We discuss aspects of thermal radiation engineering with the use of photonic structures. We show that temporal modulation of the refractive index can be used to create new thermal radiation phenomena, including coherence transfer, and near field radiative heat pump. We also show that the thermal radiation properties can be strongly influenced with unitary transformation of the external modes.
Unitary control changes the optical absorption and emission of an object by transforming the external modes. We answer two basic questions: Given an object, what absorptivity, emissivity, and their difference are attainable via unitary control? How to obtain given absorptivity, emissivity, and their difference? We show that both questions can be answered using the mathematics of majorization. We further provide explicit algorithms for the practical implementation of unitary control.
Unitary control changes the optical absorption and emission of an object by transforming the external modes. We answer two basic questions: Given an object, what absorptivity, emissivity, and their difference are attainable via unitary control? How to obtain given absorptivity, emissivity, and their difference? We show that both questions can be answered using the mathematics of majorization.
We propose the generation of 3D linear light bullets propagating in free space using a single passive optical surface. The device is a single-layer photonic crystal slab. It can automatically transform an incident conventional Gaussian pulse into a light bullet in the reflection. Our approach also provides simultaneous control of various properties including group velocity, spin, and orbital angular momentum. Our results may advance practical applications of light bullet.
We study the relation between angular spectral absorptivity and emissivity for any thermal emitter, which consists of any linear media that can be dispersive, inhomogeneous, bianisotropic, or nonreciprocal. First, we establish an adjoint Kirchhoff’s law for mutually adjoint emitters. This law is based on generalized reciprocity and is a natural generalization of conventional Kirchhoff’s law for reciprocal emitters. Using this law, we derive all the relations between absorptivity and emissivity for an arbitrary thermal emitter. We reveal that such relations are determined by the symmetries of the system, which are characterized by a Shubnikov point group. We classify all thermal emitters based on their symmetries using the known list of all three-dimensional Shubnikov point groups. Each class possesses its own set of laws that relates the absorptivity and emissivity. We numerically verify our theory for all three types of Shubnikov point groups: Grey groups, colorless groups, and black/white groups. We also verify the theory for both planar and non-planar structures with single or multiple diffraction channels. Our theory provides a theoretical foundation for further exploration of thermal radiation in general media.
We show that nanophotonics can provide a compact and versatile platform to generate controllable space-time light. We will present three of our recent works: 1. Creating structured 3D linear space-time light bullets in free space using nonlocal metasurface. 2. Creating guided light bullets in a multimode waveguide using photonic interband transitions. 3. Creating spatiotemporal optical vortices with arbitrarily oriented OAM using photonic crystal slabs. Our works illustrate the significant opportunities in creating nontrivial space-time correlations with nanophotonic devices.
We propose the generation of 3D linear light bullets propagating in free space using a single passive optical surface. The device is a single-layer photonic crystal slab. It can automatically transform an incident conventional Gaussian pulse into a light bullet in the reflection. Our approach also provides simultaneous control of various properties including group velocity, spin, and orbital angular momentum. Our results may advance practical applications of light bullet.
We discuss meta-surfaces for which the transfer function is diagonal in the waveevector space. We show that such meta-surfaces can be used to perform spatial differentiation, squeeze free space, and generate a variety of optical beams with non-trivial properties associated with orbital or spin angular momentum of light.
There are significant recent interests in using nanophotonic structures to perform computations in the optical domain. Specifically in optical image processing, there have been a number of demonstrations using nanophotonic structures to perform edge detection and spatial filtering operations on images without the bulky 4f systems. These structures have the advantage of being compact, fast and low power. However, all previous works using nanophotonic structures, can only operate with coherent light. Here we introduce a hybrid optoelectronic approach that enables one to use nanophotonic structures to perform differentiation operation with spatially incoherent light. As a demonstration we consider a photonic crystal slab structure, and show that differentiation operation with incoherent light can be achieved by subtracting the two output images at two different frequencies, after passing through the designed structure. Both second order and first order differentiation are demonstrated with corresponding structure design. Our method is robust to noise compared to exact differentiation computation, and directly integrable into existing image sensors. This approach points to a new avenue for improving image sensors using nanophotonic structures, and has potential applications in real scene image processing and object recognition.
We discuss the use of photonic crystal slab to accomplish a number of imaging processing tasks, including edge detection, image smoothing, white noise suppression and, suppression or extraction of periodic features. All these tasks involve filtering in the wavevector domain. Image filtering can be implemented electronically. However, in big-data applications requiring real-time and high-throughput image filtering, conventional digital computations become challenging. Nanophotonics-based optical analog computing may overcome this challenge by offering high-throughput low-energy-consumption filtering using compact devices. Here, we show that several types of isotropic two-dimensional image filters can be implemented with a single photonic crystal slab device. Such a device is carefully designed so that the guided resonance near the Γ point exhibits an isotropic band structure. Depending on the light frequency and the choice of transmission or reflection mode, this compact device realizes isotropic high-pass (Laplacian), low-pass, band-reject and band-pass filtering in the wavevector domain. We numerically demonstrate various important image processing tasks enabled by these filters as mentioned above. Our work points to new opportunities in optical analog computing as provided by nanophotonic structures.
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