on these results, the AlGaN and MgZnO devices are then predicted to be in principle capable of laser action without cryogenic cooling.
105(2009); http://dx.doi.org/10.1063/1.3138082View Description Hide Description
We present long-term measurements of the blinking (on/off) behavior of the glycerol microdropletRaman laser and demonstrate the dependency of the lasing bursts on the evaporation rate of the microdroplet. Electrodynamic levitation is used to study the glycerol microdroplets. Single and multimode lasing are achieved and shown here for droplets of 10.3 and in diameter, respectively. Typical threshold fluences are measured to be between 200 and . Lasing occurs in temporally separated nearly symmetric bursts which increase in frequency and decrease in duration as the evaporation rate of the droplet is increased. Hence, we conclude that the Raman lasing blinking is caused by double resonances in the evaporating droplet and show that it can be manipulated by controlling the droplet’s evaporation rate.
Luminescence enhancement and emission color adjustment of white organic light-emitting diodes with quantum-well-like structures105(2009); http://dx.doi.org/10.1063/1.3138810View Description Hide Description
In recent years, white-light organic light-emitting diodes(OLEDs) have attracted considerable attention because of their potential applications in displays, backlight units, and general lighting. In this paper, we describe the fabrication of a high-luminance white-light OLED with dual-emission layers (EMLs). The 2,9-dimethyl-4,7-diphenyl-1,10-phenanhroline hole blocking layers (HBLs) were prepared to enhance the luminance and adjust the emission color of the device. For the device with a single-HBL structure, the excess holes were accumulated at the red EML (REML)/HBL1 interface. Consequently, most of the carriers were recombined in the REML, which led to a redshifted light emission. On the contrary, the device with a dual-HBL structure showed a blueshifted light emission. The device with a quantum-well-like structure increased the carrier trapping and recombination probabilities; as a result, a lower current density and a higher luminance intensity of the device were achieved. The maximum luminance intensity was at 13 V, with CIE coordinates at (0.32, 0.32); the maximum current efficiency was 7.06 cd/A at 10 V; and the maximum power efficiency was 2.62 lm/W at 7 V and .
Efficiency enhancement in seeded and self-amplified spontaneous emission free-electron lasers by means of a tapered wiggler105(2009); http://dx.doi.org/10.1063/1.3131630View Description Hide Description
The enhancement of the efficiency in free-electron lasers(FELs) through the use of a tapered wiggler is well known. The physics of the tapered wiggler interaction has been studied in theory and simulation, and large efficiency enhancements have been observed in the laboratory in oscillators and seeded amplifiers. In this paper, we study the differences in the tapered wiggler interaction between seeded amplifiers and in FELs that start up from noise and grow to saturation in a single pass through the wiggler. This configuration is commonly referred to as self-amplified spontaneous emission (SASE). In comparison with seeded amplifiers, SASE FELs exhibit shot-to-shot fluctuations due to random phase noise in the electron bunches, and our purpose in this paper is to determine the effect of this phase noise on the tapered wiggler interaction. To this end, we study the interaction numerically using the MEDUSA simulation code for seeded and SASE FELs operating in the infrared regime. The results of the simulations indicate that the overall efficiencies of the seeded and SASE FELs are comparable for a uniform wiggler but that the output spectrum for the SASE FEL is much broader than for the seeded case. For a tapered wiggler, the efficiency enhancement in the SASE FEL is less than that found in the seeded example due to the broader excited spectrum that detunes the tapered wiggler interaction.
Out-of-plane thermal diffusivity measurement of transparent thin film by the acoustic grating excitation105(2009); http://dx.doi.org/10.1063/1.3131662View Description Hide Description
The theory of pulsed photoacoustic phase method for out-of-plane thermal diffusivitymeasurement of thin dielectric film deposited on highly absorbing substrate is developed. The method is based on spatial modulation of the intensity of excitation. Optical detection of acoustic waves enables working with acoustic frequencies higher than and allows characterization of transparent films of submicron thickness. Pilot measurements have been carried out with films on monocrystalline silicon wafer. Thermal diffusivity determined in the experiment agrees with the reference value for the bulk fused quartz.
105(2009); http://dx.doi.org/10.1063/1.3138813View Description Hide Description
Undoped and Al-, Ga-, and In-doped thin films were prepared on fused quartz substrates by chemical solution deposition. Their microstructures and optical properties were investigated by x-ray diffraction and UV-visible-NIR spectrophotometer, respectively. The optical band-gap energies, Urbach energies, and linear refractive indices of all the films are derived from the transmittance spectrum. Following the single oscillator model, the dispersion parameters such as the average oscillator energy and dispersion energy are achieved. The energy band gap and refractive indices are found to decrease with introducing the dopants of Al, Ga, and In, which is useful for the band-gap engineering and optical waveguide devices. The refractive indexdispersion parameter increases and the chemical bonding quantity decreases in all the films compared with those of bulk. It is supposed to be caused by the nanosize grains in films.
105(2009); http://dx.doi.org/10.1063/1.3139275View Description Hide Description
We have investigated the structural, optical, and electrical properties of alumina-doped zinc oxide (AZO) thin films,grown by pulsed laser deposition. The optical transmittance of the films is over 80% in the visible region, and the absorption edge shifts from about 380 nm of the undoped sample to 320 nm of the AZO film. The calculated optical band gap of AZO films shows a widening up to 3.82 eV with respect to the undoped film (3.28 eV). Higher doping concentration leads to films with larger (4.1 eV), but also epitaxial properties are affected. A further widening of the gap occurs when the AZO films are deposited by lowering the substrate temperature from 450 to . These blueshifts are respectively attributed to the increase in carrier concentration, induced by Al-donor doping, and also a lower degree of crystalline order. AZO films with doping concentration of show resistivity values of about and the local curves, measured by scanning tunneling spectroscopy, show higher tunneling current than ZnOfilm. The Al-doping route proved to be effective in tailoring the optical and electrical properties without essentially affecting the crystalline structure of the films.
Spatially resolved determination of the dark saturation current of silicon solar cells from electroluminescence images105(2009); http://dx.doi.org/10.1063/1.3132827View Description Hide Description
We present a novel method to determine spatially resolved the dark saturation current of standard silicon solar cells. For this two electroluminescence images are taken at two different voltages. From these two images, first the spatial voltage distribution can be calculated. Second by applying the Laplacian to the voltage image from Ohm’s law and the continuity equation, the current through the device at a certain position can be determined. Knowing the local current through the device, the local voltage, and the emitter sheet resistance allows to determine the local dark saturation current. The clue of this method is to cope with the noise by using an appropriate noise reduction algorithm. By simulating electroluminescence images with realistic noise and known dark saturation current we demonstrate the applicability of the method with our noise reduction algorithm. Experimentally we compare our method with spectral response light beam induced current on multicrystalline solar cell.
105(2009); http://dx.doi.org/10.1063/1.3132834View Description Hide Description
We have designed a femtosecond electron gun suitable for ultrafast electron diffraction experiments, operating in the 30–100 kV regime. The concept is based on recompression of chirped expanding electron pulses emitted from a direct current photogun using a novel dispersion-corrected reflectron concept. We show, using detailed numerical simulations, that our design is capable of producing electron pulses containing 200 000 electrons with a full width at half maximum pulse duration of 130 fs, a root mean squared (rms) pulse radius of , and transverse coherence length of 1.5 nm at 100 kV. Our analysis includes the bunch properties at the sample, as well as interactions of the main pulse of high charge density with diffracted electrons. Since our design employs only static electron optics, we believe that it will be easier to implement than concepts based on radio frequency compression.
Refraction-enhanced x-ray radiography for inertial confinement fusion and laser-produced plasma applications105(2009); http://dx.doi.org/10.1063/1.3133092View Description Hide Description
We explore various laser-produced plasma and inertial confinement fusion applications of phase-contrast x-rayradiography, and we show how the main features of these enhancements can be considered from a geometrical optics perspective as refraction enhancements. This perspective simplifies the analysis and often permits simple analytical formulas to be derived that predict the enhancements. We explore a raytrace approach to various material interface applications, and we explore a more general example of refractive bending of x rays by an implosion plasma. We find that refraction-enhanced x-rayradiography of implosions may provide a means to quantify density differences across shock fronts as well as density variations caused by local heating due to high- dopants. We also point out that refractive bending by implosions plasmas can blur fine radiograph features and can also provide misleading contrast information on area-backlit pinhole imaging experiments unless its effects are taken into consideration.
105(2009); http://dx.doi.org/10.1063/1.3133202View Description Hide Description
We investigated the energy states in compact clusters of ferromagnetic islands with perpendicular anisotropy arranged on a triangular lattice. Due to their finite nature, we were able to determine the energies of all possible cluster states using dipolar energy calculations. We employed photoemission electron microscopy to observe the magnetic states in arrays of clusters of monodomain Co/Pt multilayerislands and following demagnetization, we observed a shift in the energy distribution to lower energies as the dipolar coupling increased. These multistate islandclusters not only provide model arrangements of frustrated Ising-type nanomagnets but are also interesting for data storage applications.
Microscopic design of GaInNAs quantum well laser diodes on ternary substrates for high-speed and high-temperature operations105(2009); http://dx.doi.org/10.1063/1.3126522View Description Hide Description
Material properties of highly strained GaInNAs quantum wells grown on GaInAs or quasi-GaInAs substrates are investigated by using microscopic theory together with a band structure calculation based on ten-band theory specially formulated for highly strained materials. It is shown that the material gain of GaInNAs quantum wells is reduced by incorporating N into a well layer although the strain in the well layer becomes small. The reduction can be compensated by properly choosing barrier materials. The performance of laser diodes, such as characteristic temperatures and differential gains, is also investigated, and the present results show that very high and differential gain with moderate strain can be achieved by carefully designing quantum well structures, indicating the applicability of these lasers for high-temperature and high-speed operation.
105(2009); http://dx.doi.org/10.1063/1.3133245View Description Hide Description
An ultralarge ring He–Ne ring laser gyroscope, UG-2, with area and dimensions , has been built underground at Cashmere Cavern, Christchurch, New Zealand (latitude −43.575°). Earth rotation is sufficient to unlock it, giving a Sagnac frequency of 2.18 kHz. Supermirrors are used with transmission parts per million (ppm) and optical loss unexpectedly high at per reflection. The cavity is . Residual Sagnac frequency error caused by backscatter coupling is measured as parts in . Its best stability is achieved for an averaging time of , for which the Allan Deviation of the Sagnac frequency is 0.08 mHz, or four parts in of Earth rotation rate. The dominant processes generating the residual rotational noise are, for times , microseismic ground movements, and for times , mechanical movement of the mirror assemblies, which act to change the geometrical dimensions and tilt. At all averaging times the residual rotational noise is well above the limit imposed by quantum phase fluctuations. It is concluded from comparisons among many large ring lasers that the excess mirror losses arise from high order aberrations, and UG-2 may be larger than the optimum size.
105(2009); http://dx.doi.org/10.1063/1.3133093View Description Hide Description
The influence of elliptically and circularly polarized excitation on terahertz emission from unbiased bulk GaAs at normal incidence and room temperature is reported. Illumination of GaAs above the bandgap produces both spin-polarized electrons and shift currents. The induced currents are monitored via terahertz emission spectroscopy. The terahertz emission amplitude is compared to theoretical calculations as a function of excitation beam ellipticity. Exciting slightly above the bandgap (800 nm at room temperature) with elliptical polarization generates shift currents that deviate substantially from theoretical predictions. On the other hand, exciting either below the bandgap (835 nm at 77 K) to produce optical rectification or far above the bandgap (400 nm at room temperature) to produce shift currents generates emission in agreement with theoretical calculations. Spin-polarized electrons created by elliptically polarized excitation are the source of the observed discrepancy.
A study of background signals in terahertz apertureless near-field microscopy and their use for scattering-probe imaging105(2009); http://dx.doi.org/10.1063/1.3141727View Description Hide Description
Apertureless near-field microscopy is an imaging technique in which a small metal tip is held close to a surface, converting evanescent waves to propagating waves and permitting extreme subwavelength spatial resolution. This technique has recently been adapted for use in the terahertz region of the spectrum. Here, the interpretation of the measured signals and the suppression of background scattering can be complicated by the extremely broad bandwidth of the terahertz source and by the coherent (i.e., phase-sensitive) detection of the scatteredradiation. We have analyzed the use of tip-sample distance modulation for the removal of background signals. We find that significant background signals, originating from scattering off the probe tip, can be observed even after modulation. These background signals result from path-length difference modulation, and thus are relevant when phase-sensitive detection is used. We use a dipole antenna model to explain the spatial variation in the scattered signal. Since this signal originates from the tip only, it can be used to characterize free-space terahertz wave fronts with subwavelength resolution.
105(2009); http://dx.doi.org/10.1063/1.3125445View Description Hide Description
We have investigated the microscopic origins of the induced in two phosphate glasses: a self-prepared lanthanum phosphate glass with molar composition and a commercial sodium alumino phosphate glass (IOG-1, Schott Glass Technologies, Inc.) with molar composition . The drastic difference in alkali content in these two phosphate glass systems results in different origins of their induced . For the poled lanthanum phosphate glass, the origin of the induced , which is directly proportional to the dc field established inside the glass, is the result of charge migration. A model that uses a single-positive-charge carrier with a nonblocking cathode describes the anodic surface of thickness. For the poled sodium alumino phosphate glass, two mechanisms—dipole reorientation via the applied field and charge migration—are responsible for the origin of the bulk and the surface. Dipole reorientation via the applied field is suggested for the bulk contribution, while a charge migration model that involves multiple-charge carriers with nonblocking electrodes is appropriate for the surface.
105(2009); http://dx.doi.org/10.1063/1.3132822View Description Hide Description
The laser-induced forward transfer process of solid ceramic donor materials (gadolinium gallium oxide and ytterbium doped yttrium aluminium oxide) was studied using triazene polymer as a sacrificial layer by means of a time-resolved nanosecond-shadowgraphy technique. The dependence of the ablation dynamics and quality of the ejected donor material on the laser fluence and thickness of the sacrificial and donor layers were investigated and discussed.
105(2009); http://dx.doi.org/10.1063/1.3129311View Description Hide Description
We report experimental and theoretical results on the photoluminescence of CdTeSe nanocrystals, embedded in a silica opaline structure by infiltration of a highly diluted solution. Strong modification of emission diagrams of embedded nanocrystals have been observed in good agreement with theoretical models. At macroscopic scale, we measured the difference of nanocrystals emission lifetime embedded either in an opal for which the emission is in the gap, or in an opal of smaller balls diameter for which the emission is outside the gap. The photonic bandgap effect leads to a lifetime increase of the order of 10%. These lifetime variations are shown to be in good agreement with the calculated local density of states modification due to the pseudogap.