We demonstrate three ways in which the optical band-gap of 2-D macroporous silicon photonic crystals can be tuned. In the first method the temperature dependence of the refractive index of an infiltrated nematic liquid crystal is used to tune the high frequency edge of the photonic band gap by up to 70 nm for H-polarized radiation as the temperature is increased from 35 to 59°C. In a second technique we have optically pumped the silicon backbone using 150 fs, 800 nm pulses, injecting high density electron hole pairs. Through the induced changes to the dielectric constant via the Drude contribution we have observed shifts upt to 30 nm of the high frequency edge of the E-polarized band-gap. Finally, we show that below-band-gap radiation at 2.0 and 1.7 μm can induce changes to the optical properties of silicon via the Kerr effect and tune the band edges of the 2-D macroporous silicon photonic crystal.
We present experimental demonstration of fast all-optical switching in a one-dimensional photonic crystal nanocavity embedded in a Silicon waveguide. The transmission of the device is tuned by injecting free carriers into the nanocavity region using an optical pump beam. By strongly confining light in the photonic crystal nanocavity the sensitivity of light to small refractive index changes is enhanced. The small cavity volume (~0.1 μm3) and unpassivated sidewalls enable ultra-fast switching speeds with low pulse energies. Using a pulse energy of only 60pJ, a refractive index change of approximately 10-2 is obtained. This small index change, due to the high confinement nature of the cavity structure, leads to a strong change in transmission spectrum. Consequently, the resonance is shifted up to its full-width-at half-maximum (~7.5nm), and the transmission of the device is modulated by 71% with a time response of less than 1.5 ns. Such a device could open the door to the large-scale integration of ultra-fast modulators and switches.
Controlling the optical properties of patterned optical materials is generally useful. Much headway has been made into patterning passive optical structures and fabricating them of a variety of materials. Recent efforts have been made to tune the properties thermally, electronically and optically. Liquid crystals offer several mechanisms for varying their optical properties. They have been incorporated into patterned templates to make electro-optic and photo-optic materials. Holographic photopolymerization provides a means to form an arbitrary structure incorporating multiple material phases such as liquid crystals. Electro optic holographic polymer dispersed liquid crystals (HPDLCs) have been the subject of extensive investigation. We report photo-optic HPDLCs incorporating azobenzene derived liquid crystals (Azo-LCs).
We demonstrate an electrically-tuned nematic liquid crystal (LC) infiltrated photonic crystal (PC) laser. The PC laser is encased between two transparent indium tin oxide (ITO) glass plates which serve as the modulating electrodes and also define the LC cell. Applying a voltage across the cell realigns the LC, modifies the laser cavity's optical path length, and blue-shifts the lasing wavelength. The measured tuning threshold voltage agrees well with the experimentally determined LC threshold voltage which confirms the tuning is due to the LC realignment at the onset of the LC's Freedericksz transition. Furthermore, the electrically-tuned PC laser also demonstrates the successful integration of nonlinear optical materials, electronics, and fluidics with PCs and suggests further integration with other materials will lead to photonic devices with increased functionality and utility.
Optical interconnects have begun replacing electrical wires in long distance, backplane applications. As their switching speed and efficiency improves, optical interconnects will penetrate deeper into the device architecture for inter- and intra-chip communications where direct integration with silicon microelectronics is a necessity. Tunable 1D and 2D silicon-based photonic bandgap (PBG) structures are viable building blocks for optical interconnects because they have the capability to redirect light both in- and out-of-plane. In this work, we report on external modulation of the optical properties of 1D and 2D porous silicon PBG structures infiltrated with liquid crystals. This class of eletrooptic modulators offers an inexpensive and versatile way of integrating optical interconnects with standard microelectronic circuits.
Our calculation concerns two-dimensional photonic crystals (PCs) of hollow cylinders (in a dielectric matrix) that are infilled with a nematic liquid crystal (NLC). A static electric field, applied parallel to the cylinders, tunes the optical response of the PC. A crucial aspect is the calculation of the dielectric tensor of the NLC. Here we generalize a preliminary study , now considering three possible configurations: escaped radial, planar radial, and axial. Further, the anchoring of the molecules at the cylinder walls now has an arbitrary strength. Finally, we address the full problem of inhomogeniety and anisotropy of the NLC cylinders. A phase transition is found from the escaped radial to the axial configuration for sufficiently high field values, which depend on the cylinder radius. The Photonic Band (PB) structure reveals gaps for propagation in the  and  directions and also a PB gap in all directions (in the plane of periodicity) for modes that are approximately polarized parallel to the cylinders. Our results show that these gaps can be tuned by the applied field.
The bandstructure of photonic crystals offers intriguing
possibilities for the manipulation of electromagnetic waves.
During the last years, research has mainly focussed on the
application of these photonic crystal properties in the telecom
area. We suggest utilization of photonic crystals for sensor
applications such as qualitative and quantitative gas and liquid
analysis. Taking advantage of the low group velocity and certain
mode distributions for some k-points in the bandstructure
of a photonic crystal should enable the realization of very
compact sensor devices. We show different device configurations of
a photonic crystal based on macroporous silicon that fulfill the
demands to serve as a compact gas sensor.
The optical properties of photonic bandgap (PBG) structures are highly sensitive to the geometry and refractive index. This makes PBG structures a good host for sensor applications. The binding of target species inside the PBG structure changes the refractive index of the material, which can be detected by monitoring the optical response of the device. One-dimensional PBG biosensors based on porous silicon (PSi) have been fabricated. The device is a microcavity, made of a symmetry breaking PSi layer (defect layer) inserted between two PSi Bragg mirrors. Narrow resonances are introduced in the photoluminescence and reflectance spectra. The large internal surface of the sensor is functionalized for the capture of target biological materials. When the sensor is exposed to the target, binding to the internal surface increases the effective optical thickness of the microcavity and thus causes a red shift of the optical spectrum. The sensor's sensitivity is determined by the morphology and geometry of the device. We will present the details of the materials science, sensor fabrication and optimization, and also describe experiments performed with biological targets.
We present a review of recent studies into the tunability of 2D PC slab waveguides designs. The properties of dynamic, static and hybrid superlattice photonic crystals are reviewed and the mechanism of tunability and its impact on tuning the refractive and dispersion and propagation properties are presented.
We show that tunable photonic band gap materials offer new opportunities for device applications. Optical switches or sensors that are far more compact and sensitive, for instance, can be constructed when we introduce either optical or mechanical tunability into photonic crystal structures. Furthermore, when we tune the photonic crystal while a photon is inside the crystal, the crystal can exhibit qualitatively different optical physics effects. As a particularly exciting example, we show that light can in fact be completely stopped in a tunable photonic crystal, and the conventional delay bandwidth product limit in resonator optics can be completely overcome.
A tunable two-dimensional photonic crystal (2D-PC) design is proposed in which the background medium is composed of an electro-optic material such as lead lanthanum zirconate titanate. The lattice structure is based upon the 2D triangular lattice of holes in a dielectric background; however, holes of two different radii are used and arranged in such a way to create a superlattice structure. The optical properties of this structure are modified by applying an electrical bias so that the dielectric constant is changed from 6.2 to 6.75. Numerical calculations show the band structure of the superlattice is highly modified in comparison to the triangular lattice. In particular, the first full photonic band gap decreases in width and band splitting occurs at high symmetry points of the lattice. From the analysis of the equifrequency contours resulting from the dispersion surface, the angle of refraction of an incident beam was calculated. By changing the biasing conditions on the structure, the refracted beam can be tuned >55° at an incident angle of 14°. This represents an increase of functionality over the regular triangular lattice which is tunable over 5-10° for a 1.36 change in dielectric constant.
The dielectric distribution and polarizability of certain materials, e.g., the liquid dielectrics, change in response to the external electromagnetic field. Since their photonic properties can be adjusted by controlling the applied field, these materials can be used to construct tunable photonic band gap crystals. Due to recent advances in tunable photonic bandgap materials technology, it has become necessary to determine the properties of the propagating fields accurately. Numerical methods currently in use are quite cumbersome and place limits on the accuracy of the solutions. A numerical scheme is developed here by expressing the solution in the framework of the Feynman path integral formulation of quantum mechanics. The formulation describes the evolution of the solution in terms of a propagator, which can be determined by the method of fast Fourier transforms. The resulting numerical scheme is more efficient and reliable than other similar methods.
The optimization of the procedure to grow accurate amounts of amorphous silicon and germanium by chemical vapor deposition (CVD) free of contamination in opals has been performed. The samples have been optically characterized and results agree with theoretical calculations of band structures. Multilayer systems of both semiconductors have been fabricated. Samples have been optically characterized and observed with a scanning electron microscope. Selective removal of germanium with aqua regia has proven to be possible. Theoretical calculations show that subtle variations of the topography may give rise to important effects (flat bands, pseudogap openings, etc). As an example, a photonic band structure with a complete photonic band gap between the 5th and 6th band has been provided along with a method to obtain it. It would be impossible to discuss all the possible structures that could be obtained from samples with different number of layers and materials forming them. However, there are many interesting topographies that could be fabricated in a relatively straightforward manner following the techniques described here.
Bulk polystyrene opals have been grown. Variable incidence angle reflectance spectroscopy is used to probe their photonic band structures. Several different structures are observed and accounted for by theoretical calculations of photonic bands and density of states. The results yield a clear distinction between diffraction in the direction of propagation by the (111) family planes (leading to the formation of the stop band) and diffraction in other directions by higher-order planes (corresponding to excitation of photonic modes in the crystal).
As photonic bandgap (PBG) technology matures and practical devices are realized, the effects of environmental factors, such as ambient temperature, on PBG device operation must be considered. The position of a PBG is determined by the geometry and refractive index of the constituent materials. Therefore, a thermally induced material expansion or refractive index change will alter the location of the PBG and affect the operation of PBG devices. In order to achieve faster switching times for PBG optical interconnects, enhanced sensitivity for PBG sensors, and smaller channel spacing for PBG-based wavelength division multiplexing, increasingly narrow PBG resonances are required. The drawback for the improved device operation is increased sensitivity to small changes in environmental conditions. A method to control and eliminate thermally induced drifts of silicon-based PBG structures has been developed based on a simple oxidation treatment. Oxide coverage of the silicon matrix provides a counterforce to the effect of the temperature dependent silicon refractive index. Depending on the degree of oxidation achieved, a redshift, no shift, or a blueshift of the PBG resonance results when the silicon-based PBG structure is heated. Control over the effects of thermal fluctuations has been demonstrated for two different PBG structure designs. Extensive reflectance and x-ray diffraction measurements have been performed to understand the mechanism behind this oxidation procedure as it relates to one-dimensional silicon-based PBG microcavities.
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.
We report a tunable nanophotonic device concept based on flexible photonic crystal, which is comprised of a periodic array of high index dielectric material and a low index flexible polymer. Tunability is achieved by applying mechanical force with nano-/micro-electron-mechanical system actuators. The mechanical stress induces changes in the periodicity of the photonic crystal and consequently modifies the photonic band structure. To demonstrate the concept, we theoretically investigated the effect of mechanical stress on the anomalous refraction behavior and observed a very wide tunability in the beam propagation direction. Extensive experimental studies on fabrication and characterizations of the flexible photonic crystal structures were also carried out. High quality nanostructures were fabricated by e-beam lithography. Efficient coupling of laser beam and negative refraction in the flexible PC structures have been demonstrated. The new concept of tunable nanophotonic device provides a means to achieve real-time, dynamic control of photonic band structure and will thus expand the utility of photonic crystal structures in advanced nanophotonic systems.
In this paper, a novel Frequency Division Multiplexer (FDM) using Photonic Band Gap (PBG) cell combination concept circuit is proposed for achieving a 3-band FDM. The preliminary 3-band FDM structure is the combination of three PBG cells. The observable frequency response experimental results are presented. We also simulate and measure all the scattering parameters for the novel 3-band FDM. The disclosed method in this paper demonstrates the possibility for applying photonic bandgap structure in designing a frequency division device.
The emerging multimedia system must be able to process huge volume of information data nowadays. Optical pick up heads with small size inspired great interest. This not only reduces the size of optical drives but also improves the ruggedness and data transferring rate. A novel design based on near field optics is proposed in this paper. We bring up concept of compact optical pickup head based on SOI substrate. By using interference and photonic crystal band gap (PBG) cell, we construct a novel optical pickup head. We can get smaller spot size than current one owing to realizing the interference of two beams. The PBG cell forms the mechanism of interference. The near field analysis of the pick up heads is theoretically investigated. Device characteristics and functions are studied. The simulation results are obtained by using finite difference time domain (FDTD) method.
We present numerical results for the optical response of tunable one-dimensional photonic crystals. Two different (monolithic) superlattices (SL) where studied: InSb/air and Si/air. We explore the tuning of a defect state as a function of the temperature and of the donor concentration. The numerical calculations were performed using a realistic model for the dielectric constant that takes into account the dispersion and absorption due to electrons, holes and phonons. In order to lessen absorptive effects, we have optimized the structure so as to achieve strong sensitivity of the defect state tuning.
In this paper, we will show modeling of MEM-mirrors for tunable optical filters and present the simulation results using ConventorWare software. The gold-plated micro-mirror is placed on InP/Air pair structure separated by air-gap. A voltage induced electrostatic force displaces the micro-mirror. Very wide tuning range of 149 nm is observed for InP/air-gap filters at low actuation voltage of 2.5V. To carry out optical simulation we developed a JAVA program that characterizes the reflectance spectra and the tuning behavior. These mirror designs can be utilized for optical communication and sensing applications in tunable optical filters.