Volume 98, Issue 21, 23 May 2011
- 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:
" title="Magnetism and magnetoelectricity of a U-type hexaferrite " />
We report on structural, magnetic, and magnetoelectric (ME) properties of a U-type hexaferrite prepared by solid state reaction. Samples sintered at in oxygen contain the fewest impurity phases and show highly insulating behavior. Powderneutron diffraction results reveal that a commensurate magnetic order with a (0,0,3/2) propagation vector develops below . Corresponding to the appearance of the magnetic order, the sample shows a small ME effect. These results suggest that is a room-temperature ME material in which the origin of the ME effect is similar to those of other ME hexaferrites.
- LASERS, OPTICS, AND OPTOELECTRONICS
Cavity-enhanced direct band electroluminescence near 1550 nm from germanium microdisk resonator diode on silicon98(2011); http://dx.doi.org/10.1063/1.3592837View Description Hide Description
We electrically and optically characterize a germaniumresonatordiode on silicon fabricated by integrating a germaniumlight emitting diode with a microdisk cavity.Diode current-voltage characteristics show a low ideality factor and a high on/off ratio. The optical transmission of the resonator features whispering gallery modes with quality factors of a few hundred. Direct band gap electroluminescence under continuous current injection shows a clear enhancement of emission by the cavity. At this stage, the pumping level is not high enough to cause linewidth narrowing and invert the material. A higher n-type activated doping of germanium is necessary to achieve lasing.
98(2011); http://dx.doi.org/10.1063/1.3584034View Description Hide Description
We propose and analyze a remote atmospheric lasing configuration which utilizes a combination of an ultrashort pulse laser to form a plasma filament (seed electrons) by tunneling ionization and a heater pulse which thermalizes the seed electrons.Electrons collisionally excite nitrogen molecules and induce lasing in the ultraviolet. The lasing gain is sufficiently high to reach saturation within the length of the plasma filament. A remotely generated ultraviolet source may have applications for standoff detection of biological and chemical agents.
98(2011); http://dx.doi.org/10.1063/1.3593007View Description Hide Description
Control of single photon wave-packets is an important resource for developing hybrid quantum systems which are composed of different physical systems interacting via photons. Here, we extend this control to triggered photons emitted by a quantum dot, temporally shaping single photon wave-packets on timescales fast compared to their radiative decay by electro-optic modulation. In particular, telecommunications-band single photons resulting from the recombination of an exciton in a quantum dot with exponentially decaying wave-packets are synchronously modulated to create Gaussian-shaped single photon wave-packets. We explore other pulse shapes and investigate the feasibility of this technique for increasing the indistinguishability of quantum dotgenerated single photons.
98(2011); http://dx.doi.org/10.1063/1.3593027View Description Hide Description
We theoretically and experimentally study the side coupling between guided modes and cavity modes in a one-way waveguide that is composed of a regular photonic crystal and a gyromagnetic photonic crystal. At the cavity resonant wavelength, the backward mode can be completely blocked while the forward mode is only slightly influenced in the transmissivity for a specially designed waveguide. This unique light transport property can be exploited to construct a unidirectional band stop filter and a unidirectional channel-drop filter that can selectively process a light signal propagating only along a particular direction.
98(2011); http://dx.doi.org/10.1063/1.3593032View Description Hide Description
The lasing operation of a ZnO planar microcavity under optical pumping is demonstrated from to 300 K. At the laser threshold, the cavity switches from the strong coupling to the weak coupling regime. A gain-related transition, which appears while still observing polariton branches and, thus, with stable excitons, is observed below 240 K. This shows that exciton scattering processes, typical of II-VI semiconductors, are involved in the gain process.
98(2011); http://dx.doi.org/10.1063/1.3593134View Description Hide Description
We report the fabrication and thorough characterization of tensile strained -type SiGe quantum well infrared photodetectors(QWIPs) grown on a pseudosubstrate. The QWIPs operate from a light-hole (LH) ground state and feature responsivity peaks in both the terahertz and mid-infrared regimes with responsivity values up to 3.7 mA/W, originating from LH–LH, LH–heavy-hole, and LH–split-off-band transitions.
98(2011); http://dx.doi.org/10.1063/1.3591967View Description Hide Description
The efficiency droop in InGaN-based UVlight emitting device(LED) with AlGaN and InAlGaN barrier is investigated. Electroluminescence results indicate that the light performance of quaternary LEDs can be enhanced by 25% and 55% at 350 mA and 1000 mA, respectively. Furthermore, simulations show that quaternary LEDs exhibit 62% higher radiative recombination rate and low efficiency degradation of 13% at a high injection current. We attribute this improvement to increasing of carrier concentration and uniform redistribution of carriers.
98(2011); http://dx.doi.org/10.1063/1.3593005View Description Hide Description
We propose a form of flexible wavefront manipulation of surface plasmonpolaritons (SPPs) without any mechanical motion component. An incoming light pattern, the amplitude of which is modulated by a liquid crystalspatial light modulator, is used to illuminate a grating on an Aufilm acting as a light-SPP coupler. The SPPs launched from the grating have the same intensity distribution as the input light pattern. By changing the input image, continuously variable in-plane SPP focusing on the Aufilm is implemented. The experimental results agree well with numerical simulations.
Modulation response of quantum dot nanolight-emitting-diodes exploiting purcell-enhanced spontaneous emission98(2011); http://dx.doi.org/10.1063/1.3592571View Description Hide Description
The modulation bandwidth for a quantum dotlight-emitting device is calculated using a detailed model for the spontaneous emission including the optical and electronic density-of-states. We show that the Purcell enhancement of the spontaneous emission rate depends critically on the degree of inhomogeneous broadening relative to the cavity linewidth and can improve the modulation speed only within certain parameter regimes.
98(2011); http://dx.doi.org/10.1063/1.3593958View Description Hide Description
Hexagonal boron nitride (hBN) has emerged as an important material for various device applications and as a template for graphene electronics. Low-dimensional hBN is expected to possess rich physical properties, similar to graphene. The synthesis of wafer-scale semiconducting hBN epitaxial layers with high crystalline quality and electrical conductivity control has not been achieved but is highly desirable. Large area hBN epitaxial layers (up to 2 in. in diameter) were synthesized by metal organic chemical vapor deposition. P-type conductivity control was attained by in situMg doping. Compared to Mg-doped wurtzite AlN, which possesses a comparable energyband gap, dramatic reductions in Mg acceptor energy level and P-type resistivity (by about six to seven orders of magnitude) have been realized in hBN epilayers. The ability of conductivity control and wafer-scale production of hBN opens up tremendous opportunities for emerging applications, ranging from revolutionizing p-layer approach in III-nitride deep ultraviolet optoelectronics to graphene electronics.
98(2011); http://dx.doi.org/10.1063/1.3593128View Description Hide Description
In this letter, we report a quantum dotphotodetector enhanced by Fano-type interference in a metallic two-dimensional (2D) subwavelength hole array (2DSHA). The photocurrent enhancement wavelength shows an offset from the plasmonic resonant peak and corresponds to a dip in the transmission spectrum of the 2DSHA structure. The offset is attributed to the Fano-type interference in the 2DSHA structure. The asymmetric line shapes of the plasmonic resonance are analyzed and agree well with the two-peak Fano-type interference model. Over 100% enhancement in photodetectivity and photoresponsivity is achieved at the wavelength of the Fano dip of the first order plasmonic mode.
98(2011); http://dx.doi.org/10.1063/1.3594244View Description Hide Description
We present the design, fabrication, and characterization of a guided-mode resonance polarizer operating in the telecommunication band. This polarizer consists of a single one-dimensional silicon grating layer and a glass substrate. The device is designed using inverse mathematical methods and fabricated by thin-film deposition, holographic-interference patterning, and etching. The fabricated polarizer has high transmittance for transverse-magnetic polarization and low transmittance for transverse-electric polarization over a wavelength range. Its experimental extinction ratio is at a central wavelength . Experimental and theoretical results are in good agreement.
98(2011); http://dx.doi.org/10.1063/1.3595277View Description Hide Description
We demonstrate experimentally optical quality factor of nearly 9000 in a micromachined Fabry–Pérot resonator based on free space propagation of light and direct coupling to optical fibers. This result is obtained on long cavityresonators, a usually difficult case in terms of power loss, but very useful configuration for experiments requiring either long optical path or enough space for manipulation. The resonator architecture includes two multilayered silicon-air Bragg mirrors of cylindrical shape, combined with a fiber rod lens. The specific stability criteria are derived for the proposed resonator architecture. Dimensions of the fabricated devices are chosen accordingly.
98(2011); http://dx.doi.org/10.1063/1.3595303View Description Hide Description
We report on the optoelectronic properties of -based ultraviolet (UV)photodetectors for the application as a high current, high gain optical switch. Due to an internal gain mechanism combined with the high conductivity of the two-dimensional electron gas at the heterostructure interface, photocurrents in the milliampere-range were obtained with UVillumination. By employing a mesa structure design with meander geometry very low dark currents below 50 nA up to a bias voltage of 100 V were achieved. Optical switching with an on/off-current-ratio of five orders of magnitude was demonstrated. The response time was determined to be 6 ms and persistent photoconductivity was observed. The photodetector is visible-blind with a cut-off wavelength of 365 nm according to the band gapenergy of the GaN absorption layer. A high responsivity with a maximum of 70 A/mW at 312 nm and 100 V bias voltage was demonstrated.
98(2011); http://dx.doi.org/10.1063/1.3595305View Description Hide Description
We demonstrate a two-dimensional imaging technique that uses a tomographic image reconstruction algorithm of object projections measured with a single photodetector. By imposing a spatially varying amplitude modulation on the illumination beam, the lateral projections of the object are encoded in the electronic spectrum of the signal from a single-element photodetector.
98(2011); http://dx.doi.org/10.1063/1.3595313View Description Hide Description
This paper reports the unambiguous identification of lasing modes observed in a single-cell photonic crystalcavity using a solid angle scanning system. The polarization-resolved far-field measurements showed distinctive features of each mode and distinguished it from the other modes in contrast to conventional optical characterization methods. Monopole, quadrupole, and hexapole lasing modes were observed and identified, showing good agreement with numerical simulations. Our understanding of the lasing modes will be useful for determining practical applications as promising light sources in ultracompact photonic integrated circuits.
98(2011); http://dx.doi.org/10.1063/1.3591171View Description Hide Description
The probability of spontaneous emission from a semiconductor, per oscillation of the optical field, is proportional to the cube of the fine structure constant and the ratio of the semiconductor dipole parameter to hydrogen’s Rydberg energy. Expressions for radiative recombination rates in quantum wells and bulk semiconductors are given, including the bimolecular radiative recombination coefficient . We show that sets the natural scale for spectral density of spontaneous emission per square wavelength area of a quantum well. The fine structure constant also sets a physical limit to the Purcell enhancement factor for spontaneous emission.
98(2011); http://dx.doi.org/10.1063/1.3593499View Description Hide Description
We report on the growth, processing, and characterization of distributed feedback quantum cascade lasers emitting in single mode at . Lasers were grown in the indium phosphide material system with a bound-to-continuum active region and processed as double channel ridges with metalized top grating.Electro-optic characteristics as well as current and temperature tuning are investigated. Maximum single mode peak powers exceeding 800 mW at room temperature and intrapulse-tuning covering for a gas absorption experiment are demonstrated. Furthermore data on the far field are presented showing a slow-axis divergence angle of 28.3° (full width at half maximum).
98(2011); http://dx.doi.org/10.1063/1.3592827View Description Hide Description
Using band charge transport model, the effect of external dc electric and magnetic fields on photorefractive parameters, viz, space charge electric field and gain coefficient in steady state regime is investigated for Cr doped GaAs. In particular, space charge electric field of is obtained for and . obtained is nearly times higher than, in absence of either or . The gain coefficient exhibits considerable enhancement at above values of and .
98(2011); http://dx.doi.org/10.1063/1.3593963View Description Hide Description
We present a method for tuning the resonant wavelength of photonic crystalcavities (PCCs) around . Large tuning of the PCC mode is enabled by electromechanically controlling the separation between two parallel InGaAsP membranes. A fabrication method to avoid sticking between the membranes is discussed. Reversible red/blueshifting of the symmetric/antisymmetric modes has been observed, which provides clear evidence of the electromechanical tuning, and a maximum shift of 10 nm with applied bias has been obtained.