High-amplitude Modes Research Articles

High-frequency hydrogen combustion dynamics driven by local flame displacement and multidimensional thermoacoustic interactions

Knowledge of the underlying physical mechanisms responsible for the triggering of high-frequency transverse combustion dynamics is of fundamental importance in the development of heavy-duty gas turbine combustors, aircraft engine afterburners, and bipropellant liquid rocket engines. Detailed information about three-dimensional thermoacoustic interactions and local flame dynamics, however, remains largely unknown and unanticipated, mainly because high-amplitude transverse mode instabilities are challenging to excite and detect in well-controlled sub-scale laboratory environments. To overcome this impasse, here we exploit a spatially tailored rectangular injector assembly consisting of ten equidistant horizontal slit nozzles to eliminate the complications of out-of-plane flame dynamics characterization. A total of 56 datasets of self-induced instabilities were acquired over a wide range of operating conditions to understand spatiotemporal phase dynamics and important mode shapes, in conjunction with 2D Rayleigh angle reconstruction and phase-resolved OH PLIF-based local flame front identification. Experimentally, we show that high-frequency transverse instabilities are excited only under high temperature and high thermal power conditions, manifested as non-evanescent pressure fluctuations at 6.50 kHz strongly coupled to the second-order tangential mode of the rectangular combustion chamber. Two vertically-oriented pressure nodal planes and the characteristic phase transition perpendicular to the horizontal slit injector direction are accurately measured and reconfirmed by Helmholtz simulations in terms of their interpositions and spatial orientation. Remarkably, the periodic formation of co-propagating coherent structures and concomitant local flame displacement/pinch-off are revealed to play an important role in driving the high-frequency hydrogen combustion dynamics.

Direct numerical simulation of mechanism and control of secondary instability induced transition in a supersonic boundary layer

Mechanisms and control of secondary-instability-induced-transition in a supersonic boundary layer are studied numerically via direct numerical simulation. The aim is to investigate and compare the transition mechanisms of fundamental, subharmonic, asymmetric subharmonic, and detuned resonances, and to control these secondary instabilities using a local wall cooling strip. The results indicate that the nonlinear interaction between the high-amplitude primary mode and low-amplitude secondary modes is the main contributor to transition. The mutual- and self-interactions of the primary and secondary modes generate other harmonic modes with laminar breakdown soon appearing. The asymmetric subharmonic resonance induces the earliest transition, while the fundamental subharmonic has the latest. Wall cooling effects are also studied. The results show that a lower wall temperature significantly suppresses the secondary instabilities, and steady modes become dominant and lead to obvious streamwise vortexes. Numerical data demonstrate that all secondary-instability-induced transitions result in fully developed turbulent boundary layers, as supported by the skin friction and scaled velocity profiles. The transition control cases indicate that the local wall cooling strip can significantly delay the transition by suppressing the growth of the primary mode. An upstream control strip is found to have a more obvious suppression effect. The fundamental and asymmetric subharmonic resonances are sensitive to the location of the local wall cooling strip and show a stronger transition delaying effect.

The VELOCE modulation zoo I. Spectroscopic detection of non-radial modes in the first-overtone Cepheids BG Crucis, QZ Normae, V0391 Normae, and V0411 Lacertae

The photometric observations from the recent decade revolutionized our view on classical pulsators. Low-amplitude signals have been detected photometrically in addition to the dominant high-amplitude radial mode pulsations in many RR Lyrae stars and classical Cepheids. First-overtone pulsators with an additional low-amplitude signal at a period ratio of around 0.61 with the main mode, the so-called 0.61 stars, form the most populous group among these stars. The nature of this signal has been attributed to non-radial pulsations. Another mysterious group are stars in which the additional signal forms a period ratio of around 0.68. These are the 0.68 stars. The origin of the signal remains unknown. Here, we search for similar phenomena in spectroscopic observations of first-overtone classical Cepheids collected as part of the project. We performed a frequency analysis of several parameters derived from cross-correlation functions (CCFs), including radial velocity, the full width at half maximum, the bisector inverse span, and the CCF depth (contrast). Using standard pre-whitening, we searched for additional low-amplitude signals. We identified the location of these stars in various sequences of the Petersen diagram. We detect additional signals in four first-overtone classical Cepheids: BG Cru, QZ Nor, V0391 Nor, and V0411 Lac. We classified BG Cru, QZ Nor, and V0391 Nor as 0.61 stars based on their period ratios. V0411 Lac, however, exhibits a ratio of 0.68 between the two modes, and the additional signal has a longer period. This type of multi-periodicity remains unexplained. CCFs yield the first spectroscopic detections of non-radial pulsation modes in classical Cepheids. This opens an asteroseismic window for pursuing a more detailed understanding of these important stars. While the 0.61 signal of BG Cru, QZ Nor, and V0391 Nor is understood to originate from non-radial modes of moderate degrees, the 0.68 signal of V0411 Lac still lacks a physical explanation.

Experiments on the aeroacoustic whistling of a cylindrical cavity

An air flow going through an axisymmetric cavity can interact with the acoustic modes of this cavity and produce loud self-sustained acoustic oscillations. Acoustic measurements reveal that the nature of the acoustic mode undergoes complex dynamics, alternating between phases where we observe a spinning wave with periodic inversions of its rotation direction, and phases where a high-amplitude mode is constantly spinning in the same direction. Stereoscopic particle image velocimetry gives the 3 velocity components in a 2D plane, allowing to identify the main hydrodynamic structures involved in the instability and the interactions of these structures with the mean flow and the acoustic oscillations.

Highly efficient energy and mass transfer in bcc metals by supersonic 2-crowdions

During irradiation, bombardment with energetic particles, or plasma treatment, structural transformations occur in materials, in which point defects play a very important role. Highly mobile crowdions contribute to the transfer of mass and energy in the crystal lattice. Static and slowly moving crowdions are well studied, in contrast to crowdions moving at a speed exceeding the speed of sound. Here, for the first time, dynamics of supersonic N-crowdions (N=1,2) is investigated in bcc metals (tungsten and vanadium) with the help of molecular dynamics simulations. N-crowdions are excited by imparting sufficiently high initial velocities to N neighboring atoms located in the same close-packed atomic row along this row. Both supersonic 1- and 2-crowdions carry one interstitial atom each. Modeling shows that much less energy is required to excite a 2-crowdion compared to a 1-crowdion. Energy dissipation for a 2-crowdion is much slower than for a 1-crowdion suggesting that the supersonic 2-crowdion is an efficient mass transfer mechanism in bcc metals. The distance traveled by supersonic 1- and 2-crowdions in W is much greater than that in V. The supersonic crowdion in W transforms into a moving subsonic crowdion, and in V it transforms into a standing subsonic crowdion carrying a localized high-amplitude oscillation mode. These differences can be explained by the features of interatomic interactions in W and V.

The CubeSpec space mission

Context. There is currently a niche for providing high-cadence, high resolution, time-series optical spectroscopy from space, which can be filled by using a low-cost cubesat mission. The Belgian-led ESA/KU Leuven CubeSpec mission is specifically designed to provide space-based, low-cost spectroscopy with specific capabilities that can be optimised for a particular science need. Approved as an ESA in-orbit demonstrator, the CubeSpec satellite’s primary science objective will be to focus on obtaining high-cadence, high resolution optical spectroscopic data to facilitate asteroseismology of pulsating massive stars. Aims. In this first paper, we aim to search for pulsating massive stars suitable for the CubeSpec mission, specifically β Cep stars, which typically require time-series spectroscopy to identify the geometry of their pulsation modes. Methods. Based on the science requirements needed to enable asteroseismology of massive stars with the capabilities of CubeSpec’s spectrograph, we combined a literature study for pulsation with the analysis of recent high-cadence time-series photometry from the Transiting Exoplanet Survey Satellite (TESS) mission to classify the variability for stars brighter than V ≤ 4 mag and between O9 and B3 in spectral type. Results. Among the 90 stars that meet our magnitude and spectral type requirements, we identified 23 promising β Cep stars with high-amplitude (non-)radial pulsation modes with frequencies below 7 d−1. Using further constraints on projected rotational velocities, pulsation amplitudes, and the number of pulsation modes, we devised a prioritised target list for the CubeSpec mission according to its science requirements and the potential of the targets for asteroseismology. The full target catalogue further provides a modern TESS-based review of line profile and photometric variability properties among bright O9–B3 stars.

Designing a Facility for Measuring Higher-Order Acoustic Modes in Uniform Heated Flows

An experimental facility was recently developed at the Georgia Tech Research Institute to measure the acoustic scattering coefficients of a test article under mean flow, high-temperature, high-amplitude, and higher-order mode propagation conditions. The experimental facility acts like a two-sided impedance tube, in which the measurement section is a circular duct with 16 acoustic drivers and 16 microphones mounted on each side of the test article. Such a facility can be configured for grazing and bias flow measurements of acoustic liners with sound traveling in and against the direction of the flow. The new facility is designed to maximize the accuracy of acoustic measurements of higher-order modes in heated flows. The influence of experimental uncertainties on the acoustic measurements is analyzed using a Monte Carlo method. Measurements of half-wave resonators in quiescent air and heated flows demonstrate that the experimental uncertainty decreases as the absorption coefficient increases. High-sensitivity microphones produce extremely low uncertainty, and even lower-sensitivity pressure transducers are sufficiently accurate for many applications. This paper is intended to serve as a reference for designing and analyzing an experimental facility to measure the acoustic scattering coefficients in heated-flow and higher-order mode propagation conditions.

High-frequency transverse combustion instabilities of lean-premixed multislit hydrogen-air flames

The present experimental investigation is concerned with combustion dynamics of lean-premixed multislit hydrogen-air flames, with the aim of addressing some of the central technical problems associated with the development of low-emission carbon-free gas turbine combustion technology. To mitigate flashback risk in relatively fast premixed hydrogen flames, we use a multislit injector assembly with a slit width of 1.5 mm, of the same order of magnitude as the characteristic thickness of lean-premixed hydrogen flames. This new perspective on the characteristic nozzle dimension makes it possible to scrutinize the dynamics of multisheet hydrogen flames under extreme conditions. We carry out extensive measurements of self-excited pressure oscillations over a broad range of operating conditions between 30 and 100 kW thermal power. The experimental datasets are analyzed using the acoustic wave decomposition method, low-order thermoacoustic network modeling, and FEM-based three-dimensional Helmholtz simulations. We show that lean-premixed multislit hydrogen flames – unlike the multi-element hydrogen-air flame ensembles considered in our earlier investigations – undergo strong high-frequency pressure oscillations between 3.1 and 3.5 kHz in excess of 25 kPa, originating from the triggering of the first tangential mode of the combustion chamber. Whereas the frequency of the first tangential mode is well defined by the square root dependence of adiabatic flame temperature, the magnitude of pressure fluctuations is observed to be intensified exponentially with increasing flame temperature up to 2200 K. We demonstrate that the transverse mode is characterized by high-amplitude spinning modes in the clockwise direction under relatively short combustor length conditions, discontinuously switching to counter-clockwise spinning modes at relatively long combustor length, and eventually transitioning to a moderate standing wave mode at the same resonant frequency. These observations demonstrate the previously unidentified complex modal dynamics induced by lean-premixed multislit hydrogen-air flames of nozzle dimension comparable to the characteristic length scale of the flame.

Acoustic Resonance in an Axial Multistage Compressor Leading to Non-Synchronous Blade Vibration

Abstract Significant non-synchronous blade vibrations (NSVs) have been observed in an experimental three-stage high-speed compressor at part-speed conditions. High-amplitude acoustic modes, propagating around the circumference and originating in the highly loaded stage-3, have been observed in coherence with the structural vibration mode. In order to understand the occurring phenomena, a detailed numerical study has been carried out to reproduce the mechanism. Unsteady full-annulus Reynolds-averaged Navier–Stokes simulations of the whole setup have been performed using the solver elsA. The results revealed the development of propagating acoustic modes which are partially trapped in the annulus and are in resonance with an aerodynamic disturbance in rotor-3. The aerodynamic disturbance is identified as an unsteady separation of the blade boundary layer in rotor-3. The results indicate that the frequency and phase of the separation adapt to match those of the acoustic wave and are therefore governed by acoustic propagation conditions. Furthermore, the simulations clearly show the modulation of the propagating wave with the rotor blades, leading to a change of circumferential wave numbers while passing the blade row. To analyze if the effect is self-induced by the blade vibration, a non-coherent structural mode has been imposed in the simulations. Even at high vibration amplitude, the formerly observed acoustic mode did not change its circumferential wave number. This phenomenon is highly relevant to modern compressor designs, since the appearance of the axially propagating acoustic waves can excite blade vibrations if they coincide with a structural eigenmode, as observed in the presented experiments.

Identification of Noise Sources in a Realistic Turbofan Rotor Using Large Eddy Simulation

Large Eddy Simulation (LES) is performed using the NASA Source Diagnostic Test database at approach conditions (62% of the design speed). The simulation is performed in a periodic domain containing one single blade. The aerodynamic and acoustic results are compared with the experiment. Ffowcs Williams and Hawkings’ (FWH) analogy is used to compute the far-field noise from the solid surface of the rotor blade. The analogy is computed for the full blade and for its tip region to see the contribution of the latter. The dynamic mode decomposition is performed at different iso-radii of the computational domain. The contribution of the tip region to the far-field noise is observed around the first blade passing frequency (2.8 kHz) and 4 kHz due to the mixing process of the leading-edge and tip vortices. The rest of the blade contributes more at frequencies above 7 kHz which corresponds to the trailing-edge noise. The high-amplitude modes are observed at 75% and 90% of the spanwise length. These modes contribute to the corresponding frequency humps in the far-field spectrum.

Control of Very Lightweight 2-DOF Single-Link Flexible Robots Robust to Strain Gauge Sensor Disturbances: A Fractional-Order Approach

This article deals with the control of two degrees of freedom manipulators that have a flexible and very lightweight link. These robots have a single low-frequency and high-amplitude vibration mode. Their actuators have high friction, and their vibration sensors are often strain gauges that have offset and high-frequency noise. These problems reduce the robot precision and produce noisy control signals that saturate actuators. An efficient control system is proposed to overcome these drawbacks. Actuator friction effect is nearly removed by closing a high gain position control loop around the actuator. It causes the separation of the robot dynamics into the controlled actuator fast subsystem and the link dynamics slow subsystem. Based on that, an innovative control system is designed to remove vibrations using the singular perturbation theory combined with the input-state linearization technique. This control system includes fractional-order controllers that nearly remove unknown sensor offset and sensor ramp disturbances while reducing the high-frequency component of the control signal caused by sensor noise. Simulated and experimental results show the superior performance of these controllers over other standard integer-order controllers of similar complexity and nominal behavior.

Eulerian and Lagrangian analysis of coherent structures in separated shear flow by time-resolved particle image velocimetry

We investigate the turbulent shear flow that separates from a two-dimensional backward-facing step. We aim to analyze the unsteady separated and reattaching shear flow in both the Eulerian and Lagrangian frameworks in order to provide complementary insight into the self-sustaining coherent structures and Lagrangian transport of the entrainment process. The Reynolds number is Reh = 1.0 × 103, based on the incoming free-stream velocity and step height. The separated and reattaching shear flow as well as the recirculation region beneath is measured by time-resolved planar particle image velocimetry. As a result, time sequences of velocity vector fields in a horizontal–vertical plane in the center of the step model are obtained. In the Eulerian approach, a set of temporally orthogonal dynamic modes are extracted, and each one represents a single-frequency vortex pattern that neutrally evolves in time. The self-sustaining coherent structures are represented by reduced-order reconstruction of the identified high-amplitude dynamic modes, showing the basic unsteady flapping motion of the shear layer and the vortex rolling-up, pairing, and shedding processes superimposed on it. On the other hand, trajectories of passive fluid tracers depict the Lagrangian fluid transport by the entrainment process in the separated shear flow and identify the time-dependent vortex rolling-up process as well as complex vortex interactions. The contours of the finite-time Lyapunov exponent reveal the unsteady Lagrangian coherent structures that significantly shape the vortex patterns and contribute substantial parts to the fluid entrainment in the shear flow.

Flow visualization and supersonic combustion studies of an acoustically open strut cavity

In this study, the supersonic flow over strut cavities was experimentally studied to understand flow features. Instantaneous schlieren imaging in non-reacting flow experiments exhibited the seven types of waves associated with cavity pressure oscillations and the formation of unstable shear layers on both sides. The shear layers moved in and out in synchronous and asynchronous modes at the trailing edge of the strut cavities. The symmetrical wave structure appeared on both sides in the synchronous mode, whereas the shear layers appeared in different stages of the cavity pressure oscillation cycle in the asynchronous mode, resulting in an asymmetrical wave structure. The pressure waves generated at the trailing edge of the strut cavities perturbed the shear layers during their movement toward the leading edge, creating a wavy shear layer with alternate troughs and crests. The pressure oscillations of the strut cavities had high-amplitude cavity modes with broadband noises, and their amplitude decreased from the trailing edge to the leading edge. The estimated recovery factor using the time lag between the signals of the leading and trailing edges showed that the flow inside the strut cavities was low subsonic. The measured dominant pressure oscillation modes had a closer match with the Rossiter modes. The pressure coefficient demonstrated that fluid accumulation inside the cavity increased with an increase in the aspect ratio. Furthermore, a higher fluid mass accumulated at the trailing edge than at the leading edge, and the difference in fluid accumulation increased with an increase in the aspect ratio. Supersonic combustion experiments with strut cavities showed that the strut cavity stabilized the flame. Moreover, the addition of an acoustically open strut cavity ahead of the flame-stabilizing cavity advanced the heat release location upstream.

Flow structures and shear-stress predictions in the turbulent channel flow over an anisotropic porous wall

This article identifies the main coherent structures driving the flow dynamics in the turbulent channel flow over anisotropic porous walls. Two different cases have been analyzed where the drag increases or decreases with respect to a channel with isotropic porous walls. Higher order dynamic mode decomposition (HODMD) is applied to analyze these data, identifying 20 and 15 high amplitude modes in the drag increasing (DI) and drag reducing (DR) cases, respectively, which well reflects the largest flow complexity in the former case. The frequency of 13 modes and the three-dimensional structure of the modes are similar in the DR and DI cases, suggesting the need of using more complex analyses to deepen our physical insight of these flows. The spatio-temporal HODMD analysis identifies a periodic solution along the spanwise direction (as imposed by the boundary conditions). The wavenumbers related to the modes with highest amplitude are β = 0 and β = 3 (Lz = 2 3 π ). The rollers, groups of spanwise correlated structures, are mostly identified in the DI case near the wall, with β = 0, while the presence of the streaks, streamwise correlated structures are mostly identified in the DR case. Although, in areas far away from the wall it is possible to identify these two types of structures with β = 3 in both cases, depending on the temporal frequency of the DMD modes, the rollers and the streaks are related to high and low frequency DMD modes, respectively. Finally, a model is constructed to predict the temporal evolution of the wall shear, using the 6 most relevant DMD modes interacting near the channel wall: 6 low frequency modes for DR and 3 low and 3 high frequency modes for DI. In the DR case the wall shear is predicted for almost 300 time units with relative error ∼ 2%, however, this error is larger in the DI case, ∼ 6%, suggesting the need of using a larger number of modes to represent this more complex flow.

Coherent structures in the turbulent channel flow of an elastoviscoplastic fluid

In this numerical and theoretical work, we study the turbulent channel flow of Newtonian and elastoviscoplastic fluids. The coherent structures in these flows are identified by means of higher order dynamic mode decomposition (HODMD), applied to a set of data non-equidistant in time, to reveal the role of the near-wall streaks and their breakdown, and the interplay between turbulent dynamics and non-Newtonian effects. HODMD identifies six different high-amplitude modes, which either describe the yielded flow or the yielded–unyielded flow interaction. The structure of the low- and high-frequency modes suggests that the interaction between high- and low-speed streamwise velocity structures is one of the mechanisms triggering the streak breakdown, dominant in Newtonian turbulence where we observe shorter near-wall streaks and a more chaotic dynamics. As the influence of elasticity and plasticity increases, the flow becomes more correlated in the streamwise direction, with long streaks disrupted for short times by localised perturbations, reflected in reduced drag. Finally, we present streamwise-periodic dynamic mode decomposition modes as a viable tool to describe the highly complex turbulent flows, and identify simple well-organised groups of travelling waves.

Analysis of the possibility of excitation of soliton-like waves in crystals using the ab initio method and molecular dynamic simulation

In this article, using the first-principle approach and the method of molecular dynamics, the authors investigate the properties of a Pt3Al crystal with a view to the possibility of the existence of such waves of soliton-like waves as discrete breathers. Methodical aspects of crystal models building are presented, with the analysis of the advantages and disadvantages of each method. Such crystal properties as the density distribution of phonon states, dispersion curves, and electron density distribution are studied. Based on the density functional theory, it is shown that the spectrum of the crystal does not have a through band gap, however, along the <100> directions, it is possible to excite high-amplitude long-lived modes on Al atoms. The results indicate the benefit of the concept of discrete breathers in the investigated crystal.

Accelerating full waveform inversion via source stacking and cross-correlations

SUMMARYAccurate synthetic seismic wavefields can now be computed in 3-D earth models using the spectral element method (SEM), which helps improve resolution in full waveform global tomography. However, computational costs are still a challenge. These costs can be reduced by implementing a source stacking method, in which multiple earthquake sources are simultaneously triggered in only one teleseismic SEM simulation. One drawback of this approach is the perceived loss of resolution at depth, in particular because high-amplitude fundamental mode surface waves dominate the summed waveforms, without the possibility of windowing and weighting as in conventional waveform tomography.This can be addressed by redefining the cost-function and computing the cross-correlation wavefield between pairs of stations before each inversion iteration. While the Green’s function between the two stations is not reconstructed as well as in the case of ambient noise tomography, where sources are distributed more uniformly around the globe, this is not a drawback, since the same processing is applied to the 3-D synthetics and to the data, and the source parameters are known to a good approximation. By doing so, we can separate time windows with large energy arrivals corresponding to fundamental mode surface waves. This opens the possibility of designing a weighting scheme to bring out the contribution of overtones and body waves. It also makes it possible to balance the contributions of frequently sampled paths versus rarely sampled ones, as in more conventional tomography.Here we present the results of proof of concept testing of such an approach for a synthetic 3-component long period waveform data set (periods longer than 60 s), computed for 273 globally distributed events in a simple toy 3-D radially anisotropic upper mantle model which contains shear wave anomalies at different scales. We compare the results of inversion of 10 000 s long stacked time-series, starting from a 1-D model, using source stacked waveforms and station-pair cross-correlations of these stacked waveforms in the definition of the cost function. We compute the gradient and the Hessian using normal mode perturbation theory, which avoids the problem of cross-talk encountered when forming the gradient using an adjoint approach. We perform inversions with and without realistic noise added and show that the model can be recovered equally well using one or the other cost function.The proposed approach is computationally very efficient. While application to more realistic synthetic data sets is beyond the scope of this paper, as well as to real data, since that requires additional steps to account for such issues as missing data, we illustrate how this methodology can help inform first order questions such as model resolution in the presence of noise, and trade-offs between different physical parameters (anisotropy, attenuation, crustal structure, etc.) that would be computationally very costly to address adequately, when using conventional full waveform tomography based on single-event wavefield computations.

Experimental Investigation of Tonal Self-Noise Emission of a Vehicle Side Mirror

The tonal self-noise emission of a vehicle side mirror and the associated flowfield are investigated experimentally. The relevant surface on the model includes a region of geometry-induced laminar flow separation close to the trailing edge. In the separated shear layer, high-amplitude instability modes at the acoustic mode frequencies are identified. The distinctive pattern of tonal noise emission combined with the shear layer instability modes are characteristic for a self-excited aeroacoustic feedback loop known from investigations of airfoil tonal self-noise emission. The resonance frequencies of the loop are derived from a simplified transfer model and adapted to the present conditions. The existence of the feedback mechanism on the side mirror is finally demonstrated by applying the resonance condition, in which the calculated modes are in very good agreement with the measured modes. Even though the feedback mechanism allows for the coexistence of several modes, a special aspect of the side mirror tonal self-noise emission is that the acoustic modes can alternate in time. The selection of a particular mode is successfully triggered by excitation with an external acoustic disturbance at or near the corresponding mode frequency, which emphasizes the sensitivity of the mode selection toward external disturbances.

Three-dimensional grating nanowires for enhanced light trapping.

We propose rationally designed 3D grating nanowires for boosting light-matter interactions. Full-vectorial simulations show that grating nanowires sustain high-amplitude waveguide modes and induce a strong optical antenna effect, which leads to an enhancement in nanowire absorption at specific or broadband wavelengths. Analyses of mode profiles and scattering spectra verify that periodic shells convert a normal plane wave into trapped waveguide modes, thus giving rise to scattering dips. A 200 nm diameter crystalline Si nanowire with designed periodic shells yields an enormously large current density of ∼28 mA/cm2 together with an absorption efficiency exceeding unity at infrared wavelengths. The grating nanowires studied herein will provide an extremely efficient absorption platform for photovoltaic devices and color-sensitive photodetectors.

Novel photochemistry of molecular polaritons in optical cavities.

Violations of the Born-Oppenheimer approximation (BOA) and the consequent nonadiabatic dynamics have long been an object of intense study. Recently, such dynamics have been induced via strong coupling of the molecule to a high-amplitude (spatially confined) mode of the electromagnetic field in optical cavities. However, the effects of a cavity on a pre-existing avoided crossing or conical intersection are relatively unexplored. The dynamics of molecules dressed by cavity modes are usually calculated by invoking the rotating wave approximation (RWA), which greatly simplifies the calculation but breaks down when the cavity mode frequency is higher than the relevant material frequencies. We develop a protocol for computing curve crossing dynamics in an optical cavity by exploiting a recently-developed method of solving the quantum Rabi model without invoking the RWA. The method is demonstrated for sodium iodide.