Volume 94, Issue 4, 26 January 2009
- lasers, optics, and optoelectronics
- structural, mechanical, thermodynamic, and optical properties of condensed matter
- electronic transport and semiconductors
- magnetism and superconductivity
- dielectrics and ferroelectricity
- nanoscale science and design
- organic electronics and photonics
- device physics
- applied biophysics
- interdisciplinary and general physics
Index of content:
Peapodlike heterostructure composed of particles orderly embedded in the high-crystalline nanoribbons was fabricated via a two-step thermal reaction route. The room temperature photoluminescencespectrum of the products showed a strong and stable blue emission centered at 467 nm under excitation at 320 nm, which was attributed to the especial heterostructure. This technique also provided a general and convenient approach for preparing even more complex heterostructures and would be useful in improving the properties of materials.
- LASERS, OPTICS, AND OPTOELECTRONICS
94(2009); http://dx.doi.org/10.1063/1.3073044View Description Hide Description
Quantum cascade (QC) laser core heating is a primary impediment to high device wall plug efficiency (WPE). Here, we demonstrate that efficient photon generation produces a quantifiable reduction in heating of the QC laser core temperature. By superimposing low duty cycle current pulses on a core-heating dc baseline, we observe the instantaneous threshold current and current efficiency evolution as the dc input is varied. From these measurements we recover the laser core temperature . Results agree well with calculations of based on measured thermal resistance and WPE. Using the same thermal model for a laser with negligible WPE, we show that the large WPE of the measured device—24% for an 80 K heat sink—results in a core temperature reduction of .
Strong violet and green-yellow electroluminescence from silicon nitride thin films multiply implanted with Si ions94(2009); http://dx.doi.org/10.1063/1.3068002View Description Hide Description
Strong visible electroluminescence(EL) has been observed from a 30 nm silicon nitridethin film multiply implanted with Si ions and annealed at . The EL intensity shows a linear relationship with the current transport in the thin film at lower voltages, but a departure from the linear relationship with a quenching in the EL intensity is observed at higher voltages. The EL spectra show two primary EL bands including the predominant violet band at (415 nm) and the strong green-yellow band at (560 nm). Two weak bands including the ultraviolet band at and the near infrared band at emerge at high voltages. The evolution of each EL band with the voltage has been examined. The phenomena observed are explained, and the EL mechanisms are discussed.
94(2009); http://dx.doi.org/10.1063/1.3073860View Description Hide Description
Nonthermal rollover (or efficiency droop) of the electroluminescence(EL) efficiency has been investigated for near-UV-emitting (AlGaIn)N single-well light-emitting diodes(LED) with varying GaInN well widths grown on substrates with different dislocation densities (DDs). For each DD the well width of the mesa-LEDs has been optimized for maximum EL efficiency at high operating currents. LEDs on freestanding GaN with an 18 nm thick GaInN wide-well active region show the highest efficiency, and the output power-versus-current characteristic remains linear up to the highest pulsed current density of . In contrast, LEDs on sapphiregrown with conventional low-temperature nucleation exhibit the optimum well width at 3 nm and show significant nonthermal rollover.
94(2009); http://dx.doi.org/10.1063/1.3075055View Description Hide Description
We study coupling of laser pulses to an evanescent electric field at a nanocorrugated dielectricsurface. We find that the local electric field is increased in the surface grooves, up to a factor of 2, compared to the incident field, thus providing a positive feedback for localized subthreshold ablation, and enabling the corrugation growth.
94(2009); http://dx.doi.org/10.1063/1.3075057View Description Hide Description
Recording analog signals using photonic time-stretch technique in a mode which combines advantages of continuous signal capture, as in real-time analog-to-digital converters (ADCs), and very high bandwidth capability of (equivalent-time) sampling oscilloscopes, is proposed. It is shown that the eye diagrams of high speed serial data can be acquired at least 100 times faster than the fastest capture rates today. Unlike conventional sampling scopes, this technique can capture ultrafast dynamics of repetitive signals, nonrepetitive signals, and rare events. Experimentally, 45 Gbit/s data eye diagram measurement is demonstrated.
94(2009); http://dx.doi.org/10.1063/1.3075059View Description Hide Description
We report enhanced transmission of broadband terahertz electromagnetic waves in the frequency range from 0.2 to 3.15 THz using silicon with micropyramid surface structures. We observe a maximum 89% reduction in reflectivity of terahertz power when the sample with -period micropyramids is used, compared to a planar silicon substrate. By varying the period of micropyramid structures from 110 to , the cutoff frequency of enhanced transmission is tuned from 0.74 to 2.93 THz and the bandwidth of enhancement increases from 0.91 to 3.15 THz, respectively.
94(2009); http://dx.doi.org/10.1063/1.3070536View Description Hide Description
An optical detection technique for a flow cytometer is described, which delivers high signal-to-noise discrimination without precision optics to enable a flow cytometer that can combine high performance, robustness, compactness, low cost, and ease of use. The enabling technique is termed “spatially modulated emission” and generates a time-dependent signal as a continuously fluorescing bioparticle traverses a predefined pattern for optical transmission. Correlating the detected signal with the known pattern achieves high discrimination of the particle signal from background noise. The technique is demonstrated with measurements of fluorescent beads flowing through a microfluidic chip.
Electrowetting retroreflectors: Scalable and wide-spectrum modulation between corner cube and scattering reflection94(2009); http://dx.doi.org/10.1063/1.3073840View Description Hide Description
Corner cube and spherical retroreflectors are ubiquitous in conspicuity and range-finding applications since they reflect light back to the illumination source with unmatched efficiency. We report here a switchable electrowetting retroreflector platform that provides multiple novel features, including (a) using electrical energy to switch from a light scattering state, (b) low loss and wide spectrum as limited only by the absorptionspectrum of water, (c) use of ultrasimple self-assembly of on a polymer/Al corner-cube substrate, and (d) change in retroreflected irradiance of over a ±30° field of view.
94(2009); http://dx.doi.org/10.1063/1.3075853View Description Hide Description
Optical properties of staggered 530 nm InGaN/InGaN/GaN quantum-well (QW) light-emitting-diodes are investigated using the multiband effective mass theory. These results are compared with those of conventional 530 nm InGaN/GaN QW structures. A staggered InGaN/InGaN/GaN QW structure is shown to have much larger spontaneous emission than a conventional InGaN/GaN QW structure. This can be explained by the fact that a staggered QW structure has much larger matrix element than a conventional QW structure because a spatial separation between electron and hole wave functions is substantially reduced with the inclusion of a staggered InGaN layer. A staggered QW structure shows that the peak position at a high carrier density (530 nm) is similar to that at a noninjection level.
94(2009); http://dx.doi.org/10.1063/1.3075855View Description Hide Description
We measured the gain dynamics of the ground-state transition at 20 K in an undoped and identically fabricated -doped InAs/GaAs quantum-dot amplifier. The dynamics in the doped device is dominated by a very short and a very long time constant. These were attributed to hole and electron dynamics, respectively, and quantitatively described by a microstate model. By comparing the dynamics for the same modal gain in the two devices, the gain recovery was initially faster in the -doped sample, attributed to ultrafast hole-hole scattering, but slower at later times due to the lack of an electron reservoir.
Robust organic lasers comprising glassy-cholesteric pentafluorene doped with a red-emitting oligofluorene94(2009); http://dx.doi.org/10.1063/1.3073713View Description Hide Description
Doped with a red-emitting oligofluorene, fluid and glassy cholesteric liquid crystal (CLC) films are characterized by similar lasing thresholds and efficiencies. With picosecond excitations the output from a glassy CLC laser is temporally stable, but that from a fluid CLC laser decays with time. The difference in stability is attributable to external perturbations on supramolecular structure in the fluid but not the solid state, such as heating through optical pumping, light-induced pitch dilation, and laser-induced flow.
Role of carrier reservoirs on the slow phase recovery of quantum dot semiconductor optical amplifiers94(2009); http://dx.doi.org/10.1063/1.3073715View Description Hide Description
The gain and phase recovery dynamics of quantum-dot (QD)semiconductor optical amplifiers are calculated, including all the optical transitions involved in successive carrier recovery processes. The carrier recovery dynamics of inhomogeneously broadened QDs is simulated by solving 1088 coupled rate equations. The respective contributions of QD states and quantum-well carrier reservoirs to the gain and phase changes are identified both temporally and spectrally. We show that the slow phase recovery component of the QDground state is induced by the slow carrier dynamics of the carrier reservoir due to a slowly varying line shape function of the refractive index change.
94(2009); http://dx.doi.org/10.1063/1.3068003View Description Hide Description
We proposed a method to control the pretilt angle of liquid crystals by stacking of a vertical alignment layer on a planar alignment layer. The pretilt angle can be controlled over the full range (0°–90°) depending on the thickness of the vertical alignment layer. We also proposed a numerical model to describe the physical mechanism based on the anchoring competition between liquid crystal, planar, and vertical polyimide alignment layers.
94(2009); http://dx.doi.org/10.1063/1.3068496View Description Hide Description
A quantum cascade laser based on a three well active module and emitting at 3 THz is demonstrated. The optical transition is vertical in real space and localized in an quantum well. Maximum operating temperature of 123 K in pulsed mode is reported, with threshold current densities as low as at 10 K and at 100 K. High slope efficiency values testify the good internal quantum efficiency of the structure.
94(2009); http://dx.doi.org/10.1063/1.3072594View Description Hide Description
We introduce a method of gradient-based optimization that continuously deforms a periodic dielectric distribution to generate photonicstructures that possess any desired figure of merit expressible in terms of the electromagnetic eigenmodes. As an example, we generate forbidden regions between specified bands at extremely low dielectric contrast.
Effects of organic film morphology on the formation of Rb clusters on surface coatings in alkali metal vapor cells94(2009); http://dx.doi.org/10.1063/1.3073711View Description Hide Description
Surface relaxation rates differ for spin-polarized alkali atoms interacting with monolayer or bilayer octadecyltrichlorosilane (OTS) coatings. The morphology and composition of Rb vapor-exposed films of OTS have been studied with atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS). On OTS monolayers, numerous small ( wide) Rb containing islands nucleate at the boundaries of the wide organic domains. On OTS bilayers, singular large ( wide) Rb containing islands were found. Alkali island formation mediated by surface structure could affect the antirelaxation behavior of organic coatings used in atomic magnetometer cells.
Rapid efficiency roll-off in high-quality green light-emitting diodes on freestanding GaN substrates94(2009); http://dx.doi.org/10.1063/1.3077017View Description Hide Description
InGaN/GaN multiple-quantum-well green light-emitting diodes(LEDs) were grown on freestanding GaN and sapphire substrates. The density of microstructural defects in the LED on GaN was substantially reduced, leading to a significant reduction in defect-assisted tunnelingcurrents and an improved injection efficiency under low bias. The LED on GaN outperformed the LED on sapphire at low injection currents and exhibited a peak internal quantum efficiency. However, it suffered from even more dramatic efficiency roll-off, which occurs at a current density as low as . This behavior is explained as the combined result of efficient current injection and significant carrier overflow in a high-quality LED.
94(2009); http://dx.doi.org/10.1063/1.3077020View Description Hide Description
A transistor laser with a tunnel junction collector is demonstrated. Its optical output is sensitive to third terminal voltage control owing to the electron tunneling (photon-assisted or not assisted) from the base to collector, which acts in further support of resupply of holes for recombination in addition to the usual base Ohmic current, . Collector tunneling enhances laser operation even under a weak collector junction field and quenches it under a strong reverse-biased field. The sensitivity of the tunnel junctiontransistor laser to voltage control enables the tunnel junctiontransistor laser to be directly modulated by both current and voltage control.
94(2009); http://dx.doi.org/10.1063/1.3077152View Description Hide Description
A solution-processable erbium-ytterbium codoped complex is synthesized. The absorption and photoluminescencespectra of the complex are observed. The full width at half maximum of the emission spectrum is 80 nm around 1530 nm. A structure of embedded waveguide with the complex as core layer is designed. With an input signal power of 0.3 mW, an optical gain of 5.2 dB at a wavelength of 1550 nm is obtained in a 15-mm-long waveguide when the cladding layer is SU-8. When the cladding layer is polymethyl methacrylate, an optical gain of 6.5 dB is obtained in a 12-mm-long waveguide with an input signal power of 1 mW.
94(2009); http://dx.doi.org/10.1063/1.3072607View Description Hide Description
We report in this letter the fabrication of unique single-crystalline silver filaments with periodic, pearl-chain-like structures by electrodeposition without using any templates, surfactants, and additives. Fourier transform infrared spectroscopy, infrared focus-plane-array imaging, and numerical simulations demonstrate that the excited surface waves may sustain on the silver “pearl chains” in midinfrared range. Based on the propagation features of surface waves on the silver filaments, we suggest that such a structure can be applied for light transmission in midinfrared range.