Fei Sun

Bio: Fei Sun is an academic researcher from Taiyuan University of Technology. The author has contributed to research in topics: Transformation optics & Metamaterial. The author has an hindex of 16, co-authored 73 publications receiving 773 citations. Previous affiliations of Fei Sun include Royal Institute of Technology & Zhejiang University.

A transient thermal cloak experimentally realized through a rescaled diffusion equation with anisotropic thermal diffusivity

Abstract: Transformation optics has made a major contribution to the advancement of modern electromagnetism and related research assisted by the development of metamaterials. In this work, we applied this concept to the thermodynamics using the coordinate transformation to the time-dependent heat diffusion equation to manipulate the heat flux by predefined diffusion paths. Experimentally, we demonstrated a transient thermal cloaking device engineered with effective thermal materials and successfully hid a centimeter-sized vacuum cavity. A rescaled heat equation accounting for all the pertinent parameters of various ingredient materials was proposed to greatly facilitate the fabrication. Our results unambiguously demonstrate the practical possibility of implementing complex transformed thermal media with high accuracy and acquiring several unprecedented thermodynamic functions, which we believe will help to broaden the current research and pave a new path to manipulate heat for novel device applications. In this work, transformation optics technique was applied to the thermodynamic area by using the coordinate transformation to the time-dependent heat diffusion equation, which enables the manipulation of the heat flux by predefined diffusion paths. A transient thermal cloaking device engineered with effective thermal materials was experimentally demonstrated by hiding a centimeter-sized vacuum cavity. To facilitate the fabrication a rescaled heat equation taking into account of all the pertinent parameters of various ingredient materials was proposed to guide the practical design of complex transformed thermal devices. Metamaterials are complex materials carefully devised to exhibit specific properties that do not typically occur in nature. For example, in transformation optics, they can guide light along predefined paths to produce effects such as invisibility cloaking. In a Chinese-Swedish collaboration, Sailing He and co-workers have applied this same approach to thermodynamics to fabricate a thermal cloaking device. The researchers assembled constituent materials in a precise manner to form a centimeter-sized thermal cloak that successfully guided a heat flux along predefined diffusion paths. Such devices have previously been investigated but the calculations of these studies were based on the heat conduction equation, and the resulting heat manipulation could only be applied to materials in a thermostatic state. Instead, He and co-workers have taken into account the time-dependent rescaled heat diffusion equation, which uses anisotropic thermal diffusivity as the main factor, so that their multi-layered device can be implemented with high accuracy and works in a transient state.

Transformation Optics: From Classic Theory and Applications to its New Branches

Abstract: In the modern world, the ability to manipulate and control electromagnetic waves has greatly changed people's lives. Novel optical and electromagnetic phenomena and devices will lead to new scientific trends and techniques in the future. The exploration of new theories of optical design and new materials for optical engineering has attracted great attention in recent years. Transformation optics (TO) provides a new way to design optical devices with extraordinary predesigned functions such as invisibility cloaks and electromagnetic wormholes. As the development of artificial electromagnetic media (e.g. metamaterials and metasurfaces) progresses, many of these novel optical devices designed by TO have been experimentally demonstrated and used in specific applications. Starting from the basic theory of transformation optics, we review its applications, extensions, new branches and recent developments in this paper.

A transient thermal cloak experimentally realized through a rescaled diffusion equation with anisotropic thermal diffusivity

Abstract: Transformation optics originating from the invariance of Maxwell's equations under the coordinate mapping has enabled the design and demonstration of many fascinating electromagnetic devices that were unconceivable or deemed impossible before [1-11], and has greatly contributed to the advancement of modern electromagnetism and related researches assisted with the development of metamaterials [12-15]. This technique has been extended to apply to other partial differential equations governing different waves [16-23] or flux [24-28], and has produced various novel functional devices such as acoustic cloaks [20-23] and Schrodinger's 'hat' [19]. In the present work we applied the coordinate transformation to the time-dependent heat diffusion equation [24-28] and achieved the manipulation of the heat flux by predefined diffusion paths. In the experiment we demonstrated a transient thermal cloaking device engineered with thermal metamaterials and successfully hid a centimeter sized strong 'scatter' (thermal disturber), i.e., a vacuum cavity. To facilitate reliable fabrication we adopted the rescaled thermal diffusion equation for various ingredient materials with nearly constant product of the density and heat capacity, and took the anisotropic thermal diffusivities as the key parameters for the design. Our results unambiguously show the practical possibility to implement the complex transformed thermal media with high accuracy and acquire some unprecedented thermodynamic functions, which we believe will help to broaden the current research and pave a new way to manipulate heat for novel device applications.

Experimentally demonstrated a unidirectional electromagnetic cloak designed by topology optimization

Abstract: Electromagnetic invisible devices usually designed by transformation optics are rather complicated in material parameters and not suitable for general applications. Recently, a topology optimized cloak based on level-set method was proposed to realize nearly perfect cloaking [Fujii et al., Appl. Phys. Lett. 102, 251106 (2013)]. In this work, we experimentally implemented this idea and fabricated a unidirectional cloak with a relative large invisible region made of single dielectric material. Good cloaking performance was verified through measurement which consists very well with numerical simulation. The advantages and disadvantages of this optimization method are also discussed.

Optical Surface Transformation: Changing the optical surface by homogeneous optic-null medium at will.

Abstract: A new theory on designing electromagnetic/optical devices is proposed, namely, an optical surface transformation (OST). One arbitrary surface can establish the corresponding relationship with another surface entirely optically with an optic-null medium (ONM), (i.e. the electromagnetic wave propagates from one surface to its equivalent surface without any phase delay). Many novel optical devices can be designed by an OST with the help of an ONM. Compared with traditional devices designed by Transformation Optics, our optical surface-reshaping devices have two main advantages. Firstly, the design process is very simple (i.e. we do not need to consider any mathematics on how to make a coordinate transformation, and what we need to do is simply to design the shapes of the input and the output surfaces of the devices). Secondly, we only need one homogeneous anisotropic medium to realize all devices designed by this method. Our method will explore a new way to design novel optical devices without considering any coordinate transformations.

Handbook of Chemistry and Physics

Abstract: There is a special reason for reviewing this book at this time: it is the 50th edition of a compendium that is known and used frequently in most chemical and physical laboratories in many parts of the world. Surely, a publication that has been published for 56 years, withstanding the vagaries of science in this century, must have had something to offer. There is another reason: while the book is a standard fixture in most chemical and physical laboratories, including those in medical centers, it is not as frequently seen in the laboratories of physician's offices (those either in solo or group practice). I believe that the Handbook can be useful in those laboratories. One of the reasons, among others, is that the various basic items of information it offers may be helpful in new tests, either physical or chemical, which are continuously being published. The basic information may relate

Photon Enhanced Thermionic Emission for Solar Concentrator Systems

Abstract: Solar-energy conversion usually takes one of two forms: the 'quantum' approach, which uses the large per-photon energy of solar radiation to excite electrons, as in photovoltaic cells, or the 'thermal' approach, which uses concentrated sunlight as a thermal-energy source to indirectly produce electricity using a heat engine. Here we present a new concept for solar electricity generation, photon-enhanced thermionic emission, which combines quantum and thermal mechanisms into a single physical process. The device is based on thermionic emission of photoexcited electrons from a semiconductor cathode at high temperature. Temperature-dependent photoemission-yield measurements from GaN show strong evidence for photon-enhanced thermionic emission, and calculated efficiencies for idealized devices can exceed the theoretical limits of single-junction photovoltaic cells. The proposed solar converter would operate at temperatures exceeding 200 degrees C, enabling its waste heat to be used to power a secondary thermal engine, boosting theoretical combined conversion efficiencies above 50%.

Effective medium theory of left-handed materials

Abstract: We analyze the transmission and reflection data obtained through transfer matrix calculations on metamaterials of finite lengths, to determine their effective permittivity epsilon and permeability micro. Our study concerns metamaterial structures composed of periodic arrangements of wires, cut wires, split ring resonators (SRRs), closed SRRs, and both wires and SRRs. We find that the SRRs have a strong electric response, equivalent to that of cut wires, which dominates the behavior of left-handed materials (LHM). Analytical expressions for the effective parameters of the different structures are given, which can be used to explain the transmission characteristics of LHMs. Of particular relevance is the criterion introduced by our studies to identify if an experimental transmission peak is left or right handed.

CatBoost for big data: an interdisciplinary review

Abstract: Gradient Boosted Decision Trees (GBDT’s) are a powerful tool for classification and regression tasks in Big Data. Researchers should be familiar with the strengths and weaknesses of current implementations of GBDT’s in order to use them effectively and make successful contributions. CatBoost is a member of the family of GBDT machine learning ensemble techniques. Since its debut in late 2018, researchers have successfully used CatBoost for machine learning studies involving Big Data. We take this opportunity to review recent research on CatBoost as it relates to Big Data, and learn best practices from studies that cast CatBoost in a positive light, as well as studies where CatBoost does not outshine other techniques, since we can learn lessons from both types of scenarios. Furthermore, as a Decision Tree based algorithm, CatBoost is well-suited to machine learning tasks involving categorical, heterogeneous data. Recent work across multiple disciplines illustrates CatBoost’s effectiveness and shortcomings in classification and regression tasks. Another important issue we expose in literature on CatBoost is its sensitivity to hyper-parameters and the importance of hyper-parameter tuning. One contribution we make is to take an interdisciplinary approach to cover studies related to CatBoost in a single work. This provides researchers an in-depth understanding to help clarify proper application of CatBoost in solving problems. To the best of our knowledge, this is the first survey that studies all works related to CatBoost in a single publication.

Intelligent nanophotonics: merging photonics and artificial intelligence at the nanoscale.

Abstract: Nanophotonics has been an active research field over the past two decades, triggered by the rising interests in exploring new physics and technologies with light at the nanoscale. As the demands of performance and integration level keep increasing, the design and optimization of nanophotonic devices become computationally expensive and time-inefficient. Advanced computational methods and artificial intelligence, especially its subfield of machine learning, have led to revolutionary development in many applications, such as web searches, computer vision, and speech/image recognition. The complex models and algorithms help to exploit the enormous parameter space in a highly efficient way. In this review, we summarize the recent advances on the emerging field where nanophotonics and machine learning blend. We provide an overview of different computational methods, with the focus on deep learning, for the nanophotonic inverse design. The implementation of deep neural networks with photonic platforms is also discussed. This review aims at sketching an illustration of the nanophotonic design with machine learning and giving a perspective on the future tasks.