Volume 104, Issue 8, 24 February 2014
- 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:
Deformable electronics have found various applications and elastomeric materials have been widely used to reach flexibility and stretchability. In this Letter, we report an alternative approach to enable deformability through origami. In this approach, the deformability is achieved through folding and unfolding at the creases while the functional devices do not experience strain. We have demonstrated an example of origami-enabled silicon solar cells and showed that this solar cell can reach up to 644% areal compactness while maintaining reasonable good performance upon cyclic folding/unfolding. This approach opens an alternative direction of producing flexible, stretchable, and deformable electronics.
- PHOTONICS AND OPTOELECTRONICS
104(2014); http://dx.doi.org/10.1063/1.4866342View Description Hide Description
Incandescent radiation sources are widely used as mid-infrared emitters owing to the lack of alternative for compact and low cost sources. A drawback of miniature hot systems such as membranes is their low efficiency, e.g., for battery powered systems. For targeted narrow-band applications such as gas spectroscopy, the efficiency is even lower. In this paper, we introduce design rules valid for very generic membranes demonstrating that their energy efficiency for use as incandescent infrared sources can be increased by two orders of magnitude.
104(2014); http://dx.doi.org/10.1063/1.4866343View Description Hide Description
The attention towards light-emitting diode (LED) structures based on nanowires, surface plasmon coupled LEDs, and large-area high-power LEDs has been increasing for their potential in increasing the optical output power and efficiency of LEDs. In this work we demonstrate an alternative way to inject charge carriers into the active region of an LED, which is based on completely different current transport mechanism compared to conventional current injection approaches. The demonstrated structure is expected to help overcoming some of the challenges related to current injection with conventional structures. A functioning III-nitride diffusion injected light-emitting diode structure, in which the light-emitting active region is located outside the pn-junction, is realized and characterized. In this device design, the charge carriers are injected into the active region by bipolar diffusion, which could also be utilized to excite otherwise challenging to realize light-emitting structures.
104(2014); http://dx.doi.org/10.1063/1.4866606View Description Hide Description
We report on the quantum confinement of zero-dimensional polaritons in perovskite-based microcavity at room temperature. Photoluminescence of discrete polaritonic states is observed for polaritons localized in symmetric sphere-like defects which are spontaneously nucleated on the top dielectric Bragg mirror. The linewidth of these confined states is found much sharper (almost one order of magnitude) than that of photonic modes in the perovskite planar microcavity. Our results show the possibility to study organic-inorganic cavity polaritons in confined microstructure and suggest a fabrication method to realize integrated polaritonic devices operating at room temperature.
Complex photonic lattices embedded with tailored intrinsic defects by a dynamically reconfigurable single step interferometric approach104(2014); http://dx.doi.org/10.1063/1.4866660View Description Hide Description
We report sculptured diverse photonic lattices simultaneously embedded with intrinsic defects of tunable type, number, shape as well as position by a single-step dynamically reconfigurable fabrication approach based on a programmable phase spatial light modulator-assisted interference lithography. The presented results on controlled formation of intrinsic defects in periodic as well as transversely quasicrystallographic lattices, irrespective and independent of their designed lattice geometry, portray the flexibility and versatility of the approach. The defect-formation in photonic lattices is also experimentally analyzed. Further, we also demonstrate the feasibility of fabrication of such defects-embedded photonic lattices in a photoresist, aiming concrete integrated photonic applications.
104(2014); http://dx.doi.org/10.1063/1.4866662View Description Hide Description
A combination of deep level optical spectroscopy and lighted capacitance voltage profiling has been used to analyze the effect of N into the energy levels close to the valence band of Zn 0.9 Mg 0.1O. Three energy levels at EV + 0.47 eV, EV + 0.35 eV, and EV + 0.16 eV are observed in all films with concentrations in the range of 1015–1018 cm−3. The two shallowest traps at EV + 0.35 eV and EV + 0.16 eV have very large concentrations that scale with the N exposure and are thus potential acceptor levels. In order to correctly quantify the deep level concentrations, a metal-insulator-semiconductor model has been invoked, explaining well the resulting capacitance-voltage curves.
104(2014); http://dx.doi.org/10.1063/1.4866665View Description Hide Description
A graphene-based, multiband absorber operating in terahertz (THz) frequency range was demonstrated. Graphene film was transferred onto the top of a flexible polymer substrate backed with a gold reflector. The graphene acts as a resistive film that partially attenuates and reflects THz waves. The destructive interference between THz waves reflected from graphene and backside reflector gives rise to perfect absorbance at multiple frequencies. To enhance the absorbance on/off ratio (AR), the conductivity of graphene was varied using a chemical doping method. The resulting p-doped, graphene-based THz absorber exhibited absorbance at maxima and AR higher than 0.95 and 25 dB, respectively.
Coherent detection of metal-metal terahertz quantum cascade lasers with improved emission characteristics104(2014); http://dx.doi.org/10.1063/1.4866267View Description Hide Description
Coherent detection of emission from quantum cascade lasers with metal-metal waveguides is demonstrated through free-space coupling of a THz pulse to the sub-wavelength waveguide. We implement a simple, monolithic planar horn antenna design on the metal-metal waveguide that reduces the impedance mis-match to the waveguide. The resulting devices show up to 10 times more directed output power than conventional metal-metal waveguides. This enhanced coupling to free-space allows a more efficient injection of broad-band THz pulses into the waveguide. Through this, we are able to seed the laser emission and coherently detect the laser emission by electro-optic sampling.
104(2014); http://dx.doi.org/10.1063/1.4866582View Description Hide Description
We present a free-running single photon detector for telecom wavelengths based on a negative feedback avalanche photodiode (NFAD). A dark count rate as low as 1 cps was obtained at a detection efficiency of 10%, with an afterpulse probability of 2.2% for 20 μs of deadtime. This was achieved by using an active hold-off circuit and cooling the NFAD with a free-piston stirling cooler down to temperatures of −110 °C. We integrated two detectors into a practical, 625 MHz clocked quantum key distribution system. Stable, real-time key distribution in the presence of 30 dB channel loss was possible, yielding a secret key rate of 350 bps.
104(2014); http://dx.doi.org/10.1063/1.4866668View Description Hide Description
We report a study of refractive index of a wurtzite ZnO single crystal microwire at a temperature range from room temperature to about 400 K using optical cavity modes. The photoluminescence (PL) spectra of the ZnO microwire at different temperatures were performed using a confocal micro-photoluminescence setup. The whispering gallery modes observed in the PL spectra show a redshift both in the ultraviolet and the visible range as the temperature rises. The redshift is used to extract the refractive index of the ZnO microwire. The dispersion relations are deduced at different temperatures, and the results show that the refractive index increases with raising temperature for both transverse electric and transverse magnetic modes. The refractive index increases faster at a shorter wavelength, which is due to the fact that the shorter wavelength is closer to the resonance frequencies of ZnO microwire according to the Lorentz oscillator model.
104(2014); http://dx.doi.org/10.1063/1.4866669View Description Hide Description
The effect of a low refractive index film on plasmonic scattering from individual silver nanoparticles and nanowires is observed experimentally in Fourier space. The addition of the film over particles on a glass substrate was found to have a strong effect on the scattering pattern, due to interference between directly scattered light and light reflected from the interfaces, which depends strongly on the film thickness. Nanowires are shown to produce a much more directional scattering pattern than spherical particles, and the addition of the film above the nanowires leads to further directionality enhancements with light scattering at well-defined angles into the glass substrate. The use of the dielectric environment and particle shape to tune the directional scattering is demonstrated. These results are of interest for applications such as optical nanoantennae and plasmonically enhanced solar cells.
104(2014); http://dx.doi.org/10.1063/1.4866792View Description Hide Description
A simple and low-loss metal/semiconductor surface plasmon polariton (SPP) device consisting of a SPP waveguide and a detector is studied theoretically and experimentally. We demonstrate a simple diffraction structure (a metal grating) where the SPP couples from the waveguide to the detector. The SPP can propagate without large losses at the air/Au interface, and this interface was used for SPP waveguiding. To convert the SPP into an electric signal using internal photoemission, the propagating SPP is coupled into the Au/Si interface by the diffraction structure. The propagation direction of the coupled SPP at the Au/Si interface depends on the slit pitch of the diffraction structure, and the direction can be controlled by adjusting the pitch. The slit pitch is also modeled using a diffraction grating equation, and the results show good agreement with those of simulations using the finite-difference time-domain method. When diffraction structures consisting of a multi-slit structure and a disk array are placed at the end of the waveguide, SPP coupling into the Au/Si interface is also observed. The photocurrents detected at the Au/Si interface are much larger when compared with that detected for the device without the diffraction structure (26 times for the multi-slit structure and 10 times for the disk array). From the polarization angle dependence of the detected photocurrent, we also confirmed that the photocurrent was caused by the SPP propagating at the air/Au interface.
104(2014); http://dx.doi.org/10.1063/1.4866801View Description Hide Description
We have investigated the quality factor (Q-factor) of the band edge modes in the plasmonic crystal by a cathodoluminescence technique. We have found that the Q-factor at the Γ point depends on the terrace width (D)/period (P) ratio of the plasmonic crystal. The finite-difference time-domain methods predict that the band edge mode at D/P = 3/4 has a high-Q-factor (Q ∼ 250 by Palik's permittivity data and Q ∼ 530 by Johnson and Christy's data). The beam-scan spectral images allowed us to visualize the standing surface plasmon polariton waves at the band edge energies, and a high-Q-factor of ∼200 was observed at D/P ∼ 3/4.
104(2014); http://dx.doi.org/10.1063/1.4866805View Description Hide Description
We report on the fabrication of gallium arsenide (GaAs)/air distributed Bragg reflector microresonators with indium gallium arsenide quantum wells. The structures are studied via momentum resolved photoluminescence spectroscopy which allows us to investigate a pronounced optical mode quantization of the photonic dispersion. We can extract a length parameter from these quantized states whose upper limit can be connected to the lateral physical extension of the microcavity via analytical calculations. Laser emission from our microcavity under optical pumping is observed in power dependent investigations.
104(2014); http://dx.doi.org/10.1063/1.4866864View Description Hide Description
By combining the antireflective properties from gradual changes in the effective refractive index and cavity coupling from cone gratings and the efficient optical behavior of a tungsten film, a flexible filter showing very broad antireflective properties from the visible to short wavelength infrared region and, simultaneously, a mirror-like behavior in the mid-infrared wavelength region and long-infrared wavelength region has been conceived. Nanoimprint technology has permitted the replication of inverted cone patterns on a large scale on a flexible polymer, afterwards coated with a thin tungsten film. This optical metafilter is of great interest in the stealth domain where optical signature reduction from the optical to short wavelength infrared region is an important matter. As it also acts as selective thermal emitter offering a good solar-absorption/infrared-emissivity ratio, interests are found as well for solar heating applications.
104(2014); http://dx.doi.org/10.1063/1.4866870View Description Hide Description
The two-surface-plasmon-polariton-absorption (TSPPA) at the vacuum wavelength of 400 nm is observed, and the subwavelength lithography, by using this nonlinear phenomenon, is demonstrated. Resist patterns with the period of ∼138 nm have been obtained by exciting the SPP at the Al/resist interface with the 400 nm femtosecond laser. By altering the exposure time, the exposure linewidth reduces from ∼90 nm to ∼55 nm, which explores the ability of the TSPPA based lithography at the short wavelength. The factors limiting the performance of the proposed TSPPA based lithography are discussed in detail.
λ 3/20000 plasmonic nanocavities with multispectral ultra-narrowband absorption for high-quality sensing104(2014); http://dx.doi.org/10.1063/1.4867028View Description Hide Description
We present a method to theoretically achieve multispectral ultra-narrowband near-unity absorption in metal structure with deep-subwavelength plasmonic nanocavities (i.e., volumes < λ 3/20 000) for high-quality sensing. The concept is based on the combined excitation of multiple plasmon resonances and high optical field confinement by an array of hexagonal non-close-packed cross-shape nanocavities on an opaque metal film. A multispectral plasmonic nano-sensor with high refractive index sensitivity (538 nm/RIU) and figure of merit (58) in the optical region is obtained with the minimum bandwidth of 8 nm, which offers new perspectives for achieving ultra-compact efficient biosensors.
104(2014); http://dx.doi.org/10.1063/1.4866778View Description Hide Description
Data are presented to quantify the effect of the conduction band energy barrier (ΔEC,B) in the base region of the transistor laser on the minority carrier transport dynamics, recombination lifetime in the base region, and frequency response of the device. A greater ΔEC,B results in lower transistor current gain (β) and higher optical output power, indicating increased carrier confinement and recombination in the base. For a device with ΔEC,B = 41 meV, the measured bias-dependent optical frequency response and subsequent data fitting yield a short recombination lifetime of 30 ps in the base and a small resonance peak of 1.5 dB. A device with ΔEC,B = 82 meV exhibits a longer recombination lifetime of 70 ps and a larger resonance peak of 4 dB.
104(2014); http://dx.doi.org/10.1063/1.4866868View Description Hide Description
One salient characteristic of Quantum Cascade Laser (QCL) is its very short τ ∼ 1 ps gain recovery time that so far thwarted the attempts to achieve self-mode locking of the device into a train of single pulses. We show theoretically that four wave mixing, combined with the short gain recovery time causes QCL to operate in the self-frequency-modulated regime characterized by a constant power in time domain and stable coherent comb in the frequency domain. Coherent frequency comb may enable many potential applications of QCL's in sensing and measurement.
104(2014); http://dx.doi.org/10.1063/1.4867092View Description Hide Description
Light trapping is crucial for thin film silicon solar cells and is often achieved by expensive, clean-room intensive nano-patterning methods. In this work, nanostructured silicon thin films were realized by direct deposition on substrates with nanoscale features prepared by simple and scalable fabrication based on electrochemical methods and wet etching. A broadband and angle-insensitive absorption enhancement of thin film silicon was observed. The photocurrent density shows improvements up to ∼40% in the AM 1.5 spectrum when comparing to the same thin film silicon deposited on smooth substrates. This approach not only demonstrates the advantage of simple fabrication but also has potential for ultra thin film photovoltaic applications.
104(2014); http://dx.doi.org/10.1063/1.4867242View Description Hide Description
We have fabricated a GaAs-based triangular-barrier photodiode, in which self-assembled InGaAs quantum rods (Q-rods) are embedded in its barrier region. Transport study at 100 K has shown that electrons start to flow mainly through Q-rods when a bias is set above a threshold. Upon illumination, photo-generated holes are found to accumulate in the middle portion of Q-rods and efficiently lower the local barrier height, yielding the responsivity as high as 105 A/W at the incident light of 1 fW.