Volume 102, Issue 3, 21 January 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:
An approach for an all lattice-matched multijunction solar cell optimized design is presented with 5.807 Å lattice constant, together with a detailed analysis of its performance by means of full device modeling. The simulations show that a (1.93 eV)In0.37 Al 0.63As/(1.39 eV)In0.38Ga0.62As0.57P0.43/(0.94 eV)In0.38Ga0.62As 3-junction solar cell can achieve efficiencies >51% under 100-suns illumination (with Voc = 3.34 V). As a key proof of concept, an equivalent 3-junction solar cell lattice-matched to InP was fabricated and tested. The independently connected single junction solar cells were also tested in a spectrum splitting configuration, showing similar performance to a monolithic tandem device, with Voc = 1.8 V.
- PHOTONICS AND OPTOELECTRONICS
Quantifying the intrinsic amount of fabrication disorder in photonic-crystal waveguides from optical far-field intensity measurements102(2013); http://dx.doi.org/10.1063/1.4788709View Description Hide Description
Residual disorder due to fabrication imperfections has important impact in nanophotonics where it may degrade device performance by increasing radiation loss or spontaneously trap light by Anderson localization. We propose and demonstrate experimentally a method of quantifying the intrinsic amount of disorder in state-of-the-art photonic-crystal waveguides from far-field measurements of the Anderson-localized modes. This is achieved by comparing the spectral range where Anderson localization is observed to numerical simulations, and the method offers sensitivity down to .
102(2013); http://dx.doi.org/10.1063/1.4788753View Description Hide Description
The optical response of a heavily doped quantum well, with two occupied subbands, has been investigated as a function of the electronic density. It is shown that the two optically active transitions are mutually coupled by dipole-dipole Coulomb interaction, which strongly renormalizes their absorption amplitude. In order to demonstrate this effect, we have measured a set of optical spectra on a device in which the electronic density can be tuned by the application of a gate voltage. Our results show that the absorption spectra can be correctly described only by taking into account the Coulomb coupling between the two transitions. As a consequence, the optical dipoles originating from intersubband transitions are not independent, but rather coupled oscillators with an adjustable strength.
102(2013); http://dx.doi.org/10.1063/1.4788754View Description Hide Description
Except for the commonly used surface plasmon polaritons (SPPs), in this letter we demonstrate that waveguide modes (WMs) can realize the large area sub-wavelength gratings. Both transverse-magnetic (TM) and transverse-electric polarized beams can be used in this method, while for the lithography based on SPPs only TM polarized beam is applicable. The WMs interference lithography has the advantages of low heat loss and much suitable for thick photo-resist films. Large area gratings were inscribed on the azo polymer film at period of 187 nm and 189 nm, which are smaller than the half wavelength of the incident beam.
102(2013); http://dx.doi.org/10.1063/1.4788929View Description Hide Description
White emission produced by Ti-doped MgAl2O4 phosphor powder is reported, which is in contrast to blue emission from most Ti-doped single crystals of MgAl2O4. The white emission peak consists of four deconvoluted peaks: 440, 490, 550, and 620 nm, when was excited by 260 nm wavelength. Ti4+ in octahedral sites was found to contribute mostly to greenish blue emissions at 490 and 550 nm. The red emission at 620 nm was produced by abundant Mg 2+ and O2− vacancies in the spinel powder.
Silicon sub-bandgap photon linear detection in two-photon experiments: A photo-assisted Shockley-Read-Hall mechanism102(2013); http://dx.doi.org/10.1063/1.4788705View Description Hide Description
We investigate the linear response of silicon p-i-n diodes to sub-bandgap photons (1.4 μm-1.6 μm) that has been reported by many authors and left unexplored till then. The quantum efficiency of this mechanism is extremely low (typically 10−9) but has a drastic influence on silicon devices harnessing two-photon absorption. We show that this linear photonic current decreases with temperature, displaying an activation energy similar to the dark current one. We show that this behaviour is consistent with a photo-assisted Shockley-Read mechanism in which the occupancy factor of a defect state in the Si band gap is influenced by the sub-band gap photon flux.
102(2013); http://dx.doi.org/10.1063/1.4788750View Description Hide Description
A coupled optoelectronic simulation was used to theoretically assess the performance of core-shell GaInP nanowires array (NWA)/Si thin-film two-junction solar cell. Outstanding light harvesting rooted from the strong light trapping and the formation of Fabry-Pérot optical cavity in the NWA enables the cell to produce high photocurrent. From the detailed balance calculation, above 32% of efficiency could be obtained at the current matching geometries. Coupled optoelectronic simulation shows that current matching can be easily achieved even under relatively high recombination rate. A promising efficiency of 27.1% is obtained for the proposed cell with appropriate nanowires surface passivation.
102(2013); http://dx.doi.org/10.1063/1.4788751View Description Hide Description
Dye-doped hemispherical resonators are self-assembled on a distributed Bragg reflector based on hydrophobic effect. The size of hemispheres can be well-controlled with diameters ranging from 5 to 150 μm. Upon optical pumping, whispering gallery mode laser emission with transverse magnetic polarization is observed from the hemispheres. Application of the microlasers as refractive index gas sensors has been demonstrated by detecting the spectrum shift of the lasing mode, and the sensitivity higher than 130 nm/RIU is achieved. Our approach provides an effective technique to obtain high quality microlasers and opens an opportunity to employ the cost effective microlasers as high sensitive sensors.
102(2013); http://dx.doi.org/10.1063/1.4774385View Description Hide Description
Super-oscillatory optical lenses have recently been shown to achieve subwavelength focusing and have been used for super-resolution imaging. However, the subwavelength hotspots created by these lenses are always accompanied by sidebands containing a significant fraction of the optical energy and are highly localised in the axial direction. Here, we report a class of super-oscillatory lenses that form extended subwavelength optical needles on a 15λ field of view.
Experimental determination of electron-hole pair creation energy in 4H-SiC epitaxial layer: An absolute calibration approach102(2013); http://dx.doi.org/10.1063/1.4776703View Description Hide Description
Electron-hole pair creation energy (ε) has been determined from alpha spectroscopy using 4H-SiC epitaxial layer Schottky detectors and a pulser calibration technique. We report an experimentally obtained ε value of 7.28 eV in 4H-SiC. The obtained ε value and theoretical models were used to calculate a Fano factor of 0.128 for 5.48 MeV alpha particles. The contributions of different factors to the ultimate alpha peak broadening in pulse-height spectra were determined using the calculated ε value and Monte-Carlo simulations. The determined ε value was verified using a drift-diffusion model of variation of charge collection efficiency with applied bias.
102(2013); http://dx.doi.org/10.1063/1.4788752View Description Hide Description
We report photochromism (PC) in Sr2SnO4:Eu3+ with layered perovskite-related structure. The Sr2SnO4:Eu3+ turned purple upon irradiation with UV light (λ < 350 nm), and the colored Sr2SnO4:Eu3+ returned to its initial colorless state when visible light (λ = 400–700 nm) was irradiated. Furthermore, the PC was strongly dependent on the firing temperature; purple color upon UV irradiation can be enhanced by increasing the firing temperature, which was attributed to an increase of the Sr vacancies in the host lattice from the results of crystal structure analysis. This suggests that controlling the lattice defect plays an important role for enhancing the PC performance.
102(2013); http://dx.doi.org/10.1063/1.4788915View Description Hide Description
This work reports the design, microfabrication, and characterization of highly reflective electrostatic mirrors with sub-wavelength holes for scanning terahertz (THz) waves. The mirror consists of an aluminum coated silicon mirror plate precisely assembled on the top of two axis torsional microactuators. The mirror plate with sub-wavelength microholes not only provides high reflectivity over 98% at THz waves by decoupling the surface plasmon resonance but also reduces air damping by allowing air to flow through the mirror plate during the mirror scanning. The device can provide many opportunities for miniaturized THz time domain spectroscopic imaging systems.
102(2013); http://dx.doi.org/10.1063/1.4789366View Description Hide Description
We present a simple method to generate spectrally uniform wideband chaos by injecting chaotic laser into a fiber ring resonator. The resonator is a single-coupler ring equipped with an optical filter and amplifier, which adjust the optical field circulating in the ring. The incoherent interference of the circulating fields produces wideband chaos with uniform power spectrum density distribution. We experimentally achieved a chaotic spectrum that extends over 26.5 GHz (limited by measurement bandwidth) and fluctuates within ±1.5 dB. In addition, tuning the filter frequency can control the spectral profile so as to meet different application needs.
102(2013); http://dx.doi.org/10.1063/1.4789370View Description Hide Description
The thermal expansion mismatch of thermal grown silica on a silicon wafer is well known to induce compressive stress upon cooling from the growth temperature to room temperature. In this Letter, we investigate how this stress impacts silica disk structures by comparison of measurements with both a finite element and an analytical model. The disk structures studied are also whispering gallery optical resonators, and proper control of stress is critical to obtain high-Q resonances. Based on our analysis, thicker oxide layers and proper control of undercut enable ultra-high-Q optical performance and mechanical stability.
102(2013); http://dx.doi.org/10.1063/1.4789393View Description Hide Description
We demonstrate two-photon-absorption photodiodes in Si photonic-crystal waveguides, which shows wideband low-dispersion slow light. The device was fabricated on SOI substrate by CMOS-compatible process. The responsivity was improved by higher group indexes of slow light up to 0.052 A/W for pulses at wavelengths around 1550 nm with a 2.7 ps width and sub-watt peak powers. We applied this device to an optical correlator and dispersion detector. In the former, the correlation waveforms of 0.7−10 ps pulses were observed with small errors. In the latter, photocurrents inversely proportional to the pulse width were detected.
Impact of intermediate localized states on nonlinear optical absorption of Ga-Ge-Se nanocolloidal solutions102(2013); http://dx.doi.org/10.1063/1.4789436View Description Hide Description
We present the linear and nonlinear optical studies on nanocolloidal solutions of Ga9Ge27Se64 glass with varying concentrations. Optical bandgap of the material is found to vary with respect to the concentration of the solute in the solution. An intermediate peak in the band tail of the absorption spectra is observed due to the presence of energy band in the forbidden gap. The existence of fluorescence emission confirms the above argument. Nonlinear absorption is studied using open aperture Z-scan technique. The mechanism behind nonlinear absorption is predicted as two photon as well as two step photon absorption. Nonlinearity increases with decrease in optical bandgap which in turn depends on the concentration of the nanocolloidal solutions.
Improved photovoltaic effects in InGaN-based multiple quantum well solar cell with graphene on indium tin oxide nanodot nodes for transparent and current spreading electrode102(2013); http://dx.doi.org/10.1063/1.4789502View Description Hide Description
We implemented graphene network on indium tin oxide (ITO) nanodots to a transparent and current-spreading electrode in InGaN-based solar cell to improve power conversion efficiency. The external quantum efficiency and short circuit current density (Jsc) of solar cells with graphene network on ITO nanodots were enhanced compared to those of solar cells with ITO and bare graphene film. The increase of the power conversion efficiency is attributed to the high transmittance, internal light-scattering effect, and effective carrier absorption of ITO nanodots.
A light emitting device made from thin zirconium-doped hafnium oxide high-k dielectric film with or without an embedded nanocrystal layer102(2013); http://dx.doi.org/10.1063/1.4789531View Description Hide Description
A solid state light emitting device composed of the 10 nm thickness zirconium-doped hafnium oxide high-k gate dielectric with or without an embedded nanocrystalline ZnO layer has been fabricated and studied. The emission spectrum, which extended from visible light to IR, was broadened and the intensity was increased with the embedding of a nanocrystalline ZnO layer. The mechanisms of light emission and enhancement were investigated and explained with defect generation process in the film. This kind of device is easily prepared by the IC compatible process. There are many potential applications of this kind of device.
102(2013); http://dx.doi.org/10.1063/1.4789533View Description Hide Description
We report on slow-light GaInAs/GaAs electro-absorption modulators with a Bragg reflector waveguide. We fabricated 20∼100 μm long compact modulators composed of triple GaInAs/GaAs quantum wells sandwiched by highly reflective Bragg reflectors. A large group index of 20 enables us to reduce the size of the modulators. We demonstrated 6 dB intensity modulation with a voltage swing Vpp below 0.5 V for 50 μm long devices. Shorter devices, for example with a length of only 20 μm, also showed an extinction ratio over 4 dB for sub-volt driving. Characterizations on wavelength dependence were also carried out experimentally.
102(2013); http://dx.doi.org/10.1063/1.4789535View Description Hide Description
We demonstrate single-mode surface-emitting terahertz frequency quantum cascade lasers utilising non-uniform second-order distributed feedback concentric-circular-gratings. The grating is designed for single-mode operation and surface emission for efficient and directional optical power out-coupling. The devices exhibit single-mode operation over the entire dynamic range with a side-mode-suppression-ratio of around 30 dB at 78 K, and a six-fold rotationally symmetric far-field pattern. In addition, the devices show a peak output power approximately three times higher than in ridge-waveguide lasers of similar size, whilst maintaining similar threshold current densities for the 3.8 THz emission and without remarkably sacrificing the maximum temperature operation performance. Owing to the high symmetry of the structure and the broad area light emission from surface, the devices are potentially very suitable for use as single-mode, high power emitters for integration into two-dimensional laser arrays.