We demonstrate a novel artificial optical material, the “photonic hyper-crystal”, which combines the most interesting
features of hyperbolic metamaterials and photonic crystals. Similar to hyperbolic metamaterials, photonic hyper-crystals
exhibit broadband divergence in their photonic density of states due to the lack of usual diffraction limit on the photon
wave vector. On the other hand, similar to photonic crystals, hyperbolic dispersion law of extraordinary photons is
modulated by forbidden gaps near the boundaries of photonic Brillouin zones. Three dimensional self-assembly of
photonic hyper-crystals has been achieved by application of external magnetic field to a cobalt nanoparticle-based
ferrofluid. Unique spectral properties of photonic hyper-crystals lead to extreme sensitivity of the material to monolayer
coatings of cobalt nanoparticles, which should find numerous applications in biological and chemical sensing.
Despite strong experimental and theoretical evidence supporting superresolution imaging based on microlenses, imaging
mechanisms involved are not well understood. Based on the transformation optics approach, we demonstrate that
microlenses may act as two-dimensional fisheye or Eaton lenses. An asymmetric Eaton lens may exhibit considerable
image magnification, which has been confirmed experimentally.
Surface plasmon polariton enhanced fluorescence from CdSe/ZnS quantum dots (QD) deposited onto patterned gold/PMMA substrates has been observed. Comparison of gold, chromium, and ITO substrates is used to verify restriction of enhancement effect to surface plasmon polariton supporting materials. Enhanced fluorescence is observed
by using one dimensional gratings defined by electron beam lithography to provide phase matched coupling into surface plasmon polaritons on the metal film surface. Controlled variation of grating parameters is used to examine the dependence of this effect on the periodicity and shape of the coupling grating. Analysis of the results indicates that
strong enhancement requires optimization of both grating periodicity and structure for specific wavelengths and
Plasmonic metamaterials provide a convenient experimental platform for demonstration of principles of transformational
optics. Design and performance of two-dimensional focusing and imaging devices based on plasmonic metamaterials are
described. Depending on frequency, these devices may operate both in the normal lens and the superlens mode. Negative
index imaging and guiding of surface waves using layered plasmonic metamaterials is demonstrated. ε near zero metamaterial is realized.
Superlens imaging based on negative refractive index behavior of surface plasmon polaritons is described. The design of
a magnifying superlens is based on two-dimensional plasmonic metamaterials consisting of alternating layers of positive
and negative refractive index.
Fabrication of three-dimensional negative refractive index materials in the visible range faces numerous technological
challenges. On the other hand, many new concepts and device ideas in the optics of metamaterials may be demonstrated
much easier in two spatial dimensions using surface plasmon polaritons. Here we demonstrate these concepts and
devices using novel multilayer two-dimensional photonic materials consisting of alternating layers of positive and
negative refractive index. By changing composition and geometry of the layers, the effective anisotropic refractive index
of the material may be continuously varied locally from large negative to large positive values. This approach may find
applications in many novel nanophotonic devices, which enable efficient control of light propagation on submicrometer
scale. This becomes possible because of the considerably improved figure of merit of the plasmonic metamaterials
compared to the typical figures of merit of their three-dimensional counterparts.
While many parallel computers have been built, it has generally been too difficult to program them. Now, all computers
are effectively becoming parallel machines. Biannual doubling in the number of cores on a single chip, or faster, over the
coming decade is planned by most computer vendors. Thus, the parallel programming problem is becoming more
critical. The only known solution to the parallel programming problem in the theory of computer science is through a
parallel algorithmic theory called PRAM. Unfortunately, some of the PRAM theory assumptions regarding the
bandwidth between processors and memories did not properly reflect a parallel computer that could be built in previous
decades. Reaching memories, or other processors in a multi-processor organization, required off-chip connections
through pins on the boundary of each electric chip. Using the number of transistors that is becoming available on chip,
on-chip architectures that adequately support the PRAM are becoming possible. However, the bandwidth of off-chip
connections remains insufficient and the latency remains too high. This creates a bottleneck at the boundary of the chip
for a PRAM-On-Chip architecture. This also prevents scalability to larger "supercomputing" organizations spanning
across many processing chips that can handle massive amounts of data. Instead of connections through pins and wires,
power-efficient CMOS-compatible on-chip conversion to plasmonic nanowaveguides is introduced for improved latency
and bandwidth. Proper incorporation of our ideas offer exciting avenues to resolving the parallel programming problem,
and an alternative way for building faster, more useable and much more compact supercomputers.
Fluorescence from a layer of Rhodamine 6G (R6G) is observed to be enhanced strongly if a dielectric grating deposited onto a gold film is used as a substrate. The fluorescence enhancement has been studied as a function of the grating periodicity and the angle of incidence of the excitation light. The enhancement mechanism is consistent with excitation of surface-plasmon-polaritons on the metal film surface. The observed phenomenon may be promising in sensing applications.
We discuss various designs of two-dimensional dielectric optical elements, which enable efficient coupling of external
light to surface plasmon polaritons, and allow us to guide and redistribute surface plasmon energy in the plane of
propagation. Examples of these 2D dielectric optical elements include lenses, mirrors, waveguides, 2D plasmonic
crystals, etc. The effective refractive index of the plasmonic crystals may be either positive or negative. These simple
elements enable us to create compound 2D optical devices, such as microscopes and waveguide couplers. These devices
exhibit diffraction-limited resolution, which is considerably better than the resolution of usual three-dimensional light
Atmospheric turbulence is caused by inhomogeneities in the temperature and pressure of the atmosphere, resulting in random variations of the refractive index. A laser beam propagating through such turbulences experiences random amplitude and phase fluctuations, which can severely degrade the performance of free space optical (FSO) communication systems. In our time delayed diversity (TDD) technique, we transmit twice and take advantage of the fact that propagation along an atmospheric path is statistically uncorrelated with an earlier-time path for a time interval greater than the atmospheric turbulence correlation time. Communications performance is improved because the joint probability of error is less than the probability of error from individual channels. In this paper, we describe the theoretical and experimental analyses of FSO systems implementing this novel scheme in various performance scenarios. Theoretical models and performance of TDD systems are derived and characterized. The experimental performance results obtained under weak turbulence conditions are shown to be in good agreement with the theory. Related system design and implementation issues, such as: atmospheric turbulence statistics, laser beam depolarization, and diversity receiver architecture are also discussed.
We have observed experimentally the anomalously large light transmission through a continuous gold film with various PMMA surface dielectric gratings deposited on top of the film. The spectra of these samples have been measured for different incident and scattered angles. Enhanced transmission through the film is attributed to the excitation of various surface plasmon modes. Both symmetrical and anti-symmetrical surface plasmon dispersion relations have been applied to analyze the transmission spectrum. Similar anomalous transmission effects have been observed in light transmission through gold-chalcogenide glass (As<sub>2</sub>S<sub>3</sub>) interfaces after grating formation in the chalcogenide glass using two-beam interference with strong pump light. Enhanced transmission is demonstrated using a weak probe beam. These observations demonstrate the possibility of all-optical signal processing using enhanced anomalous light transmission through metal films.
The enhanced transmission of square arrays of nanoholes in thin and thick metal films has been studied. We show that normal incidence transmission spectra of an array of elliptical nanoholes in a 220 nm thick gold films have reduced symmetry with respect to the four-fold symmetry found in an equivalent array of circular nanoholes. Elliptical nanoholes milled in a 40 nm thick gold film show complex oscillatory behaviour of the transmission spectrum that has properties similar to those of a two-dimensional birefingent crystal. The transmission spectrum may also be controlled by polarisation selection due to the different degrees of the elliptical polarisation of the transmitted light. The enhanced transmission through 1D arrays of stripes is studied for a range of incident angles with a polarisation perpendicular to the stripe length. Increasing the incident angle increases the number of observed peaks, and changes their spectral positions. Changing the polarisation or the angle of incidence in a 1D array of stripes or a 2D array of reduced symmetry motifs allows control of the enhanced transmission spectrum and shows great potential for numerous applications in photonic and opto-electronic devices.
Atmospheric turbulence causes fluctuations in both the intensity and phase of the received signal in an optical wireless communication link. These fluctuations, often referred to as scintillation noise, lead to signal fading, which increase bit errors in digital communication links using intensity modulation and direct detection. The performance of an optical link can be improved by the use of a time delayed diversity technique, which takes advantage of the fact that the atmospheric path from transmitter to receiver is statistically independent for time intervals beyond the correlation time of the intensity fluctuations. We have designed and constructed a prototype optical wireless system using this scheme. Bit-error-rate measurements have been used to characterize the link performance for different delay periods under conditions of controlled simulated turbulence. It has been determined that link performance improves significantly, especially in strong turbulence. In addition, we have implemented orthogonal polarization modulation, which works especially well in optical wireless systems. In contrast to fiber optic communications, the polarization state of a laser beam is well preserved on a free space optical path.
Polarization properties of the enhanced light transmission through a thin gold film perforated with an array of subwavelength elliptical holes have been studied. It is shown that broadband optical transmission can be achieved through such nanostructures due to the complex nature of the SPP Bloch modes related to a periodic lattice with a low symmetry primitive cell. The optical transmission is dependent on both the incident and transmitted light polarization states even at normal incidence. Using this feature it is therefore possible to tune the transmission spectrum by selecting the polarization of the incident and/or transmitted light. It is shown that such a nanostructure acts as a thin two-dimensional birefringent crystal with wavelength dependent principal optical axes: the property which is not encountered in natural crystals. A rotation of the polarization of the light transmitted through an array of subwavelength holes strongly depends on the thin layer of chiral material placed upon the nanostructured surface. The polarization rotation effect of the chiral molecules appeared to be coupled with the polarization properties of the metallic structure related to SPP excitations. Optical components based on nanostructured metallic systems can find numerous photonic applications space-based and terrestrial systems in extreme ambient conditions.
We describe our latest experimental and theoretical results on two promising nanophotonics geometries for sensor applications. These geometries are based on various combinations of nanohole and/or microdroplet arrays on the surfaces of metal films which support propagation of surface plasmon-polaritons. These novel geometries exhibit large enhancements of local electromagnetic field, which can be used in various nonlinear optical sensing arrangements. For example, liquid microdroplets on the gold film surface support surface plasmon whispering gallery modes. Local field enhancement due to excitation of such modes is determined by combination of both cavity electrodynamics and surface plasmon-polariton related effects. In addition, individual microdroplets have interesting imaging properties, which may be used in high-resolution visualization of individual viruses and cells.
Free space, dynamic, optical wireless communications will require topology control for optimization of network performance. Such networks may need to be configured for bi- or multiple-connectedness, reliability and quality-of-service. Topology control involves the introduction of new links and/or nodes into the network to achieve such performance objectives through autonomous reconfiguration as well as precise pointing, acquisition, tracking, and steering of laser beams. Reconfiguration may be required because of link degradation resulting from obscuration or node loss. As a result, the optical transceivers may need to be re-directed to new or existing nodes within the network and tracked on moving nodes. The redirection of transceivers may require operation over a whole sphere, so that small-angle beam steering techniques cannot be applied. In this context, we are studying the performance of optical wireless links using lightweight, bi-static transceivers mounted on high-performance stepping motor driven stages. These motors provide an angular resolution of 0.00072 degree at up to 80,000 steps per second. This paper focuses on the performance characteristics of these agile transceivers for pointing, acquisition, and tracking (PAT), including the influence of acceleration/deceleration time, motor angular speed, and angular re-adjustment, on latency and packet loss in small free space optical (FSO) wireless test networks.
There is recent interest from the US Department of Defense in free space optical communication networks involving aircraft flying at various altitudes. The optical links between these aircraft may be as long as 100km, and involve communication between network nodes that are moving at sub-sonic speeds. An unresolved issue for links of this kind between pairs of aircraft is the effect of boundary layer turbulence near each aircraft, as well as along the atmospheric path between them. The deployment of optical wireless links in several different scenarios will be described. These include links near to the ground for which the turbulence parameter C<sub>n</sub><sup>2</sup> varies along the path between transmitter (TX) and receiver (RX), high altitude links between aircraft, and ground to aircraft links. The last two of these may involve boundary layer turbulence near the aircraft node where the turbulence is localized either at the TX or at the RX. Some of the theoretical approaches to examining these situations will be described, as well as an ongoing program of research to examine these situations experimentally. Ways to mitigate the effects of node motion, and scintillation at the RX will be discussed, including the use of non-imaging concentrators at the RX.
Free space optics (FSO) is one solution to the bandwidth bottleneck resulting from increased demand for broadband access. It is well known that atmospheric turbulence distorts the wavefront of a laser beam propagating through the atmosphere. This research investigates methods of reducing the effects of intensity scintillation and beam wander on the performance of free space optical communication systems, by characterizing system enhancement using either aperture averaging techniques or nonimaging optics. Compound Parabolic Concentrators, nonimaging optics made famous by Winston and Welford, are inexpensive elements that may be easily integrated into intensity modulation-direct detection receivers to reduce fading caused by beam wander and spot breakup in the focal plane. Aperture averaging provides a methodology to show the improvement of a given receiver aperture diameter in averaging out the optical scintillations over the received wavefront.
Recent measurements of photon tunneling through nanoholes in a gold film covered with a layer of the nonlinear polymer polydiacetylene have provided strong evidence for “photon blockade” effects similar to the Coulomb blockade phenomena observed in single-electron tunneling experiments. We will report on the control of photon tunneling at one wavelength through nonlinear nanoholes by a second, different, wavelength. Our observations suggest the possibility of building a new class of “gated” photon tunneling devices for all-optical signal processing and computation.
The worldwide demand for broadband communications is being met in many places through the use of installed single-mode fiber networks. However, there is still a significant 'first-mile' problem, which seriously limits the availability of broadband Internet access. Free-space optical wireless communication has emerged as a technique of choice for bridging gaps in the existing high data rate communication networks, and as a backbone for rapidly deployable mobile wireless communication infrastructure. Because free space laser communication links can be easily and rapidly redirected, optical wireless networks can be autonomously reconfigured in a multiple-connected topology to provide improved network performance. In this paper we describe research designed to improve the performance of such networks. Using topology control algorithms, we have demonstrated that multiply-connected, rapidly reconfigurable optical wireless networks can provide robust performance, and a high quality of service at high data rates (up to and beyond 1 Gbps). These systems are also very cost-effective. We have designed and tested on the University of Maryland campus a prototype four-node optical wireless network, where each node could be connected to the others via steerable optical wireless links. The design and performance of this network and the topology control is discussed.
Free space optical wireless communication is an attractive way of connecting vast numbers of urban area customers to the fiber optic communication network. We have designed and tested a prototype 2 km long 1.2 Gb/s optical wireless link operating at 1550 nm. An EDFA amplified signal from a standard fiber optic transmitter unit was sent via a small telescope to a 5 inch corner cube mounted on the roof of a building located over 1 km from the transmitter. An estimated 10 mWatt incident on the corner cube was reflected back to the transmitter/receiver unit, where the signal was successfully recovered. Using this test range we have tested the two-fold time-delayed diversity scheme. Diversity delays of 5 ms, and 10 ms show significant reductions in the probability of a joint fade at a particular level. Delays beyond about 10 ms do not significantly improve link performance. The system we have developed allows straightforward DWDM and polarization diversity extensions. Design issues for such optical wireless systems are discussed. We believe that such optical wireless transmitter/receiver units, which operate as an extension of the fiber network, offer a reliable and inexpensive solution for the 'last mile' problem in optical communications.
Atmospheric turbulence produces scintillation at an optical receiver, which leads to fading of the received signal. This fading affects the bit-error-rate (BER) of a digital signal in a way that depends on the depth of the fade, the decision threshold at the receiver, and the average signal-to-noise ratio. The degree of fading can be dramatically reduced by the use of a time-delayed diversity technique, which involves retransmission of the data stream after a short delay, and resynchronization of the received data streams.
Proc. SPIE. 3666, International Conference on Fiber Optics and Photonics: Selected Papers from Photonics India '98
KEYWORDS: Ferroelectric materials, Second-harmonic generation, Ferromagnetics, Microscopy, Ceramics, Near field scanning optical microscopy, Near field, Harmonic generation, Near field optics, Diffraction gratings
Near-field scanning optical microscopy (NSOM) allows simultaneous mapping of both the topography and optical properties of a surface with resolution below the diffraction limit. Second harmonic generation (SHG) always occurs at a surface, even for centro-symmetric media, because of symmetry breaking. By combining NSOM and SHG we can study local variations in symmetry breaking, caused for example by ferroelectric and ferromagnetic domains, and can correlate them with surface topography. We report NSOM/SHG measurements made on piezoelectric ceramics, ferromagnetic materials and periodic structures.
New experimental technique for near-field observation of second-harmonic generation is discussed. This technique gives the unique possibility for the investigation of local optical nonlinear processes with subwavelength resolution and will be useful for the characterization of materials as well as for fabrication and utilization of optical nonlinear devices at nanometer scale. Peculiarities of optical nonlinear processes in near-field region have been studied. The difference in the mechanisms of second-harmonic generation for different polarizations of excitation light has been demonstrated for rough metal surfaces. The dependence of second-harmonic intensity on tip-surface distance has been examined verifying the presence of strong evanescent SH field components. The technique has been applied for characterization of optical nonlinear crystals (LiNbO<SUB>3</SUB> and KDP), ferromagnetic and ferroelectric materials. The spatial resolution of the microscope in the SH light collection mode has been determined to be better than 150 nm.