Volume 96, Issue 5, 01 February 2010
- lasers, optics, and optoelectronics
- plasmas and electrical discharges
- 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
- biophysics and bio-inspired systems
- interdisciplinary and general physics
Index of content:
The authors report on “graphene-like” exfoliation of the large-area crystalline films and ribbons of bismuth telluride with the thicknesses of a few atoms. It is demonstrated that crystal can be mechanically separated into its building blocks—Te–Bi–Te–Bi–Te atomic fivefolds—with the thickness of and even further—to subunits with smaller thicknesses. The atomically-thin films can be structured into suspended crystalline ribbons providing quantum confinement in two dimensions. The quasi two-dimensional crystals of bismuth telluride revealed high electrical conductivity and low thermal conductivity. The proposed atomic-layer engineering of bismuth telluride opens up a principally new route for drastic enhancement of the thermoelectric figure of merit.
- LASERS, OPTICS, AND OPTOELECTRONICS
96(2010); http://dx.doi.org/10.1063/1.3299257View Description Hide Description
We report the development of 480 nm cyan and 520 nm green light emitting diodes(LEDs) with a highly stable emission wavelength. The shift is less than 3 nm when the drive current density is changed from 0.1 to . LEDs have been obtained in GaInN-based homoepitaxy on nonpolar -plane GaN bulk substrates. For increasing emission wavelength we find a large number of additional dislocations generated within the quantum wells ( to ) and a decrease in the electroluminescence efficiency. This suggests that the strain induced generation of defects plays a significant role in the performance limitations.
Enormously high-peak-power optical pulse generation from a single-transverse-mode GaInN blue-violet laser diode96(2010); http://dx.doi.org/10.1063/1.3299261View Description Hide Description
We have demonstrated extraordinary optical pulse generation with a peak-power of 55 W and pulse duration of 15 ps by intense electrical pulse excitation of a 401 nm GaInN laser diode(LD). Electrical pulse excitation of a GaInN LD which contained a thicker electron blocking layer gave rise to abnormal behavior with a several nanosecond-long delay and apparent Q-switching under intense excitation. Operation of this LD under such excitation was found to produce highly intense optical pulses even in semiconductor lasers with a single-transverse-mode.
Binary liquid crystal alignments based on photoalignment in azo dye-doped liquid crystals and their application96(2010); http://dx.doi.org/10.1063/1.3299268View Description Hide Description
This work demonstrates the feasibility of binary liquid crystal(LC) alignments, in which two forms of LC alignment are in a single pixel or a specific area using a surface-treated alignment layer and a photoalignment film of the adsorption of azo dyes onto the polymer surface in azo dye-doped liquid crystals. Binary LC alignments that involve two of common LC alignments, which are homogeneous, homeotropic, hybrid, and twisted nematic alignments, are initially demonstrated. Then, a binary LC alignment that comprises hybrid and homeotropic alignments is adopted to fabricate a viewing-angle-dependent liquid crystal display, which displays two images by changing viewing angles.
96(2010); http://dx.doi.org/10.1063/1.3302406View Description Hide Description
Directional emission InP/AlGaInAs square-resonator microlasers with a side length of are fabricated by standard photolithography and inductively coupled-plasma etching technique. Multimode resonances with about seven distinct mode peaks in a free-spectral range are observed from 1460 to 1560 nm with the free-spectral range of 12.1 nm near the wavelength of 1510 nm, and the mode refractive index versus the photon energy (eV) as are obtained by fitting the laser spectra with an analytical mode wavelength formula derived by light ray method. In addition, mode field pattern is simulated for cold cavity by two dimensional finite-difference time-domain technique.
96(2010); http://dx.doi.org/10.1063/1.3279154View Description Hide Description
This work demonstrates the feasibility of a terahertz time-domain spectrometer based on a subwavelength-diameter plastic wire (SPW) for sensing applications. The dispersion property of the SPW is experimentally and theoretically studied. The SPW exhibits a low and controllable waveguide dispersion, which can be engineered by changing the core diameter, the core index, and the cladding index of the wire. Two white powders, tryptophan and polyethylene, deposited on the bottom of the wire can be successfully distinguished based on the waveguide dispersion of SPW. The SPW would be a promising candidate for combination with biochips for sensing minute molecules.
Optical gain improvement in type-II InGaN/GaNSb/GaN quantum well structures composed of InGaN/and GaNSb layers96(2010); http://dx.doi.org/10.1063/1.3300840View Description Hide Description
Optical gain characteristics of type-II InGaN/GaNSb quantum well(QW) structure are investigated by using the multiband effective mass theory. These results are compared with those of conventional InGaN/GaN QW structures. The transition wavelength rapidly increases with increasing the Sb composition in GaNSb layer while it is less sensitive to the In composition in InGaN layer. Hence, longer wavelength QW structures with a relatively lower In composition can be easily obtained by controlling Sb composition, compared to the conventional type-I InGaN/GaN QW structures. The optical gain and the differential gain of a type-II QW structure are shown to be much larger than that of a conventional QW structure in an investigated range of carrier densities. This is due to the reduction in the effective well width, in addition to the increase in the optical matrix element.
The impact of piezoelectric polarization and nonradiative recombination on the performance of (0001) face GaN/InGaN photovoltaic devices96(2010); http://dx.doi.org/10.1063/1.3301262View Description Hide Description
The impact of piezoelectricpolarization and nonradiative recombination on the short-circuit current densities of (0001) face GaN/InGaN photovoltaic devices is demonstrated. diodes consisting of 170 nm thick intrinsic layers sandwiched by GaN layers exhibit low . The piezoelectricpolarization at the GaN/InGaN heterointerfaces creates drift currents opposite in direction needed for efficient carrier collection. Also, nonradiative recombination centers produce short carrier lifetimes, limiting . Alternative structures with intrinsic InGaN layers sandwiched by -type InGaN or graded layer and a -type layer have favorable potentials, longer carrier lifetimes, and improve to .
Hollow-core resonator based on out-of-plane two-dimensional photonic band-gap crystal cladding at microwave frequencies96(2010); http://dx.doi.org/10.1063/1.3303857View Description Hide Description
We report on the demonstration of a resonator based on electromagnetic field confinement in a hollow-core by implementing an out-of-plane two-dimensional (2D) photonic band-gap(PBG) crystal cladding. In contrast with in-plane 2D PBG crystal devices, the PBG crystal studied here is perpendicular to the propagation axis. A resonator was constructed with silica rods to prove the concept at frequencies around 30 GHz. We show that the technique has the potential to reach quality factors (Q) of .
Improved crystal quality and performance of GaN-based light-emitting diodes by decreasing the slanted angle of patterned sapphire96(2010); http://dx.doi.org/10.1063/1.3304004View Description Hide Description
Periodic triangle pyramidal array patterned sapphire substrates (PSSs) with various slanted angles were fabricated by wet etching. It was found beside normal wurtzite GaN,zinc blende GaN was found on the sidewall surfaces of PSS. The crystal quality and performance of PSS-LEDs improved with decrease in slanted angle from 57.4° to 31.6°. This is because most of the growth of GaN was initiated from -planes. As the growth time increased, GaN epilayers on the bottom -plane covered these pyramids by lateral growth causing the threading dislocation to bend toward the pyramids.
96(2010); http://dx.doi.org/10.1063/1.3304118View Description Hide Description
We report and characterize second-harmonic generation from a single wurtzite GaAs nanoneedle. The wurtzite crystal structure of the nanoneedle relaxes the strict nonlinear selection rules of normal zincblende GaAs while maintaining its strong nonlinear optical coefficients. The ability to growGaAs nanoneedles without catalysts on (111) Si makes them particularly attractive as nonlinear optoelectronic media compatible with complementary metal-oxide-semiconductortechnology.
96(2010); http://dx.doi.org/10.1063/1.3295699View Description Hide Description
We demonstrate saturable absorption operation of single-walled carbon nanotubes(CNTs)deposited by an efficient electrospray process that provides target-localized deposition with a homogeneous dispersion of the nanomaterials. An improvement in deposition efficiency of 95% is achieved. CNT dispersion/deposition conditions are optimized for suppressed CNT agglomeration, thereby maximizing the nonlinear absorption of CNTs. An ultrafast saturable absorber is formed with side-polished fiber as a substrate for evanescent field interaction of propagated light with CNTs. A resultant fiber mode-locked laser is realized. The laser has output with an estimated pulse duration and repetition rate of 190.9 fs and 3.95 MHz, respectively.
Room temperature operation of photonic-crystal distributed-feedback quantum cascade lasers with single longitudinal and lateral mode performance96(2010); http://dx.doi.org/10.1063/1.3295704View Description Hide Description
We demonstrate room temperature operation of photonic-crystal distributed-feedback quantum cascade lasers emitting at . A rectangular photonic crystal lattice perpendicular to the cleaved facet was defined using holographic lithography. The anticrossing of the index- and Bragg-guided dispersions of rectangular lattice forms the band-edge mode with extended mode volume and reduced group velocity. Utilizing this coupling mechanism, single mode operation with a near-diffractive-limited divergence angle of 12° is obtained for wide devices in a temperature range of 85–300 K. The reduced threshold current densities and improved heat dissipation management contribute to the realization of devices’ room temperature operation.
Effect of Mg doping in the barrier of InGaN/GaN multiple quantum well on optical power of light-emitting diodes96(2010); http://dx.doi.org/10.1063/1.3302458View Description Hide Description
We report on Mgdoping in the barrier layers of InGaN/GaN multiple quantum wells(MQWs) and its effect on the properties of light-emitting diodes(LEDs).Mgdoping in the barriers of MQWs enhances photoluminescence intensity, thermal stability, and internal quantum efficiency of LEDs. The light output power of LEDs with Mg-doped MQW barriers is higher by 19% and 27% at 20 and 200 mA, respectively, than that of LEDs with undoped MQW barriers. The improvement in output power is attributed to the enhanced hole injection to well layers in MQWs with Mg-doped barriers.
96(2010); http://dx.doi.org/10.1063/1.3303980View Description Hide Description
We present the integration of a self-aligned microtip on a vertical-cavity surface-emitting laser (VCSEL) by near infrared photopolymerization. This one-step fabrication process is triggered by the laser source itself. It is based on the use of photopolymers sensitive at the lasing wavelength and can be applied to VCSEL devices after their process fabrication. We have characterized the fabricated microtips and shown that they focus laser light at few micrometers from the device. The applications of this simple method may concern VCSEL beam shaping as well as the fabrication of microprobes for near-field optical microscopy.
96(2010); http://dx.doi.org/10.1063/1.3304021View Description Hide Description
In contrast to earlier classical studies that analyze the surface Bloch modes supported by metallic gratings composed of slits as collective surface-plasmon-polaritons (SPPs) resonances (or poles) of the entire periodic problem, we study the normalized rate of SPPs that are locally launched on every individual ridge of metallic lamellar gratings. With this “microscopic” description at the unit-cell level, we further explain how these individual SPPs constructively interfere to build up the classical collective resonances. The approach, which combines analytical treatments and fully-vectorial computations, shines new light on an important and classical phenomenon of grating diffraction.
96(2010); http://dx.doi.org/10.1063/1.3294628View Description Hide Description
A microscopically motivated nonequilibrium theory is applied to study the power characteristics of an in-well pumped vertical external cavity surface emitting Laser for varying pump energies. Dynamic simulations yield steady state nonequilibrium carrier distributions resulting in gain reduction due to kinetic hole burning. Pauli blocking effects become prominent for more resonant pumping and increased pump powers. The reduced pump absorption results in a sublinear input-output power characteristics even for the optimized case where heating of the active mirror plays no role.
96(2010); http://dx.doi.org/10.1063/1.3304787View Description Hide Description
Recently, Wolf has shown that the phase measurement associated with fields that are not monochromatic, which is relevant for all x-ray structure investigations, must be properly defined via a cross-spectral density function under full spatial coherence conditions; otherwise, the problem is meaningless and has no solution [E. Wolf, Phys. Rev. Lett.103, 075501 (2009)]. We propose an experimental realization for retrieving the phase across the entire field of an image. The demonstration is performed using broadband optical fields, but can be extended to other electromagnetic radiation, including x-rays.
rf linewidth reduction in a quantum dot passively mode-locked laser subject to external optical feedback96(2010); http://dx.doi.org/10.1063/1.3299714View Description Hide Description
The effect of external optical feedback on an InAs/GaAs quantum dot passively mode-locked laser is investigated. The rf linewidth narrows from 8 KHz in the free-running situation to a value as low as 350 Hz under relatively low feedback. The rf linewidth characterization under resonant feedback at a multiple of the laser cavity length validates the prediction of a previous numerical simulation. It is also confirmed that the integrated rms timing jitter varies as the square root of the rf linewidth. The results are promising for the development of compact, monolithic semiconductormode-locked lasers as low noise optoelectronic oscillators.
96(2010); http://dx.doi.org/10.1063/1.3294626View Description Hide Description
We demonstrate surface plasmon (SP) excitation in silver nanowires directly deposited on the emission facet of a laser diode (LD) chip. Evident light output from the silver nanowires is observed. The output is linear-polarized and is strongly dependent on the nanowire orientation. SP excitation at the central part of a silver nanowire is also observed. The possibility of direct SP excitation on an LD chip may open opportunities for realizing plasmonic and photonic circuits or components with high compactness.
96(2010); http://dx.doi.org/10.1063/1.3304783View Description Hide Description
We describe a heterogeneous terahertz (THz) quantum cascade laser that is composed of two different active region designs. This device emits simultaneously at around 2.5 and 2.9 THz with certain frequency tunability by applied current. We also investigate the spectral gain in the structure by THz time-domain spectroscopy and correlate the gain spectral bandwidth with the alignment and wavelength emission behavior of the two stack device.