Chalcogenide phase change materials based on germanium-antimony-tellurides (GST-PCMs) have shown outstanding properties in non-volatile memory (NVM) technologies due to their high write and read speeds, reversible phase transition, high degree of scalability, low power consumption, good data retention, and multi-level storage capability. However, GST-based PCMs have shown recent promise in other domains, such as in spatial light modulation, beam steering, and neuromorphic computing. This paper reviews the progress in GST-based PCMs and methods for improving the performance within the context of new applications that have come to light in recent years.
We demonstrate a classical analogue of electromagnetically induced transparency (EIT) in a highly flexible planar terahertz metamaterial (MM) comprised of three-gap split ring resonators. The keys to achieve EIT in this system are the frequency detuning and hybridization processes between two bright modes coexisting in the same unit cell as opposed to bright-dark modes. We present experimental verification of two-bright mode coupling for a terahertz EIT-MM in the context of numerical results and theoretical analysis based on a coupled Lorentz oscillator model. In addition, a hybrid variation of the EIT-MM is proposed and implemented numerically in order to dynamically tune the EIT window by incorporating photosensitive silicon pads in the split gap region of the resonators. As a result, this hybrid MM enables the potential active optical control of a transition from the on-state (EIT mode) to the off-state (dipole mode). PACS numbers: 81.05.Xj, 78.67.PtMetamaterials (MMs) offer versatile and remarkableways to manipulate electromagnetic waves for extraordianary applications such as sub-diffraction focusing 1 , sensing 2,3 , electro-magnetic cloaking 4 , near-perfect absorption 5,6 , etc. In recent years, the concept of electromagnetically induced transparency (EIT) has attracted significant interest due to its potential applications in sensing, controllable delay lines, optical buffers, slow light devices and nonlinear effects 7-10 . EIT is a quantum phenomenon that arises from the destructive interference between different excitation pathways in a three level atomic system, making an initially opaque medium transparent to a probe laser beam 11,12 . However, practical applications of EIT are limited by severe experimental conditions such as high intensity lasers and cryogenic temperatures. The emergence of MMs as classical analogues of EIT have paved the way for the development of new applications such as sensing, THz modulators and slow light metadevices for operation at room temperature.The generation of an EIT analogue in MMs is achieved by two different approaches: (i) bright-dark mode coupling and (ii) bright-bright mode coupling. The first approach typically involves a bright mode resonator coupled to the incident wave directly which is highly radiative and exhibits a low Q-factor. By contrast, the dark mode resonator is characterized by a high Q-factor, and could not be excited by the incident wave directly. However, this dark mode is activated by the bright mode resonator via near-field coupling. In such systems, the necessary condition to achieve EIT is to couple the bright and quasidark resonances at the same resonance frequency with each possessing contrasting linewidths [13][14][15][16][17][18][19][20] .The second approach based on the frequency detuning and hybridization of two bright modes placed in a close proximity of one another has been rarely reported in literature. Such reported works for bright-bright mode coupling in the terahertz frequency regime is very limited 21-24 . In one example, the a...
Generalized spectroscopic ellipsometry determines the principal monoclinic optical constants of thin films consisting of slanted titanium nanocolumns deposited by glancing angle deposition under 85° incidence and tilted from the surface normal by 47°. Form birefringence measured for wavelengths from 500 to 1000 nm renders the Ti nanocolumns monoclinic absorbing crystals with c-axis along the nanocolumns, b-axis parallel to the film interface, and 67.5° monoclinic angle between the a- and c-axes. The columnar thin film reveals anomalous optical dispersion, extreme birefringence, strong dichroism, and differs completely from bulk titanium. Characteristic bulk interband transitions are absent in the spectral range investigated.
Generalized spectroscopic ellipsometry is used to determine the form-induced birefringence and monoclinic optical constants of chromium columnar thin films. The slanted nanocolumns were deposited by glancing angle deposition under 85 degrees incidence and are tilted from the surface normal. Dichroism measured for wavelengths from 400 to 1000 nm renders the Cr nanocolumns monoclinic absorbing crystals with c axis along the nanocolumns axis, b axis parallel to the film interface, and 74.8 degrees monoclinic angle between a and c axes. The columnar thin film reveals anomalous optical dispersion, extreme birefringence, and strong dichroism and differs entirely from bulk chromium.
Extrinsic or pseudo-chiral (meta)surfaces have an achiral structure, yet they can give rise to circular dichroism when the experiment itself becomes chiral. Although these surfaces are known to yield differences in reflected and transmitted circularly polarized light, the exact mechanism of the interaction has never been directly demonstrated. Here we present a comprehensive linear and nonlinear optical investigation of a metasurface composed of tilted gold nanowires. In the linear regime, we directly demonstrate the selective absorption of circularly polarised light depending on the orientation of the metasurface. In the nonlinear regime, we demonstrate for the first time how second harmonic generation circular dichroism in such extrinsic/pseudo-chiral materials can be understood in terms of effective nonlinear susceptibility tensor elements that switch sign depending on the orientation of the metasurface. By providing fundamental understanding of the chiroptical interactions in achiral metasurfaces, our work opens up new perspectives for the optimisation of their properties.
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