Dark Mode Research Articles

A flexible bilayer p-NiO/n-ZnO films with photodetecting properties in self power mode

We report highly efficient flexible p-NiO/n-ZnO heterojunction UV photodetectors. NiO and ZnO thin films were grown on plastic polyethylene terephthalate (PET) substrates by dc magnetron sputtering for constructing high quality interfaces. The diode has shown an excellent rectification ratio i.e. 106 under dark mode and observed more than 80% transparency in the visible region. Investigated structural, compositional, and optical properties of the film to correlate the UV photodetector transport characteristics. Further, UV illumination devices exhibit an excellent responsivity of 0.24 A W−1 with a detectivity of 1.25 × 1011 jones and the highest external quantum efficiency of 83.14% achieved at −5 V of bias. The device shows the fastest speed of response with rise/fall times of 86.10 ms/106.60 ms, accomplished at −1 V with the lowest optical power density. The photodiode has shown incredible characteristics that are mainly attributed to the built-in potential and the transport mechanism at the interface.

Anisotropic metallic heterotrimer systems for an ultrahigh plasmonic-based improvement of hyper-Raman scattering signal

An anisotropic metallic trimer is proposed as an active plasmonic substrate for an ultrahigh enhancement in the spectroscopic signal of the hyper-Raman scattering (HRS) process. The suggested three-particle system is composed from non-aligned asymmetric nanoparticles of a cubic shape. The interacting resonators are made of gold material and illuminated by a longitudinally polarized light. The non-alignment condition in the heterotrimer is achieved by shifting the intermediate cube transversely away from the interparticle axis. Optical cross-section, nearfield distribution and charge density are calculated by using the finite-difference time-domain electrodynamic simulation tool. The enhancement factor of the HRS is calculated theoretically from the nearfield intensity associated with the resonance phenomenon of the considered trimer. The extinction profile of the illuminated system exhibits the excitation of two plasmonic modes. A superradiant mode observed in the longer wavelength region which resulted from the in-phase coupling between the plasmonic modes excited in each one of the three resonators. The second mode is a subradiant band emerged from the interference between bright and dark modes. The resonance wavelength of these two modes matches the excitation one and the second-order Stockes condition, respectively. After optimizing the value of both the transverse shift and the gap spacing, the enhancement factor of the HRS can reach as high as a value never reported before of 1 × 1018.

Broadband Plasmon‐Induced Transparency to a Electromagnetically Induced Absorption Conversion Metastructure Based on Germanium

AbstractA Germanium (Ge) metastructure with switching features from broadband plasmon‐induced transparency (PIT) and electromagnetically induced absorption (EIA) is presented, which forms a broadband PIT through the near‐field coupling between the localized plasmon resonance and the Mie resonance employing four gold cut wires as bright plasmon resonators and four C‐shaped dielectric rings as dark plasmon resonators. A wide transparent window above 0.9 is achieved covering from 0.622 to 0.823 THz with a relative bandwidth of 27.8%. In addition, through phase modulation, a magnetic dipole is generated by the constructive interference of the near‐field coupling of the three resonators to realize the PIT to EIA conversion. In addition, a grid‐like metastructure is introduced to realize the PIT to EIA conversion by phase modulation. To enhance the EIA bandwidth, another layer of C‐shaped reflection plate resonator is introduced to increase the dark mode loss, and a layer of frequency selective surface composed of cross‐shaped resonators is also used. Thus under the dual effect, a wide absorption window above 0.75 is achieved covering from 0.647 to 0.756 THz with a relative bandwidth of 15.5%.

Thermal-noise-resistant optomechanical entanglement via general dark-mode control

Quantum entanglement not only plays an important role in the study of the fundamentals of quantum theory, but also is considered as a crucial resource in quantum information science. The generation of macroscopic entanglement involving multiple optical and mechanical modes is a desired task in cavity optomechanics. However, the dark-mode effect is a critical obstacle against the generation of quantum entanglement in multimode optomechanical systems consisting of multiple degenerate or near-degenerate mechanical modes coupled to a common cavity mode. Here we propose an auxiliary-cavity-mode method to enhance optomechanical entanglement in a multimode optomechanical system by breaking the dark-mode effect. We find that the introduction of the auxiliary cavity mode not only assists the entanglement creation between the cavity mode and the mechanical modes, but also improves the immunity of the optomechanical entanglement to the thermal excitations by about three orders of magnitude. We also study the optomechanical entanglement in the network-coupled optomechanical system consisting of two mechanical modes and two cavity modes. By analyzing the correspondence between the optomechanical entanglement and the dark-mode effect, we find that optomechanical entanglement can be largely enhanced once the dark mode is broken. In addition, we study the mechanical entanglement and find that it is negligibly small. We also present some discussions on the experimental implementation with a microwave optomechanical setup, on the relationship between the dark-mode-breaking mechanism and the center-of-mass and relative coordinates, and on the explanation of the important role of the dark-mode breaking in the enhancement of optomechanical entanglement. Our results pave the way towards the preparation of entangled optomechanical networks and noise-resistant quantum resources.

Cavity-enabled enhancement of ultrafast intramolecular vibrational redistribution over pseudorotation.

Vibrational strong coupling (VSC) between molecular vibrations and microcavity photons yields a few polaritons (light-matter modes) and many dark modes (with negligible photonic character). Although VSC is reported to alter thermally activated chemical reactions, its mechanisms remain opaque. To elucidate this problem, we followed ultrafast dynamics of a simple unimolecular vibrational energy exchange in iron pentacarbonyl [Fe(CO)₅] under VSC, which showed two competing channels: pseudorotation and intramolecular vibrational-energy redistribution (IVR). We found that under polariton excitation, energy exchange was overall accelerated, with IVR becoming faster and pseudorotation being slowed down. However, dark-mode excitation revealed unchanged dynamics compared with those outside of the cavity, with pseudorotation dominating. Thus, despite controversies around thermally activated VSC modified chemistry, our work shows that VSC can indeed alter chemistry through a nonequilibrium preparation of polaritons.

All-dielectric Si metamaterials with electromagnetically induced transparency and strong gap-mode electric field enhancement

We designed all-dielectric π-shaped Si metamaterials and numerically demonstrated their electromagnetically induced transparency (EIT) behavior. Each unit of the metamaterial comprises one horizontal Si nanobar and two vertical Si nanobars with feed-nanogaps between them. A narrow EIT peak with a transmittance of 99% and a Q-factor of 2436 is observed due to the destructive interference between the horizontal bright and vertical dark mode resonators. The EIT can be tunable by changing the nanogap width, and the narrowest EIT window with the largest Q-factor is obtained as the nanogap width is 20 nm. Strong electric field enhancement exceeding 104 in magnitude is also realized inside the 20-nm nanogap. Theoretical fittings according to the coupled Lorentz oscillator model exhibit a well consistency with the simulation spectra, validating the accuracy of the numerical simulation. The designed metamaterial is sensitive to the surround refractive index change with a figure-of-merit of 460, which far exceeds Fano-resonant plasmonic metal sensors. The designed Si metamaterial with a sharp EIT response and a strong electric field enhancement is an ideal platform for applications in optical sensing, low-loss slow-light devices, fluorescence enhancement, and nonlinear optics.

Scattering by circularly symmetric structured optical fields in stratified media

Structured optical fields (SOFs) with spatially inhomogeneous phase, amplitude, or polarization can excite substantially different optical responses in nanoscale scattering particles. Inhibition of low-order multipolar resonances, excitation of dark modes and anapoles, and spin and orbital angular momenta dichroism have been demonstrated, broadening the scope of nanoscale manipulations of optical resonances by using these engineered external SOFs. However, studying scattering effects illuminated by SOFs is more complicated than conventional plane wave illuminations, and the difficulty increases for scattering particles on a substrate. In this paper, we present explicit expressions of SOFs propagating through stratified media, or multilayered systems, and provide their beam-shape coefficients. Specifically, simple analytic expressions are provided for circularly symmetric Bessel beams passing through an interface. Then, scattering calculations for a dielectric sphere on a substrate illuminated by an evanescent optical vortex are performed via the $T$-matrix method, which yields accurate and efficient results as compared with finite element analysis. This paper will find practical applications in modeling SOFs for realistic configurations of scattering of particles on a substrate at nonplanar nonparaxial external light source illuminations.

SPP standing waves within plasmonic nanocavities.

Surface plasmons usually take two forms: surface plasmon polaritons (SPP) and localized surface plasmons (LSP). Recent experiments demonstrate an interesting plasmon mode within plasmonic gaps, showing distinct characters from the two usual forms. In this investigation, by introducing a fundamental concept of SPP standing wave and an analytical model, we reveal the nature of the recently reported plasmon modes. The analytical model includes SPP propagating and SPP reflection within a metal-insulator-metal (MIM) cavity, which is rechecked and supplemented by numerical simulations. We systematically analyze SPP standing waves within various nanocavities. During the discussion, some unusual phenomena have been explained. For example, the hot spot of a nanodimer could be off-tip, depending on the order of standing wave mode; and that a nanocube on metal film can be viewed as a nanocube dimer with the same separation. And many other interesting phenomena have been discussed, such as dark mode of SPP standing wave and extraordinary optical transmission. The study gives a comprehensive understanding of SPP standing waves, and may promote the applications of cavity plasmons in ultrasensitive bio-sensings.

Freestanding Metal Nanomembranes and Nanowires by Template Transfer with a Soluble Adhesive.

The fabrication of nanostructures usually involves chemical processes that have in certain steps. Especially, it is necessary to use the chemical etching method to release the as-patterned structures from the substrate in most of the transfer techniques. Here, a novel scheme of template transfer as developed for the fabrication of freestanding Au nanomembranes and nanowires by using a soluble PVP adhesive. The nanomembranes feature the periodic nanohole arrays with high uniformity. Without the substrates, these plasmonic nanohole arrays show symmetric and antisymmetric resonance modes with bright and dark spectral features, respectively, in transmission. Through the spectral analysis for reflection, we have disclosed that the usual dark mode in transmission is not really dark, but it reveals a distinct feature in reflection. Two coupling modes present distinct spectral features in transmission and reflection due to their different loss channels. To show their versatility, the freestanding nanomembranes were also employed as secondary templates to form Si nanowire arrays by the metal-assisted chemical etching method.

Remote near-field spectroscopy of vibrational strong coupling between organic molecules and phononic nanoresonators

Phonon polariton (PhP) nanoresonators can dramatically enhance the coupling of molecular vibrations and infrared light, enabling ultrasensitive spectroscopies and strong coupling with minute amounts of matter. So far, this coupling and the resulting localized hybrid polariton modes have been studied only by far-field spectroscopy, preventing access to modal near-field patterns and dark modes, which could further our fundamental understanding of nanoscale vibrational strong coupling (VSC). Here we use infrared near-field spectroscopy to study the coupling between the localized modes of PhP nanoresonators made of h-BN and molecular vibrations. For a most direct probing of the resonator-molecule coupling, we avoid the direct near-field interaction between tip and molecules by probing the molecule-free part of partially molecule-covered nanoresonators, which we refer to as remote near-field probing. We obtain spatially and spectrally resolved maps of the hybrid polariton modes, as well as the corresponding coupling strengths, demonstrating VSC on a single PhP nanoresonator level. Our work paves the way for near-field spectroscopy of VSC phenomena not accessible by conventional techniques.

Multiband tunable exciton-induced transparencies: Exploiting both strong and intermediate coupling in a nanocube-hexagonal-nanoplate heterodimer J-aggregates hybrid.

Understanding and mastering the light-light and light-matter interactions in coupled structures have become significant subjects, as they provide versatile tools for manipulating light in both classical and quantum regimes. Mimicking quantum interference effects in pure photonic nanostructures, from weak Fano dip to intense electromagnetically induced transparency, usually requires strong asymmetries in complex geometries and larger interactions between resonances, i.e., in the intermediate coupling regime. Here, we numerically demonstrate a simple and chemically feasible plasmonic nanocube-hexagonal-nanoplate heterodimer with a strong, tunable self-induced transparency window created by the intermediate coupling between the near-degenerate dark and bright hybridized modes. Further assisted by the strong coupling introduced by the J-aggregate excitons covering the heterodimer, three evident exciton-induced transparency windows were observed. These multiband transparencies in a single-particle-level subwavelength configuration, could on one hand enrich the toolbox of multi-frequency light filtering, slowing and switching beyond the diffraction limit, and on the other hand, work as a fundamental testbed for investigating multiscale light-matter interactions at the nanoscale.

Dual Tunable Electromagnetically Induced Transparency Based on a Grating-Assisted Double-Layer Graphene Hybrid Structure at Terahertz Frequencies.

We propose a graphene plasmonic structure by applying two graphene layers mingled with a thin gold layer in a silicon grating. By utilizing the finite-difference time-domain (FDTD) method, we investigate the optical response of the system, and observe that the design achieves dual tunable electromagnetically induced transparency (EIT)-like effect at terahertz frequencies. The EIT-like effect arises from the destructive interference between the grapheme-layer bright modes and the gold-layer dark mode. The EIT-like phenomenon can be adjusted by the Fermi level, which is related to the applied voltage. The results show that the group delay of the present structure reaches 0.62 ps in the terahertz band, the group refractive index exceeds 1200, the maximum delay-bandwidth product is 0.972, and the EIT-like peak frequency transmittance is up to 0.89. This indicates that the device has good slow light performance. The proposed structure might enable promising applications in slow-light devices.

Dark modes governed by translational-symmetry-protected bound states in the continuum in symmetric dimer lattices

Creating nonradiating dark modes is key to achieving high-Q resonance in dielectric open cavities. The concept of photonic bound states in the continuum (BIC) offers an efficient method to suppress radiative loss through symmetry engineering. Structural reflection symmetry (RS) has been widely utilized to construct BICs in asymmetric metasurfaces. In this paper, we show that the radiation channel of translational-symmetry (TS) protected BIC in 1D symmetric dimer lattice could be unlocked by dimer spacing perturbation. A semi-analytical coupled mode analysis reveals that the total radiation suppression of the TS-BIC is due to the elimination of the first Fourier harmonic component in the lattice parameters. TS-BIC mechanism could also be applied in a 2D symmetric dimer lattice, and BICs protected by TS are robust to RS breaking, and vice versa, providing a promising way to independently control the quality factor of two interacting BIC resonances. Our results suggest a new degree of freedom to engineer BICs as well as their interactions in dimer lattices tailored by different symmetries, and could provide new insight for realizing practical applications requiring high-Q resonances.

Radially Polarized Second-Harmonic Generation from a Single-Element Nanoantenna via Dark Plasmon Coupling of Nonlinear Polarization

Controlling the process of second-harmonic (SH) generation at the nanoscale is important in photonic applications but remains challenging in nanophotonics. Herein, based on theoretical and experimental studies, we found that a cross-shaped single-element plasmonic nanoantenna resonating with bright and dark modes at the fundamental (excitation) and SH wavelengths, respectively, generates radially polarized ring-shaped SH radiations with axial symmetry by exciting the bright dipole mode with linearly polarized light. Mode expansion analysis of the generated SH radiation revealed that the radial polarization arises from a dark breathing mode in which coherent dipoles are radially distributed along the arms of the cross-shaped antenna. This study reveals that surface SH polarization generated by the electric near-field of the bright mode at the fundamental wavelength efficiently is coupled to the dark modes rather than the bright mode because its spatial distribution has the same symmetry as the dark modes. By engineering bright and dark modes at the fundamental and SH wavelengths, respectively, novel plasmonic nanodevices that simultaneously perform polarization control and efficient wavelength conversion are expected.

Simultaneous ground-state cooling of multiple degenerate mechanical modes through the cross-Kerr effect.

Simultaneous ground-state cooling of multiple degenerate mechanical modes is a difficult issue in optomechanical systems, owing to the existence of the dark mode effect. Here we propose a universal and scalable method to break the dark mode effect of two degenerate mechanical modes by introducing cross-Kerr (CK) nonlinearity. At most, four stable steady states can be achieved in our scheme in the presence of the CK effect, unlike the bistable behavior of the standard optomechanical system. Under a constant input laser power, the effective detuning and mechanical resonant frequency can be modulated by the CK nonlinearity, resulting in an optimal CK coupling strength for cooling. Similarly, there will be an optimal input laser power for cooling when the CK coupling strength stays fixed. Our scheme can be extended to break the dark mode effect of multiple degenerate mechanical modes by introducing more than one CK effect. To fulfill the requirement of the simultaneous ground-state cooling of N multiple degenerate mechanical modes, N - 1 CK effects with different strengths are needed. Our proposal provides new, to the best of our knowledge. insights into dark mode control and might pave the way to manipulating multiple quantum states in a macroscopic system.

Actively Controllable Terahertz Metal-Graphene Metamaterial Based on Electromagnetically Induced Transparency Effect.

A metal–graphene metamaterial device exhibiting a tunable, electromagnetically induced transparency (EIT) spectral response at terahertz frequencies is investigated. The metamaterial structure is composed of a strip and a ring resonator, which serve as the bright and dark mode to induce the EIT effect. By employing the variable conductivity of graphene to dampen the dark resonator, the response frequency of the device shifts dynamically over 100 GHz, which satisfies the convenient post-fabrication tunability requirement. The slow-light behavior of the proposed device is also analyzed with the maximum group delay of 1.2 ps. The sensing performance is lastly studied and the sensitivity can reach up to 100 GHz/(RIU), with a figure of merit (FOM) value exceeding 4 . Therefore, the graphene-based metamaterial provides a new miniaturized platform to facilitate the development of terahertz modulators, sensors, and slow-light applications.

High-sensitivity metamaterial sensor based on electromagnetically induced transparency (EIT) effect

We propose a metamaterial sensor based on electromagnetically induced transparency (EIT) effect, which is composed of two elliptical structures with splits. Based on the coupling of bright and dark modes, a low-loss transparent window is realized at 1.88 THz. Surface current distribution and equivalent circuit model are used to analyze the physical mechanism of the EIT effect. In addition, the sensitivity of the sensor is investigated for different thickness and refractive index for analyte. The simulation results show that when the analyte thickness is 15 μm and the refractive index is between 1 and 1.5, the sensitivity of the sensor is as high as 450.7 GHz/RIU, and the figure of merit (FOM) value is 7.7. And when the refractive index of analyte is 1.5 and its thickness is between 0 and 5 μm, the sensor can also be used for thickness sensing with a sensitivity of 40.8 GHz μm−1. In addition, the proposed metamaterial sensor can be used to simulate the detection of sodium chloride solution, and the sensitivity is 66.2 GHz/. These above features indicate that the proposed structure paves the way for designing high-sensitivity EIT sensors in the terahertz region, which plays important roles in promoting terahertz sensing and detection technology.

The Edinburgh Companion to Anthony Trollope, edited by Frederik Van Dam, David Skilton, and Ortwin de Graef

Reviewed by: The Edinburgh Companion to Anthony Trollope ed. by Frederik Van Dam, David Skilton, and Ortwin de Graef Deborah Denenholz Morse (bio) The Edinburgh Companion to Anthony Trollope, edited by Frederik Van Dam, David Skilton, and Ortwin de Graef; pp. xii + 396. Edinburgh: Edinburgh University Press, 2019, $200.00. There are many fine essays in The Edinburgh Companion to Anthony Trollope, some of which I heard in earlier versions at the 2015 Trollope bicentenary in Leuven, Belgium. All of the contributors are accomplished Victorianists. However, unlike the volume's editors—Frederik Van Dam, David Skilton, and Ortwin de Graef—few of them are known primarily as Trollope scholars, although some—Helen Blythe, Sophie Gilmartin, and Lauren Goodlad, among others—have written brilliantly on Trollope for years. Few contributors have written a major study on Trollope; John McCourt is a luminous exception, with his essential book on Trollope and Ireland. This editorial choice is both a strength and a weakness of the volume—a strength with regard to the scope of inquiry and the [End Page 463] wonderful minds thinking about Trollope; a weakness because capacious knowledge of Trollope's fiction is requisite to full interpretation of his novels, as the great Trollopian James Kincaid once claimed. There is at times a related tendency not to acknowledge important scholarship on the very subject at hand that I fear will seem ungenerous to scholars steeped in Trollope. That said, there is a plethora of exciting scholarship here. Claire Jarvis's compelling "Almost Trollope" opens the volume's first section, "Style," arguing that Trollope's "hesitant" style, marked by his pervasive use of "almost," is intentional rather than a sign of haste. Trollope's "almostness," in which the narrator enters only partially into characters' minds, issues in a new realism in which there is both sympathy and separation between narrator and character (22). Patrick Fessenbecker is concerned with the intersection of narration and morality, as he plumbs Trollope's language in light of moral philosophy. There is much to admire in his readings of The Small House at Allington (1862) and The Claverings (1866), and in his determination "to treat Trollope as substantively engaging philosophical questions" (41). Gilmartin's "The Physiology of the Everyday" adds to her distinguished work on Trollope and the body. Gilmartin analyzes Trollope's mastery of free indirect discourse as it represents everyday sensory responses in novels from Framley Parsonage (1861) to The Way We Live Now (1875). Blythe's impressive essay centering on An Old Man's Love (1884) extends her work on Trollope's style in lesser-known works. Blythe argues that Trollope's allusions to his own earlier work and to Latin tags demonstrate his sophistication in interrogating Victorian historical and cultural as well as readerly expectation and experience. Eminent Trollopian Skilton reads An Autobiography (1883) as advice literature, interpreting Trollope's infamous accounting of monies earned as "a faithful servant's report to that Great Office in the sky" (89). Goodlad's original essay considers Trollope's novel chronicles in relation to contemporary serial television. Goodlad's previous innovative work enriches this consideration of transtemporal and transnational Trollope. Her erudite historical, critical, and theoretical contexts focus here on speculative financial conditions that encourage "Bigger Stories"—an infinitely expanding genre (107). Section 2, "Circulation," opens with distinguished Trollope editor Steve Amarnick's engaging critique of Harry Heathcote of Gangoil (1874) as revising the Dickensian Christmas genre in a dark Australian mode. Readers of An Autobiography and Trollope's letters will know of Trollope's Australian sheepfarmer son Fred's influence upon the character of Harry. John Bowen's brilliant essay focuses upon Elizabeth Bishop's dramatic monologue "From Trollope's Journal" (1965) and its relation to the travel book North America (1859), while also exploring Trollope's influence upon Anthony Powell's A Dance to the Music of Time series (1951–75). The range of this essay—from Charles Darwin to Wilfred Owen—is impressive. Luca Caddia has written incisively on Trollope for many years. Here he trenchantly explores Trollope's interiority, using Philip Roth's term "counterlife" to consider twentieth- and twenty-first-century texts that reference and critique Trollope's work (143). Caddia...

Robust Fano resonance between mechanical first- and second-order topological states

The Fano resonance, as a phenomenon of wave scattering, is based on the interaction between a “bright mode” and a “dark mode” giving rise to an asymmetric and ultrasharp spectral peak. However, the Fano resonant frequency is sensitive to structural imperfections such as defects or disorders, which will shift the resonant peak, or even damage the Fano line shape. Here, we harness the coupling between the first-order and the higher-order topological insulators to overcome this shortcoming. We construct a first-order topological edge state to serve as a bright mode, and a second-order topological corner state to be the dark mode using the same base configuration. As a result, a topological Fano resonance is expected to occur around the resonant frequency of the dark mode. The robustness of the Fano resonance is verified by deliberately introducing various types of imperfections into the system. Our findings may further enhance confidence in using the resonance such as acoustic switching, refractive index sensing, high quality factor filters, and accurate interferometers.

Coherent emission from surface Josephson plasmons in striped cuprates

The interplay between charge order and superconductivity remains one of the central themes of research in quantum materials. In the case of cuprates, the coupling between striped charge fluctuations and local electromagnetic fields is especially important, as it affects transport properties, coherence, and dimensionality of superconducting correlations. Here, we study the emission of coherent terahertz radiation in single-layer cuprates of the La2-xBaxCuO4 family, for which this effect is expected to be forbidden by symmetry. We find that emission vanishes for compounds in which the stripes are quasi-static but is activated when c-axis inversion symmetry is broken by incommensurate or fluctuating charge stripes, such as in La1.905Ba0.095CuO4 and in La1.845Ba0.155CuO4. In this case,terahertz radiation is emitted by surface Josephson plasmons, which are generally dark modes, but couple to free space electromagnetic radiation because of the stripe modulation.