Free-space Photons Research Articles

Enhancement of emission and directionality of Light Emitting Diode using Surface Plasmon Resonance

Emission of the LED can be enhanced by Surface Plasmons Resonance (SPRs), which are electron oscillations that can exist at the interface between a metal and dielectric materials. The mechanism is reliant upon the energy coupling effect between the emitted photons from the semiconductor and arranged metallic nanoparticles. The focus of this theoretical investigation is study the interaction between light and the metallic film, explicitly Ag and Au, deposited onto the surface of a light emitting diode (LED), which is proposed to allow coupling between surface plasmons and free-space photons. Moreover, it’s also demonstrated how the optical emission from a typical light of LED device can be spatially controlled through the deposition of a periodically structured metallic film. This theoretical investigation indicated that high-efficiency and spatially controlled direction light emission can be predicted for light emitters. This can be done through the best selection of the proposed parameters (λ=547.6nm and ʌ=660nm) and area of overlapping the circles of wave vectors for light in free space and surface plasmons. It is also appeared that the coupling between the free photons and surface plasmons is optimumfor the proposed wavelengths

Quantum random number generation using an on-chip plasmonic beamsplitter

We report an experimental realisation of a quantum random number generator using a plasmonic beamsplitter. Free-space single photons are converted into propagating single surface plasmon polaritons on a gold stripe waveguide via a grating. The surface plasmons are then guided to a region where they are scattered into one of two possible outputs. The presence of a plasmonic excitation in a given output determines the value of a random bit generated from the quantum scattering process. Using a stream of single surface plasmons injected into the beamsplitter we achieve a quantum random number generation rate of 2.37 Mbits s−1 even in the presence of loss. We characterise the quality of the random number sequence generated, finding it to be comparable to sequences from other quantum photonic-based devices. The compact nature of our nanophotonic device makes it suitable for tight integration in on-chip applications, such as in quantum computing and communication schemes.

Enhanced Performance Analysis of Inter-aircraft Optical Wireless Communication Link (IaOWC) Using EDFA Pre-amplifier

Optical wireless communication (OWC) also known as free space photonics or free space optics is a data transmission technology in which information is transmitted between two points separated by a certain distance is space using optical signals as the information carrier and free space/air as the propagation medium. OWC technology can be deployed to achieve inter-satellite links, inter-aircraft links and terrestrial links. In this paper, the performance analysis of an inter-aircraft optical wireless communication (IaOWC) link has been reported under different system parameters. Also, an improved performance analysis of IaOWC link consisting of two aircraft separated by a distance of 100 km and exchanging information at 1.25 Gbps has been reported using OPTISYSTEM simulation software.

Coupled One-Dimensional Plasmons and Two-Dimensional Phonon Polaritons in Hybrid Silver Nanowire/Silicon Carbide Structures.

Surface plasmons (SPs) and phonon polaritons (PhPs) are two distinctive quasiparticles resulting from the strong coupling of photons with electrons and optical phonons, respectively. In this Letter, we investigate the interactions between one-dimensional (1D) plasmons in silver nanowires with two-dimensional (2D) surface phonon polaritons of the silicon carbide (SiC) substrate. Using near-field infrared spectroscopy of the silver nanowire-SiC heterostructure at wavelengths close to the phonon resonance of SiC, we observe that the 1D plasmon dispersion is strongly modified by the 2D phonon polaritons in SiC. In particular, we observe for the first time well-defined 1D plasmon oscillations with the plasmon wavelengths longer than the free-space photon wavelengths due to the 1D plasmon-2D phonon polariton coupling. Our work demonstrates that unusual polariton behavior can emerge from interactions between polariton excitons of different dimensionality, which can enable new ways to engineer plasmons in hybrid structures.

Modeling of High Speed Free Space Optics System to Maintain Signal Integrity in Different Weather Conditions; System Level

Free space optical (FSO) also known as free space photonics (FSS) is a technology widely deployed in Local Area Network (LAN), Metro Area Network (MAN), and in Inter & Intra chip communications. However satellite to satellite and other space use of FSO requires further consideration. Although FSO is highly beneficial due to its easy deployment and high security in narrow beam as well as market demand for 10 GB+, some factors especially rain, snow and fog attenuation causes signal integrity problem in FSO. To get better Signal Integrity in FSO we need to consider all components while designing the system. In this paper a comparative analysis has been performed on 10 GB and 40 GB FSO system over 1 Km. Firstly for selecting suitable modulation technique we compared NRZ and RZ modulation and get spectrum analysis. NRZ modulation was found more data efficient. Signal Integrity in FSO system with 10 GB/s was analyzed by eye diagram and Q-Factor of both APD and PIN Photo detector was presented in graph. Same experiment was repeated with 40 GB/s and Bit error rate of both photo detectors were presented.

Ultraconfined Plasmonic Hotspots Inside Graphene Nanobubbles.

We report on a nanoinfrared (IR) imaging study of ultraconfined plasmonic hotspots inside graphene nanobubbles formed in graphene/hexagonal boron nitride (hBN) heterostructures. The volume of these plasmonic hotspots is more than one-million-times smaller than what could be achieved by free-space IR photons, and their real-space distributions are controlled by the sizes and shapes of the nanobubbles. Theoretical analysis indicates that the observed plasmonic hotspots are formed due to a significant increase of the local plasmon wavelength in the nanobubble regions. Such an increase is attributed to the high sensitivity of graphene plasmons to its dielectric environment. Our work presents a novel scheme for plasmonic hotspot formation and sheds light on future applications of graphene nanobubbles for plasmon-enhanced IR spectroscopy.

Hong–Ou–Mandel interference without beam splitters

We propose a new interferometric setup which displays a completely destructive generalized N-photon Hong–Ou–Mandel interference. The key property of this scheme is that it does not require any optical elements like beam splitters or integrated waveguide structures. The interference is intrinsically produced by the evolution of N photons in free space when emitted by N identical single photon sources and measured by N detectors in the far field. In this sense, the setup is a most simple and natural implementation of the Hong–Ou–Mandel interference effect, i.e. of a completely destructive multi-photon interference produced by independent incoherent sources.

Amplitude- and Phase-Controlled Surface Plasmon Polariton Excitation with Metasurfaces

Surface plasmon polaritons (SPPs) have shown high potential for various applications in various fields, ranging from physics, chemistry, and biology to integrated photonic circuits due to their the strong confinement of light to the metal surface. Exciting an SPP from a free-space photon in a controllable manner is an essential step toward more complex and integrated applications. Methods for coupling photons to SPPs are numerous, but in order to control the amplitude and phase of an SPP, most of these methods require bulky or multiple optical components or sensitive adjustments that are difficult to control. Here we present a novel approach for an independent control of the amplitude and phase of an SPP excited by a normally incident beam using a metasurface. The full control in amplitude and phase is achieved via the polarization state and polarization orientation angle of the electrical field of the incoming light. We experimentally demonstrate the functionality of such a metasurface consisting of peri...

Antenna-coupled photon emission from hexagonal boron nitride tunnel junctions.

The ultrafast conversion of electrical signals to optical signals at the nanoscale is of fundamental interest for data processing, telecommunication and optical interconnects. However, the modulation bandwidths of semiconductor light-emitting diodes are limited by the spontaneous recombination rate of electron-hole pairs, and the footprint of electrically driven ultrafast lasers is too large for practical on-chip integration. A metal-insulator-metal tunnel junction approaches the ultimate size limit of electronic devices and its operating speed is fundamentally limited only by the tunnelling time. Here, we study the conversion of electrons (localized in vertical gold-hexagonal boron nitride-gold tunnel junctions) to free-space photons, mediated by resonant slot antennas. Optical antennas efficiently bridge the size mismatch between nanoscale volumes and far-field radiation and strongly enhance the electron-photon conversion efficiency. We achieve polarized, directional and resonantly enhanced light emission from inelastic electron tunnelling and establish a novel platform for studying the interaction of electrons with strongly localized electromagnetic fields.

Excitation of Propagating Plasmons in Semi-Infinite Graphene Layer by Free Space Photons

By investigating the diffraction of plane waves by a semi-infinite graphene layer, we present a rigorous solution for propagating surface plasmons in graphene, which can be excited by incident plane waves through the graphene edge. The theoretical results are confirmed by numerical simulations. Our results reveal a convenient way to excite propagating surface plasmons in graphene where the graphene edge plays an important role.

Improvement of light extraction efficiency and reduction of driving voltage in organic light emitting diodes using a plasmonic crystal

Two-dimensional periodic corrugation was introduced into the surface of metallic cathodes of organic light-emitting diodes (OLEDs) to extract surface plasmon energy, which is trapped in that surface, as free-space photons. The dependence of the improvement factor of the emission efficiency on the modulation depth of the corrugation was systematically investigated. The corrugation was fabricated by using a colloidal lithography technique, which can be easily applied to a wide area. The obtained maximum improvement factor in current efficiency was 1.67 for an OLED with a 40 nm modulation depth, whereas the improvement in power efficiency was 2.35 for an OLED with a 60 nm modulation depth. We attributed the former improvement factor purely to optical effects and the latter to both optical and electrical effects, namely, a reduction of the electrical resistance of the organic layers due to the introduced corrugation.

Plasmonic nanopatch array for optical integrated circuit applications

Future plasmonic integrated circuits with the capability of extremely high-speed data processing at optical frequencies will be dominated by the efficient optical emission (excitation) from (of) plasmonic waveguides. Towards this goal, plasmonic nanoantennas, currently a hot topic in the field of plasmonics, have potential to bridge the mismatch between the wave vector of free-space photonics and that of the guided plasmonics. To manipulate light at will, plasmonic nanoantenna arrays will definitely be more efficient than isolated nanoantennas. In this article, the concepts of microwave antenna arrays are applied to efficiently convert plasmonic waves in the plasmonic waveguides into free-space optical waves or vice versa. The proposed plasmonic nanoantenna array, with nanopatch antennas and a coupled wedge plasmon waveguide, can also act as an efficient spectrometer to project different wavelengths into different directions, or as a spatial filter to absorb a specific wavelength at a specified incident angle.

A bit error rate analysis for TCP traffic over parallel free space photonics

Inter-satellite links (ISL) are a useful technology to transmit data to space stations and to communicate between satellites. However, there are serious limitations due to long delays and poor channel performance, resulting in high bit error rates (BER). In this paper, parallel transmission and the scaling of the Transport Control Protocol (TCP) window in free space optics (FSO) communications are analyzed in order to overcome these disadvantages in optical inter-satellite links. Latency and BER are the dominant effects that determine link performance. Thus, a physical, link, network and transport cross-layer analysis for FSO over ISL is presented in this paper. This analysis shows the advantages and disadvantages of using optical parallel transmission and TCP window scaling for free space optical links between stations and satellite constellations. The key contribution of this work is to simulate the effects of the BER and to link the results to packet error rate (PER) to determine the goodput for TCP transmissions by using a cross-layering approach. The results give evidence that wavelength division multiplexing (WDM) can mitigate the effects of long delay and high BER for a FSO communication using TCP.

Helicity dependent directional surface plasmon polariton excitation using a metasurface with interfacial phase discontinuity

Surface plasmon polaritons (SPPs) have been widely exploited in various scientific communities, ranging from physics, chemistry to biology, due to the strong confinement of light to the metal surface. For many applications, it is important that the free space photon can be coupled to SPPs in a controllable manner. In this Letter, we apply the concept of interfacial phase discontinuity for circularly polarizations on a metasurface to the design of a novel type of polarization-dependent SPP unidirectional excitation at normal incidence. Selective unidirectional excitation of SPPs along opposite directions is experimentally demonstrated at optical frequencies by simply switching the helicity of the incident light. This approach, in conjunction with dynamic polarization modulation techniques, opens gateway towards integrated plasmonic circuits with electrically reconfigurable functionalities.

A submicron plasmonic dichroic splitter

Spectral imaging and sensing techniques, new solar cell designs and wavelength-division multiplexing in optical communication rely on structures that collect and sort photons by wavelength. The strong push for chip-scale integration of such optical components has necessitated ultracompact, planar structures, and fomented great interest in identifying the smallest possible devices. Consequently, novel micro-ring, photonic crystal and plasmonic solutions have emerged. Meanwhile, the optical coupling of subwavelength plasmonic structures supporting a very limited number of modes has also enabled new functionalities, including Fano resonances and structural electromagnetically-induced transparency. Here we show how two similarly sized subwavelength metal grooves can form an ultracompact submicron plasmonic dichroic splitter. Each groove supports just two electromagnetic modes of opposite symmetry that allows independent control of how a groove collects free-space photons and directs surface plasmon polaritons. These results show how the symmetry of electromagnetic modes can be exploited to build compact optical components.

Analytical properties of the plasmon decay profile in a periodic metal-nanoparticle chain

We show that the spatial decay of plasmons in a periodic metal-nanoparticle chain is composed of exponential and power-law decays. Our results show a high level of similarity between the absorptive and radiative decay channels. By analyzing the poles (and the corresponding residues) of the generating function for the lattice Green's function, we explain the details of the spatial decay profile. We also present an analytical formula for the decay profile.

Plasmonic Luneburg and Eaton lenses

Plasmonics takes advantage of the properties of surface plasmon polaritons, which are localized or propagating quasiparticles in which photons are coupled to the quasi-free electrons in metals. In particular, plasmonic devices can confine light in regions with dimensions that are smaller than the wavelength of the photons in free space, and this makes it possible to match the different length scales associated with photonics and electronics in a single nanoscale device. Broad applications of plasmonics that have been demonstrated to date include biological sensing, sub-diffraction-limit imaging, focusing and lithography and nano-optical circuitry. Plasmonics-based optical elements such as waveguides, lenses, beamsplitters and reflectors have been implemented by structuring metal surfaces or placing dielectric structures on metals to manipulate the two-dimensional surface plasmon waves. However, the abrupt discontinuities in the material properties or geometries of these elements lead to increased scattering of surface plasmon polaritons, which significantly reduces the efficiency of these components. Transformation optics provides an alternative approach to controlling the propagation of light by spatially varying the optical properties of a material. Here, motivated by this approach, we use grey-scale lithography to adiabatically tailor the topology of a dielectric layer adjacent to a metal surface to demonstrate a plasmonic Luneburg lens that can focus surface plasmon polaritons. We also make a plasmonic Eaton lens that can bend surface plasmon polaritons. Because the optical properties are changed gradually rather than abruptly in these lenses, losses due to scattering can be significantly reduced in comparison with previously reported plasmonic elements.

Enhancement of near-band edge emission of Au/ZnO composite nanobelts by surface plasmon resonance

In this paper, ZnO nanobelts were synthesized by chemical vapor deposition. And then Au nanoparticles were sputtered on the ZnO nanobelts. Enhancement of the near-band edge emission of Au/ZnO composite nanobelts was observed from photoluminescence measurement due to the coupling between the defect emission from ZnO and surface plasmons of Au nanoparticles, compared to bare ZnO nanobelts. The coupled energy could be transferred into free space photons when the interface is sufficiently rough to scatter the surface plasmons. The enhancement ratio of the near-band edge emission of Au/ZnO is 11-fold higher than that of bare ZnO. The enhancement mechanism was proposed based on scattering and absorption by Au nanoparticles.

Generalized free-space diffuse photon transport model based on the influence analysis of a camera lens diaphragm

The camera lens diaphragm is an important component in a noncontact optical imaging system and has a crucial influence on the images registered on the CCD camera. However, this influence has not been taken into account in the existing free-space photon transport models. To model the photon transport process more accurately, a generalized free-space photon transport model is proposed. It combines Lambertian source theory with analysis of the influence of the camera lens diaphragm to simulate photon transport process in free space. In addition, the radiance theorem is also adopted to establish the energy relationship between the virtual detector and the CCD camera. The accuracy and feasibility of the proposed model is validated with a Monte-Carlo-based free-space photon transport model and physical phantom experiment. A comparison study with our previous hybrid radiosity-radiance theorem based model demonstrates the improvement performance and potential of the proposed model for simulating photon transport process in free space.

Optical properties of metal-dielectric-metal microcavities in the THz frequency range

We present an experimental and theoretical study of the optical properties of metal-dielectric-metal structures with patterned top metallic surfaces, in the THz frequency range. When the thickness of the dielectric slab is very small with respect to the wavelength, these structures are able to support strongly localized electromagnetic modes, concentrated in the subwavelength metal-metal regions. We provide a detailed analysis of the physical mechanisms which give rise to these photonic modes. Furthermore, our model quantitatively predicts the resonance positions and their coupling to free space photons. We demonstrate that these structures provide an efficient and controllable way to convert the energy of far field propagating waves into near field energy.