Coexisting nonequilibrium condensates with long-range spatial coherence in semiconductor microcavities

Coherent light sources have been demonstrated based on a wide range of nanostructures, however, little effort has been devoted to probing their underlying coherence properties. Here, we report long-range spatial coherence of lattice plasmon lasers constructed from a periodic array of gold nanoparticles and a liquid gain medium at room temperature. By combining spatial and temporal interferometry, we demonstrate millimeter-scale (∼1 mm) spatial coherence and picosecond (∼2 ps) temporal coherence. The long-range spatial coherence occurs even without the presence of strong coupling with the lattice plasmon mode extending over macroscopic distances in the lasing regime. This plasmonic lasing system thus provides a platform for understanding the emergence of long-range coherence from collections of nanoscale resonators and points toward novel types of distributed lasing sources.

Inconel alloys including IN 718 alloy are widely used in turbomachinery industry due to their superior mechanical properties. Inconel alloys are very difficult to machine using cutting and grinding. Wire electrical discharge machining (W-EDM) is an alternative process to manufacture complex Inconel parts. However, little research has been done on surface integrity by W-EDMed IN 718. This study focuses on surface integrity of IN 718 by W-EDM at different modes of discharge energy. The results show that the EDMed surface topography shows dominant coral reef microstructures at high energy mode, while random microvoids are dominant at low energy modes. The average roughness can be significantly reduced at low energy mode. A thick white layer is predominantly discontinuous and non-uniform at relative high energy modes. Microvoids are confined within the thick white layers and no microcracks were found in the subsurface. A thin white layer by trim cut at low energy mode becomes continuous, uniform, and is free of voids. Compared to the bulk, white layers have dramatic reduction in microhardness. In addition, surface alloying from wire electrode and water dielectric is obvious in main cut, but it can be minimized in trim cuts.

Long-range spatial coherence can be induced in incoherent thermal emitters by embedding a periodic grating within a material supporting propagating polaritons or dielectric modes. However, only a single spatially coherent mode is supported by purely periodic thermal emitters. While various designs have been proposed for the purpose of allowing arbitrary emission profiles, the limitations associated with the partial spatial coherence of thermal emitters are not known. Here, we explore superstructure gratings (SSGs) to control the spatial and spectral properties of thermal emitters. SSGs have long-range periodicity but employ a unit cell that provides multiple Bragg vectors to interact with light. These Bragg vectors allow simultaneous launching of polaritons with different frequencies/wavevectors in a single grating, manifesting as additional spatial and spectral modes in the thermal emission profile. However, SSGs still have a well-defined period, which allows us to assess the role that finite spatial coherence plays in thermal emitters. We find that the spatial coherence length defines the maximum possible SSG period that can be used. This provides a fundamental limit on the degree of spatial coherence that can be induced in a thermal emitter and has broader implications for the use of techniques such as inverse design for structure optimization.

We describe recent continuous wave studies of stimulated polariton scattering in semiconductor microcavities, with emphasis on the variety of new physics which has been observed in the last few years. The ability to manipulate both exciton and photon properties, and how they interact together to form strongly coupled exciton–photon coupled modes, exciton polaritons, leads to a number of novel phenomena, previously unreported in bulk semiconductors or quantum wells. The unusual dispersion of the lower polariton branch, which allows energy and momentum conserving polariton–polariton parametric scattering under resonant excitation, is shown to be a key factor enabling much of the new physics. A classical theoretical treatment of the phenomena which draws parallels between the equivalent pictures of stimulated polariton scattering and those of a triply resonant optical parametric oscillator is discussed. Higher order parametric phenomena and the factors which control the threshold for stimulation are also described. The results obtained under resonant excitation are contrasted with phenomena which occur under conditions of non-resonant excitation. Prospects to achieve ‘polariton lasing’ under non-resonant excitation are discussed.

Temperature effect on vibrational properties of crystalline p-quaterphenyl.

A method of determining the temperature of the nonradiative reservoir in a microcavity exciton-polariton system is developed. A general relation for the homogeneous polariton linewidth is theoretically derived and experimentally used in the method. In experiments with a GaAs microcavity under nonresonant pulsed excitation, the reservoir temperature dynamics is extracted from the polariton linewidth. Within the first nanosecond the reservoir temperature greatly exceeds the lattice temperature and determines the dynamics of the major processes in the system. It is shown that, for nonresonant pulsed excitation of GaAs microcavities, the polariton Bose-Einstein condensation is typically governed by polariton-phonon scattering, while interparticle scattering leads to condensate depopulation.

Research in microfabrication is important to many areas of science and technology. An emerging set of microlithographic techniques, known as soft lithography, uses elastomeric elements for microcontact printing (μCP), molding, or near field photolithography. We describe the use of two soft lithographic techniques to build DFB lasers with narrow emission linewidths. This demonstrates the suitability of soft lithography for building structures that have (i) feature sizes (∼300 nm) significantly less than 1 μm, and (ii) long-range (∼1 mm) spatial coherence. The lasers use optically pumped gain material deposited on a DFB resonator formed from periodic relief on the surface of a transparent substrate. The required relief was generated on glass and polymer surfaces with μCP and replica molding respectively. μCP produced lines of gold on a glass slide; RIE using the gold as a resist generated periodic relief (600 nm period, 50 nm depth) on the glass surface. Polymer gratings with similar dimensions were formed by curing a polyurethane film against an elastomeric mold. The good performance of the lasers suggest the potential utility of soft lithographic techniques for low-cost fabrication of high-performance optoelectronic devices. Other types of resonators, including distributed Bragg reflector and quasi two-dimensional photonic bandgap based resonators can also be fabricated with the techniques demonstrated here.

Local vibrational spectra of meta-dichlorobenzene molecules adsorbed on different parts of the Au(111) reconstruction are investigated using a low-temperature scanning tunneling microscope. The spectra show substantial variations on subnanometer length scale. While for the molecule physisorbed on either the hcp or the fcc domain of the reconstruction only low-energy modes are beyond the detection limit, higher-energy modes are observed for the molecule chemisorbed at the elbow site. The different adsorption strengths of the molecules manifest themselves in an energy shift of the modes. These shifts are used to identify through which part the molecule is bonded to the surface.

A new electrostatic positron beam has been constructed and tested. The energy range spans more than three decades, from less than 50 eV to more than 50 keV. The machine can operate in a low-energy mode (50 eV to 2 keV) with a beam diameter (FWHM) from 2.9 to 1.7 mm or in a high-energy mode (1 keV to 50 keV) with a FWHM from 1.4 to 1.0 mm. The transverse spot position on the target is very stable, with wandering being much smaller than the beam diameter. The apparatus transmission function is very flat, with an intensity change of only 30% when going from 10 eV to 50 keV. The product of moderation and optical transmission efficiency is , corresponding to 800-1000 with a 1 mCi source. The apparatus shows low noise in S-parameter measurements. The background of the high-energy wing of the annihilation line is also one of the best reported. Optical and constructional details are given.

Monitoring some sites using a wireless sensor network (WSN) may be hampered by the difficulty of recharging or renewing the batteries of the sensing devices. Mechanisms aiming at improving the energy usage at any moment while fulfilling the application requirements are thus key for maximizing the lifetime of such networks. Among the different methods for achieving such a goal, we focus on energy management methods based on duty-cycling allowing the sensors to switch between two modes: a high-energy mode (active) and a low-energy mode (sleep). In this paper we propose two new scheduling heuristics for addressing the problem of maximizing the lifetime of a WSN under the constraint of coverage of a subset of fixed targets. The first one is a stochastic greedy algorithm and the second one is based on applying Simulated Annealing (SA). Both heuristics use a specific knowledge about the problem. Experimental results show that while both algorithms perform well, greedy algorithm is preferable for small and medium sizes networks, and SA algorithm has competitive advantages for larger networks.

Anisotropic polariton scattering and spin dynamics of cavity polaritons

We study the optical properties of hybrid gold nanodisk and nanohole arrays and present experimental evidence of nanoparticle trapping using these devices. The fabrication procedure using electron beam lithography (EBL) is also discussed. This hybrid design exhibits a splitting of the resonance modes (low and high energy modes) due to the coupling of the electromagnetic interaction between nanohole and nanodisk plasmons. The devices demonstrate high plasmon resonance tunabilities from the visible to the near-infrared region (NIR) by varying the dimensions of the features of this design. This enhancement in the NIR is highly desirable for the purposes of biological sample manipulation where photo damage should be low. Additionally, these devices consist of grooves connecting the hybrid structures to each other. These regions provide further enhancement of the local electric fields and play the role of the trapping sites. We demonstrate multiple dielectric nanoparticle trapping in these grooves while the devices are excited by evanescent fields via the Kretschmann configuration. The results provide good evidence of the potential of this design to be used for the manipulation of biological samples with sub-diffraction limit sizes.

The research focuses on Raman effects from vibrational interactions for mixed solute and solvent in binary mixed liquid for vibrational exchange from anharmonic interactions from three and four body collisions of mixed solvents from shields and cages about excited molecule of solute. The Raman linewidth were measured and theoretically fitted by varying the mole fraction (X) of solute proportions with mixed solvents, one acting as a solute and the other as a solvent. The excitation of CH3 modes to the daughter modes and other mixed molecule modes show the effect of anharmonic many body collisions, dephasing and vibrational energy transfer. The Raman linewidth and intensity of the higher vibrational modes of solvents: Acetone and Methanol can rise in the presence of a second solvent of lower vibrational modes act as shield. Using various combinations of the solvents as acetone and methanol with carbon disulfide, and carbon tetrachloride can alter relaxation pathways due to the interruption of intermolecular energy decay routes of acetone and methanol by the low energy modes of carbon tetrachloride and carbon disulfide act as shields to energy transfer. Carbon tetrachloride is a stronger shielding agent than carbon disulfide for both acetone’s high and low energy modes due to acetone’s larger amphiphilic structure that is more resilient to structural hindrance of the linear carbon disulfide as opposed to the larger spherical carbon tetrachloride. Carbon disulfide is a stronger shielding agent than carbon tetrachloride for methanol except in the case of 1456 cm−1 energy mode due to the increased structural hindrance of carbon disulfide’s linear structure, which methanol is more susceptible to by its smaller molecular structure, preventing intermolecular interactions with other methanol molecules.

We study energy relaxation in thermalized one-dimensional nonlinear arrays of the Fermi-Pasta-Ulam type. The ends of the thermalized systems are placed in contact with a zero-temperature reservoir via damping forces. Harmonic arrays relax by sequential phonon decay into the cold reservoir, the lower-frequency modes relaxing first. The relaxation pathway for purely anharmonic arrays involves the degradation of higher-energy nonlinear modes into lower-energy ones. The lowest-energy modes are absorbed by the cold reservoir, but a small amount of energy is persistently left behind in the array in the form of almost stationary low-frequency localized modes. Arrays with interactions that contain both a harmonic and an anharmonic contribution exhibit behavior that involves the interplay of phonon modes and breather modes. At long times relaxation is extremely slow due to the spontaneous appearance and persistence of energetic high-frequency stationary breathers. Breather behavior is further ascertained by explicitly injecting a localized excitation into the thermalized arrays and observing the relaxation behavior.

In this chapter we address a complex set of optical phenomena linked to the spin dynamics of exciton polaritons in semiconductor microcavities. When optically created, polaritons inherit the spin and dipole moment from the exciting light. Their state can be fully characterized by a so‐called “pseudospin” accounting for both spin and dipole moment orientation. However, from the very beginning of their life in a microcavity, polaritons start changing their pseudospin state under effect of effective magnetic fields of different nature and due to scattering with acoustic phonons, defects, and other polaritons. This makes pseudospin dynamics of exciton polaritons rich and complex. It manifests itself in non‐trivial changes in polarization of light emitted by the cavity versus time, pumping energy, pumping intensity and polarization. During the first years of theoretical research on exciton‐polariton relaxation the polarization has been simply neglected. Later it has been understood that the energy and momentum‐relaxation of exciton polaritons are spin‐dependent. It is typically the case in the regime of stimulated scattering when the spin polarizations of initial and final polariton state have a huge effect on the scattering rate between these states. It appeared that critical conditions for polariton Bose‐condensation are also polarization‐dependent. In particular, the stimulation threshold (i.e. the pumping power needed to have a population exceeding 1 at the ground state of the lower polariton branch) has been experimentally shown to be lower under linear than under circular pumping. These experimental observations have stimulated the theoretical research toward understanding of mutually dependent polarization‐ and energy‐relaxation mechanisms in microcavities. The authors of this chapter have been working on theoretical description of different specific effects of polariton spin‐dynamics in microcavities for years. Here we attempted to put together all fragments and to formulate a general approach to the problem that would allow then to consider a variety of particular cases. We start from reminding the main spin‐relaxation mechanisms known for free carriers and excitons. We then overview the most essential experimental results in this field before to present our original formalism which allowed us to interpret the key experimental findings. We are going to discuss only the strong coupling regime leaving aside all polarization effects in VCSELs. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)