Chapter 2 - Invisibility Physics: Past, Present, and Future

Optical invisibility, which started in the pages of fiction before becoming an intriguing quest of humankind for over a century, has blossomed into a remarkable scientific journey toward reality over the last two decades. Perfect optical cloaking requires the total scattering of electromagnetic waves around an object at all angles, all polarizations, over a wide frequency range, irrespective of the medium. Such a device is still far-fetched, requiring the transformation of space around a cloaked region such that the phase velocity is faster than other areas to preserve the phase relationships. However, by simplifying the invisibility requirements, pioneering work on spherical transformation cloaks, carpet cloaks, plasmonic cloaks, and mantle cloaks has been realized in narrowband microwave, infrared, and even optical wavelengths. In this Tutorial, we review the theoretical basis for invisibility cloaking, from spherical transformational optics to non-Euclidian cases, and discuss their limitations. Subsequently, we highlight the recent trends in realizing reconfigurable intelligent cloaks to overcome the traditional limitations of wideband operation and parallel efforts in unidirectional cloaking. Because the human eye is insensitive to the phase and polarization of visible light, a class of ray optics cloaking devices has been recently developed by eliminating phase preservation requirements. Notably, we focus on the recent progress achieved on invisibility cloaks that function in natural incoherent light and can be realized using standard optical components. We conclude this Tutorial with a prospective of potential applications and the practicality of optical cloaks in everyday life.

A thermal invisibility cloak, as inspired by optical invisibility cloaks, is a device which can steer the conductive heat flux around an isolated object without changing the ambient temperature distribution so that the object can be “invisible” to external thermal environment. While designs of thermal invisibility cloaks inherit previous theories from optical cloaks, the uniqueness of heat diffusion leads to more achievable implementations. Thermal invisibility cloaks, as well as the variations including thermal concentrator, rotator, and illusion devices, have potentials to be applied in thermal management, sensing and imaging applications. Here, we review the current knowledge of thermal invisibility cloaks in terms of their design and implementation in cloaking studies, and their extension as other functional devices.

Invisibility has long been a staple in science fiction. The idea of not being seen has enchanted people for centuries. Recent examples in popular literature include H. G. Wells’s The Invisible Man, Wonder Woman’s invisible plane, and the invisibility cloak featured in several Harry Potter books. While advances in optical cloaking have improved the likelihood of realizing invisibility, classroom demonstrations involving a vanishing object immersed in a liquid have been popular with students and teachers alike. In these demonstrations, the refractive indices of the materials and liquid are very close, and the invisibility effect is observed using white light or a broadband light source. However, since the index of refraction is a function of wavelength, any dependence of invisibility on wavelength would not be observed.

We propose the design theory of nonreciprocal invisibility cloaking for an optical camouflage device with unidirectional transparency in which a person in the cloak can see the outside but cannot be seen from the outside. Existing theories of designing invisibility cloaks cannot be used for this purpose, because they are based on the transformation optics that uses the Riemannian metric tensor independent of direction. To realize nonreciprocal cloaking, we propose the theory of effective electromagnetic field for photons, which is an extended version of the theory of effective magnetic field for photons. The effective electromagnetic field can be generated using a photonic resonator lattice. The Hamiltonian for a photon in this field has a similar form to that of the Hamiltonian for a charged particle in an electromagnetic field. Incident photons, therefore, experience a Lorentz-like force and a Coulomb-like force and show nonreciprocal movement depending on their traveling direction. We design an actual invisibility cloaking system on the basis of this theory and, with the aid of computer simulation, confirm the nonreciprocal propagation of light needed for nonreciprocal cloaking.

This article explores women’s attitudes regarding the feminist movement in science, feminist identification, and the ‘women in science’ label. The data was gathered through in-depth interviews with women studying and researching in the fields of physics and physical sciences at four Dublin universities. Previous studies have not looked into women’s participation in feminist collective action in science, as well as their perceptions of the movement and its impact on their science identities. This study sheds light on how women from undergraduate to postdoctoral levels in the fields of physics and physical sciences in Irish higher education where the gender gap is the highest of all science disciplines in Ireland, think about the relationship between science and feminism through their experiences and perspectives. The findings reveal women’s support for feminist goals such as gender equality and encouraging women to pursue careers in science. However, how they define and label ‘feminist’, ‘feminism’ and ‘women in science’ varies based on their social circle, involvement in the feminist movement, and their (gendered) experiences within their scientific community.

Transformation optics enables one to guide and control light at will using metamaterials. However the designed device is deterministic and not flexible for different objects. Based on force-loaded transformation optics we propose a force-induced transformational device, which can realize dynamic escalator metamorphosing continuously between optical elevator and invisibility cloak. This escalator can visually lift up and down the perceived height of a plane fixed in space by controlling the forces loaded in different directions. Or conversely, the escalator can physically lift up and down a plane while visually maintaining the same height to an outside observer. One can quickly adjust this device to the required demand without changing the background index, while the usual transformation cloak will be detectable due to the lateral shift from mismatched background. The schematic is self-adaptive, multi-functional, and free of metamaterial or nanofabrication. Our work opens a new perspective in controlling light dynamically and continuously, empowering unprecedented applications in military cloak, optic communication, holographic imaging, and phase-involved microtechnique.

A new invisibility cloak was recently proposed for hiding objects in front of a highly reflecting mirror. This cloak requires only modest values of optical constants with minimal anisotropy and thus can be implemented by using non-resonant dielectric materials, making it an ideal system for optical frequency operation. We implemented the cloak using an array of silicon nanorods fabricated by electron-beam lithography. We then directly visualized the cloaking effect by monitoring the light propagation inside the device using the near-field optical microscopy.

Editorial: theme issue on complex rheology in biological systems

It is well known that the physical sciences and data processing are not only helpful in all fields of science and technology, but also contribute to the improvement of the quality of human life to a large extent. Thus, more and more scholars focus on physical sciences and data processing. It is obvious that the related fields have become the frontier and hot topic of scientific research. In this context, it is urgent to build an academic exchange platform to promote the sharing of research results and theoretical exchanges in related research fields. Therefore, the International Conference on Advances in Physical Sciences and Data Processing comes into being.APSDP 2020 is held at the Qingdao, China in Video presentation mode on 31st December 2020, which is sponsored by Shandong University(at Qingdao) and Qingdao University. It aims to promote integration and innovation in the fields of physical sciences and data processing and provide researchers, policymakers and industry practitioners with an excellent opportunity to exchange ideas, technologies and related applications, and share successful experiences and the latest research results.The global outbreak of COVID-19 in 2020 poses a serious threat to social and economic development, cultural exchanges and people’s lives and health. Governments around the world have taken active and effective measures to prevent the spread of the epidemic, such as international traffic control, keeping social distancing, wearing masks at all times, and so on, which have effectively prevented the spread of COVID-19 but to a certain extent affected international academic exchanges. How to realize academic exchanges while ensuring the life and health safety of participants has become a problem that we must solve. In any case, life and health always come first. With the help of the development of network technology, we have decided to hold this conference in a video presentation way rather than physical face-to-face, which not only realizes the purpose of academic exchange, but also reduces the risk of the spread of the epidemic. Our decision is praised unanimously by the participants and this online conference has achieved great results.We have received a total of more than 40 contributions. Through strict peer review, 14 articles are accepted. Authors whose articles are accepted are required to record 10-15 minutes of video to present their research findings. All videos are uploaded to a web server (Google Drive and Bilibili) where other participants can log in and watch the videos at any time. If they have any questions, they can communicate with the presenters via email. These videos will exist for a long time and people can access to them at any time. By doing so, we not only break the limitation of place and time, but also realize the purpose of communication and learning. More importantly, we can prevent the spread of COVID-19 and ensure the health and safety of the participants.We would like to express our sincere thanks to Prof. Dr. Osman ADIGUZEL for his selfless help in organizing the conference and publishing articles. We would also like to thank all members of the organizing committee for their efforts in organizing this conference under such epidemic spreading circumstances. Thanks to all the peer reviewers for their hard work in controlling the quality of the articles. It is expected that the results of the conference will inspire and help scholars in the field of physics and information processing. APSDP 2021 conference is under active preparation and we look forward to your continued participation.The Conference Committees

Supporting Undergraduates in Conducting Field-Based Research: A Perspective from On-Site Faculty and Staff

Metamaterials: Optical, acoustic, elastic, heat, mass, electric, magnetic, and hydrodynamic cloaking

There are notably fewer women than men, worldwide, in the physical sciences and engineering. Numbers also decrease markedly with each step up the career ladder, in both the academic and research (industrial and government laboratories) environments. In this study, academic performance of secondary‐school and university females in the mathematical and physical sciences was analyzed. The choice of careers for a group of secondary‐school females was also studied. A positive correlation between the choice of career and academic performance among the secondary‐school females was observed. The correlation was, however, not obvious for the female university students. This study presents possible reasons for poor performance and lack of interest in physics, and suggests ways of attracting and keeping females in the field of physics and its related sciences.

The challenge of building a good recommendation system is deeply connected to missing data---unknown features and labels to suggest the most valuable items to the user. The mysterious properties of the power law distributions that generally arises out of recommender (and social systems in general) create skewed and long-tailed consumption patterns that are often still puzzling to many of us. Missing data and skewed distributions create not just accuracy and recall problems, but also capacity allocation problems, which are at the roots of recent debate on inclusiveness and responsibility. So how do we move forward in the face of these immense conceptual and practical issues? In our work, we have been asking ourselves ways to deriving insights from first principles and drawing inspiration from fields like statistical physics. Surprised, one might ask---what does the field of physics has to do with missing data in ranking and recommendations? As we all know, in the field of information systems, concepts like information entropy and probability have a rich intellectual history. This history is deeply connected to the greatest discoveries of science in the 19th century---statistical mechanics, thermodynamics, and specific concepts like thermal equilibrium. In this talk, I will take us on a journey connecting Boltzmann distribution and partition functions from statistical mechanics with importance weighting for learning better softmax functions, and then further to reinforcement learning, where we can plan better explorations using off-policy correction with policy gradient approaches. As I shall show, these techniques enable us to reason about missing data features, labels, and time dynamic patterns from our data.

Electromagnetic cloaking, as challenging as it may be to the physicist and the engineer has become a topical subject over the past decade. Thanks to the transformations optics (TO) invisibility devices are in sight even though quite drastic limitations remain yet to be lifted. The extreme material properties which are deduced from TO can be achieved in practice using dispersive metamaterials. However, the bandwidth over which a metamaterial cloak is efficient is drastically limited. We design and simulate a spherical cloak which takes into account the dispersive nature of relative permittivity and permeability tensors realized by plasma-like metamaterials. This spherical cloak works over a broad frequency-band even though these materials are of a highly dispersive nature. We establish two equations of state that link the eigenvalues of the permittivity and permeability tensors in every spherical cloak regardless of the geometrical transformation. Frequency dispersive properties do not disrupt cloaking as long as the equations of states are satisfied in the metamaterial cloak.

In this paper, we investigate the nonreciprocity of reflection in parity-time−symmetric (PT-symmetric) Cantor photonic crystals (PCs). Two one-dimensional PCs abiding by the Cantor sequence are PT-symmetric about the center. The PT symmetry and defect cavities in Cantor PCs can induce optical fractal states which are transmission modes. Subsequently, the left and right reflectionless states are located on both sides of a transmission peak. The invisible effect depends on the incident direction and the invisible wavelength can be modulated by the gain–loss factor. This study has potential applications in tunable optical reflectors and invisible cloaks.