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Detection, Estimation, and Modulation Theory

In this paper we present the efficient design of functional thin-film metamaterial devices with the effective surface conductivity approach. As an example, we demonstrate a graphene based perfect absorber. After formulating the requirements to the perfect absorber in terms of surface conductivity we investigate the properties of graphene wire medium and graphene fishnet metamaterials and demonstrate both narrowband and broadband tunable absorbers.

/pdf/graphene-metamaterials-based-tunable-terahertz-absorber-7dt82exsc1.pdf

Graphene metamaterials based tunable terahertz absorber: effective surface conductivity approach

Terahertz time-domain spectroscopy (THz-TDS) is a powerful technique for material’s characterization and process control. It has been used for contact-free conductivity measurements of metals, semiconductors, 2D materials, and superconductors. Furthermore, THz-TDS has been used to identify chemical components such as amino acids, peptides, pharmaceuticals, and explosives, which makes it particularly valuable for fundamental science, security, and medical applications. This tutorial is intended for a reader completely new to the field of THz-TDS and presents a basic understanding of THz-TDS. Hundreds of articles and many books can be consulted after reading this tutorial. We explore the basic concepts of TDS and discuss the relationship between temporal and frequency domain information. We illustrate how THz radiation can be generated and detected, and we discuss common noise sources and limitations for THz-TDS. This tutorial concludes by discussing some common experimental scenarios and explains how THz-TDS measurements can be used to identify materials, determine complex refractive indices (phase delay and absorption), and extract conductivity.

Tutorial: An introduction to terahertz time domain spectroscopy (THz-TDS)

Several classes of non-planar metallic and dielectric waveguides have been proposed in the literature for guidance of terahertz (THz) or T-ray radiation. In this review, we focus on the development of dielectric waveguides, in the THz regime, with reduced loss and dispersion. First, we examine different THz spectroscopy configurations and fundamental equations employed for characterization of THz waveguides. Then we divide THz dielectric waveguides into three classes: solid-core, hollow-core, and porous-core waveguides. The guiding mechanism, fabrication steps, measured loss, and dispersion are presented for the waveguides in each class in chronological order. The goal of this review is to compare and contrast the current solutions for guiding THz radiation.

Terahertz dielectric waveguides

Tunable localized surface plasmon graphene metasurface for multiband superabsorption and terahertz sensing

We report broad bandwidth, 0.1–10 THz time-domain spectroscopy of linear and electro-optic polymers. The common THz optical component materials high-density polyethylene, polytetrafluoroethylene, polyimide (Kapton), and polyethylene cyclic olefin copolymer (Topas) were evaluated for broadband THz applications. Host polymers polymethyl methacrylate, polystyrene, and two types of amorphous polycarbonate were also examined for suitability as host for several important chromophores in guest-host electro-optic polymer composites for use as broadband THz emitters and sensors.

/pdf/broadband-terahertz-characterization-of-the-refractive-index-4m1y1fojz0.pdf

Broadband terahertz characterization of the refractive index and absorption of some important polymeric and organic electro-optic materials

We have investigated the broadband terahertz (THz) optical properties of nanoporous silicon samples with different porosities and the ultrafast carrier dynamics of photogenerated charge carriers in these materials. Following photoexcitation, we observe a fast carrier recovery time consisting of two dominant recombination processes with decay constants below ~10 ps. All samples exhibit initially low THz absorption that increases at higher frequencies, and is likely due to contributions from phonon bands and oxidation of the porous surface. The refractive index depends on porosity but shows little frequency dependence. These properties indicate that nanoporous silicon is a useful material for fast, ultrabroadband THz applications (e.g. intensity modulation).

Ultrafast carrier dynamics and optical properties of nanoporous silicon at terahertz frequencies

We use coupled mode theory, adequately incorporating optical losses, to model ultra-broadband terahertz (THz) waveguide emitters (0.1-20 THz) based on difference frequency generation of femtosecond infrared (IR) optical pulses. We apply the model to a generic, symmetric, five-layer, metal/cladding/core waveguide structure using transfer matrix theory. We provide a design strategy for an efficient ultra-broadband THz emitter and apply it to polymer waveguides with a nonlinear core composed of a poled guest-host electro-optic polymer composite and pumped by a pulsed fiber laser system operating at 1567 nm. The predicted bandwidths are greater than 15 THz and we find a high conversion efficiency of 1.2 × 10−4 W−1 by balancing both the modal phase-matching and effective mode attenuation.

Design of ultra-broadband terahertz polymer waveguide emitters for telecom wavelengths using coupled mode theory

espanolEl objetivo del presente trabajo es hacer una introduccion a la teoria basica utilizada para extraer informacion y como hacer un procesamiento de senales optimo para sistemas SoNAR activos o pasivos. En especial, se revisa como se catalogan los problemas tipicos que se tratan en sistemas tipo sonar. Se revisa la teoria estadistica que se usa en el procesamiento de senal acustica y se estudia el comportamiento de las curvas caracteristicas de operacion cuando se varia la varianza de las distribuciones de probabilidad de ruido y de senal en presencia del ruido. Por ultimo, se revisa la teoria de fondo de los problemas inversos y el procesamiento de campo ajustado para aplicarlo en un futuro a un sistema de visualizacion acuatica en condiciones de poca visibilidad. EnglishThe present work introduces the reader to the basic theory used to optimally extract and process information in passive and active sonar systems. In particular, the typical problems addressed when implementing sonar technology systems are reviewed. The work also covers the basic statistical theory that is used in acoustic signal processing and the behavior of the receiving operating characteristics (ROC) when dealing with probability density functions for noise and signal in presence of noise with varying standard deviations. Finally, we review the underlying theory of inverse problems and model based matched field processing that will be implemented in a future visualization system for under water applications.

https://dialnet.unirioja.es/descarga/articulo/6333257.pdf

Detección, estimación y problemas inversos con sistemas sonar

We present a simplified coupled mode theory (CMT), suited for high losses, to describe ultra-broadband THz generation through optical rectification (OR) of fs infrared pulses in waveguides. We derive a new expression that incorporates loss effects into the coherence length for OR. The simplified approach reproduces the results of a computationally rigorous integral CMT that must be used for broadband THz generation. With the new model we perform a parametric study to establish the optimal conditions for OR in symmetric, five-layer, metal/cladding/core structures with electro optic polymer cores. We find conversion efficiencies as high as 35 × 10−4 W−1 and bandwidths up to 20 THz when pumping at 1900 nm. We find that low-loss-cladding layers enhance the efficiency for phase-matched structures, increase the interaction length, and improve the stability of the efficiency with respect to variations in waveguide parameters.

Simplified model for optical rectification of broadband terahertz pulses in lossy waveguides including a new generalized expression for the coherence length.

Felipe A. Vallejo

Papers

Broadband terahertz characterization of the refractive index and absorption of some important polymeric and organic electro-optic materials

Ultrafast carrier dynamics and optical properties of nanoporous silicon at terahertz frequencies

Design of ultra-broadband terahertz polymer waveguide emitters for telecom wavelengths using coupled mode theory

Detección, estimación y problemas inversos con sistemas sonar

Simplified model for optical rectification of broadband terahertz pulses in lossy waveguides including a new generalized expression for the coherence length.