Volume 103, Issue 6, 05 August 2013
- photonics and optoelectronics
- surfaces and interfaces
- structural, mechanical, optical, and thermodynamic properties of advanced materials
- magnetics and spintronics
- superconductivity and superconducting electronics
- dielectrics, ferroelectrics, and multiferroics
- nanoscale science and technology
- organic electronics and photonics
- device physics
- biophysics and bio-inspired systems
- energy conversion and storage
- interdisciplinary and general physics
Index of content:
Experimental results are presented for heat flux cloaking, focusing, and reversal in ultra-thin anisotropic composites. A two-material system is utilized in the device design, which features an annular region for heat flow control. The effective thermal conductivity layout of the composite is specified through logical combination of the base material constituents. Heat transfer considering conduction-convection is numerically predicted and experimentally verified via infrared thermography. A Biot number analysis reveals the significance of high rates of convection for large-area planar devices, while the experimental results indicate the feasibility of such heat flow control techniques for advanced electronics applications involving natural convection.
- PHOTONICS AND OPTOELECTRONICS
103(2013); http://dx.doi.org/10.1063/1.4817672View Description Hide Description
We present a feasible method that can make quantum key distribution (QKD), both ultra-long-distance and immune, to all attacks in the detection system. This method is called measurement-device-independent QKD (MDI-QKD) with entangled photon sources in the middle. By proposing a model and simulating a QKD experiment, we find that MDI-QKD with one entangled photon source can tolerate 77 dB loss (367 km standard fiber) in the asymptotic limit and 60 dB loss (286 km standard fiber) in the finite-key case with state-of-the-art detectors. Our general model can also be applied to other non-QKD experiments involving entanglement and Bell state measurements.
103(2013); http://dx.doi.org/10.1063/1.4817754View Description Hide Description
The optical gain spectra of InGaN-based multiple-quantum-well (MQW) laser diodes (LDs) grown by plasma-assisted molecular beam epitaxy are compared for different emission wavelengths. Two AlGaN cladding free LDs with similar epitaxial structures but with different In compositions in MQW were grown to study the dependence of material gain on lasing wavelength. As the emission wavelength increased from 432 to 458 nm, the differential modal gain decreased from 5.7 to 4.7 cm/kA, and the optical losses increased from 40 to 46 cm−1 resulting in an increase in threshold current density. This dependence is attributed to lower optical mode confinement of LD emitting at longer wavelength. We found a strong decrease of confinement factor with increasing wavelength.
64 μW pulsed terahertz emission from growth optimized InGaAs/InAlAs heterostructures with separated photoconductive and trapping regions103(2013); http://dx.doi.org/10.1063/1.4817797View Description Hide Description
We present results on optimized growth temperatures and layer structure design of high mobility photoconductive Terahertz (THz) emitters based on molecular beam epitaxy grown In0.53Ga0.47As/In0.52Al0.48As multilayer heterostructures (MLHS). The photoconductive antennas made of these MLHS are evaluated as THz emitters in a THz time domain spectrometer and with a Golay cell. We measured a THz bandwidth in excess of 4 THz and average THz powers of up to 64 μW corresponding to an optical power-to-THz power conversion efficiency of up to 2 × 10−3.
Enhanced overall efficiency of GaInN-based light-emitting diodes with reduced efficiency droop by Al-composition-graded AlGaN/GaN superlattice electron blocking layer103(2013); http://dx.doi.org/10.1063/1.4817800View Description Hide Description
Al x Ga1− x N/GaN superlattice electron blocking layers (EBLs) with gradually decreasing Al composition toward the p-type GaN layer are introduced to GaInN-based high-power light-emitting diodes (LEDs). GaInN/GaN multiple quantum well LEDs with 5- and 9-period Al-composition-graded Al x Ga1− x N/GaN EBL show comparable operating voltage, higher efficiency as well as less efficiency droop than LEDs having conventional bulk AlGaN EBL, which is attributed to the superlattice doping effect, enhanced hole injection into the active region, and reduced potential drop in the EBL by grading Al compositions. Simulation results reveal a reduction in electron leakage for the superlattice EBL, in agreement with experimental results.
103(2013); http://dx.doi.org/10.1063/1.4817818View Description Hide Description
Infrared photoconductive and photovoltaic effects are observed in Er-doped Si nanoclusters incorporated in a silicon p-i-n slot-waveguide device. These effects are ascribed to deep gap states of Si nanoclusters. The room temperature open circuit voltage of the devices is 290 mV under transmission of guided light at 1.5 μm. A power dependence, with the exponent close to 0.5 and 1 for forward and reverse bias, respectively, has been observed for the photocurrent versus light intensity characteristic. The former is attributed to bimolecular recombination (empty deep gap states) and the latter to linear recombination with the states being populated with electrons.
103(2013); http://dx.doi.org/10.1063/1.4817732View Description Hide Description
We quantify the mechanisms that govern the lasing wavelength in edge-emitting InP/AlGaInP quantum dot (QD) lasers, by characterising the constituent factors controlling the temperature dependence of the gain peak wavelength. We show that a regime exists where the temperature coefficient of the bandgap can be compensated by the increasing wavelength-shift associated with state-filling in the QD ensemble, necessary to recover the gain peak magnitude. We demonstrate cleaved-facet edge-emitting lasers with a wavelength temperature dependence of 0.03 nm/K, similar to the temperature dependence of a Bragg stack fabricated in this material and approximately a sixth of the dependence of the bandgap.
103(2013); http://dx.doi.org/10.1063/1.4817816View Description Hide Description
By focusing femtosecond pulses on the front and rear surface of a fused silica coverslip, we desorb 8-nm thick polymer films at submicron scale. To determine the role of the substrate in the desorption process, we measure the threshold for nonlinear absorption in fused silica and compare it to the threshold for desorption, taking into account the enhancement of the field at the dielectric-air interface. The results indicate that absorption of energy only occurs in the film. We then measure the beam radius in situ by the knife-edge technique and characterize the desorption by atomic force microscopy. The radius of the laser desorbed area is determined by the desorption threshold intensity and can be a factor of 5 smaller than the beam waist.
Compact and broadband directional coupling and demultiplexing in dielectric-loaded surface plasmon polariton waveguides based on the multimode interference effect103(2013); http://dx.doi.org/10.1063/1.4817860View Description Hide Description
We theoretically, numerically, and experimentally demonstrate that a directional coupling function can be realized with a wide bandwidth (greater than 200 nm) in dielectric-loaded surface plasmon polariton waveguides based on the multimode interference effect. The functional size of the structures is in the range of several micrometers, which is much shorter than traditional directional couplers consisting of two parallel dielectric or plasmonic metallic waveguides. In addition, 1 × 2 beam splitting and demultiplexing function was realized. Such devices with wide bandwidth and small size indicate potential applications in high density lab-on-chip photonic integration and circuits.
103(2013); http://dx.doi.org/10.1063/1.4817973View Description Hide Description
We investigate the unexplored physics of slow light effect in resonance induced transparent grating waveguide structures. We show that with a simple three-layer thin-film structure, a narrow transparent window can be obtained, enabling substantially slow down the speed of out-of-plane propagation of light. Further, we numerically demonstrate an active slow light device that potentially achieves high-speed control of slow light at the optical communication band. This work paves a way for the design of functional devices, such as slow-light chips, switches, and modulators functioning in optical and infrared regimes.
Tunable sideband laser from cascaded four-wave mixing in thin glass for ultra-broadband femtosecond stimulated Raman spectroscopy103(2013); http://dx.doi.org/10.1063/1.4817915View Description Hide Description
We demonstrate the generation of broadband up-converted multicolor array (BUMA) in a thin BK7 glass slide using two noncollinear weak near-IR laser pulses with various crossing angles. The BUMA signal arises from cubic nonlinear χ(3):χ(3) processes via cascaded four-wave mixing of the two incident beams. Broad and continuous tunability of BUMA is simply achieved by varying the time delay between the two pulses. We implement one of the BUMA sidebands as the probe pulse for femtosecond stimulated Raman spectroscopy and collect a solvent mixture anti-Stokes Raman spectrum with an ultrabroad detection range of ca. 100–4000 cm−1.
103(2013); http://dx.doi.org/10.1063/1.4817975View Description Hide Description
The evolution of THz waveform generated in air plasma provides a sensitive probe to the variation of the carrier envelope phase (CEP) of propagating intense few-cycle pulses. Our experimental observation and calculation reveal that the number and positions of the inversion of THz waveform are dependent on the initial CEP, which is near 0.5π constantly under varied input pulse energies when two inversions of THz waveform in air plasma become one. This provides a method of measuring the initial CEP in an accuracy that is only limited by the stability of the driving few-cycle pulses.
Observation of hybrid state of Tamm and surface plasmon-polaritons in one-dimensional photonic crystals103(2013); http://dx.doi.org/10.1063/1.4817999View Description Hide Description
Experimental observation of hybrid mode of Tamm plasmon-polariton and surface plasmon-polariton is reported. The hybrid state is excited in one-dimensional photonic crystal terminated by semitransparent metal film under conditions of total internal reflection for transverse-magnetic-polarized light. Coupling between Tamm and surface plasmon-polaritons leads to repulsion of their dispersion curves controlled by metal film thickness.
Broadband enhancement and inhibition of single quantum dot emission in plasmonic nano-cavities operating at telecommunications wavelengths103(2013); http://dx.doi.org/10.1063/1.4818131View Description Hide Description
A plasmonic microcavity providing broadband control of spontaneous emission for large and sparse semiconductor quantum dots emitting at telecommunications wavelengths is proposed. By designing and fabricating such a cavity, we demonstrate a broadband Purcell effect with spontaneous emission enhancement over a broad spectral range of nm with a 3.9-fold maximum enhancement, as well as inhibition over nm around 1.3 μm. The broadband feature relaxes the constraint on spectral matching between the dot emission and the cavity mode, favourable for implementing efficient non-classical light sources or nanoscale lasers.
Single-photon emission in telecommunication band from an InAs quantum dot grown on InP with molecular-beam epitaxy103(2013); http://dx.doi.org/10.1063/1.4817940View Description Hide Description
We report on the experimental demonstration of a single-photon source based on an InAs quantum dot (QD) on InP grown by molecular-beam epitaxy emitting in the telecommunication band. We develop a method to reduce the QD density to prevent inter-dot coupling via tunneling through coupled excited states. A single InAs QD embedded in an as-etched pillar structure exhibits intense and narrow emission lines. Photon antibunching is clearly observed using superconducting single-photon detectors with high sensitivity, and further improvement of the generated single-photon purity is demonstrated with below-barrier-bandgap excitation.
103(2013); http://dx.doi.org/10.1063/1.4818143View Description Hide Description
The generation of adhesive regions on a z-cut lithium niobate crystal without an additional voltage supply is demonstrated. We show that the origin of the attractive force in the respective solvent is electrophoresis, which can selectively trap charged particles in illuminated regions. Using digital holographic microscopy to measure the space-charge field in a y-cut crystal, we demonstrate the difference between electrophoretic and dielectrophoretic particle manipulation. The suggested method enables the creation of arbitrary two-dimensional patterns, circumventing restrictions originating from the crystal asymmetry. Furthermore, it allows the discrimination between charged particles of different signs, thus acting as a charge sensor.
103(2013); http://dx.doi.org/10.1063/1.4818457View Description Hide Description
We report a highly tunable graphene embedded waveguide which overall modal index is in linear relationship with the in-plane permittivity of graphene and the electro-refraction effect has been significantly enhanced after graphene is embedded. An eight-layer graphene embedded Mach-Zender modulator has been theoretically demonstrated with the advantage of ultra-compact footprint (4 × 30 μm2), high modulation efficiency (20 V·μm), fast modulation speed, and large extinction ratio (35 dB). Our results may promote various on-chip active components, boosting the utilization of graphene in optical applications.
Influence of film thickness in THz active metamaterial devices: A comparison between superconductor and metal split-ring resonators103(2013); http://dx.doi.org/10.1063/1.4817814View Description Hide Description
We experimentally demonstrate thickness-dependent resonance tuning in planar terahertz superconducting metamaterials. Inductive-capacitive resonance of arrays of split-ring resonators fabricated from 50, 100, and 200 nm thick YBa2Cu3O7−δ (YBCO) and gold films were characterized and compared as a function of temperature. In the YBCO metamaterials the resonance frequency strongly depends on the thickness, and they show high thermal tunability in both resonance strength and frequency below the superconducting transition temperature, where the imaginary conductivity varies by three orders of magnitude. In contrast, the resonance in the gold metamaterials exhibits little thickness-dependence and very small tunability.
Impacts of ambipolar carrier escape on current-voltage characteristics in a type-I quantum-well solar cell103(2013); http://dx.doi.org/10.1063/1.4818510View Description Hide Description
We study the current-voltage characteristics of a GaAs/AlGaAs quantum well solar cell (QWSC) at different temperatures. The photocurrent of the QWSC decreases from the short-circuit level with increasing forward bias, resulting in a low fill factor of the cell. The photocurrent reduction is attributed to the carrier confinement in the QW, which is investigated in detail by changing the temperature and excitation wavelength. We observe two reduction steps in the photocurrent at low temperature, and find that the different hole and electron tunneling rates are responsible for the two reductions.
103(2013); http://dx.doi.org/10.1063/1.4817248View Description Hide Description
We introduce a sensitive method for laser based standoff detection of chemicals based on stimulated Raman scattering. Selective excitation of a particular Raman transition is detected by measuring the diffusely reflected laser light from a distant surface. The method simultaneously measures stimulated Raman loss and gain within a single laser shot and is insensitive to the optical properties (reflectivity/absorptivity) of the substrate. We demonstrate the specificity and sensitivity by detecting and imaging nanogram analyte micro-crystals on paper, fabric, and plastic substrates at 1 to 10 m standoff distance using only 10 mW of laser power from a single femtosecond laser.
Time-resolved measurement of pulse-to-pulse heating effects in a terahertz quantum cascade laser using an NbN superconducting detector103(2013); http://dx.doi.org/10.1063/1.4818584View Description Hide Description
Joule heating causes significant degradation in the power emitted from terahertz-frequency quantum-cascade lasers (THz QCLs). However, to date, it has not been possible to characterize the thermal equilibration time of these devices, since THz power degradation over sub-millisecond time-scales cannot be resolved using conventional bolometric or pyroelectric detectors. In this letter, we use a superconducting antenna-coupled niobium nitride detector to measure the emission from a THz QCL with a nanosecond-scale time-resolution. The emitted THz power is shown to decay more rapidly at higher heat-sink temperatures, and in steady-state the power reduces as the repetition rate of the driving pulses increases. The pulse-to-pulse variation in active-region temperature is inferred by comparing the THz signals with those obtained from low duty-cycle measurements. A thermal resistance of 8.2 ± 0.6 K/W is determined, which is in good agreement with earlier measurements, and we calculate a 370 ± 90-μs bulk heat-storage time, which corresponds to the simulated heat capacity of the device substrate.