Superradiant Modes Research Articles

(In,Ga)N-GaN resonant Bragg structures with single and double quantum wells in the unit supercell

We study the formation of a superradiant optical mode in the room temperature reflection spectra from resonant Bragg structures (RBSs) composed of single and double (In,Ga)N quantum wells (QWs) in the unit cell. The appearance of the mode manifests itself by a significant increase in the resonant optical reflectivity due to the electromagnetic coupling of quasi-two-dimensional excitons in the QWs. The implementation of the supercells with double (In,Ga)N QWs results in an increase in the oscillator strength of the quasi-2D excitons and corresponding rise of the radiative broadening parameter to the value as high as 0.3 ± 0.02 meV. We also show that the supercells with double QWs are preferable for RBS with large number of periods due to better tolerance to deviations from the exact periodicity.

Elucidating refractive index sensitivity on subradiant, superradiant, and fano resonance modes in single palladium-coated gold nanorods

Herein, we investigated the distinctive scattering properties exhibited by single gold nanorods coated with palladium (AuNRs@Pd), with variations in the Pd shell thicknesses and morphologies. AuNRs@Pd were synthesized through bottom-up epitaxial Pd growth using two different concentrations of Pd precursor. These single AuNRs@Pd displayed the characteristic of subradiant and superradiant localized surface plasmon resonance peaks, characterized by a noticeable gap marked by a Fano dip. We revealed the effect of local refractive index (RI) on the subradiant and superradiant peak energies, as well as the Fano dip in the scattering spectra of AuNRs@Pd with different Pd shell thicknesses. We demonstrated the applicability of the inflection points (IFs) method on detecting peaks and dip changes across different RIs. Thin AuNRs@Pd1mM displayed more pronounced sensitivity to peak shifts in response to variations in local RIs compared to thick AuNRs@Pd2mM. In contrast, thick AuNRs@Pd2mM exhibited greater sensitivity to changes in curvature near the subradiant and superradiant peak energies rather than peak shift sensitivity across different local RIs. Moreover, the Fano dip shift was more noticeable in thick AuNRs@Pd2mM compared to thin AuNRs@Pd1mM across different local RIs. Therefore, we provided new insight into the RI sensitivity on subradiant, superradiant, and Fano resonance modes in single AuNRs@Pd.

Germanium metasurface near-infrared high-q absorber with symmetry-protected bound states in the continuum

The all-dielectric germanium nanohole (GNH) metasurface with a sub-wavelength thickness supports simultaneous excitation of quasi bound state in the continuum (BIC) and super radiant mode. By selecting the different hole depths in a germanium slab, we present a trade-off metasurface between high Q-factor and high absorption in the photonic system. The presented device demonstrated absorption of super-radiant mode ∼98.5% and quasi-BIC ∼93% without back-metal reflector at the telecommunication wavelength. The numerical results, obtained by the finite difference time domain (FDTD) method are explained in the framework of temporal coupled mode theory (TCMT).

Experimental demonstration of a Fano resonant hybrid plasmonic metasurface absorber for the O and E bands of the optical communication window

Plasmonic metasurface absorbers are capable of absorbing the incident light at wavelengths corresponding to the excitation of Fano resonant modes. Absorption of the incident light is possible because of its confinement near the edges of the plasmonic nanostructure. Confinement of light takes place because of the coupling of superradiant and subradiant modes near the edges of the plasmonic metasurface. Superradiant and subradiant modes are excited for the oblique angle incidence of transverse magnetic (TM)-polarized light. The incidence of TM-polarized light supports the excitation of surface plasmon modes at the metal–dielectric interface. For the oblique angle incidence, surface plasmon modes couple with the incident light and generate the superradiant and subradiant modes near the plasmonic metasurface. We experimentally demonstrate the absorption of near-infrared light in the O and E optical communication band by a one-dimensional (1D) hybrid plasmonic metasurface. A low-cost, and flexible, 1D hybrid plasmonic metasurface absorber (HPMA) was obtained by extracting an Ag-coated, flexible, and 1D patterned polycarbonate layer from a digital versatile disc (DVD). The DVD consists of an Ag layer sandwiched between two 1D patterned polycarbonate layers. A large-area HPMA of 3cm2 in size was fabricated for optical characterization. Control experiments on the variation of the angle of incidence of light were performed to achieve the maximum light absorption of 79%. The effect of transverse electric (TE)- and TM-polarized light on the HPMA was studied. The effect of the thickness of the polymer layer on the HPMA, and per unit change of refractive index (RIU) of the analyte medium, were also investigated. HPMA supports refractive index sensing characteristics with a maximum sensitivity of 954 nm/RIU. Electric field profiles at different incidence angles were simulated using the finite element method on COMSOL Multiphysics software to explain the underlying physics of Fano resonance. HPMA can be used to develop cost-effective photonic devices such as sensors, spectral filters, photodetectors, heat-absorbing protective photonic covers, etc.

Resonant optical reflection from a GaN/(Al,Ga)N excitonic Bragg structure

We experimentally demonstrate the formation of a superradiant optical mode in the room-temperature reflection spectra from a resonant Bragg structure composed of 30 equidistant GaN quantum wells separated by (Al,Ga)N barriers. The mode arises when the condition of the Bragg diffraction is fulfilled at the wavelength corresponding to the energy of the quasi-two-dimensional excitons in the quantum wells. It manifests itself as a significant increase in the amplitude and a change in the shape of the resonant optical reflection due to the electromagnetic coupling of the excitons. By modeling of the optical spectra, we evaluate the radiative and non-radiative broadening parameters of the excitonic states in the GaN quantum wells, which appear to be 0.4 ± 0.02 and 40 ± 5 meV, correspondingly, for the resonant exciton energy of 3.605 eV. The resonant Bragg structure based on the periodic sequence of the GaN quantum wells demonstrates an efficient coupling of photons and excitons at room temperature, which makes it promising for device applications.

Super-Radiant SERS Enhancement by Plasmonic Particle Gratings.

Despite recent developments, surface-enhanced Raman spectroscopy (SERS) applications face challenges in achieving both high sensitivity and uniform Raman signals over a large area. Using the directional self-assembly of plasmonic nanoparticles in lattice structures, we show how one can increase the SERS signal 43-fold over randomly aligned gold nanoparticles without relying on the photoluminescence of Rhodamine 6G. For this study, we have chosen the lattice constant for an off-resonant case that matches the lattice resonance and super-radiant plasmon mode along the particle chain. Supported by electromagnetic simulations, we systematically analyze the radiative components of the plasmon modes by varying the particle size while keeping the lattice periodicity constant. We perform polarization-dependent SERS measurements and compare them with other standard SERS excitation wavelengths. Using the self-assembled plasmonic particle lattice, we have developed an effective SERS substrate that provides a significantly higher signal with 73% less surface coverage. This colloidal approach enables the cost-effective and scalable fabrication of highly sensitive, uniform, and polarization-dependent SERS substrates.

Superradiance instabilities of charged black holes in Einstein-Maxwell-scalar theory

We study time evolutions of charged scalar perturbations on the background of a charged hairy black hole, in which the perturbations can be governed by a double-peak effective potential. By extracting quasinormal modes from the waveform of scalar perturbations, we discover that some quasinormal modes, which are trapped in a potential well between two potential peaks, can be superradiantly amplified. These superradiant modes make the hairy black hole unstable against charged scalar perturbations. Moreover, it is found that the superradiant modes arise from the competition between the superradiant amplification caused by tunneling through the outer potential barrier and the leakage of modes through the inner potential barrier into the black hole.

Superradiance and quantum states on black hole space-times

We consider the definition of the Boulware and Hartle-Hawking states for quantum fields on black hole space-times. The properties of these states on a Schwarzschild black hole have been understood for many years, but neither of these states has a direct analogue on a Kerr black hole. We show how superradiant modes play an important role in the definition of quantum states on Kerr. Superradiance is also present on static black hole space-times, in particular for a charged scalar field on a Reissner-Nordström black hole. We explore whether analogues of the Boulware and Hartle-Hawking states exist in this situation.

Coaction effect of radiative and non-radiative damping on the lifetime of localized surface plasmon modes in individual gold nanorods.

Revealing the coaction effect of radiative and non-radiative damping on the lifetime of the localized surface plasmon resonance (LSPR) mode is a prerequisite for the applications of LSPR. Here, we systematically investigated the coaction effect of radiative and non-radiative damping on the lifetime of the super-radiant and sub-radiant LSPR modes of gold nanorods using time-resolved photoemission electron microscopy (TR-PEEM). The results show that the lifetime of the LSPR mode depends on the length of the gold nanorod, and the different variation behavior of an LSPR mode lifetime exists between the super-radiative mode and the sub-radiative one with the increase of nanorod length (volume). Surprisingly, it is found that the lifetime of the super-radiant LSPR mode can be comparable to or even longer than that of the sub-radiant LSPR mode, instead of the usual claim that a sub-radiant LSPR mode has a longer life than the super-radiant mode. Those TR-PEEM experimental results are supported by finite-difference time-domain simulations and are well explained by the coaction effect with the calculation of the radiative and non-radiative damping rate with the increase of the nanorod volume. We believe that this study is beneficial to build a low-threshold nano-laser and ultrasensitive molecular spectroscopy system.

Impact of plasmonic substrate of different geometries on the enhancement factor of hyper‐Raman scattering process

Effective plasmonic heterotrimers of different geometry have been proposed as an active substrate to substantially enhance the scattered signal of hyper-Raman scattering process. The three-particle systems are constructed independently from asymmetric nanoparticles of spherical, cylindrical, and spheroidal shapes. The interacting metallic characters in each one of the three trimers are arranged collinearly in a J-aggregate configuration and illuminated by a longitudinally polarized light. The finite-difference time-domain electrodynamic simulation tool is employed to simulate optical cross-section and nearfield intensity associated with the excited plasmonic bands. The extinction profile of each one of the three trimers exhibits the excitation of two plasmonic modes. The first is a superradiant mode, which resulted from the in-phase coupling between the dipole moments induced in each one of the three plasmonic elements. The second mode excited at a shorter wavelength, which is a combination between bright and dark modes (Fano interference). The resonance wavelength of the former mode matches the incident one; meanwhile, the resonance energy of the Fano profile is equivalent to the second-order Stokes condition. From the values of the highly enhanced nearfield associated with the observation of these modes, the enhancement factor of the hyper-Raman scattering process is calculated. The simulation results highlight the influence of the selected particle's geometry on both the quality of plasmonic coupling and calculated enhancement factor. The maximum value of the enhancement factor can reach as high as 3.83 × 1017 when the heterotrimer is constructed from spheroids. For nanoparticles of spherical shape, the calculated value of the factor is comparable to the maximum limit; meanwhile, the rod shape demonstrates the minimum limit of the enhancement factor.

Gregory-Laflamme encounters Superradiance

We investigate the effect of superradiant scattering of gravitational perturbations on the stability of rotating black strings, focusing on the six dimensional equal-spinning Myers-Perry black string. We find that rapidly rotating black strings are unstable to gravitational superradiant modes within a bounded range of string lengths. The instability occurs because momentum along the string direction creates a potential barrier that allows for the confinement of superradiant modes. Yet, five dimensional Myers-Perry black holes do not have stable particle orbits so, unlike other known superradiant systems, these black strings remain stable to perturbations with sufficiently high azimuthal mode number — this is a ‘finite-m’ superradiant instability. For some parameters, this instability competes with the Gregory-Laflamme instability, but otherwise exists independently. The onset of this instability is degenerate and branches to multiple steady-state solutions. This paper is the first of a trilogy: in the next two, we construct two distinct families of rotating strings emerging from the superradiant onset (the ‘black resonator strings’ and ‘helical black strings’). We argue that similar physics is present in 5-dimensional Kerr black strings, but not in D > 6 equal-spinning Myers-Perry black strings.

Quantization of a charged scalar field on a charged black hole background

We study the canonical quantization of a massless charged scalar field on a Reissner-Nordstr\"om black hole background. Our aim is to construct analogs of the standard Boulware, Unruh and Hartle-Hawking quantum states which can be defined for a neutral scalar field, and to explore their physical properties by computing differences in expectation values of the scalar field condensate, current and stress-energy tensor operators between two quantum states. Each of these three states has a non-time-reversal-invariant past and future charged field generalization, whose properties are similar to those of the corresponding past and future states for a neutral scalar field on a Kerr black hole. In addition, we present some tentative, time-reversal-invariant, equilibrium states. The first is a ``Boulware''-like state which is as empty as possible at both future and past null infinity. Second, we posit a ``Hartle-Hawking''-like state which may correspond to a thermal distribution of particles. The construction of both these latter states relies on the use of nonstandard commutation relations for the creation and annihilation operators pertaining to superradiant modes.

Dissipative quantum vortices and superradiant scattering

Inspired by Analogue Gravity, superradiance has been previously investigated in Bose-Einstein condensates (BECs). In this work, we revisit this problem by introducing dissipation to the system. After establishing the possibility of quantum vortices in dissipative BECs, we analyze the propagation of elementary excitations, and we demonstrate the existence of superradiant modes, which can be interpreted in terms of the dissipation of ``antiparticles.'' Our findings support the possibility of superradiant scattering around dissipative quantum vortices, and they pave the way for future experimental realization of the phenomenon.

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.

Superradiance in stars:non-equilibrium approach to damping of fields in stellar media

Superradiance in black holes is well-understood but a general treatment for superradiance in stars has until now been lacking. This is surprising given the ease with which we can observe isolated neutron stars and the array of signatures which would result from stellar superradiance. In this work, we present the first systematic pipeline for computing superradiance rates in rotating stars. Our method can be used with any Lagrangian describing the interaction between the superradiant field and the constituents of the star. Our scheme falls into two parts: firstly we show how field theory at finite density can be used to express the absorption of long wavelength modes into the star in terms of microphsyical scattering processes. This allows us to derive a damped equation of motion for the bosonic field. We then feed this into an effective theory for long wavelengths (the so-called worldline formalism) to describe the amplification of superradiant modes of arbitrary multipole moment for a rapidly rotating star. Our method places stellar superradiance on a firm theoretical footing and allows the calculation of the superradiance rate arising from any interaction between a bosonic field and stellar matter.

Coupling‐Induced Tunable Broadband Superradiance in 2D Metal‐Dielectric‐Metal Nanocavity Arrays (Laser Photonics Rev. 16(11)/2022)

Tunable Broadband Superradiance In article number 2200393, Zhengchen Liang, Hong Zhang, Wei Wang, and colleagues experimentally demonstrate a tunable broadband superradiance in 2D metal-dielectric-metal nanocavity arrays. Rationally designed 3-mode hybridization enables an emissive superradiant hybrid mode over a broadband visible range, with greatly shortened polariton lifetimes down to 4 femtoseconds. This may provide a potential strategy to promote efficient manipulation of light emission for applications requiring bright, fast-emitting photon sources.

Concentrated subradiant modes in a one-dimensional atomic array coupled with chiral waveguides

Non-Hermitian systems have recently attracted broad interest and exhibited intriguing physical phenomena, in which the non-Hermitian skin effect is one of the most remarkable quantum phenomena desiring detailed investigations and has been widely studied in various fermionic and bosonic systems. Here we propose a non-Hermitian atom-waveguide system composed of a tilted one-dimensional atomic array coupled with two identical waveguides with opposite chiralities. Such system creates an effective lattice model including nonreciprocal long-range hoppings through the chiral-waveguide photon-mediated interactions. We find the excitation of the collective atomic states concentrates in the middle interface, pointing towards the non-Hermitian skin effect associated with subradiant modes, while, on the contrary, superradiant modes exhibit extended features. Simulation results present subradiant funneling effect, with robustness against small atomic position disorders. Our work underpins the fundamental comprehension towards the non-Hermitian skin effect in open quantum systems and also provide prospective paths to study non-Hermitian systems in the area of quantum optics.

Turning rotating D-branes and black holes inside out their photon-halo

We extend our investigation on Couch-Torrence conformal inversions of black holes (BHs) and D-branes in various directions. We analyze asymptotically flat rotating charged BHs in $D=4$, in particular extremal rotating BHs in STU supergravity, and find invariance for special choices of the charges. Due to the dependence of the critical radii on the impact parameter(s), the relation between the scattering angle for geodesics outside the photon-halo and the inspiraling angle for geodesics inside the photon-halo is modified by the inclusion of a boundary term. We also consider rotating BHs in $D=5$ and rotating D3-branes and find invariance under generalized Couch-Torrence inversions for special choices of the angular momenta. Alas we don't find any similar symmetry for smooth horizonless geometries. Moreover, relying on the surprising connection between classical BH perturbation theory and quantum Seiberg-Witten curves for $\mathcal{N}=2$ SYM theories, we study scalar wave equations in these backgrounds and identify the near superradiant modes produced in near-extremal BH mergers. Finally, we study scalar fluctuations around Kerr-Newman BHs in ${\mathrm{AdS}}_{4}$ and find stringent conditions for generalized Couch-Torrence symmetry that are relaxed in the extremal case or by allowing a rescaling of the wave function.

Coupling‐Induced Tunable Broadband Superradiance in 2D Metal‐Dielectric‐Metal Nanocavity Arrays

AbstractSubradiance/superradiance, cooperative effects causing suppressed/enhanced radiative decay, are of particular interest in plasmonic systems as they play a very important role in modulating dampings and optical properties of resonant systems. However, subradiance/superradiance are generally limited in narrow spectral range with inaccessible tunability. Realizing broadband subradiant and superradiant plasmon modes with flexible tuning is still challenging. Here, a 2D periodic multilayer metal‐dielectric‐metal (MDM) nanostructure is rationally designed and fabricated to realize a tunable superradiant mode over a broad visible range. Angle‐resolved spectroscopy combined with full quantum calculations reveal a sufficient hybridization of delocalized guided plasmons with localized plasmons and a plasmonic cavity mode, leading to an emissive superradiant hybrid mode over a broadband visible range, which can be readily tuned by controlling the spectral three‐mode overlap. Greatly shortened polariton lifetimes down to 4 fs are achieved as direct consequence of the Rabi phases and considerable incoherent coupling strengths between interacting subsystems. Such a control of plasmon damping by cooperative mode interactions paves the way toward efficient manipulation of light emission for applications requiring bright, fast‐emitting photon sources.

Six-dimensional non-extremal Reissner-Nordstrom black hole, charged massive scalar perturbation and black hole bomb

The superradiant stability of higher dimensional non-extremal Reissner-Nordstrom black hole under charged massive scalar perturbation is analytically studied. We extend our previous studies of four- and five-dimensional non-extremal Reissner-Nordstrom black hole cases to six-dimensional case. By analyzing the derivative of the effective potential with an analytical method, we find that no potential well exists outside the outer horizon of the black hole for the superradiant scalar modes. This means that there is no black hole bomb for the system consisting of six-dimensional Reissner-Nordstrom black hole and charged massive scalar perturbation and the system is superradiantly stable.