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Controlling the coherence properties of rare earth emitters in solid-state platforms in the absence of an optical cavity is highly desirable for quantum light-matter interfaces and photonic networks. Here, we demonstrate the possibility of generating directional and spatially coherent light from Nd3+ ions coupled to the longitudinal plasmonic mode of a chain of interacting Ag nanoparticles. The effect of the plasmonic chain on the Nd3+ emission is analyzed by Fourier microscopy. The results reveal the presence of an interference pattern in which the Nd3+ emission is enhanced at specific directions, as a distinctive signature of spatial coherence. Numerical simulations corroborate the need of near-field coherent coupling of the emitting ions with the plasmonic chain mode. The work provides fundamental insights for controlling the coherence properties of quantum emitters at room temperature and opens new avenues towards rare earth based nanoscale hybrid devices for quantum information or optical communication in nanocircuits.

New York, NY, USA, 27 Sept.-3 Oct. 2020, We investigate the quantum optical behavior of quantum emitters coupled to a Weyl-like photonic environment. For the single emitter case, we will report on the formation of a novel photon-atom bound state featuring a tunable power-law localization of the excitation around the emitter. When two emitters are considered we show that the emergent bound state can mediate tunable and robust long-range coherent interactions., We acknowledge financial support from the Spanish MCIU under grant RTI2018-098452-B-I00 (MCIU/AEI/FEDER, UE) and the “Marıa de Maeztu” programme for Units of Excellence in R&D (MDM-2014-0377 and CEX2018-000805- M). AGT acknowledges support from CSIC Research Plat form on Quantum Technologies PTI-001 and from Spanish project PGC2018-094792-B-100 (MCIU/AEI/FEDER, EU). IGE acknowledges financial support from the Ministry of Science and Innovation of Spain (FPU grant AP-2018-027).

Next-generation optoelectronic devices and photonic circuitry will have to incorporate on-chip compatible nanolaser sources. Semiconductor nanowire lasers have emerged as strong candidates for integrated systems with applications ranging from ultrasensitive sensing, to data communication technologies. Despite significant advances in their fundamental aspects, the integration within scalable photonic circuitry remains challenging. Here we report on the realization of hybrid photonic devices consisting of nanowire lasers integrated with wafer-scale lithographically designed V-groove plasmonic waveguides. We present experimental evidence of the lasing emission and coupling into the propagating modes of the V-grooves, enabling on-chip routing of coherent and sub-diffraction confined light with room temperature operation. Theoretical considerations suggest that the observed lasing is enabled by a waveguide hybrid photonic-plasmonic mode. This work represents a major advance towards the realization of application-oriented photonic circuits with integrated nanolaser sources., 24 pages, 4 figures

Nanoscale laser sources based on single metallic nanoparticles (spasers) have attracted significant interest for their fundamental implications and technological potential. Here we theoretically investigate the spatio-temporal dynamics of lasing action in core–shell metallic nanoparticles that include optically pumped four-level gain media. By using detailed semiclassical simulations based on a time-domain generalization of the finite-element method, we study the evolution of the lasing dynamics when going from a spherical case to an elongated nanorod configuration. Our calculations show that there exists an optimal nanoparticle elongation that exhibits significantly improved lasing threshold and slope efficiency over those obtained for its spherical counterpart. These results are accounted for in terms of a coupled-mode theory analysis of the variation with elongation of the light confinement properties of localized surface plasmons. This work could be of importance for further development of nanoscale light sources based on localized surface plasmon resonances.

Significance Optically pumped far-infrared (OPFIR) lasers are one of the most powerful continuous-wave terahertz sources. However, such lasers have long been thought to have intrinsically low efficiency and large sizes. Moreover, all previous theoretical models failed to predict even qualitatively the experimental performance at high pressures. Here, we have developed an innovative model that captures the full physics of the lasing process and correctly predicts the behavior in the high-pressure regime. Validated against experiments, our model shows that nearly all previous OPFIR lasers were operating in the wrong regime and that 10 × greater efficiency is possible by redesigning the terahertz cavity. Our results reintroduce the use of OPFIR lasers as a powerful and compact source of terahertz radiation.

The demonstration of plasmon-assisted lasing by associating optical gain media with plasmonic nanostructures has led to a new generation of nanophotonic devices with unprecedented performances. However, despite the variety of designs demonstrated so far, the operation of these systems is in most cases limited to a single output wavelength, and some reports on multiline emission refer to mixing single nanolasers with the subsequent limitation in compactness. Here, we show multiline operation from a single plasmon-assisted nonlinear solid-state laser on which a linear chain of Ag nanoparticles is deposited. The system provides lasing at 1.08 μm, which is self-converted to the visible range through different parametric frequency-mixing processes generated at metal-dielectric interfaces. Near infrared and simultaneously green and tunable blue radiation with a subwavelength confinement in the direction perpendicular to the nanoparticle chain, are obtained at room temperature in CW regime. The results demonstrate the possibility of multifunctional operation from a single plasmon-assisted laser and offer new avenues for the development of highly integrable sources of coherent radiation, This work has been supported by the Spanish Ministry of Economy and Competitiveness (MINECO) under Projects MAT201343301-R, MAT2015-66128-R and MAT2016-76106-R and by Comunidad de Madrid under Grant S2013/MIT-2740

It is shown that lasing action at subwavelength scales can be achieved in realistic plasmonic systems supporting long-range surface plasmons (LRSPPs). To this end, a general numerical framework has been developed that is able to accurately account for the full spatio-temporal lasing dynamics and the vastly different length- and time-scales featured by this class of systems. Starting from a loss compensation regime for propagating LRSPPs, it is shown how the introduction of an optical feedback mechanism induces the formation of a self-sustained laser oscillation at moderate pump intensities. The simplicity of the proposed subwavelength scale laser offers significant potential as a novel class of planar light sources in complex plasmonic circuits.

We study theoretically nonlinear propagation of light in a graphene monolayer. We show that the large intrinsic nonlinearity of graphene at optical frequencies enables the formation of quasi one-dimensional self-guided beams (spatial solitons) featuring subwavelength widths at moderate electric-field peak intensities. We also demonstrate a novel class of nonlinear self-confined modes resulting from the hybridization of surface plasmon polaritons with graphene optical solitons.

Extracting the light trapped in a waveguide, or the opposite effect of trapping light in a thin region and guiding it perpendicular to its incident propagation direction, is essential for optimal energetic performance in illumination, display or light harvesting devices. Here we demonstrate that the paradoxical goal of letting as much light in or out while maintaining the wave effectively trapped can be achieved with a periodic array of interpenetrated fibers forming a photonic fiber plate. Photons entering perpendicular to that plate may be trapped in an intermittent chaotic trajectory, leading to an optically ergodic system. We fabricated such a photonic fiber plate and showed that for a solar cell incorporated on one of the plate surfaces, light absorption is greatly enhanced. Confirming this, we found the unexpected result that a more chaotic photon trajectory reduces the production of photon scattering entropy., SCOPUS: ar.j, info:eu-repo/semantics/published

Graphene plasmons are collective excitations of charges within the 2D sheet of carbon atoms. A strong energy confinement, low group velocities and high tunability, make graphene plasmons exciting candidates for plasmonic applications in the THz to mid-IR range. In contrast to most metallic and semiconductor materials graphene plasmons can strongly couple to the particle/hole plasma due to a unique combination of 2D plasmons dispersion, gapless bandstructure and the extreme confinement of the plasmons to the sheet of carbon atoms. In equilibrium this coupling is observable as strong absorption at frequencies above to the Fermi energy, an effect known as Landau damping. When the carrier system is inverted by photoexcitation this process is reversed and plasmons below the inversion line are spontaneously emitted and amplified [1]. Based on the random-phase approximation we calculate the nonequilibrium polarizability of the Dirac plasma, and determine the exact complex-frequency plasmon dispersion (see Fig. 1) and gain/loss spectra under realistic conditions (temperature, collision loss) [2]. With spontaneous emission rates in the 100-fs range, our results predict that plasmon emission is a dominant channel for carrier recombination [3] explaning the rapid decay of inversion in photo-excited graphene as observed by femtosecond pump-probe and time- and angle-resolved photo-emission spectroscopy.

The fast decay of carrier inversion in photoexcited graphene has been attributed to optical phonon emission and Auger recombination. Plasmon emission provides another pathway that, as we show here, drives the carrier relaxation dynamics on ultrafast time scales. In studying the nonequilibrium relaxation dynamics we find that plasmon emission effectively converts inversion into hot carriers, whose energy is then extracted by optical phonon emission. This mechanism not only explains the observed femtosecond lifetime of inversion but also offers the prospect for atomically thin ultrafast plasmon emitters, This work has been funded by the Engineering and Physical Sciences Research Council (United Kingdom), the Leverhulme Trust (United Kingdom), the European Research Council (ERC- 2011-AdG Proposal No. 290981) and the Spanish MINECO (Grant No. MAT2011-28581-C02-01)

In this paper, we review various numerical methods currently used to model nonlinear optical processes in nanophotonics. Among the different theoretical frameworks that have been used to study nonlinear photonic structures, we particularly focus on the application of both perturbation theory and coupled-mode theory to the analysis of complex nonlinear nanophotonic devices. This description is illustrated on several examples of how these techniques can be used to design photonic-crystal-based nonlinear devices. In addition, in all these examples, we show that the predictions made by the two mentioned techniques are in a good agreement with the numerical results obtained from a nonlinear finite-difference-time-domain approach to these problems.

We present our recent advances in nanoscale lasing action enabled by plasmonic resonances. First, we study lasing action in plasmonic crystals incorporating optically pumped four-level gain media. We show that the excitation of dark plasmonic resonances (via the gain medium) enables accessing the optimal lasing characteristics of this class of systems. Second, we analyze the conditions for the emergence of self-sustained laser oscillations in an optically-pumped long-range surface-plasmon structure that includes plasmonic Bragg mirrors as optical feedback mechanism.

We theoretically investigate lasing action in plasmonic crystals incorporating optically pumped four-level gain media. By using detailed simulations based on a time-domain generalization of the finite-element method, we show that the excitation of dark plasmonic resonances (via the gain medium) enables accessing the optimal lasing characteristics of the considered class of systems. Moreover, our study reveals that, in general, arrays of nanowires feature lower lasing thresholds and larger slope efficiencies than those corresponding to periodic arrays of subwavelength apertures. These findings are of relevance for further engineering of active devices based on plasmonic crystals

In this paper, we show that the phenomenon of extraordinary transmission through arrays of subwavelength holes, present in the optical regime, is also present in the millimeter wave range. After presentation of the theoretical foundations of the enhanced transmission, measurements of the transmission response have been performed on different samples by using a millimeter wave quasioptical vector network analyzer in the range between 45 and 110 GHz. The prototypes have been fabricated in Aluminum plates with several thickness and different hole diameters drilled by using a laser machine. Good agreement between theory and experiment is obtained, with clear signals of the existence of resonant transmission at wavelengths close to the period of the array. Possible applications in frequency selective surfaces and near-field imaging are envisaged.

A structure based on photonic crystals for optical wavelength demultiplexing is proposed and analyzed. This structure consists of two coupled-cavity waveguides, each one tuned at a different frequency transmission band. A model based on the tight-binding method taken from solid-state physics is used to design the working frequencies of the waveguides. The demultiplexing behavior is demonstrated by means of simulations employing a finite-difference time-domain method. It is concluded that the demultiplexing mechanism presented here may be used as a building block in multifunctional lightwave integrated circuits based on photonic crystals.

We theoretically study the emergence of strong coupling in the interaction between quantum emitters and the localized surface plasmons of a metal nanoparticle. Owing to their quasidegenerate nature, the continuum of multi-poles is shown to behave as a pseudomode strongly coupled to single emitters instead of as a Markovian bath. We demonstrate that the corresponding capping of the induced loss rate enables collective strong coupling to the dipole mode. Numerical simulations and analytical modeling are applied to several configurations of increasing complexity to grasp the relevant physics. In particular, the emitters closest to the nanoparticle surface are proven to contribute the most to the build-up of the plasmon-exciton polaritons, in contrast with the weak-coupling picture of quenching.

The crystal symmetry in materials such as GaAs can enable quasi-phasematching for efficient optical frequency conversion without poling, twinning or other engineered domain inversions. symmetry means that a 90° rotation is equivalent to a crystallographic inversion. Therefore, when light circulates about the axis, as in GaAs whispering-gallery-mode microdisks, it encounters effective domain inversions that can produce quasi-phasematching. Microdisk resonators also offer resonant field enhancement, resulting in highly efficient frequency conversion in micrometre-scale volumes. These devices can be integrated in photonic circuits as compact frequency convertors, sources of radiation or entangled photons. Here we present the first experimental observation of second-harmonic generation in a whispering-gallery-mode microcavity utilizing -quasi-phasematching. We use a tapered fibre to couple into the 5-μm diameter microdisk resonator, resulting in a normalized conversion efficiency η≈5 × 10−5 mW−1. Simulations indicate that when accounting for fibre-cavity scattering, the normalized conversion efficiency is η≈3 × 10−3 mW−1., Second-harmonic generation is used in photonics applications to convert the frequency of light. Here, Kuo et al. demonstrate second-harmonic generation of light from a gallium arsenide microdisk resonator sufficiently compact to be suitable for on-chip optical circuits.

In this work we use multiple scattering in conjunction with a genetic algorithm to reliably determine the optimized photonic-crystal-based structure able to perform a specific optical task. The genetic algorithm operates on a population of candidate structures to produce new candidates with better performance in an iterative process. The potential of this approach is illustrated by designing a spot size converter that has a very low F-number (F=0.47) and a conversion ratio of 11:1. Also, we have designed a coupler device that introduces the light from the optical fiber into a photonic-crystal-based waveguide with a coupling efficiency over 87% for a wavelength that can be tuned to 1.5 microns., Comment: 4 pages, 4 figures

Many of graphene’s unique electronic properties emerge from its Dirac-like electronic energy spectrum. Similarly, it is expected that a nanophotonic system featuring Dirac dispersion (two conical bands touching at a single point, the so-called Dirac point) will open a path to a number of important research avenues. To date, however, all proposed realizations of a photonic analog of graphene lack fully omnidirectional out-of-plane light confinement, which has prevented creating truly realistic implementations of this class of systems able to mimic the two-dimensional transport properties of graphene. Here we report on a novel route to achieve all-dielectric three-dimensional photonic materials featuring Dirac-like dispersion in a quasi-two-dimensional system. We further discuss how this finding could enable a dramatic enhancement of the spontaneous emission coupling efficiency (the β-factor) over large areas, defying the common wisdom that the β-factor degrades rapidly as the size of the system increases. These results might enable general new classes of large-area ultralow-threshold lasers, single-photon sources, quantum information processing devices and energy harvesting systems.

et al., We theoretically investigate the amplification of extraordinary optical transmission (EOT) phenomena in periodic arrays of subwavelength apertures incorporating gain media. In particular, we consider a realistic structure consisting of an opaque silver film perforated by a periodic array of slits and clad on each side by an optically pumped dielectric thin film containing rhodamine dye molecules. By solving the semiclassical electronic rate equations coupled to rigorous finite-element simulations of the electromagnetic fields, we show how the resonant electric-field enhancement associated with EOT properties enables complete ohmic loss compensation at moderate pump intensity levels. Furthermore, our calculations show that, as a consequence of the strong spatial hole-burning effects displayed by the considered structures, three separate regimes of operation arise: the system can behave as an absorber, an optical amplifier or a laser, depending on the value of the pump intensity. A discussion on the feasibility of reaching the lasing regime in the considered class of structures is also presented. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft., This work was sponsored by the Spanish Ministry of Science and Innovation under the projects MAT2009-06609-C02 and CSD2007-046-NanoLight and by the Ramón-y-Cajal program (grant no. RyC-2009-05489).

In this chapter, we describe a physical picture of nonlinear frequency mixing processes in resonant cavities inspired on a classical analog of the quantum Purcell effect. Applying this general framework to one particularly important system, we show how this approach allows identifying the optimal conditions enabling low-power efficient terahertz (THz) generation via nonlinear frequency mixing. We also address the practical implementation of these general principles. Using a detailed numerical analysis, we demonstrate how the unique properties of photonic crystals to achieve simultaneous spectral and spatial electromagnetic mode engineering enable a dramatic reduction of the pump powers required to reach quantum-limited THz conversion efficiencies using realistic nonlinear optical material systems.

We report on the experimental observation of speckle formation from a transparent crystal formed by a random distribution of nonlinear domains. The angular distribution of second-harmonic light generated by a transparent strontium barium niobate crystal is measured for different diameters of the fundamental beam and crystal thicknesses. Distinct manifestations of speckle pattern formation are found in these experiments. By using a theoretical Green’s function formalism, we explain the reported observations as a result of the linear interference among the second-harmonic waves generated in all directions by each of the nonlinear domains forming the nonlinear crystal.

The second-harmonic generation from transparent strontium-barium niobate crystals is experimentally studied and explained from the emission patterns of randomly scattered nonlinear domains and the far field interference of the light generated from such domains.

We propose a scheme for efficient cavity-enhanced nonlinear THz generation via difference-frequency generation (DFG) processes using a triply resonant system based on photonic crystal cavities. We show that high nonlinear overlap can be achieved by coupling a THz cavity to a doubly-resonant, dual-polarization near-infrared (e.g. telecom band) photonic-crystal nanobeam cavity, allowing the mixing of three mutually orthogonal fundamental cavity modes through a chi(2) nonlinearity. We demonstrate through coupled-mode theory that complete depletion of the pump frequency - i.e., quantum-limited conversion - is possible in an experimentally feasible geometry, with the operating output power at the point of optimal total conversion efficiency adjustable by varying the mode quality (Q) factors.

We develop a theoretical framework for second-order nonlinear difference frequency generation in triply resonant wavelength-scale cavities, demonstrating stable CW conversion with quantum-limited efficiency. We propose a scheme for efficient THz generation based on this framework.

We show that Weyl points can be realized in all-dielectric superlattices based on three-dimensional (3D) layered photonic crystals. Our approach is based on creating an inversion-breaking array of weakly-coupled planar defects embedded in a periodic layered structure with a large omnidirectional photonic band gap. Using detailed band structure calculations and tight-binding theory arguments, we demonstrate that this class of layered systems can be tailored to display 3D linear point degeneracies between two photonic bands, without breaking time-reversal symmetry and using a configuration that is readily-accessible experimentally. These results open new prospects for the observation of Weyl points in the near-infrared and optical regimes and for the application of Weyl-physics in integrated photonic devices., Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies (W911NF-13-D-0001), United States. Dept. of Energy. Office of Basic Energy Sciences (DE- SC0001299), National Science Foundation (U.S.) (DMR-1419807

4 pages, 4 figures.-- et al., The extraordinary optical transmission through periodic arrays of subwavelength holes has been studied extensively since it was first reported in 1998, owing to both its fundamental implications and its technological potential. The picture of the underlying mechanism that emerges from most of the theoretical studies is a resonant process assisted by surface electromagnetic modes, such as surface plasmons. However, these studies consider an infinite array of holes. By combining experiment and theory, we have analysed the influence of the inherent finite size of the arrays and report here the unexpected spatial distribution of light as it emerges from the arrays. This distribution is strongly anisotropic and extremely sensitive to the angle of incidence of the impinging light. The behaviour can be explained by a model that takes into account the asymmetry induced by the array edges, and the effects this has on the emission pattern across the array., Financial support by the EC under Project Nos. FP6-NMP4-CT-2003-505699 (Surface Plasmon Photonics) and FP6-2002-IST-1-507879 (Plasmo-Nano-Devices) is gratefully acknowledged.

Surface plasmons (SPs) have long been known to be able to guide light on the surface of a metal and to concentrate light in subwavelength volumes. But another functionality of SPs was added to the previous list in 1998, when Ebbesen and coworkers found that SPs could enhance the transmission of light passing through subwavelength holes. Seminal paper reported that, when a metal film is perforated with subwavelength holes and these are arranged in a two-dimensional (2D) periodic array, the transmission of light is greatly enhanced at some particular wavelengths. The experimental spectral location of transmission peaks was found to be related to the dispersion relation of SPs modes running on the metal surface. This chapter establishes a close connection between the extraordinary optical transmission (EOT) and the excitation of SPs. Since 1998, several experimental and theoretical groups around the world have reproduced the main features present in the first set of experiments. The influence of the metal forming the structure and the dependence of EOT with the lattice symmetry (square or triangular), hole shape (circular, elliptical, square, or rectangular) and frequency regime (optical, THz, or microwave) have been thoroughly analyzed. This chapter summarizes the theoretical results on the EOT (in both 2D hole arrays and single apertures) and beaming from single apertures, concentrating on the basic physics behind these phenomena.

Lattice surface plasmons enable large-area unidirectional emission of coherent light generated at deep subwavelength scales.

In this Letter we show that the extraordinary optical transmission phenomenon found before in 2D hole arrays is already present in a linear chain of subwavelength holes, which can be considered as the basic geometrical unit showing this property. In order to study this problem, we have developed a new theoretical framework, able to analyze the optical properties of finite collections of subwavelength apertures and/or dimples (of any shape and placed in arbitrary positions) drilled in a metallic film.

In this paper we explore the transmission properties of single subwavelength apertures perforated in thin metallic films flanked by asymmetric configurations of periodic arrays of indentations. It is shown how the corrugation in the input side can be used to transmit selectively only two different wavelengths. Also, by tuning the geometrical parameters defining the corrugation of the output side, these two chosen wavelengths can emerge from the structure as two very narrow beams propagating at well-defined directions. This new ability of structured metals can be used as a base to build micron-sized wavelength de-multiplexers., Comment: Accepted for publication in Photonics and Nanostructures

In this work we explore the transmission properties of a single slit in a metallic screen. We analyze the dependence of these properties on both slit width and angle of incident radiation. We study in detail the crossover between the subwavelength regime and the geometrical-optics limit. In the subwavelength regime, resonant transmission linked to the excitation of waveguide resonances is analyzed. Linewidth of these resonances and their associated electric field intensities are controlled by just the width of the slit. More complex transmission spectra appear when the wavelength of light is comparable to the slit width. Rapid oscillations associated to the emergence of different propagating modes inside the slit are the main features appearing in this regime., Comment: Accepted for publication in Phys. Rev. E

In this letter we explore, both experimentally and theoretically, the existence in the millimeter wave range of the phenomenon of extraordinary light transmission though arrays of subwavelength holes. We have measured the transmission spectra of several samples made on aluminum wafers by using an AB Millimetre Quasioptical Vector Network Analyzer in the wavelength range between 4.2mm to 6.5mm. Clear signals of the existence of resonant light transmission at wavelengths close to the period of the array appear in the spectra., Comment: To be published in Optics Letters

Transmission of light through linear defects in two-dimensional (2D) photonic crystals has been already successfully demonstrated in two ways: numerical simulations and experimental measurements. Recently, novel waveguides have been proposed in which the propagation of photons is performed via hopping due to overlapping between nearest-neighbors defect cavities. These waveguides are commonly referred to as coupled-cavity waveguides (CCW). In this work, we present a comprehensive analysis of the light transmission (TM modes) in CCW's created in hexagonal 2D photonic crystals made of high-index dielectric rods. Numerical simulations of the transmission are performed using a 2D Finite-Difference Time-Domain method. A plane wave algorithm and a simple one-dimensional (1D) tight-binding model are employed to describe the miniband which allows the light transport. It is shown that modifying the individual cavities along the CCW one can control the average frequency and the dispersion relation of the miniband. The results also show that this novel guiding method can be used to develop 1310nm/1550nm Coarse-WDM optical demultiplexers employing bended waveguides.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In this Letter we introduce a novel route for achieving negative-group-velocity waveguiding at deep-subwavelength scales. Our scheme is based on the strong electromagnetic coupling between two conformal surface plasmon structures. Using symmetry arguments and detailed numerical simulations, we show that the coupled system can be geometrically tailored to yield negative-index dispersion. A high degree of subwavelength modal confinement, of λ/10 in the transversal dimensions, is also demonstrated. These results can assist in the development of ultrathin surface circuitry for the low-frequency region (microwave and terahertz regimes) of the electromagnetic spectrum.

Organic dye molecules offer significant potential as gain media in the emerging field of optical amplification and lasing at subwavelength scales. Here, we investigate the laser dynamics in systems comprising subwavelength-structured cavities that incorporate organic dyes. To this end, we have developed a comprehensive theoretical framework able to accurately describe the interaction of organic molecules with any arbitrary photonic structure to produce single-mode lasing. The model provides explicit analytic expressions of the threshold and slope efficiency that characterize this class of lasers, and also the duration over which lasing action can be sustained before the dye photobleaches. Both the physical properties of the dyes and the optical properties of the cavities are considered. We also systematically studied the feasibility of achieving lasing action under continuous-wave excitation in optically pumped monolithic organic dye lasers. This study suggests routes to realize an organic laser that can potentially lase with a threshold of only a few W cm⁻². Our work puts forward a theoretical formalism that could enable the advancement of nanostructured organic-based light emitting and sensing devices., National Science Foundation (U.S.) (DMR-0819762), Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies (W911NF-13-D-0001), Solid-State Solar-Thermal Energy Conversion Center (DE-SC0001299)

Diffusive random lasers, whose mechanism has been elusive, are now explained by a general theory that encompasses conventional lasers.

Photonic crystal resonators present unique properties for confining light in volumes much smaller than the wavelength. The ultrafast dynamic change of these properties is an important step towards the complete control of light.

We investigate laser emission from optically-pumped rotationally excited molecular gases confined in a metallic cavity. To this end, we have developed a theoretical framework able to accurately describe, both in the spatial and temporal domains, the molecular collisional and diffusion processes characterizing the operation of this class of lasers. The effect on the main lasing features of the spatial variation of the electric field intensity and the ohmic losses associated to each cavity mode are also included in our analysis. Our simulations show that, for the exemplary case of methyl fluoride gas confined in a cylindrical copper cavity, the region of maximum population inversion is located near the cavity walls. Based on this fact, our calculations show that the lowest lasing threshold intensity corresponds to the cavity mode that, while maximizing the spatial overlap between the corresponding population inversion and electric-field intensity distributions, simultaneously minimizes the absorption losses occurring at the cavity walls. The dependence of the lasing threshold intensity on both the gas pressure and the cavity radius is also analyzed and compared with experiment. We find that as the cavity size is varied, the interplay between the overall gain of the system and the corresponding ohmic losses allows for the existence of an optimal cavity radius which minimizes the intensity threshold for a large range of gas pressures. The theoretical analysis presented in this work expands the current understanding of lasing action in optically-pumped far-infrared lasers and, thus, could contribute to the development of a new class of compact far-infrared and terahertz sources able to operate efficiently at room temperature.

We present a comprehensive study of second-order nonlinear difference frequency generation in triply resonant cavities using a theoretical framework based on coupled-mode theory. We show that optimal quantum-limited conversion efficiency can be achieved at any pump power when the powers at the pump and idler frequencies satisfy a critical relationship. We demonstrate the existence of a broad parameter range in which all triply-resonant DFG processes exhibit monostable conversion. We also demonstrate the existence of a geometry-dependent bistable region., Comment: 10 pages, 3 figures

pa href="http://oe.osa.org/virtual_issue.cfm?vid=36"Focus Serial: Frontiers of Nonlinear Optics/a/pNonlinear photonic-crystal microresonators offer unique fundamental ways of enhancing a variety of nonlinear optical processes. This enhancement improves the performance of nonlinear optical devices to such an extent that their corresponding operation powers and switching times are suitable for their implementation in realistic ultrafast integrated optical devices. Here, we review three different nonlinear optical phenomena that can be strongly enhanced in photonic crystal microcavities. First, we discuss a system in which this enhancement has been successfully demonstrated both theoretically and experimentally, namely, a photonic crystal cavity showing optical bistability properties. In this part, we also present the physical basis for this dramatic improvement with respect to the case of traditional nonlinear devices based on nonlinear Fabry-Perot etalons. Secondly, we show how nonlinear photonic crystal cavities can be also used to obtain complete second-harmonic frequency conversion at very low input powers. Finally, we demonstrate that the nonlinear susceptibility of materials can be strongly modified via the so-called Purcell effect, present in the resonant cavities under study.

We present a theoretical analysis of lasing action in photonic crystal surface-emitting lasers (PCSELs). The semiclassical laser equations for such structures are simulated with three different theoretical techniques: exact finite-difference time-domain calculations, an steady-state ab-initio laser theory and a semi-analytical coupled-mode formalism. Our simulations show that, for an exemplary four-level gain model, the excitation of dark Fano resonances featuring arbitrarily large quality factors can lead to a significant reduction of the lasing threshold of PCSELs with respect to conventional vertical-cavity surface-emitting lasers. Our calculations also suggest that at the onset of lasing action, most of the laser power generated by finite-size PCSELs is emitted in the photonic crystal plane rather than the vertical direction. In addition to their fundamental interest, these findings may affect further engineering of active devices based on photonic crystal slabs. Published version