28 research outputs found
Negative Refractive Index in Optics of Metal-Dielectric Composites
Specially designed metal-dielectric composites can have a negative refractive
index in the optical range. Specifically, it is shown that arrays of single and
paired nanorods can provide such negative refraction. For pairs of metal rods,
a negative refractive index has been observed at 1.5 micrometer. The inverted
structure of paired voids in metal films may also exhibit a negative refractive
index. A similar effect can be accomplished with metal strips in which the
refractive index can reach -2. The refractive index retrieval procedure and the
critical role of light phases in determining the refractive index is discussed.Comment: 39 pages, 17 figures, 24 equation
Always Keep your Target in Mind: Studying Semantics and Improving Performance of Neural Lexical Substitution
Lexical substitution, i.e. generation of plausible words that can replace a
particular target word in a given context, is an extremely powerful technology
that can be used as a backbone of various NLP applications, including word
sense induction and disambiguation, lexical relation extraction, data
augmentation, etc. In this paper, we present a large-scale comparative study of
lexical substitution methods employing both rather old and most recent language
and masked language models (LMs and MLMs), such as context2vec, ELMo, BERT,
RoBERTa, XLNet. We show that already competitive results achieved by SOTA
LMs/MLMs can be further substantially improved if information about the target
word is injected properly. Several existing and new target word injection
methods are compared for each LM/MLM using both intrinsic evaluation on lexical
substitution datasets and extrinsic evaluation on word sense induction (WSI)
datasets. On two WSI datasets we obtain new SOTA results. Besides, we analyze
the types of semantic relations between target words and their substitutes
generated by different models or given by annotators.Comment: arXiv admin note: text overlap with arXiv:2006.0003
Sub-diffraction light propagation in fibers with anisotropic dielectric cores
We present a detailed study of light propagation in waveguides with
anisotropic metamaterial cores. We demonstrate that in contrast to conventional
optical fibers, our structures support free-space-like propagating modes even
when the waveguide radius is much smaller than the wavelength. We develop
analytical formalism to describe mode structure and propagation in strongly
anisotropic systems and study the effects related to waveguide boundaries and
material composition
Meta-material photonic funnels for sub-diffraction light compression and propagation
We present waveguides with photonic crystal cores, supporting energy
propagation in subwavelength regions with a mode structure similar to that in
telecom fibers. We design meta-materials for near-, mid-, and far-IR
frequencies, and demonstrate efficient energy transfer to and from regions
smaller than 1/25-th of the wavelength. Both positive- and negative-refractive
index light transmissions are shown. Our approach, although demonstrated here
in circular waveguides for some specific frequencies, is easily scalable from
optical to IR to THz frequency ranges, and can be realized in a variety of
waveguide geometries. Our design may be used for ultra high-density energy
focusing, nm-resolution sensing, near-field microscopy, and high-speed
all-optical computing.Comment: 4 pages, 3 figures, texify read
Gain-assisted slow to superluminal group velocity manipulation in nano-waveguides
We study the energy propagation in subwavelength waveguides and demonstrate
that the mechanism of material gain, previously suggested for loss
compensation, is also a powerful tool to manipulate dispersion and propagation
characteristics of electromagnetic pulses at the nanoscale. We show
theoretically that the group velocity in lossy nano-waveguides can be
controlled from slow to superluminal values by the material gain and waveguide
geometry and develop an analytical description of the relevant physics. We
utilize the developed formalism to show that gain-assisted dispersion
management can be used to control the transition between ``photonic-funnel''
and ``photonic-compressor'' regimes in tapered nano-waveguides. The phenomenon
of strong modulation of group velocity in subwavelength structures can be
realized in waveguides with different geometries, and is present for both
volume and surface-modes.Comment: Some changes in the abstract and Fig.1. No results affecte
Active metamaterials: sign of refraction index and gain-assisted dispersion management
We derive an approach to define the causal direction of the wavevector of
modes in optical metamaterials, which in turn, determines signs of refractive
index and impedance as a function of {\it real and imaginary} parts of
dielectric permittivity and magnetic permeability. We use the developed
technique to demonstrate that the interplay between resonant response of
constituents of metamaterials can be used to achieve efficient dispersion
management. Finally we demonstrate broadband dispersion-less index and
impedance matching in active nanowire-based negative index materials. Our work
opens new practical applications of negative index composites for broadband
lensing, imaging, and pulse-routing
Revisiting Mahalanobis Distance for Transformer-Based Out-of-Domain Detection
Real-life applications, heavily relying on machine learning, such as dialog
systems, demand out-of-domain detection methods. Intent classification models
should be equipped with a mechanism to distinguish seen intents from unseen
ones so that the dialog agent is capable of rejecting the latter and avoiding
undesired behavior. However, despite increasing attention paid to the task, the
best practices for out-of-domain intent detection have not yet been fully
established.
This paper conducts a thorough comparison of out-of-domain intent detection
methods. We prioritize the methods, not requiring access to out-of-domain data
during training, gathering of which is extremely time- and labor-consuming due
to lexical and stylistic variation of user utterances. We evaluate multiple
contextual encoders and methods, proven to be efficient, on three standard
datasets for intent classification, expanded with out-of-domain utterances. Our
main findings show that fine-tuning Transformer-based encoders on in-domain
data leads to superior results. Mahalanobis distance, together with utterance
representations, derived from Transformer-based encoders, outperforms other
methods by a wide margin and establishes new state-of-the-art results for all
datasets.
The broader analysis shows that the reason for success lies in the fact that
the fine-tuned Transformer is capable of constructing homogeneous
representations of in-domain utterances, revealing geometrical disparity to out
of domain utterances. In turn, the Mahalanobis distance captures this disparity
easily.Comment: to appear in AAAI 202
GEC-DePenD: Non-Autoregressive Grammatical Error Correction with Decoupled Permutation and Decoding
Grammatical error correction (GEC) is an important NLP task that is currently
usually solved with autoregressive sequence-to-sequence models. However,
approaches of this class are inherently slow due to one-by-one token
generation, so non-autoregressive alternatives are needed. In this work, we
propose a novel non-autoregressive approach to GEC that decouples the
architecture into a permutation network that outputs a self-attention weight
matrix that can be used in beam search to find the best permutation of input
tokens (with auxiliary {ins} tokens) and a decoder network based on a
step-unrolled denoising autoencoder that fills in specific tokens. This allows
us to find the token permutation after only one forward pass of the permutation
network, avoiding autoregressive constructions. We show that the resulting
network improves over previously known non-autoregressive methods for GEC and
reaches the level of autoregressive methods that do not use language-specific
synthetic data generation methods. Our results are supported by a comprehensive
experimental validation on the ConLL-2014 and Write&Improve+LOCNESS datasets
and an extensive ablation study that supports our architectural and algorithmic
choices.Comment: ACL 202
Plasmonic Nanolayer Composites: Coupled Plasmon Polaritons, Effective-Medium Response, and Subdiffraction Light Manipulation
We analyze the evolution of the modes in nanoplasmonic multilayered structures and study the transition of the optical properties of these systems to the effective-medium regime. We derive the effective-medium parameters and study the validity of our analytical results with exact numerical solutions of Maxwell
equations. Finally, we explore the applications of multilayered systems for subwavelength light confinement in planar and circular waveguides