Review Articles|14 Article(s)
Coherent free-electron light sources|Story Video , On the Cover
Dongdong Zhang, Yushan Zeng, Ye Tian, and Ruxin Li
Free-electron light sources feature extraordinary luminosity, directionality, and coherence, which has enabled significant scientific progress in fields including physics, chemistry, and biology. The next generation of light sources has aimed at compact radiation sources driven by free electrons, with the advantages of reduction in both space and cost. With the rapid development of ultra-intense and ultrashort lasers, great effort has been devoted to the quest for compact free-electron lasers (FELs). This review focuses on the current efforts and advancements in the development of compact FELs, with a particular emphasis on two notable paths: the development of compact accelerators and the construction of micro undulators based on innovative materials/structures or optical modulation of electrons. In addition, the physical essence of inverse Compton scattering is discussed, which offers remarkable capability to develop an optical undulator with a spatial period that matches the optical wavelength. Recent scientific developments and future directions for miniaturized and integrated free-electron coherent light sources are also reviewed. In the future, the prospect of generating ultrashort electron pulses will provide fascinating means of producing superradiant radiation, promising high brilliance and coherence even on a micro scale using optical micro undulators.
Photonics Insights
  • Publication Date: Sep. 27, 2023
  • Vol. 2, Issue 3, R07 (2023)
Epitaxial quantum dots: a semiconductor launchpad for photonic quantum technologies|Story Video
Xiaoyan Zhou, Liang Zhai, and Jin Liu
Epitaxial quantum dots formed by III–V compound semiconductors are excellent sources of non-classical photons, creating single photons and entangled multi-photon states on demand. Their semiconductor nature allows for a straightforward combination with mature integrated photonic technologies, leading to novel functional devices at the single-photon level. Integrating a quantum dot into a carefully engineered photonic cavity enables control of the radiative decay rate using the Purcell effect and the realization of photon–photon nonlinear gates. In this review, we introduce the basis of epitaxial quantum dots and discuss their applications as non-classical light sources. We highlight two interfaces—one between flying photons and the quantum-dot dipole, and the other between the photons and the spin. We summarize the recent development of integrated photonics and reconfigurable devices that have been combined with quantum dots or are suitable for hybrid integration. Finally, we provide an outlook of employing quantum-dot platforms for practical applications in large-scale quantum computation and the quantum Internet.
Photonics Insights
  • Publication Date: Jan. 18, 2023
  • Vol. 1, Issue 2, R07 (2022)
Topological photonics in metamaterials|Story Video
Shaojie Ma, Biao Yang, and Shuang Zhang
Originally a pure mathematical concept, topology has been vigorously developed in various physical systems in recent years, and underlies many interesting phenomena such as the quantum Hall effect and quantum spin Hall effect. Its widespread influence in physics led the award of the 2016 Nobel Prize in Physics to this field. Topological photonics further expands the research field of topology to classical wave systems and holds promise for novel devices and applications, e.g., topological quantum computation and topological lasers. Here, we review recent developments in topological photonics but focus mainly on their realizations based on metamaterials. Through artificially designed resonant units, metamaterials provide vast degrees of freedom for realizing various topological states, e.g., the Weyl point, nodal line, Dirac point, topological insulator, and even the Yang monopole and Weyl surface in higher-dimensional synthetic spaces, wherein each specific topological nontrivial state endows novel metamaterial responses that originate from the feature of some high-energy physics.
Photonics Insights
  • Publication Date: Aug. 03, 2022
  • Vol. 1, Issue 1, R02 (2022)
Terahertz spin dynamics in rare-earth orthoferrites|Story Video , On the Cover
Xinwei Li, Dasom Kim, Yincheng Liu, and Junichiro Kono
Recent interest in developing fast spintronic devices and laser-controllable magnetic solids has sparked tremendous experimental and theoretical efforts to understand and manipulate ultrafast dynamics in materials. Studies of spin dynamics in the terahertz (THz) frequency range are particularly important for elucidating microscopic pathways toward novel device functionalities. Here, we review THz phenomena related to spin dynamics in rare-earth orthoferrites, a class of materials promising for antiferromagnetic spintronics. We expand this topic into a description of four key elements. (1) We start by describing THz spectroscopy of spin excitations for probing magnetic phase transitions in thermal equilibrium. While acoustic magnons are useful indicators of spin reorientation transitions, electromagnons that arise from dynamic magnetoelectric couplings serve as a signature of inversion-symmetry-breaking phases at low temperatures. (2) We then review the strong laser driving scenario, where the system is excited far from equilibrium and thereby subject to modifications to the free-energy landscape. Microscopic pathways for ultrafast laser manipulation of magnetic order are discussed. (3) Furthermore, we review a variety of protocols to manipulate coherent THz magnons in time and space, which are useful capabilities for antiferromagnetic spintronic applications. (4) Finally, new insights into the connection between dynamic magnetic coupling in condensed matter and the Dicke superradiant phase transition in quantum optics are provided. By presenting a review on an array of THz spin phenomena occurring in a single class of materials, we hope to trigger interdisciplinary efforts that actively seek connections between subfields of spintronics, which will facilitate the invention of new protocols of active spin control and quantum phase engineering.
Photonics Insights
  • Publication Date: Jan. 18, 2023
  • Vol. 1, Issue 2, R05 (2022)
Classical and generalized geometric phase in electromagnetic metasurfaces
Yinghui Guo, Mingbo Pu, Fei Zhang, Mingfeng Xu, Xiong Li, Xiaoliang Ma, and Xiangang Luo
The geometric phase concept has profound implications in many branches of physics, from condensed matter physics to quantum systems. Although geometric phase has a long research history, novel theories, devices, and applications are constantly emerging with developments going down to the subwavelength scale. Specifically, as one of the main approaches to implement gradient phase modulation along a thin interface, geometric phase metasurfaces composed of spatially rotated subwavelength artificial structures have been utilized to construct various thin and planar meta-devices. In this paper, we first give a simple overview of the development of geometric phase in optics. Then, we focus on recent advances in continuously shaped geometric phase metasurfaces, geometric–dynamic composite phase metasurfaces, and nonlinear and high-order linear Pancharatnam–Berry phase metasurfaces. Finally, conclusions and outlooks for future developments are presented.
Photonics Insights
  • Publication Date: Aug. 03, 2022
  • Vol. 1, Issue 1, R03 (2022)
Emerging low-cost, large-scale photonic platforms with soft lithography and self-assembly
Hyunjung Kang, Dohyeon Lee, Younghwan Yang, Dong Kyo Oh, Junhwa Seong, Jaekyung Kim, Nara Jeon, Dohyun Kang, and Junsuk Rho
Advancements in micro/nanofabrication have enabled the realization of practical micro/nanoscale photonic devices such as absorbers, solar cells, metalenses, and metaholograms. Although the performance of these photonic devices has been improved by enhancing the design flexibility of structural materials through advanced fabrication methods, achieving large-area and high-throughput fabrication of tiny structural materials remains a challenge. In this aspect, various technologies have been investigated for realizing the mass production of practical devices consisting of micro/nanostructural materials. This review describes the recent advancements in soft lithography, colloidal self-assembly, and block copolymer self-assembly, which are promising methods suitable for commercialization of photonic applications. In addition, we introduce low-cost and large-scale techniques realizing micro/nano devices with specific examples such as display technology and sensors. The inferences presented in this review are expected to function as a guide for promising methods of accelerating the mass production of various sub-wavelength-scale photonic devices.
Photonics Insights
  • Publication Date: May. 22, 2023
  • Vol. 2, Issue 2, R04 (2023)
Revolutionary meta-imaging: from superlens to metalens|Story Video
Tao Li, Chen Chen, Xingjian Xiao, Ji Chen, Shanshan Hu, and Shining Zhu
The refractive-lens technique has been well developed over a long period of evolution, offering powerful imaging functionalities, such as microscopes, telescopes, and spectroscopes. Nevertheless, the ever-growing requirements continue to urge further enhanced imaging capabilities and upgraded devices that are more compact for convenience. Metamaterial as a fascinating concept has inspired unprecedented new explorations in physics, material science, and optics, not only in fundamental researches but also novel applications. Along with the imaging topic, this paper reviews the progress of the flat lens as an important branch of metamaterials, covering the early superlens with super-diffraction capability and current hot topics of metalenses including a paralleled strategy of multilevel diffractive lenses. Numerous efforts and approaches have been dedicated to areas ranging from the new fascinating physics to feasible applications. This review provides a clear picture of the flat-lens evolution from the perspective of metamaterial design, elucidating the relation and comparison between a superlens and metalens, and addressing derivative designs. Finally, application scenarios that favor the ultrathin lens technique are emphasized with respect to possible revolutionary imaging devices, followed by conclusive remarks and prospects.
Photonics Insights
  • Publication Date: Mar. 31, 2023
  • Vol. 2, Issue 1, R01 (2023)
Optical manipulation: from fluid to solid domains|Story Video , On the Cover
Qiannan Jia, Wei Lyu, Wei Yan, Weiwei Tang, Jinsheng Lu, and Min Qiu
Light carries energy and momentum, laying the physical foundation of optical manipulation that has facilitated advances in myriad scientific disciplines, ranging from biochemistry and robotics to quantum physics. Utilizing the momentum of light, optical tweezers have exemplified elegant light–matter interactions in which mechanical and optical momenta can be interchanged, whose effects are the most pronounced on micro and nano objects in fluid suspensions. In solid domains, the same momentum transfer becomes futile in the face of dramatically increased adhesion force. Effective implementation of optical manipulation should thereupon switch to the “energy” channel by involving auxiliary physical fields, which also coincides with the irresistible trend of enriching actuation mechanisms beyond sole reliance on light-momentum-based optical force. From this perspective, this review covers the developments of optical manipulation in schemes of both momentum and energy transfer, and we have correspondingly selected representative techniques to present. Theoretical analyses are provided at the beginning of this review followed by experimental embodiments, with special emphasis on the contrast between mechanisms and the practical realization of optical manipulation in fluid and solid domains.
Photonics Insights
  • Publication Date: Jun. 13, 2023
  • Vol. 2, Issue 2, R05 (2023)
Microcavity exciton polaritons at room temperature|Story Video , On the Cover
Sanjib Ghosh, Rui Su, Jiaxin Zhao, Antonio Fieramosca, Jinqi Wu, Tengfei Li, Qing Zhang, Feng Li, Zhanghai Chen, Timothy Liew, Daniele Sanvitto, and Qihua Xiong
The quest for realizing novel fundamental physical effects and practical applications in ambient conditions has led to tremendous interest in microcavity exciton polaritons working in the strong coupling regime at room temperature. In the past few decades, a wide range of novel semiconductor systems supporting robust exciton polaritons have emerged, which has led to the realization of various fascinating phenomena and practical applications. This paper aims to review recent theoretical and experimental developments of exciton polaritons operating at room temperature, and includes a comprehensive theoretical background, descriptions of intriguing phenomena observed in various physical systems, as well as accounts of optoelectronic applications. Specifically, an in-depth review of physical systems achieving room temperature exciton polaritons will be presented, including the early development of ZnO and GaN microcavities and other emerging systems such as organics, halide perovskite semiconductors, carbon nanotubes, and transition metal dichalcogenides. Finally, a perspective of outlooking future developments will be elaborated.
Photonics Insights
  • Publication Date: Aug. 03, 2022
  • Vol. 1, Issue 1, R04 (2022)