We propose a method of achieving large temperature T sensitivity in the Casimir force that involves measuring the stable separation between dielectric objects immersed in a fluid. We study the Casimir force between slabs and spheres using realistic material models, and find large >2 nm/K variations in their stable separations (hundreds of nanometers) near room temperature. In addition, we analyze the effects of Brownian motion on suspended objects, and show that the average separation is also sensitive to changes in T. Finally, this approach also leads to rich qualitative phenomena, such as irreversible transitions, from suspension to stiction, as T is varied.

We present a method to study current paths through quantum cascade lasers (QCLs). The temperature dependence of the current is measured at a fixed voltage. At low temperatures we find activation energies that correspond to the energy difference between the injector ground state and the upper laser level. At higher temperatures additional paths with larger activation energies are found. Application of this method to high performance QCLs based on strained InGaAs/InAlAs quantum wells and barriers with different band-offsets allows us to identify individual parasitic current paths through the devices. The results give insight into the transport properties of quantum cascade lasers thus providing a useful tool for device optimization. (C)2010 Optical Society of America

Surface plasmons have found a broad range of applications in photonic devices at visible and near-infrared wavelengths. In contrast, longer-wavelength surface electromagnetic waves, known as Sommerfeld or Zenneck waves(1,2), are characterized by poor confinement to surface and are therefore difficult to control using conventional metallo-dielectric plasmonic structures. However, patterning the surface with subwavelength periodic features can markedly reduce the asymptotic surface plasmon frequency, leading to `spoof' surface plasmons(3,4) with subwavelength confinement at infrared wavelength and beyond, which mimic surface plasmons at much shorter wavelength. We demonstrate that by directly sculpting designer spoof surface plasmon structures that tailor the dispersion of terahertz surface plasmon polaritons on the highly doped semiconductor facets of terahertz quantum cascade lasers, the performance of the lasers can be markedly enhanced. Using a simple one-dimensional grating design, the beam divergence of the lasers was reduced from similar to 180 degrees to similar to 10 degrees, the directivity was improved by over 10 decibels and the power collection efficiency was increased by a factor of about six compared with the original unpatterened devices. We achieve these improvements without compromising high-temperature performance of the laser.

The impact of upper state lifetime and spatial hole burning on pulse shape and stability in actively mode locked QCLs is investigated by numerical simulations. It is shown that an extended upper state lifetime is necessary to achieve stable isolated pulse formation per roundtrip. Spatial hole burning helps to reduce the pulse duration by supporting broadband multimode lasing, but introduces pulse instabilities which eventually lead to strongly structured pulse shapes that further degrade with increased pumping. At high pumping levels gain saturation and recovery between pulses leads to suppression of mode locking. In the absence of spatial hole burning the laser approaches single-mode lasing, while in the presence of spatial hole burning the mode locking becomes unstable and the laser dynamics does not reach a steady state anymore. (C) 2010 Optical Society America

We study the emission properties of electrically pumped triangular-shaped microlasers with rounded corners. We find no signs of directional emission for the relatively large cavities (dimension similar to 100 mu m) used in our experiments, in full agreement with ray simulation results. The broad emission characteristics that we observe can be fine-tuned by adjusting the resonator geometry as is verified through simulations which might prove useful for applications in optical devices. (C) 2010 Optical Society of America

This paper describes the fabrication of arrays of nanostructures (rings, crescents, counterfacing split rings, cylinders, coaxial cylinders, and other structures) by a four-step process: (i) molding an array of epoxy posts by soft lithography, (ii) depositing thin films on the posts, (iii) embedding the posts in epoxy, and (iv) sectioning in a plane parallel to the plane defined by the array of posts, into slabs, with an ultramicrotome (''nanoskiving''). This work demonstrates the combination of four capabilities: (i) formation of structures that are submicrometer in all dimensions; (ii) fabrication of 3D structures, and arrays of structures, with gradients of height; (iii) patterning of arrays containing two or more materials, including metals, semiconductors, oxides, and polymers; and (iv) generation of as many as 60 consecutive slabs bearing contiguous arrays of nanostructures. These arrays can be transferred to different substrates, and arrays of gold rings exhibit plasmonic resonances in the range of wavelengths spanning 2-5 mu m.

Clusters of plasmonic nanoparticles and nanostructures support Fano resonances. Here we show that this spectral feature, produced by the interference between bright and dark modes of the nanoparticle cluster, is strongly dependent upon both geometry and local dielectric environment. This permits a highly sensitive tunability of the Fano dip in both wavelength and amplitude by varying cluster dimensions, geometry, and relative size of the individual nanocluster components. Plasmonic nanoclusters show an unprecedented sensitivity to dielectric environment with a local surface plasmon resonance figure of merit of 5.7. the highest yet reported for localized surface plasmon resonance sensing in a finite nanostructure.

Assemblies of strongly interacting metallic nanoparticles are the basis for plasmonic nanostructure engineering. We demonstrate that clusters of four identical spherical particles self-assembled into a close-packed asymmetric quadrumer support strong Fano-like interference. This feature is highly sensitive to the polarization of the incident electric held due to orientation-dependent coupling between particles in the cluster. This structure demonstrates how careful design of self-assembled colloidal systems can lead to the creation of new plasmonic modes and the enabling of interference effects in plasmonic systems.

We report the design and performance of GaAs/Al0.15Ga0.85As terahertz quantum cascade lasers with double-phonon resonant depopulation and a vertical lasing transition. Devices were processed into gold-clad double-metal waveguides. Lasing at 3 THz was observed up to a heat-sink temperature of 172 K, which compares favorably with the performance of single-phonon resonant depopulation devices based on vertical lasing transitions. These results demonstrate that terahertz quantum cascade lasers based on double-phonon depopulation designs may be a viable alternative to single-phonon depopulation designs for achieving high-temperature operation. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3496035]

We investigated dual wavelength mid-infrared quantum cascade lasers based on heterogeneous cascades. We found that due to gain competition laser action tends to start in higher order lateral modes. The mid-infrared mode with the lower threshold current reduces population inversion for the second laser with the higher threshold current due to stimulated emission. We developed a rate equation model to quantitatively describe mode interactions due to mutual gain depletion. (C) 2010 Optical Society of America

A strain-balanced, AlInAs/InGaAs/InP quantum cascade laser structure, designed for light emission at 4.0 mu m using nonresonant extraction design approach, was grown by molecular beam epitaxy. Laser devices were processed in buried heterostructure geometry. An air-cooled laser system incorporating a 10-mm x 11.5-mu m laser with antireflection-coated front facet and high-reflection-coated back facet delivered over 2 W of single-ended optical power in a collimated beam. Maximum continuous-wave room temperature wall plug efficiency of 5.0% was demonstrated for a high-reflection-coated 3.65-mm x 8.7-mu m laser mounted on an aluminum nitride submount.

The design and operating principles of quantum cascade lasers (QCLs) are reviewed along with recent developments in high-power cw and broadband devices. Cw power levels of several watts at room temperature have been achieved at 4.6-mu m wavelength; broadband single-mode tuning (approximate to 400 cm(-1)) has been achieved using an external-cavity QCL with a grating as a tuning element. An alternative approach, consisting of a monolithically integrated array of single-mode QCLs individually currentdriven by a microcontroller, has led to broadband single-mode tuning over a range of 200 cm(-1) without requiring the use of moving parts. This spectrometer on a chip holds promise for high-brightness compact trace-gas sensors capable of operating in harsh environments. (C) 2010 Society of Photo-Optical Instrumentation Engineers. [DOI: 10.1117/1.3505844]

Dark field microspectroscopy is the primary method for the study of plasmon modes of individual metallic nanostructures. Light from a plasmonic nanostructure typically scatters with a strong angular and modal dependence, resulting in significant variations in the observed spectral response depending on excitation and collection angle and polarization of incident light. Here we examine how spectrally dependent radiation patterns arising from an individual plasmonic nanoparticle, positioned on a dielectric substrate, affect the detection of its plasmon modes. Careful consideration of excitation and collection geometry is of critical concern in quantitative studies of the optical response of these nanoparticle systems. (C) 2010 Optical Society of America

Enhancing nonlinear processes at the nanoscale is a crucial step toward the development of nanophotonics and new spectroscopy techniques Here we demonstrate a novel plasmonic structure called plasmonic nanocavtiy grating which is shown to dramatically enhance surface nonlinear optical processes It consists of resonant cavities that are periodically arranged to combine local and grating resonances The four wave mixing signal generated in our gold nanocavity grating is enhanced by a factor up to approximate to 2000 2 orders of magnitude higher than that previously reported

We analyze the use of layered superconductors as strongly anisotropic metamaterials, which can possess negative-refractive-index in a wide frequency range. Superconductors are of particular interest because they have the potential to support low losses, which is critical for applications such as super-resolution imaging. We show that low-T(c) (s-wave) superconductors can be used to construct layered heterostructures with low losses for T << T(c). However, the real part of their in-plane effective permittivity is very large, making coupling into the structure difficult. Moreover, even at low temperatures, layered high-T(c) superconductors have a large in-plane normal conductivity, producing large losses (due to d-wave symmetry). Therefore, it is difficult to enhance the evanescent modes in either low-T(c) or high-T(c) superconductors.

We study the multimode operation regimes of midinfrared quantum cascade lasers (QCLs), taking into account nonlinear phase-sensitive interactions between transverse modes. We show the possibility of the coherent coupling of several transverse modes, which results in a number of interesting effects including frequency and phase locking between transverse modes, bistability, and beam steering. We present an analytical model for the modal dynamics and its numerical analysis. Effects of amplitude and phase fluctuations on the modal stability are explored. The theoretical results are in agreement with our experimental measurements of buried heterostructure QCLs. (C) 2010 Society of Photo-Optical Instrumentation Engineers. [DOI: 10.1117/1.3498773]

We analyze the dynamics of broad-area mid-infrared quantum cascade lasers (QCLs). We show the possibility of the coherent coupling of several transverse modes which results in several interesting effects including frequency and phase locking between transverse modes, bistability, and beam steering. We present an analytical model for the modal dynamics and its numerical analysis. Effects of amplitude and phase fluctuations on the modal stability are explored. We compare our theoretical results with our experimental measurements of buried heterostructure QCLs.

We present a scheme for obtaining stable Casimir suspension of dielectric nontouching objects immersed in a fluid, validated here in various geometries consisting of ethanol-separated dielectric spheres and semi-infinite slabs. Stability is induced by the dispersion properties of real dielectric (monolithic) materials. A consequence of this effect is the possibility of stable configurations (clusters) of compact objects, which we illustrate via a molecular two-sphere dicluster geometry consisting of two bound spheres levitated above a gold slab. Our calculations also reveal a strong interplay between material and geometric dispersion, and this is exemplified by the qualitatively different stability behavior observed in planar versus spherical geometries.

This paper reviews several topics related to optically pumped ZnO nanowire lasers. A systematic study of the various properties of a device as it evolves from the regime of amplified spontaneous emission to laser oscillation above threshold is presented. The key dependence of the laser threshold on nanowire diameter is demonstrated and explained by means of a thorough study of guided modes in semiconducting nanowires for a nanowire-on-substrate geometry. A `head on' detection geometry is used to measure the far-field profile of a nanowire laser and thus identify the modes responsible for lasing. Finally, the temperature behavior of a nanowire laser is reported, and possible mechanisms that may be responsible for gain are suggested.

This paper reviews our recent work on laser beam shaping using plasmonics. We demonstrated that by integrating properly designed plasmonic structures onto the facet of semiconductor lasers, their divergence angle can be dramatically reduced by more than one orders of magnitude, down to a few degrees. A plasmonic collimator consisting of a slit aperture and an adjacent 1-D grating can collimate laser light in the laser polarization direction; a collimator consisting of a rectangular aperture and a concentric ring grating can reduce the beam divergence both perpendicular and parallel to the laser polarization direction, thus achieving collimation in the plane perpendicular to the laser beam. The devices integrated with plasmonic collimators preserve good room-temperature performance with output power comparable to that of the original unpatterned lasers. A collimator design for one wavelength can be scaled to adapt to other wavelengths ranging from the visible to the far-IR regimes. Plasmonic collimation offers a compact and integrated solution to the problem of laser beam collimation and may have a large impact on applications such as free-space optical communication, pointing, and light detection and ranging. This paper opens up major opportunities in wavefront engineering using plasmonic structures.