I analyze new phenomena arising from embedding active materials inside of photonic crystal structures. These structures strongly modify the photonic local density of states (LDOS), leading to quantitative and qualitative changes in the behavior of active materials. First, I show that the emission spectrum of pointlike sources inside an ``omniguide'' is strongly modified by features resembling one-dimensional van Hove singularities in the LDOS. The resulting overall enhancement of the LDOS causes radiating dipoles to emit more rapidly than in vacuum (known as the Purcell effect). Second, I study optically pumped lasing in three model systems: a Fabry-Perot cavity, a line of defects in a two-dimensional square lattice of rods, and a cylindrical photonic crystal. It is shown that high conversion efficiency can be achieved for large regions of active material in the cavity, as well as for a single fluorescent atom in a hollow-core cylindrical photonic crystal, suggesting designs for ultra-low-threshold lasers and ultra-sensitive biological sensors. Third, I consider a photonic crystal-based light-trapping scheme, capable of compensating for weak optical absorption of crystalline silicon solar cells in the near infrared. For a 2 micron-thick cell, relative efficiency enhancements as high as 35% are expected. Fourth, I explore a way to achieve full plus-or-minus 90 degree electronically-controlled beam steering using a linear array of one-dimensionally periodic elements containing electro-optic materials. Fifth, I consider switching of a single signal photon by a single gating photon of a different frequency, via a cross-phase modulation generated by electromagnetically-induced transparency atoms embedded in photonic crystals. The exact solution shows that the strong coupling regime is required for lossless two-photon quantum entanglement. Finally, I demonstrate that the Purcell effect can be used to tailor the effective Kerr nonlinear optical susceptibility. Using this effect for frequencies close to an atomic resonance can substantially influence the resultant Kerr nonlinearity for light of all (even highly detuned) frequencies. For example, in realistic physical systems, enhancement of the Kerr coefficient by one to two orders of magnitude could be achieved.
The front-coating (FC) of a solar cell controls its efficiency, determining admission of light into the absorbing material and potentially trapping light to enhance thin absorbers. Single-layer FC designs are well known, especially for thick absorbers where their only purpose is to reduce reflections. Multilayer FCs could improve performance, but require global optimization to design. For narrow bandwidths, one can always achieve nearly 100% absorption. For the entire solar bandwidth, however, a second FC layer improves performance by 6.1% for 256 micron wafer-based cells, or by 3.6% for 2 micron thin-film cells, while additional layers yield rapidly diminishing returns.
Herein the authors report the experimental application of a powerful light trapping scheme, the textured photonic crystal (TPC) backside reflector, to thin film Si solar cells. TPC combines a one-dimensional photonic crystal as a distributed Bragg reflector with a diffraction grating. Light absorption is strongly enhanced by high reflectivity and large angle diffraction, as designed with scattering matrix analysis. 5 micron-thick monocrystalline thin-film Si solar cells integrated with TPC were fabricated through an active layer transfer technique. Measured short circuit current density Jsc was increased by 19%, compared to a theoretical prediction of 28%.
We develop a coupled mode theory (CMT) model of the behavior of a polarization source in a general photonic structure, and obtain an analytical expression for the resulting generated electric field; loss, gain and/or nonlinearities can also be modeled. Based on this treatment, we investigate the criteria needed to achieve an enhancement in various nonlinear effects, and to produce efficient sources of terahertz radiation, in particular. Our results agree well with exact finite-difference time-domain (FDTD) results. Therefore, this approach can also in certain circumstances be used as a potential substitute for the more numerically intensive FDTD method.
Nonlinear photonic-crystal nanoresonators offer unique, fundamental ways of enhancing a variety of nonlinear optical processes. This enhancement improves the performance of nonlinear optical devices to such an extent that their corresponding operating powers and switching times are suitable for implementation in realistic ultrafast, integrated optical devices. Here, we review three different nonlinear optical phenomena that can be strongly enhanced in photonic crystal nanocavities. First, we discuss in detail a system in which this enhancement has been succesfully demonstrated both theoretically and experimentally, namely, a photonic crystal cavity showing optical bistability properties. In this part, we also present the physical basis for this dramatic improvement with respect to traditional nonlinear devices based on nonlinear Fabry-Perot etalons. Secondly, we show how nonlinear photonic crystal cavities can also be used to obtain complete second harmonic frequency converstion at very low input powers. Finally, we demonstrate that the nonlinear susceptibility of materials can be strongly modified via the so-called Purcell effect, present in the resonant cavities under study.
We predict that the effective nonlinear optical susceptibility can be tailored using the Purcell effect. While this is a general physical principle that applies to a wide variety of nonlinearities, we specifically investigate the Kerr nonlinearity. We show theoretically that using the Purcell effect for frequencies close to an atomic resonance can substantially influence the resultant Kerr nonlinearity for light of all (even highly detuned) frequencies. For example, in realistic physical systems, enhancement of the Kerr coefficient by one to two orders of magnitude could be achieved.
A novel on-chip Bragg cladding waveguide is designed and fabricated using conventional CMOS techniques. This optical waveguide has a low refractive index core surrounded by high index-contrast cladding bilayers. Polysilicon (n = 3.5) and silicon nitride (n=2.0) are used for high index-contrast Bragg layers, where index difference is as high as 1.5. Our simulation shows that sharp bending in low index core materials can be achieved, which is not possible using index guiding mechanism. Within our approach, various on-chip applications are expected such as optical integration, high power transmission, biosensor/microelectromechanical system and so on.
In this work, a textured photonic crystal is used as a novel backside reflector for mono- and poly-crystalline Si thin film solar cells. The backside reflector has two components, a grating and a distributed Bragg reflector (DBR), both of which enhance light-trapping for the near-infrared region of crystalline silicon. Simulations based on the scattering matrix method were used to systematically optimize all the key parameters to achieve the highest efficiency for a given solar cell thickness. We found that the optimal length scales in the problem, namely the period of the grating, the etch depth of the grating, the Bragg wavelength of the DBR, and the anti-reflection coating thickness, all decrease linearly as the absorption layer becomes thinner. The optimal value for the dimensionless duty cycle of the grating is found to be around 0.5 for all cell thicknesses. For a 2 micron-thick cell, the efficiency enhancement relative to a cell with un-patterned backside can be as high as 53% using the optimized design.
This manuscript demonstrates the possibility of single-photon optical switching in a waveguide-cavity QED framework using electromagnetically induced transparency (EIT). An analytical model of a system consisting of a photonic crystal (PhC) waveguide, a microcavity, and a four-level EIT atom is solved exactly and analyzed using experimentally accessible parameters. It is found that a large Rabi-splitting is preferred for switching applications.
Finite-difference time-domain (FDTD) methods suffer from reduced accuracy when modeling discontinuous dielectric materials, due to the inhererent discretization (pixelization). We show that accuracy can be significantly improved by using a subpixel 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 with 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.
Most photovoltaic (solar) cells are made from crystalline silicon (c-Si), which has an indirect bandgap. This gives rise to weak absorption of one-third of usable solar photons. Therefore, improved light trapping schemes are needed, particularly for c-Si thin film solar cells. Here, a photonic crystal-based light-trapping approach is analyzed and compared to previous approaches. For a solar cell made of a 2 micron thin film of c-Si and a 6 bilayer distributed Bragg reflector (DBR) in the back, power generation can be enhanced by a relative amount of 24.0% by adding a 1D grating, 26.3% by replacing the DBR with a six-period triangular photonic crystal made of air holes in silicon, 31.3% by a DBR plus 2D grating, and 26.5% by replacing it with an eight-period inverse opal photonic crystal.
A calculational scheme is presented to model the interaction of light with active dielectric media, represented by four-level atomic materials, surrounded by photonic crystals. Optically pumped lasing is studied in three model systems: a Fabry-Perot cavity, a line of defects in a two-dimensional square lattice of rods, and a cylindrical photonic crystal. Field profiles and conversion efficiencies are calculated for these systems. It is shown that high conversion efficiency can be achieved for large regions of active material in the cavity, as well as for a single fluorescent atom in a hollow-core cylindrical photonic crystal, suggesting designs for ultralow-threshold lasers and ultrasensitive biological sensors.
A new silicon based waveguide with full CMOS compatibility is developed to fabricate an on-chip Bragg cladding waveguide that has an oxide core surrounded by a high index contrast cladding layers. The cladding consists of several dielectric bilayers, where each bilayer consists of a high index-contrast pair of layers of Si and Si3N4. This new waveguide guides light based on omnidirectional reflection, reflecting light at any angle or polarization back into the core. Its fabrication is fully compatible with current microelectronics processes. In principle, a core of any low-index material can be realized with our novel structure, including air. Potential applications include tight turning radii, high power transmission, and dispersion compensation.
The behavior of pointlike electric dipole sources enclosed by an axially uniform, cylindrically symmetric waveguide of omnidirectionally reflecting material is analyzed. It is found that the emission spectrum of a source inside the waveguide is strongly modified by features resembling one-dimensional Van Hove singularities in the local density of states (LDOS). Additionally, more than 100% of the power radiated by a dipole in vacuum can be captured at the end of the waveguide, owing to the overall enhancement of the LDOS (the Purcell effect). The effect of varying the positions and orientations of electric dipole sources is also studied.
A Si-based tunable omnidirectional reflecting photonic band gap structure with a relatively large air gap defect is fabricated and measured. Using only one device, low-voltage tuning around two telecom wavelengths of 1.55 and 1.3 µm by electrostatic force is realized. Four widely spaced resonant modes within the photonic band gap are observed, which is in good agreement with numerical simulations. The whole process is at low temperature and can be compatible with current microelectronics process technology. There are several potential applications of this technology in wavelength division multiplexing devices.
A one-dimensional Si/SiO2 photonic crystal with a large, tunable air defect cavity is fabricated. Multiple resonant modes are observed within the photonic band gap. The free spectral range (FSR) is large compared to other resonant structures, with more than 100nm bandwidth. Simultaneous low voltage tuning around two telecom wavelengths, 1.55 microns and 1.3 microns, is realized using electrostatic force. The whole process is at low temperature and can be CMOS compatible. Potential applications include switching, modulation and wavelength conversion devices, particularly WDM devices.