Showing papers in "Advanced Optical Materials in 2020"

Laser‐Driven Strong‐Field Terahertz Sources

Abstract: DOI: 10.1002/adom.201900681 reason for this interest relies on the fact that THz radiation can couple resonantly to numerous fundamental motions of ions, electrons, and electron spins in all phases of matter. For example, in solids, the THz range overlaps with the frequency of lattice vibrations (phonons), the collision rates of conduction electrons, the binding energy of bound electron–hole pairs (excitons), and the precession frequency of spin waves (magnons). Consequently, THz radiation, both continuous-wave and pulsed, has been used for characterization of and gaining insight into elementary processes in complex materials. The majority of these studies used relatively weak THz fields and, thus, probed the linear response of the material, without inducing notable material modifications. Only recently, however, completely new avenues in THz science were opened up by triggering nonlinear THz responses of materials.[3–13] Instead of using weak fields to primarily observe selected THz modes such as phonons or magnons, strong fields allow one to actively drive them to unprecedentedly large amplitudes, potentially thereby resulting in novel states of matter.[6,8,11] For example, simulations suggest that exciting matter with intense THz transients may lead to massive modifications of electrically[14] or magnetically[15] ordered domains and enable the acceleration of free ions to ≈1 MeV,[16] and postacceleration to 50–100 MeV energies.[17] Remarkable experimental results such as switching of magnetic order,[18,19] parametric amplification of optical phonons,[20] novel insights into spin-lattice coupling,[21,22] and acceleration of free electrons in a THz linear accelerator[23] were achieved only recently. This progress has been made possible by the development of laser-driven table-top THz sources routinely providing pulses with unprecedented energies and peak electric and magnetic field strengths throughout the entire THz spectral range. Different laser-based THz pulse generation techniques can be used to access different parts of the spectral range extending from 0.1 to 10 THz. Some of the recently developed technologies enable the generation of radiation with even larger bandwidth or tuning range up to 100 THz and beyond, which lead to an extension of what is called the THz spectral range. An overview of the approximate spectral coverage and the achieved highest pulse energies and peak electric-field strengths of various laserdriven technologies is given in Figure 1.

Terahertz Sensing Based on Metasurfaces

Abstract: This work is supported by the Ministerio de Ciencia, Innovacion y Universidades, RTI2018-094475-B-I00.

Toward Intelligent Metasurfaces: The Progress from Globally Tunable Metasurfaces to Software-Defined Metasurfaces with an Embedded Network of Controllers

Abstract: This work was supported by the European Union’s Horizon 2020 research and innovation programme-Future Emerging Topics (FETOPEN) under grant agreement No 736876 (VISORSURF). Financial support by the National Priorities Research Program grant No. NPRP9-383-1-083 from the Qatar National Research Fund is also acknowledged. O.T. acknowledges the financial support of the Stavros Niarchos Foundation within the framework of the project ARCHERS (“Advancing Young Researchers’ Human Capital in Cutting Edge Technologies in the Preservation of Cultural Heritage and the Tackling of Societal Challenges”).

Improving Processability and Efficiency of Resonant TADF Emitters: A Design Strategy

Abstract: This work is funded by the EC through the Horizon 2020 Marie Sklodowska-Curie ITN project TADFlife. The St Andrews team would also like to thank the Leverhulme Trust (RPG-2016- 047) and EPSRC (EP/P010482/1) for financial support. Computational resources have been provided by the Consortium des Equipements de Calcul Intensif (CECI), funded by the Fonds de la Recherche Scientifiques de Belgique (F.R.S.-FNRS) under Grant No. 2.5020.11, as well as the Tier-1 supercomputer of the Federation Wallonie-Bruxelles, infrastructure funded by the Walloon Region under the grant agreement n1117545. AP acknowledges the financial support from the Marie Curie Fellowship (MILORD project, N°. 748042). DB is a FNRS Research Director. We thank Franck-Julian Kahle for support with data analysis.

A Tunable Metasurface with Switchable Functionalities: From Perfect Transparency to Perfect Absorption

Abstract: DOI: 10.1002/adom.201901548 energy with 100% efficiency can be very useful in energy harvesting and invisibility-related applications.[2–4] Moreover, it is even more fascinating if one can combine those two functionalities into one single device, which can behave either as an ideal filter (as shown in Figure 1a) or a perfect absorber (as shown in Figure 1b), controlled dynamically by certain external knobs. Such tunable devices can be potentially useful in many application scenarios (e.g., smart radar radomes and solar cells), but are extremely challenging to realize since the two EM-wave properties (i.e., scattering and absorption) of an ultrathin slab are strongly coupled with each other. In fact, early theoretical studies have demonstrated that the largest absorption enabled by an ultrathin screen is 50% (as shown in Figure 1c,d),[5] not mentioning further difficulties in making the device actively tunable. Recent developments on metasurface (particularly on tunable metasurfaces) provide possible solutions to make such optical devices. Metasurfaces,[6–12] ultrathin metamaterials composed by planar meta-atoms with tailored EM properties, have exhibited strong abilities to control EM waves,[6,7,13–16] with perfect transmission[17] and absorption[18] of EM waves both realized. With active elements integrated, one can further construct tunable metasurfaces with dynamically controlled functionalities[19–29] including tunable absorbers and filters. However, most tunable metadevices realized so far are based on reflection geometry[20–23,26–29] which are relatively easy to design, since only two channels exist in such systems for EM waves to radiate or dissipate (i.e., the reflection port and absorption).[30] In contrast, high-performance tunable metadevices in transmission modes are rarely seen, not mentioning those exhibiting dynamically switched dual functionalities. The intrinsic physics is that a transmissive metasurface has three channels (i.e., transmission and reflection ports, and absorption) to transport/dissipate energy, which must be carefully balanced to yield the desired functionalities (i.e., perfect transmission or perfect absorption). However, the competitions between these three channels are very complicated, which make a transmission-absorption switchable metadevice very difficult to realize, especially on an ultrathin platform. In this paper, we design and experimentally realize such a metadevice with ultrathin thickness. In Section 2, we start from examining the EM properties a trilayer metasurface with PIN diodes loaded and controlled by the same voltages, and show that such a device cannot exhibit freely controlled transmission and absorption properties. We next employ the coupled-mode A thin screen exhibiting dynamically switchable transmission/absorption functionalities is highly desired in practice. However, a trilayer transmissive metasurface with active elements controlled in a uniform manner cannot exhibit independently controlled transmission and absorption properties due to intriguing interplays between the scattering and absorbing properties in systems exhibiting inversion symmetries. This motivates to employ the coupled-mode theory to establish a generic phase diagram for such transmissive metasurfaces with active elements loaded in different layers tuned independently and to guide researchers design tunable metadevices with completely decoupled transmission/absorption responses. Based on such a phase diagram, a microwave metasurface is designed/fabricated with PIN diodes incorporated, and it is experimentally demonstrated that its functionality can switch from perfect transparency to perfect absorption, controlled by external voltages applied across the diodes. In addition to finding immediate applications in practice (e.g., smart radomes), the results of this study also provide a new type of tunable meta-atom for building metasurfaces with flexible wave-front control abilities.

Leveraging of MEMS Technologies for Optical Metamaterials Applications

Abstract: In recent years, metamaterials with artificially engineered sub-wavelength structure have shown great advancement in numerous interesting electromagnetic (EM) properties such as artificial magnetism,[1,2] negative refractive index,[3–7] metalenses,[8–13] wavelength selective absorption,[14–21] slow light behavior,[22–28] and chirality.[29–32] To actively control the metamaterial, various efforts have been developed such as the optically pumped photoconductive materials,[33,34] electrically controlled refractive index of liquid crystals,[35,36] biasing of doped semiconductor devices[37–40] or graphene,[41–43] thermally controlled refractive index of materials,[44,45] conductivity control in phase change materials,[46,47] magnetically controlled active materials,[48,49] and so on.[50–53] However, the intrinsic frequencydependent property of these materials hinders the spectral scalability. Some exotic materials are not complementary metal-oxide-semiconductor (CMOS) compatible and require bulky equipment for external stimulus, which limits commercialization and miniaturization. On the other hand, the most ideal and straightforward method for the reconfiguration is to geometrically modify the parameters of the unit cell, which is also called the meta-atom that determines the property of metamaterials. Furthermore, in terms of feature size, the microelectromechanical system (MEMS) and micro/ nanofluidics enable micro/nanoscale mechanical manipulation and are suitable for meta-atom in terahertz (THz) and IR region, which brings the diversified applications in metamaterial functional device. The advancement in MEMS and micro/nanofluidics offers a wide palette of actuators and liquid channels to enable both in-plane and out-of-plane reconfigurations with varying performance characteristics that could be realized based on the application requirements, ranging from fundamental functions, such as the modulation of intensity, frequency, bandwidth, and electromagnetically induced transparency (EIT) phenomenon, to more sophisticated devices, such as tunable waveplate, logic operation, and resonant cloaking. Beyond tunability, novel chemical sensing platforms in terms of gas, liquid, and thin film sensing of biomolecules can be realized through metamaterials resonators or the hybrid sensing platforms Tunable metamaterial devices have experienced explosive growth in the past decades, driving the traditional electromagnetic (EM) devices to evolve into diversified functionalities by manipulating EM properties such as amplitude, frequency, phase, polarization, and propagation direction. However, one of the bottlenecks of these rapidly developed metamaterials technologies is limited tunability caused by the intrinsic frequencydependent property of exotic tunable material. To overcome such limitation, the microelectromechanical system (MEMS) enabling micro/nanoscale manipulation is developed to actively control “meta-atom” in terahertz and infrared region, which brings frequency-scalable tunability and complementary metal-oxide-semiconductor-compatible functional metadevices. Beyond tunability, novel chemical sensing platforms of molecular identification and dynamic monitoring of the biochemical process can be achieved by integrating micro/nanofluidics channels with metamaterial resonators. Additionally, incorporating metamaterial absorbers with MEMS resonators brings another research interest in MEMS zero-power devices and radiation sensors. Furthermore, moving from 2D metasurfaces to 3D metamaterials, enhanced EM properties like novel resonance mode, giant chirality, and 3D manipulation reinforce the application in biochemical and physical sensors as well as functional meta-devices, paving the way to realize multi-functional sensing and signal processing on a hybrid smartsensor microsystem for booming healthcare, environmental monitoring, and the Internet of Things applications.

Terahertz Nonlinear Optics of Graphene: From Saturable Absorption to High‐Harmonics Generation

Abstract: Graphene has long been predicted to show exceptional nonlinear optical properties, especially in the technologically important terahertz (THz) frequency range. Recent experiments have shown that this atomically thin material indeed exhibits possibly the largest nonlinear coefficients of any material known to date, paving the way for practical graphene-based applications in ultrafast (opto-)electronics operating at THz rates. Here the advances in the booming field of nonlinear THz optics of graphene are reported, and the state-of-the-art understanding of the nature of the nonlinear interaction of graphene with the THz fields based on the thermodynamic model of electron transport in graphene is described. A comparison between different mechanisms of nonlinear interaction of graphene with light fields in THz, infrared, and visible frequency ranges is also provided. Finally, the perspectives for the expected technological applications of graphene based on its extraordinary THz nonlinear properties are summarized. This report covers the evolution of the field of THz nonlinear optics of graphene from the very pioneering to the state-of-the-art works. It also serves as a concise overview of the current understanding of THz nonlinear optics of graphene and as a compact reference for researchers entering the field, as well as for the technology developers.