Near-field nanophotonics offers the promise of orders-of-magnitude enhancements for phenomena ranging from spontaneous-emission engineering to Casimir forces via zero-point quantum fluctuations. An increasing variety of approaches — photonic crystals, metamaterials, metasurfaces, antennas, and more — has underscored our lack of understanding as to how large these effects can be. We provide a general answer to this question, deriving the first sum rule for near-field optical response as well as general upper bounds for any bandwidth, i.e. power–bandwidth limits. Within such a unified framework valid for structures of arbitrary shape and size, we approach single-frequency limits as bandwidth goes to zero and the sum rule as bandwidth goes to infinity. Power–bandwidth limits are derived from energy-conservation principles and depend on the susceptibility at the frequency of interest, and the sum rule arises from the requirement of causality and only depends on susceptibility at zero frequency. We explore to what extent power–bandwidth bounds can be attained for real materials and how the sum rule can be realized for canonical geometries. We further prove a "monotonicity" theorem that enables us to bound the integrated frequency response of any complicated structure in terms of the response of simple geometries. Our framework provides a universal measure of intrinsic optical-response characteristics that helps identify optimal nanophotonic materials for any combination of frequency and bandwidth, leading to wide-ranging applications in medical imaging and thermophotovoltaics.
For 75 years it has been known that radiative heat transfer can exceed far-field blackbody rates when two bodies are separated by less than a thermal wavelength. Yet an open question has remained: what is the maximum achievable radiative transfer rate? Here we describe basic energy-conservation principles that answer this question, yielding upper bounds that depend on the temperatures, material susceptibilities, and separation distance, but which encompass all geometries. The simple structures studied to date fall far short of the bounds, offering the possibility for significant future enhancement, with ramifications for experimental studies as well as thermophotovoltaic applications.
Nanophotonic techniques can enable numerous novel and exciting phenomena. However, in order to make use of these opportunities for many applications of interest (e.g. energy, or displays), one has to have the ability to implement nanophotonic structures over large scales. In this talk, I will present some of our recent theoretical and experimental progress in exploring these opportunities.
Photonic crystals provide superb opportunities for tailoring of the photonic density of states. This ability can in turn be
explored to control radiation into far-field, enhance fluorescent light emission, as well as optimize laser emission. In
order to make these phenomena useful for large macroscopic devices, large-area nano-fabrication techniques have to be
successfully implemented. In this talk, I will present some of our recent theoretical and experimental progress in
exploring these opportunities.
We present two photonic crystal enabled platforms, exhibiting novel active optical
phenomena. First, using a detailed theoretical and numerical analysis, we show how
a Purcell-effect inspired nonlinear nanophotonic scheme could enable optimal and
compact THz sources via optical difference frequency generation. Second, we show
how electromagnetic one-way edge modes analogous to quantum Hall edge states,
originally predicted by Raghu and Haldane in gyroelectric photonic crystals, can
appear in more general settings. In gyromagnetic YIG photonic crystals operating at
microwave frequencies, time-reversal breaking is strong enough that the effect is
readily observable. We present our experimental results on this novel
We present analytical results that shed new light on the properties of photonic-crystal fibers (optical fibers with
periodic structures in their cladding). First, we discuss a general theorem, applicable to any periodic cladding
structure, that gives rigorous conditions for the existence of cutoff-free guided modes-it lets you look at a
structure, in most cases without calculation, and by inspection give a rigorous guarantee that index-guiding
will occur. This theorem especially illuminates the long-wavelength limit, which has proved diffcult to study
numerically, to show that the index-guided modes in photonic-crystal fibers (like their step-index counterparts)
need not have any theoretical cutoff for guidance. Second, we look in the opposite regime, that of very short
wavelengths. As previously identified by other authors, there is a scalar approximation that becomes exact in
this limit, even for very high contrast fibers. We show that this "scalar" limit has consequences for practical
operation at finite wavelengths that do not seem to have been fully appreciated: it tells you when band gaps
arise and between which bands, reveals the symmetry and "LP" degeneracies of the modes, and predicts the
scaling of cladding-related losses (roughness, absorption, etc.) as the size of a hollow core is increased.
We investigate the extension of optical micromanipulation to integrated optics. In particular, we consider whether propagating light signals can cause mechanical reconfiguration of a device. While such forces are intrinsically weak, we predict theoretically that significant displacements can be achieved using various enhancement mechanisms. These include the use of high-index materials, high-Q (cavity quality factor) enhancement, and slow light in photonic crystals. Silicon optical waveguides have a considerable refractive index contrast with the surrounding air, with a ratio of roughly 3.45/1 at optical communications wavelengths. We show that the strong confinement of light to silicon magnifies optical forces arising from overlap in the guided modes of neighboring waveguides. Silica microsphere resonators are known to have extremely high cavity quality factors, in excess of 108. We show that the quality factor of the resonator magnifies the optical force due to modal overlap between two neighboring spheres. Thirdly, we investigate slow-light enhancement of optical forces using photonic-crystal devices. We show that slow-light velocities give rise to larger forces for the same amount of signal power, enhancing optomechanical coupling effects. In addition to being of fundamental interest, our work suggests that optical manipulation may ultimately provide a route to all-optical conformational control and switching.
Finite-difference time-domain (FDTD) methods suffer from reduced accuracy when modeling
discontinuous dielectric materials, due to the inhererent discretization ("pixellization"). We show
that accuracy can be significantly improved by using a sub-pixel smoothing of the dielectric function,
but only if the smoothing scheme is properly designed. We develop such a scheme based on a
simple criterion taken from perturbation theory, and compare it to other published FDTD smoothing
methods. In addition to consistently achieving the smallest errors, our scheme is the only one
that attains quadratic convergence with resolution for arbitrarily sloped interfaces. Finally, we
discuss additional difficulties that arise for sharp dielectric corners.
We study tunable time-delay devices in which the time delay is tuned by changing the group velocity of the propagating signal. The device is designed to place the operating frequency near a photonic band edge. This enhances the change in delay for a given tuning range of the device refractive index. Here we provide an extended explanation of mode symmetry, nomenclature, and the complete band structure of a sample, integrated device to aid the understanding of our previously published work.
We show that two-dimensional photonic crystals can be designed to have dispersion relations with an extended ultra-flat cross-section, meaning that for a fixed wave vector component kx the frequency of a band is almost constant when the other wave vector component, ky, takes all possible values. These ultra-flat bands are the result of a non-trivial saddle point in the dispersion relation located in the interior of the Brillouin zone. Interesting consequences include 1D-like behavior, improved super-collimation, and enhanced density of states.
Supercontinuum based sources and measurement techniques are developed, enabling optical ultra-broadband studies of nano-scale photonic crystal devices and integrated photonic circuits over 1.2 - 2.0 micron wavelength range. Experiments involving 1-D periodic photonic crystal microcavity waveguides and 3-D periodic photonic crystals with embedded point defects are described. Experimental findings are compared with rigorous electromagnetic simulations.
We describe the use of high-index-contrast, photonic-crystal wavegides for tunable time delays. The waveguide is designed such that the operating frequency is near a photonic band edge. In this slow light region, a small change in index yields a large change in group velocity, and consequently in time delay. Figures of merit for tunable time delay devices are introduced, including sensitivity, length, and dispersion.
We show that a simple quadratic band model is a good predictor of the figures of merit for realistic, 3D, high-index-contrast structures. By cascading two grated waveguides, we can obtain a flat tunable time delay across the operating bandwidth.
The majority of photonic crystals developed till-date are not dynamically tunable, especially in silicon-based structures. Dynamic tunability is required not only for reconfiguration of the optical characteristics based on user-demand, but also for compensation against external disturbances and relaxation of tight device fabrication tolerances. Recent developments in photonic crystals have suggested interesting possibilities for static small-strain modulations to affect the optical characteristics [1-3], including a proposal for dynamic strain-tunability . Here we report the theoretical analysis, device fabrication, and experimental measurements of tunable silicon photonic band gap microcavities in optical waveguides, through direct application of dynamic strain to the periodic structures . The device concept consists of embedding the microcavity waveguide  on a deformable SiO2 membrane. The membrane is strained through integrated thin-film piezoelectric microactuators. We show a 1.54 nm shift in cavity resonances at 1.56 um wavelengths for an applied piezoelectric strain of 0.04%. This is in excellent agreement with our modeling, predicted through first-order semi-analytical perturbation theory  and finite-difference time-domain calculations. The measured microcavity transmission shows resonances between 1.55 to 1.57 um, with Q factors ranging from 159 to 280. For operation at infrared wavelengths, we integrate X-ray and electron-beam lithography (for critical 100 nm feature sizes) with thin-film piezoelectric surface micromachining. This level of integration permits realizable silicon-based photonic chip devices, such as high-density optical filters and spontaneous-emission enhancement devices with tunable configurations.
In this work we present an introduction to photonic crystals by discussing the basic concepts and principles behind these artificial materials, as well as their abilities to control light and enable unusual optical phenomena. We will focus on specific examples including (1) negative refraction of light, (2) the superprism effect (anomalous electromagnetic dispersion), and (3) the possibility of superlensing (subwavelength focusing). These are very general results based on direct solutions of Maxwell’s equations, and can consequently be of relevance to many areas of science and technology.
The ability of photonic crystals to mold the flow of light in new ways can lead to a variety of novel and improved designs of optical nano-components and nano-devices in photonics. Two examples will be presented: a) Using linear materials, a polarization independent waveguide is designed in a 3D photonic crystal. It is demonstrated that this system provides lossless guiding of light at length-scales approaching the wavelength of the light itself, offering a promising platform for the design of integrated high performance polarization-insensitive waveguide networks. b) Using nonlinear materials, a cylindrical photonic crystal fiber is designed that can exhibit all-optical switching without the need for an axial periodicity. It is shown that this property stems from the unique structure of the cylindrical photonic crystal guided-mode dispersion relation, and can lead to significant improvements in manufacturing ease, operating power usage, and device size requirements, making such a system ideal for integrated all-optical signal processing.
We demonstrate optical bistability in a class of non-linear photonic crystal devices, through the use of detailed numerical experiments, and analytical theory. Our devices are shorter than the wavelength of light in length, they can operate with only a few mW of power, and can be faster than 1ps.
We demonstrate how dramatic increases in the induced phase shifts caused by small changes in the index of refraction can be achieved by using very slow group velocities of light, which are readily achievable in photonic crystal systems. Combined with the fact that small group velocity greatly decreases the power requirements needed to operate a device, enhanced phase sensitivity may be used to decrease the size and power requirements of many devices, including switches, routers, all-optical logical gates, wavelength converters, etc. We demonstrate how these advantages can be used to design switches smaller than 20*200 square microns in size, using readily available materials, and at modest levels of power. With this approach, one could have "4O such devices on a surface 2*2 square cm, making it an important step towards large-scale all-optical integration.
A new micro-cavity design is proposed and structures are realized using a 2D photonic-crystal slab. The cavity consists of seven defect holes that encompass a hexagon and is designed to reduce vertical light leakage. From a direct transmission measurement, a Q-value of 816+/- 30 is achieved at (lambda) =1.55micrometers . This high-Q cavity will enable realistic realization of spontaneous emission modification and on-off optical switches.
We argue that OmniGuide fibers, which guide light within a hollow core by concentric multilayer films having the property of omnidirectional reflection, have the potential to lift several physical limitations of silica fibers. We show how the strong confinement in OmniGuide fibers greatly suppresses the properties of the cladding materials: even if highly lossy and nonlinear materials are employed, both the intrinsic losses and nonlinearities of silica fibers can be surpassed by orders of magnitude. This feat, impossible to duplicate in an index-guided fiber with existing materials, would open up new regimes for long-distance propagation and dense wavelength-division multiplexing (DWDM). The OmniGuide-fiber modes bear a strong analogy to those of hollow metallic waveguides; from this analogy, we are able to derive several general scaling laws with core radius. Moreover, there is strong loss discrimination between guided modes, depending upon their degree of confinement in the hollow core: this allows large, ostensibly multi-mode cores to be used, with the lowest-loss TE01 mode propagating in an effectively single-mode fashion. Finally, because this TE01 mode is a cylindrically symmetrical ('azimuthally' polarized) singlet state, it is immune to polarization-mode dispersion (PMD), unlike the doubly-degenerate linearly-polarized modes in silica fibers that are vulnerable to birefringence.
We describe a new photonic-crystal structure with a complete three-dimensional photonic band gap (PBG) and its potential application to integrated optics. The structure not only has a large band gap and is amenable to layer-by-layer litho-fabrication, but also introduces the feature of high-symmetry planar layers resembling 2D photonic crystals. This feature enables integrated optical devices to be constructed by modification of only a single layer, and supports waveguide and resonant-cavity modes that strongly resemble the corresponding modes in the simpler and well-understood 2D systems. In contrast to previous attempts to realize 2D crystals in 3D via finite-height 'slabs', however, the complete PBG of the new system eliminates the possibility of radiation losses. Thus, it provides a robust infrastructure within which to embed complex optical networks, combining elements such as compact filters, channel-drops and waveguide bends/junctions that have previously been proposed in 2D photonic crystals.
Using a 2D photonic-crystal slab structure, we have demonstrated a strong 2D photonic band gap with the capability of fully controlling light in all three dimensions. Our demonstration confirms the predictions on the possibility of achieving 3D light control using 2D band gaps, with strong index guiding providing control in the third dimension, and raise the prospect of being able to realize novel photonic-crystal devices. Based on such slab structure with triangular lattice of holes, a 60 degree photonic-crystal waveguide bend is fabricated. The intrinsic bending efficiency is measured within the photonic band gap. As high as 90 percent bending efficiency is observed at some frequencies.
The advent of large-scale, free-space, opto-electronic interconnections, as demonstrated in recent system prototypes, requires new sampling methods to reveal diagnostic information. Several factors contribute to the difficulty of probing optical communications channels without disrupting their operation. High-speed electronic connections to the chip periphery are not available in sufficient number and would contribute an undesirable thermal load. Electronic and optical physical contact probes would obscure many of the optical channels that are relayed to a common surface of the chip in current systems. Optical sampling provides the better method although many standard techniques are either too time consuming or complex to implement. We describe a tool we developed that delivers diagnostic information on a large number of high-speed, optical data channels simultaneously and operates analogously to the conventional sampling electronic oscilloscope. The optical oscilloscope is constructed using CCD cameras and video capture boards that are controlled by a software application resident in a personal computer. Sampling is based on a stroboscopic method of using short pulsed laser probe beam synchronized to a data stream to illuminate optical modulators within the optoelectronic circuit. We have demonstrated and discuss the tool's capability of simultaneously monitoring arrays of broadband optoelectronic devices operating at speeds from several hundred Megabit/s to a few Gigabit/s.
Active and Passive Optical Components for Communications VII
11 September 2007 | Boston, MA, United States
Active and Passive Optical Components for Communications VI
3 October 2006 | Boston, Massachusetts, United States
Active and Passive Optical Components for WDM Communications V
24 October 2005 | Boston, MA, United States
Active and Passive Optical Components for WDM Communications IV
25 October 2004 | Philadelphia, Pennsylvania, United States
Active and Passive Optical Components for WDM Communications III
8 September 2003 | Orlando, Florida, United States
SC608: Photonic Crystals: A Crash Course, from Bandgaps to Fibers
This half-day course will survey basic principles and developments in the field of photonic crystals, nano-structured optical materials that achieve new levels of control over optical phenomena. This leverage over photons is primarily achieved by the photonic band gap: a range of wavelengths in which light cannot propagate within a suitably designed crystal, forming a sort of optical insulator.
The course will begin with an introduction to the fundamentals of wave propagation in periodic systems, Bloch's theorem and band diagrams, and from there moves on to the origin of the photonic band gap and its realization in practical structures. After that we will cover a number of topics and applications important for understanding the field and its future.
Topics will include: the introduction of intentional defects to create waveguides, cavities, and ideal integrated optical devices in a crystal; exploitation of exotic dispersions for negative-refraction, super-prisms, and super-lensing; the combination of photonic band gaps and conventional index guiding to form easily fabricated hybrid systems (photonic-crystal slabs); the origin and control of losses in hybrid systems; photonic band gap and microstructured optical fibers; and computational approaches to understanding these systems (from brute-force simulation to semi-analytical techniques).