Photodetection with Active Optical Antennas

We present a bottom-up approach to fabricate single active optical antennas as well as antenna arrays consisting of semiconductor quantum dots and plasmonic nanostructures. It is based on a two-step lithographic process in combination with chemical functionalization of the substrate and allows for the deposition of quantum dots with high accuracy relative to arbitrary metal nanostructures. Examples of such active optical antennas are presented and proof of their functionality via polarization measurements is given.

Spiers Memorial Lecture.Introductory lecture: Hot-electron science and microscopic processes in plasmonics and catalysis.

Nanometallic optical antennas are rapidly gaining popularity in applications that require exquisite control over light concentration and emission processes. The search is on for high-performance antennas that offer facile integration on chips. Here we demonstrate a new, easily fabricated optical antenna design that achieves an unprecedented level of control over fluorescent emission by combining concepts from plasmonics, radiative decay engineering and optical beaming. The antenna consists of a nanoscale plasmonic cavity filled with quantum dots coupled to a miniature grating structure that can be engineered to produce one or more highly collimated beams. Electromagnetic simulations and confocal microscopy were used to visualize the beaming process. The metals defining the plasmonic cavity can be utilized to electrically control the emission intensity and wavelength. These findings facilitate the realization of a new class of active optical antennas for use in new optical sources and a wide range of nanoscale optical spectroscopy applications.

In the future, future heterogeneous communications networks may use visible light communications (VLC) as a supplementary technology to WiFi. The signal to noise ratio (SNR) of the received signal determines a VLC system’s channel capacity, just like it does for any other communications system. The channel estimate for VLC communication proposed in this study is based on spectral domain wavelet transform. Due to their excellent spectrum and energy efficiency for VLC MIMO communication systems, SMTs have become more and more common. This work focuses on SMTs, or entirely generalized spatial modulation, in which multiple active optical antennas and a constant number of antennae are used to transmit data symbols for VLC at any time interval. In this study, an adaptive channel estimation approach over a time-varying MIMO channel is proposed using the Spectral Domain Wavelet Transform. Furthermore, the broadcast data and optical antenna indices are identified using a maximum likelihood (ML) decoder based on the received signal and the predicted VLC-MIMO channel. Matlab software is used in the implementation of this project.

In this review, we focus on the experimental demonstration of enhanced emission from single plasmonic tunneling junctions consisting of coupled nano antennas or noble metal tips on metallic substrates in scanning tunneling microscopy. Electromagnetic coupling between resonant plasmonic oscillations of two closely spaced noble metal particles leads to a strongly enhanced optical near field in the gap between. Electron beam lithography or wet chemical synthesis enables accurate control of the shape, aspect ratio, and gap size of the structures, which determines the spectral shape, position, and width of the plasmonic resonances. Many emerging nano-photonic technologies depend on the careful control of such localized resonances, including optical nano antennas for high-sensitivity sensors, nanoscale control of active devices, and improved photovoltaic devices. The results discussed here show how optical enhancement inside the plasmonic cavity can be further increased by a stronger localization via tunneling. Inelastic electron tunneling emission from a plasmonic junction allows for new analytical applications. Furthermore, the reviewed concepts represent the basis for novel ultra-small, fast, optically, and electronically switchable devices and could find applications in high-speed signal processing and optical telecommunications.

Transformation optics (TO) is conventionally based on real-valued coordinate transformations and, therefore, cannot naturally handle metamaterials featuring gain and/or losses. Motivated by the growing interest in non-Hermitian optical scenarios featuring spatial modulation of gain and loss, and building upon our previous studies, we explore here possible extensions of the TO framework relying on complex-valued coordinate transformations. We show that such extensions can be naturally combined with well-established powerful tools and formalisms in electromagnetics and optics, based on the ‘complexification’ of spatial and spectral quantities. This enables us to deal with rather general non-Hermitian optical scenarios, while retaining the attractive characteristics of conventional (real-valued) TO in terms of physically incisive modeling and geometry-driven intuitive design. As representative examples, we illustrate the manipulation of beam-like wave-objects (modeled in terms of ‘complex source points’) as well as radiating states (‘leaky waves’, modeled in terms of complex-valued propagation constants). Our analytical results, validated against full-wave numerical simulations, provide useful insight into the wave propagation in non-Hermitian scenarios, and may indicate new directions in the synthesis of active optical devices and antennas.

A distributed system for remote generation and detection of terahertz pulses using an active optical fiber link and photoconductive antennas optimized for telecom wavelengths is experimentally demonstrated. Frequencies beyond 1 THz after 1×16 division and 100 m fiber transmission are obtained. It shows the capability of fiber-based spectroscopy setups for cost-reduction in facilities where several sensing platforms are needed such as industrial in-line product inspection, transport security checkpoints, etc.

The strong coupling between electronic transitions and resonant cavity modes, facilitated by coherent energy transfer, presents unprecedented opportunities for tailoring the photoelectronic properties of constituent components. Here, the concept of Kerker effect is leveraged to demonstrate the dynamic control of scattering directionality in dielectric nanostructures by tuning the exciton‐photon coupling. First, theoretical evidence for a significant modification of the scattering directionality of a dielectric metastructure engineered by excitonic polaritons is provided. As a proof of concept, self‐coupled metasurfaces composed of bulk MoS2, which exhibit a forward/backward scattering ratio up to 20, are constructed. Importantly, tunable directionality is achieved by thermally controlling the excitonic coupling to the Mie modes. The simulated results are in good agreement with the experimental measurements, and the subsequent multipole decompositions effectively elucidate the underlying mechanism, attributed to the interplay between electric and magnetic dipole modes that are modified by excitons. The findings shed light on the control of light flow in the far field through coherent light–matter interactions, thereby opening up numerous possibilities for active optical antennas and quantum emitters on a nanoscale.

Directional harmonic generation is an important property characterizing the ability of nonlinear optical antennas to diffuse the signal in a well-defined region of space. Herein, we show how sub-wavelength facets of an organic molecular mesowire crystal can be utilized to systematically vary the directionality of second-harmonic generation (SHG) in the forward-scattering geometry. We demonstrate this capability on crystalline diamonoanthraquinone (DAAQ) mesowires with sub-wavelength facets. We observed that the radial angles of the SHG emission can be tuned over a range of 130deg. This angular variation arises due to spatially distributed nonlinear dipoles in the focal volume of the excitation as well as the geometrical cross section and facet orientation of the mesowire. Numerical simulations of the near-field excitation profile corroborate the role of the mesowire geometry in localizing the electric field. In addition to directional SHG from the mesowire, we experimentally observe optical waveguiding of the nonlinear two-photon excited fluorescence (TPEF). Interestingly, we observed that for a given pump excitation, the TPEF signal is isotropic and delocalized, whereas the SHG emission is directional and localized at the location of excitation. All the observed effects have direct implications not only in active nonlinear optical antennas but also in nonlinear signal processing.

: This report covers the third year of progress of the 1997 MURI on RF Photonics for Antenna Arrays at the University of Colorado, Montana State University, George Mason University, and the University of California Davis. Novel techniques for optical control and processing of the wideband RF and microwave signals encountered in phased array antennas are being developed, guided by research in spatio-temporal adaptive processing algorithms and active quasi-optical RF antenna arrays. The primary goal of this research is to develop enabling optical techniques that provide dramatic improvements in antenna array performance over conventional RF, optical, and digital techniques, allowing the efficient processing of large broadband antenna arrays. Coherent modulation and detection is made robust and practical by the use of dynamic holography in photorefractive and optical coherent transient media. This report summarizes the teams management, educational, and outreach activities, as well as the 3rd years technical progress on the constituent projects - broadband adaptive optical array processing, spatio-temporal array-processing algorithms, coherent-transient true-time-delay, photorefractive signal extraction, optical antenna control, and polymer in-line fiber modulators.

Spatial modulation (SM) improves spectral efficiency (SE) by creating the constellation of active optical antenna indices. However, quantum properties of light are not exploited to improve the SE. In this letter, multi-plane diffraction (MPD) set-up creating exponentially increasing number of propagation paths for classical sources is exploited to introduce quantum spatial modulation (QSM). Exponential number of paths and quantum superposition-based classical symbol generation for transmitting simultaneous multiple classical symbols are exploited for linear and exponential quantum boosting of SM, respectively. It is theoretically modeled, numerically analyzed, and the challenges of joint design of source-channel coding and QSM are discussed.