Volume 103, Issue 20, 11 November 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:
The Leidenfrost effect is undesirable in cooling applications as the vapor layer on which the liquid levitates acts as a heat transfer barrier. Here, we report on increasing the Leidenfrost temperature by surface textures that can promote droplet wetting at high superheat via capillary wicking. Counterintuitively, we find that sparser rather than denser textures increase the Leidenfrost temperature. Our experimental results are consistent with a physical model that balances capillary wetting pressures with dewetting pressures exerted by the escaping vapor. The physical mechanism suggests that hierarchical textures have a higher Leidenfrost temperature compared to single-length-scale textures, which is confirmed experimentally.
- PHOTONICS AND OPTOELECTRONICS
103(2013); http://dx.doi.org/10.1063/1.4829742View Description Hide Description
A scalable and robust methodology for writing cycloidal modulation patterns of optical axis orientation in photosensitive surface alignment layers is demonstrated. Counterpropagating circularly polarized beams, generated by reflection of the input beam from a cholesteric liquid crystal, direct local surface orientation in a photosensitive surface. Purposely introducing a slight angle between the input beam and the photosensitive surface normal introduces a grating period/orientation that is readily controlled and templated. The resulting cycloidal diffractive waveplates offer utility in technologies requiring diffraction over a broad range of angles/wavelengths. This simple methodology of forming polarization gratings offers advantages over conventional fabrication techniques.
103(2013); http://dx.doi.org/10.1063/1.4829743View Description Hide Description
Photonic molecules (PMs) based on multiple inner coupled microring resonators allow to surpass the fundamental constraint between the total quality factor (QT), free spectral range (FSR), and resonator size. In this work, we use a PM that presents doublets and triplets resonance splitting, all with high QT. We demonstrate the use of the doublet splitting for 34.2 GHz signal extraction by filtering the sidebands of a modulated optical signal. We also demonstrate that very compact optical modulators operating 2.75 times beyond its resonator linewidth limit may be obtained using the PM triplet splitting, with separation of ∼55 GHz.
103(2013); http://dx.doi.org/10.1063/1.4829756View Description Hide Description
We report on the fabrication of graphene contact to GaN nanowire ensemble and on the demonstration of photodetectors using chemical vapor deposition-grown few-layered graphene as a transparent electrode. The optimization of the transfer method allowed to form a continuous contact to the nanowires over a large area. The adhesion energy of the graphene sheet to the nanowire ensemble is estimated to be 0.3–0.7 J/m2. Ultraviolet photodetectors with a room-temperature responsivity of ∼25 A/W at 357 nm were fabricated. The photocurrent spectrum shows that the device has a strong response up to 4.15 eV confirming a good transparency of the top graphene contact.
103(2013); http://dx.doi.org/10.1063/1.4830006View Description Hide Description
Coupled surface plasmon modes of graphene on a plasma layer are studied. Three coupled plasmon modes are found, and analytical equations for their dispersion characteristics are given in the long-wavelength limit ( ). Symmetric and antisymmetric modes are identified; between two strongly coupled modes, the antisymmetric mode is found to be more energetic than the symmetric mode. The great tunability of plasmon characteristics is shown by changing the surrounding dielectric constant, the coupling distance, and the plasma frequency of the substrate.
103(2013); http://dx.doi.org/10.1063/1.4830007View Description Hide Description
We propose an all-optical nonlinear router based on a double barrier gate connected to periodically modulated guides. A semiconductor microcavity is driven nonresonantly in-between the barriers to form an exciton-polariton condensate on a discrete state that is subject to the exciton blueshift. The subsequent coherent optical signal is allowed to propagate through a guide provided that the condensate energy is resonant with a miniband or is blocked if it faces a gap. While a symmetric sample operates as an optical switch, its asymmetric counterpart embodies a router turned to be polarization selective under applied magnetic field.
103(2013); http://dx.doi.org/10.1063/1.4830092View Description Hide Description
We report on the ultrastrong-coupling between localized plasmons of a planar terahertz metamaterial and intersubband plasmons in a modulation doped quantum well sample. Such a system exhibits the formation of a lower and an upper polariton branch when the metamaterial eigenfrequency is tuned close to resonance with the intersubband transition. We achieve a normalized polariton splitting of 22% and a polaritonic gap of 2.4% of the intersubband transition frequency. In addition to the usual geometrical scaling, we demonstrate the effective tuning of the metamaterial resonance by dry etching with a tuning range of more than 1 THz.
103(2013); http://dx.doi.org/10.1063/1.4830360View Description Hide Description
It is shown from accurate on-wafer measurement that a low-temperature-grown GaAs photoconductor using a metallic mirror Fabry-Perot cavity can serve as highly efficient optoelectronic heterodyne mixer in the terahertz frequency range. Conversion losses of 22 dB at 100 GHz and ∼27 dB at 300 GHz were measured, which is an improvement by a factor of about 40 dB as compared with the previous values obtained with photoconductors. Experimental results are interpreted satisfactorily by means of a simple electrical model of the optoelectronic mixing process.
103(2013); http://dx.doi.org/10.1063/1.4830370View Description Hide Description
We investigate characteristics of THz waves radiated from differently doped graphite samples excited by femtosecond laser pulses. Between n-type single-crystalline graphite and p-type polycrystalline graphite films, we observe the phase reversal of THz waves regardless of excitation energy variations around K-valley. In addition, variations in other parameters such as excitation fluence and azimuthal angle produce no changes in the phase of THz waves, which correlate well with the opposite dipole polarization between differently doped samples rather than unidirectional diffusive transport.
103(2013); http://dx.doi.org/10.1063/1.4831768View Description Hide Description
The concept of loss-compensated broadband epsilon-near-zero metamaterials consisting of step-like metal-dielectric multilayer structures doped with gain media is proposed based on the combination of the Milton representation of the effective permittivity and the optical nonlocality due to the metal-dielectric multilayer structures. With the loss compensation by gain media, broadband epsilon-near-zero metamaterials possesses significantly low material loss in optical frequency range, leading to superior broadband electromagnetic properties for realizing unique functional optical devices, such as the demonstrated prisms for broadband directional emission and S-shaped lenses for phase front shaping.
103(2013); http://dx.doi.org/10.1063/1.4829745View Description Hide Description
The quantum efficiencies of both the band edge and deep-level defect emission from annealed ZnO powders were measured as a function of excitation fluence and wavelength from a tunable sub-picosecond source. A simple model of excitonic decay reproduces the observed excitation dependence of rate constants and associated trap densities for all radiative and nonradiative processes. The analysis explores how phosphor performance deteriorates as excitation fluence and energy increase, provides an all-optical approach for estimating the number density of defects responsible for deep-level emission, and yields new insights for designing efficient ZnO-based phosphors.
103(2013); http://dx.doi.org/10.1063/1.4830417View Description Hide Description
The performances of quartz tuning forks (QTF) used in infrared spectroscopy for pollutant detection are investigated. The transduction between light and QTF vibration is elucidated, thanks to QTF encapsulation under vacuum. From the sensitivity enhancement which is obtained, we conclude that their interaction is photo-thermoelastic rather than photo-thermoacoustic. A mapping of the local sensitivity of the QTF is obtained by scanning its faces with the excitation probe beam. The comparison between the signal mapping and the theoretical strain mapping indicates that the most efficient areas of the QTF correspond to the areas where the strain or stress is the highest.
GaInN light-emitting diodes using separate epitaxial growth for the p-type region to attain polarization-inverted electron-blocking layer, reduced electron leakage, and improved hole injection103(2013); http://dx.doi.org/10.1063/1.4829576View Description Hide Description
A GaInN light-emitting diode (LED) structure is analyzed that employs a separate epitaxial growth for the p-type region, i.e., the AlGaN electron-blocking layer (EBL) and p-type GaN cladding layer, followed by wafer or chip bonding. Such LED structure has a polarization-inverted EBL and allows for uncompromised epitaxial-growth optimization of the p-type region, i.e., without the need to consider degradation of the quantum-well active region during p-type region growth. Simulations show that such an LED structure reduces electron leakage, reduces the efficiency droop, improves hole injection, and has the potential to extend high efficiencies into the green spectral region.
103(2013); http://dx.doi.org/10.1063/1.4831797View Description Hide Description
We applied a recent electromagnetic model to design the resonator-quantum well infrared photodetector (R-QWIP). In this design, we used an array of rings as diffractive elements to diffract normal incident light into parallel propagation and used the pixel volume as a resonator to intensify the diffracted light. With a proper pixel size, the detector resonates at certain optical wavelengths and thus yields a high quantum efficiency (QE). To test this detector concept, we fabricated a number of R-QWIPs with different quantum well materials and detector geometries. The experimental result agrees satisfactorily with the prediction, and the highest QE achieved is 71%.
103(2013); http://dx.doi.org/10.1063/1.4830377View Description Hide Description
Mechanically stressed nanomembranes are used to demonstrate mid-infrared interband light emission from Ge within the 2.1–2.5 μm atmospheric transmission window. Large biaxial tensile strain is introduced in these samples to convert Ge into a (near-) direct-bandgap semiconductor and to red-shift its luminescence. A diffractive array of Ge pillars is used to outcouple the long-wavelength interband radiation, which is otherwise primarily emitted in the sample plane. An order-of-magnitude strain-induced enhancement in radiative efficiency is also reported, together with the observation of luminescence signatures associated with photonic-crystal cavity modes. These results are promising for the development of silicon-compatible lasers for mid-infrared optoelectronics applications.
103(2013); http://dx.doi.org/10.1063/1.4830432View Description Hide Description
Coherently coupled vertical-cavity surface-emitting laser arrays offer unique advantages for nonmechanical beam steering applications. We have applied dynamic coupled mode theory to show that the observed temporal phase shift between vertical-cavity surface-emitting array elements is caused by the detuning of their resonant wavelengths. Hence, a complete theoretical connection between the differential current injection into array elements and the beam steering direction has been established. It is found to be a fundamentally unique beam-steering mechanism with distinct advantages in efficiency, compactness, speed, and phase-sensitivity to current.
In-situ characterization of femtosecond laser-induced crystallization in borosilicate glass using time-resolved surface third-harmonic generation103(2013); http://dx.doi.org/10.1063/1.4831655View Description Hide Description
Coherent phonon dynamics in condensed-phase medium are responsible for important material properties including thermal and electrical conductivities. We report a structural dynamics technique, time-resolved surface third-harmonic generation (TRSTHG) spectroscopy, to capture transient phonon propagation near the surface of polycrystalline CaF2 and amorphous borosilicate (BK7) glass. Our approach time-resolves the background-free, high-sensitivity third harmonic generation (THG) signal in between the two crossing near-IR pulses. Pronounced intensity quantum beats reveal the impulsively excited low-frequency Raman mode evolution on the femtosecond to picosecond timescale. After amplified laser irradiation, danburite-crystal-like structure units form at the glass surface. This versatile TRSTHG setup paves the way to mechanistically study and design advanced thermoelectrics and photovoltaics.
103(2013); http://dx.doi.org/10.1063/1.4831984View Description Hide Description
Second-order nonlinear optical properties of sputter-deposited Ge-doped SiO2 thin films were investigated. It was shown that the second-order nonlinearity of SiO2, which vanishes in the electric-dipole approximation due to the centrosymmetric structure, can be significantly enhanced by Ge doping. The observed maximum value of d 33 was 8.2 pm/V, which is 4 times larger than d 22 of β-BaB2O4 crystal. Strong correlation was observed between the d eff values and the electron spin resonance signals arising from GePb centers, suggesting that GePb centers are the most probable origin of the large second-order nonlinearity.
Fabrication and optical properties of non-polar III-nitride air-gap distributed Bragg reflector microcavities103(2013); http://dx.doi.org/10.1063/1.4832069View Description Hide Description
Using the thermal decomposition technique, non-polar III-nitride air-gap distributed Bragg reflector (DBR) microcavities (MCs) with a single quantum well have been fabricated. Atomic force microscopy reveals a locally smooth DBR surface, and room-temperature micro-photoluminescence measurements show cavity modes. There are two modes per cavity due to optical birefringence in the non-polar MCs, and a systematic cavity mode shift with cavity thickness was also observed. Although the structures consist of only 3 periods (top) and 4 periods (bottom), a quality factor of 1600 (very close to the theoretical value of 2100) reveals the high quality of the air-gap DBR MCs.
103(2013); http://dx.doi.org/10.1063/1.4832070View Description Hide Description
The authors propose and numerically demonstrate an ultra-high resolution (wavelength/50∼40 nm at wavelength ), high-throughput (∼66%), and non-destructive optical lens with a large contrast-to-noise ratio, based on the notion of correlated nano-torches formed in a subwavelength metallic grating. The correlations between the torches also allow the determination of the complex refractive index of the sample.
Lead-chalcogenide mid-infrared vertical external cavity surface emitting lasers with improved threshold: Theory and experiment103(2013); http://dx.doi.org/10.1063/1.4830276View Description Hide Description
Mid-infrared Vertical External Cavity Surface Emitting Lasers (VECSEL) based on narrow gap lead-chalcogenide (IV-VI) semiconductors exhibit strongly reduced threshold powers if the active layers are structured laterally for improved optical confinement. This is predicted by 3-d optical calculations; they show that lateral optical confinement is needed to counteract the anti-guiding features of IV-VIs due to their negative temperature dependence of the refractive index. An experimental proof is performed with PbSe quantum well based VECSEL grown on a Si-substrate by molecular beam epitaxy and emitting around 3.3 μm. With proper mesa-etching, the threshold intensity is about 8-times reduced.