An insect-scale robot reveals the effects of different body dynamics regimes during open-loop running in feature-laden terrain

The transition from the lab to natural environments is an archetypal challenge in robotics. While larger robots can manage complex limb-ground interactions using sensing and control, such strategies are difficult to implement on small platforms where space and power are limited. The Harvard Ambulatory Microrobot (HAMR) is an insect-scale quadruped capable of effective open-loop running on featureless, hard substrates. Inspired by the predominantly feedforward strategy of rapidly-running cockroaches on uneven terrain [Sponberg, 2007], we used HAMR to explore open-loop running on two 3D printed heterogeneous terrains generated using fractional Brownian motion. The ``pocked'' terrain had foot-scale features throughout while the ``jagged'' terrain features increased in height in the direction of travel. We measured the performance of trot and pronk gaits while varying limb amplitude and stride frequency. The frequencies tested encompassed different dynamics regimes: body resonance (10-25~Hz) and kinematic running (30-40~Hz), with dynamics typical of biological running and walking, respectively, and limb-transmission resonance (45-60~Hz). On the featureless and pocked terrains, low mechanical cost-of-transport (mCoT) kinematic running combinations performed best without systematic differences between trot and pronk; indicating that if terrain features are not too tall, a robot can transition from homo- to heterogeneous environments in open-loop. Pronk bypassed taller features than trot on the jagged terrain, and higher mCoT, lower frequency running was more often effective. While increasing input power to the robot improved performance in general, lower frequency pronking on jagged terrain allowed the robot to bypass taller features compared with the same input power at higher frequencies. This was correlated with the increased variation in center-of-mass orientation occurring at frequencies near body resonance. This study established that appropriate choice of robot dynamics, as mediated by gait, frequency, and limb amplitude, can expand the terrains accessible to microrobots without the addition of sensing or closed-loop control.

Cascaded PT-Symmetric Artificial Sheets: Multimodal Manipulation of Self-Dual Emitter-Absorber Singularities, and Unidirectional and Bidirectional Reflectionless Transparencies

We herein introduce cascaded parity-time (PT)-symmetric artificial sheets (e.g., metasurfaces or frequency selective surfaces) that may exhibit multiple higher-order laser-absorber modes and bidirectional reflectionless transmission resonances within the PT-broken phase, as well as a unidirectional reflectionless transmission resonance associated with the exceptional point (EP). We derive the explicit expressions of the gain-loss parameter required for obtaining these modes and their intriguing physical properties. By exploiting the cascaded PT structures, the gain-loss threshold for the self-dual laser-absorber operation can be remarkably lowered, while the EP remains unaltered. We further study interferometric sensing based on such a multimodal laser-absorber and demonstrate that its sensitivity could be unprecedentedly high and proportional to the number of metasurfaces along the light propagation direction.

Reflectionless excitation of arbitrary photonic structures: a general theory

We outline and interpret a recently developed theory of impedance matching or reflectionless excitation of arbitrary finite photonic structures in any dimension. The theory includes both the case of guided wave and free-space excitation. It describes the necessary and sufficient conditions for perfectly reflectionless excitation to be possible and specifies how many physical parameters must be tuned to achieve this. In the absence of geometric symmetries, such as parity and time-reversal, the product of parity and time-reversal, or rotational symmetry, the tuning of at least one structural parameter will be necessary to achieve reflectionless excitation. The theory employs a recently identified set of complex frequency solutions of the Maxwell equations as a starting point, which are defined by having zero reflection into a chosen set of input channels, and which are referred to as R-zeros. Tuning is generically necessary in order to move an R-zero to the real frequency axis, where it becomes a physical steady-state impedance-matched solution, which we refer to as a reflectionless scattering mode (RSM). In addition, except in single-channel systems, the RSM corresponds to a particular input wavefront, and any other wavefront will generally not be reflectionless. It is useful to consider the theory as representing a generalization of the concept of critical coupling of a resonator, but it holds in arbitrary dimension, for arbitrary number of channels, and even when resonances are not spectrally isolated. In a structure with parity and time-reversal symmetry (a real dielectric function) or with parity–time symmetry, generically a subset of the R-zeros has real frequencies, and reflectionless states exist at discrete frequencies without tuning. However, they do not exist within every spectral range, as they do in the special case of the Fabry–Pérot or two-mirror resonator, due to a spontaneous symmetry-breaking phenomenon when two RSMs meet. Such symmetry-breaking transitions correspond to a new kind of exceptional point, only recently identified, at which the shape of the reflection and transmission resonance lineshape is flattened. Numerical examples of RSMs are given for one-dimensional multimirror cavities, a two-dimensional multiwaveguide junction, and a multimode waveguide functioning as a perfect mode converter. Two solution methods to find R-zeros and RSMs are discussed. The first one is a straightforward generalization of the complex scaling or perfectly matched layer method and is applicable in a number of important cases; the second one involves a mode-specific boundary matching method that has only recently been demonstrated and can be applied to all geometries for which the theory is valid, including free space and multimode waveguide problems of the type solved here.

UNDERWATER ACOUSTIC CHANNEL MODELING PROPOSAL FOR SHALLOW WATER COMMUNICATION LINK OPTIMIZATION

ABSTRACT.Since 80’s, underwater acoustic digital communication has being one of the principal topic in underwater acoustics research. Besides, underwater waveguide modeling also has been a subject of intensive research taking benefit of the increase in available computational power and data volume provided by the new generation of ocean data measurements instruments. In shallow water specific scenario normal-mode acoustic propagation models are still based on classic Pekeris two layers ocean model. Our paper also starts from Pekeris but uses some different approaches based on waveguide coupling and in generalized adiabatic coupled mode theory. In other words, we used the property that all waveguides could be acoustically excited by transmission resonance of consecutive modes, considering an adiabatic invariant channel. Synthetic results are compared with transmission loss measurements in field experiments trying to validate the model.Keywords: waveguide, adiabatic, couple modes, resonance.RESUMO.Desde da década de 80 a comunicação digital tem sido um dos principais tópicos nos estudos de acústica subaquática. Além disso, a modelagem do guia de ondas submarino também tem sido objeto de pesquisa intensa aproveitando-se do aumento do poder computacional disponível e do volume de dados fornecido pela nova geração de instrumentos de coleta de dados oceânicos. Em um cenário específico para águas rasas, os modelos de propagação acústica de modo normal ainda tem como base o modelo clássico de oceano em duas camadas de Pekeris. Nosso artigo também começa a partir de Pekeris mas usando algumas abordagens diferentes, baseadas no acoplamento dos guias de onda e na teoria de modos adiabáticos acoplados. Em outras palavras, usamos a propriedade em que os guias de ondas podem ser acusticamente excitados por transmissão ressonante de modos consecutivos, considerando um canal adiabático invariante. Os resultados sintéticos são comparados com medidas de perda de transmissão em um experimento de campo buscando a validação do modelo.Palavras-chave: guia de ondas, adiabático, modos acoplados, ressonância.

Chiral Optical Tamm States at the Interface between an All-Dielectric Polarization-Preserving Anisotropic Mirror and a Cholesteric Liquid Crystal

As a new localized state of light, the chiral optical Tamm state exists at the interface between a polarization-retaining anisotropic mirror and a substance with optical activity. Considering a hybrid structure comprising a metal-free polarization-preserving mirror and a cholesteric liquid crystal, we highlight the high Q factor arising from the all-dielectric framework. The intensity of localized light decreases exponentially with increasing distance from the interface. The penetration of the field into the cholesteric liquid crystal is essentially prohibited for wavelengths lying in the photonic bandgap and close to the cholesteric pitch length. The dielectric mirror has its own photonic bandgap. The energy transfer along the interface can be effectively switched off by setting the tangential wave vector to zero. The spectral behavior of the chiral optical Tamm state is observed both as reflection and transmission resonance. This Fano resonance is analogous to the Kopp–Genack effect. Our analytics are well in line with precise calculations, which may pave a new route for the future development of intelligent design for laser and sensing applications.