Volume 103, Issue 18, 28 October 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:
Multiphoton microscopy (MPM) has become a powerful, important tool for tissues imaging at the molecular level. In this paper, this technique was extended to histological investigations, differentiating carcinoma in situ (CIS) lesion from normal oesophagus by imaging histological sections without hematoxylin and eosin (H&E) staining. The results show that the histology procedures of dehydration, paraffin embedding, and de-paraffinizing highlighted two photon excited fluorescence of cytoplasm and nucleolus of epithelial cell and collagen in stroma. MPM has the ability to identify the characteristics of CIS lesion including changes of squamous cells and full epithelium, identification of basement membrane, especially prominent nucleolus. The studies described here show that MPM has the potential for future retrospective studies of tumor staging by employing on histological section specimens without H&E staining.
- PHOTONICS AND OPTOELECTRONICS
Detection of microscope-excited surface plasmon polaritons with Rayleigh scattering from metal nanoparticles103(2013); http://dx.doi.org/10.1063/1.4827264View Description Hide Description
We propose a mapping tool of surface plasmon polaritons (SPPs) excited using an optical microscope. By combining dark-field and confocal microscopy, we can efficiently extract metal nanoparticle-induced Rayleigh scattering from background radiation, thereby leading to state-of-the-art SPP measurements. The method is verified to be sensitive to the dominant perpendicular field component of SPPs and be of high accuracy. We also use this method to reveal the conversion of spin angular momentum of light to the orbital angular momentum of SPPs under tight-focusing conditions.
103(2013); http://dx.doi.org/10.1063/1.4827338View Description Hide Description
GaN nanowires containing AlN/GaN distributed Bragg reflector (DBR) heterostructures have been grown on (001) silicon substrate by molecular beam epitaxy. A peak reflectance of 70% with normal incidence at 560 nm is derived from angle resolved reflectance measurements on the as-grown nanowire DBR array. The measured peak reflectance wavelength is significantly blue-shifted from the ideal calculated value. The discrepancy is explained by investigating the reflectance of the nanoscale DBRs with a finite difference time domain technique. Ensemble nanowire microcavities with In0.3Ga0.7N nanowires clad by AlN/GaN DBRs have also been characterized. Room temperature emission from the microcavity exhibits considerable linewidth narrowing compared to that measured for unclad In0.3Ga0.7N nanowires. The resonant emission is characterized by a peak wavelength and linewidth of 575 nm and 39 nm, respectively.
103(2013); http://dx.doi.org/10.1063/1.4827637View Description Hide Description
An add-drop filter (ADF) fabricated using a whispering gallery mode resonator has different crosstalks for add and drop functions due to non-zero intrinsic losses of the resonator. Here, we show that introducing gain medium in the resonator and optically pumping it below the lasing threshold allows not only loss compensation to achieve similar and lower crosstalks but also tunability in bandwidth and add-drop efficiency. For an active ADF fabricated using an erbium-ytterbium co-doped microsphere, we achieved 24-fold enhancement in the intrinsic quality factor, 3.5-fold increase in drop efficiency, bandwidth tunability of 34 MHz, and a crosstalk of only 2%.
103(2013); http://dx.doi.org/10.1063/1.4826924View Description Hide Description
We demonstrate highly efficient coupling of light from an optical fiber to a silicon photonic crystal optomechanical cavity. The fiber-to-cavity coupling utilizes a compact (L ≈ 25 μm) intermediate adiabatic coupler. The optical coupling is lithographically controlled, broadband, relatively insensitive to fiber misalignment and allows for light to be transferred from an optical fiber to, in principle, any photonic chip with refractive index greater than that of the optical fiber. Here we demonstrate single-sided cavity coupling with a total fiber-to-cavity optical power coupling efficiency of 85%.
A nanoelectromechanical systems actuator driven and controlled by Q-factor attenuation of ring resonator103(2013); http://dx.doi.org/10.1063/1.4827096View Description Hide Description
In this Letter, an optical gradient force driven Nanoelectromechanical Systems (NEMS) actuator, which is controlled by the Q-factor attenuation of micro-ring resonator, is demonstrated. The actuator consists of a tunable actuation ring resonator, a sensing ring resonator, and a mechanical actuation arc. The actuation displacement can reach up to 14 nm with a measured resolution of 0.8 nm, when the Q-factor of the ring resonator is tuned from 15 × 103 to 6 × 103. The potential applications of the NEMS actuator include single molecule manipulation, nano-manipulation, and high sensitivity sensors.
Spectroscopic investigation of coupling among asymmetric InGaN/GaN multiple quantum wells grown on non-polar a-plane GaN substrates103(2013); http://dx.doi.org/10.1063/1.4827536View Description Hide Description
Low defect density asymmetric multiple quantum wells (MQWs) of InGaN/GaN grown on non-polar a-plane GaN substrates were investigated using time-integrated and time-resolved photoluminescence spectroscopy. Comparison of these spectra with the predicted emission energies reveals that these QWs may be spectrally resolved at low temperatures. However, a combination of thermal activation and resonant tunneling of carriers increasingly coupled the QWs, favoring emission from the lowest energy QWs with increasing temperature in a manner analogous to MQWs composed of other non-polar semiconductor materials but unlike most InGaN MQWs grown on polar substrates and influenced by the strong polarization-dependent effects.
103(2013); http://dx.doi.org/10.1063/1.4827538View Description Hide Description
We report on a wide-range efficient method for optical second harmonic generation based on a whispering gallery mode resonator made from crystalline beta barium borate. In this single resonator, we were able to generate second harmonic fields for four different pump wavelengths ranging from the infrared (1557 nm) to the visible (634 nm) regime. The highest conversion efficiencies achieved in this whispering gallery mode resonator are as high as 4.6% (mW)−1. This conversion process is based on type-I phase matching with continuously varying optical axis orientation in an xy-cut configuration of the resonator. In such a geometry, the second harmonic whispering gallery mode experiences an oscillatory modulation of the refractive index. This enables wide-range cyclic phase matching along the circumference of the disk resonator.
Free-standing semipolar III-nitride quantum well structures grown on chemical vapor deposited graphene layers103(2013); http://dx.doi.org/10.1063/1.4827539View Description Hide Description
We report the synthesis and optical characterization of semipolar-oriented III-nitride quantum well (QW) structures obtained by growth on chemical vapor deposited graphene layers using metalorganic vapor phase epitaxy. Various multi-quantum well stacks of GaN(QW)/AlGaN(barrier) and InGaN (QW)/GaN (barrier) were grown. Growth on graphene not only helps achieve a semipolar orientation but also allows facile transfer of the QW multilayer stack to other cheap, flexible substrates. We demonstrate room-temperature photoluminescence from layers transferred to flexible Kapton films.
103(2013); http://dx.doi.org/10.1063/1.4827810View Description Hide Description
This work presents an optical on-a-chip humidity sensor based on the hydroscopic behavior of an infiltrated liquid into the sub-micron holes of a silicon photonic crystal. Direct measurements of the liquid refractive index in combination with numerical simulations show that the sensitivity of the device is due to changes of both the liquid's refractive index and volume. We report humidity sensing with a response time of 0.1 ms and study the stability and reversibility of the sensor. This demonstration highlights the sensitivity offered by optofluidics in photonic crystal circuits and the potential for realizing ultra-compact integrated humidity sensing components.
Band offsets and carrier dynamics of type-II InAs/GaSb superlattice photodetectors studied by internal photoemission spectroscopy103(2013); http://dx.doi.org/10.1063/1.4827881View Description Hide Description
We use internal photoemission spectroscopy to determine the conduction band offset of a type-II InAs/GaSb superlattice (T2SL) pBp photodetector to be eV at 78 K, confirming its unipolar operation. It is also found that phonon-assisted hole transport through the B-region disables its two-color detection mode around 140 K. In addition, photoemission yield shows a reduction at about an energy of longitudinal-optical phonon above the threshold, confirming carrier-phonon scattering degradation on the photoresponse. These results may indicate a pathway for optimizing T2SL detectors in addition to current efforts in material growth, processing, substrate preparation, and device passivation.
103(2013); http://dx.doi.org/10.1063/1.4827885View Description Hide Description
We built a mid-infrared time-domain spectroscopy (TDS) system optimized for the 8–12 μm spectral range based on a compact ultrafast Erbium:fiber laser that enables measurements of phase-resolved optical field transients directly in the time domain with high stability and spectral brightness. We achieved long term (>10 h) stability of the TDS signal by using a carrier-envelope-phase locking technique to reduce the timing jitter caused by environmental changes. Time domain measurements of the mid-infrared beam were achieved via electro-optic sampling in a GaSe crystal, using an ultrashort (∼15 fs) output of the fiber laser. Here, we present a full characterization of our TDS system including the dependence of the amplitude and pulse shape of the detected infrared waveforms on the thickness of the GaSe crystals.
103(2013); http://dx.doi.org/10.1063/1.4827886View Description Hide Description
We demonstrate coherent terahertz (THz) frequency imaging using the self-mixing effect in a quantum cascade laser (QCL). Self-mixing voltage waveforms are acquired at each pixel of a two-dimensional image of etched GaAs structures and fitted to a three-mirror laser model, enabling extraction of the amplitude and phase parameters of the reflected field. From the phase, we reconstruct the depth of the sample surface, and we show that the amplitude can be related to the sample reflectance. Our approach is experimentally simple and compact, and does not require frequency stabilization of the THz QCL.
AlxGa1−xN-based solar-blind ultraviolet photodetector based on lateral epitaxial overgrowth of AlN on Si substrate103(2013); http://dx.doi.org/10.1063/1.4828497View Description Hide Description
We report on Al xGa1−xN-based solar-blind ultraviolet (UV) photodetector (PD) grown on Si(111) substrate. First, Si(111) substrate is patterned, and then metalorganic chemical vapor deposition is implemented for a fully-coalesced ∼8.5 μm AlN template layer via a pulsed atomic layer epitaxial growth technique. A back-illuminated p-i-n PD structure is subsequently grown on the high quality AlN template layer. After processing and implementation of Si(111) substrate removal, the optical and electrical characteristic of PDs are studied. Solar-blind operation is observed throughout the array; at the peak detection wavelength of 290 nm, 625 μm2 area PD showed unbiased peak external quantum efficiency and responsivity of ∼7% and 18.3 mA/W, respectively, with a UV and visible rejection ratio of more than three orders of magnitude. Electrical measurements yielded a low-dark current density below 1.6 × 10−8 A/cm2 at 10 V reverse bias.
103(2013); http://dx.doi.org/10.1063/1.4826139View Description Hide Description
Polarization dependence analysis of back-gated graphene field-effect transistor terahertz responsivity at frequencies ranging from 1.63 to 3.11 THz reveals two independent mechanisms of THz detection by graphene transistor: plasmonic, associated with the transistor nonlinearity, and bolometric, caused by graphene sheet temperature increase due to THz radiation absorption. In the bolometric regime, electron and hole branches demonstrate a very different response to THz radiation, which we link to the asymmetry of the current-voltage characteristics temperature dependence with respect to the Dirac point. Obtained results are important for development of high-efficiency graphene THz detectors.
103(2013); http://dx.doi.org/10.1063/1.4828500View Description Hide Description
We report thermally tunable optical bandpass filters based on long-range surface plasmon polariton waveguides. A thin gold stripe in the waveguide core is surrounded by dielectric layers with dissimilar refractive index dispersions and dissimilar thermo-optic coefficients. High filter transmission is achieved for a wavelength at which the refractive indices of the upper and lower cladding layers are identical, and this spectral point may be changed by varying the filter temperature. Experimentally, over 220 nm of bandpass tuning is achieved around 1550 nm wavelength by varying the device temperature from 19 to 27 °C.
Resonance broadening and tuning of split ring resonators by top-gated epitaxial graphene on SiC substrate103(2013); http://dx.doi.org/10.1063/1.4828499View Description Hide Description
Split ring resonators (SRRs) are subwavelength structures that are able to localize and enhance the electromagnetic wave. Controlling the plasmonic resonance behavior of metallic nanostructures, such as SRRs, plays an important role in optoelectronics and nanophotonics applications. Electrically tunable carrier concentration of graphene provides hybrid devices, where the plasmonic structures and graphene are combined. In this paper, we report the design, fabrication, and measurement of a device comprising a SRR array on epitaxial graphene. We obtained resonance broadening and tuning of split ring resonators by utilizing an epitaxial graphene transistor with transparent top-gate.
Modulating optical polarization properties of Al-rich AlGaN/AlN quantum well by controlling wavefunction overlap103(2013); http://dx.doi.org/10.1063/1.4828667View Description Hide Description
Using modified k·p perturbation method, the optical polarization properties of Al-rich AlGaN/AlN quantum wells (QWs) are studied. It is found that change of wavefunction overlaps between conduction band and valance subbands of heavy hole, light hole, and crystal-field split off hole is different. Such difference leads to the overturn of polarization degree and modulates optical polarization properties as well width and strain vary. This prompts that changing wavefunction overlaps of electron and hole can lead to a way to modulate optical polarization properties of Al-rich AlGaN/AlN QWs, on no condition that valence band order changes.
103(2013); http://dx.doi.org/10.1063/1.4829001View Description Hide Description
A hybrid plasmonic THz fiber featuring two metallic wires in a porous dielectric cladding is studied for resonant sensing applications. In our design, introduction of even lossless analytes into the fiber core leads to significant changes in the modal losses, which is used as a transduction mechanism.
103(2013); http://dx.doi.org/10.1063/1.4826679View Description Hide Description
We report on controlling the spontaneous emission (SE) rate of a molybdenum disulfide (MoS2) monolayer coupled with a planar photonic crystal (PPC) nanocavity. Spatially resolved photoluminescence (PL) mapping shows strong variations of emission when the MoS2 monolayer is on the PPC cavity, on the PPC lattice, on the air gap, and on the unpatterned gallium phosphide substrate. Polarization dependences of the cavity-coupled MoS2 emission show a more than 5 times stronger extracted PL intensity than the un-coupled emission, which indicates an underlying cavity mode Purcell enhancement of the MoS2 SE rate exceeding a factor of 70.
Heterodyne pump probe measurements of nonlinear dynamics in an indium phosphide photonic crystal cavity103(2013); http://dx.doi.org/10.1063/1.4828355View Description Hide Description
Using a sensitive two-color heterodyne pump-probe technique, we investigate the carrier dynamics of an InP photonic crystal nanocavity. The heterodyne technique provides unambiguous results for all wavelength configurations, including the degenerate case, which cannot be investigated with the widely used homodyne technique. A model based on coupled mode theory including two carrier distributions is introduced to account for the relaxation dynamics, which is assumed to be governed by both diffusion and recombination.