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An athermal optical sensor using intensity detection based on silicon microring resonators (MRs) is presented. The thermally independent sensing is realized in the sensing zone within the temperature range \pm5~K. The significant phase difference in the presence of an analyte between the low-Q coupled MRs is further sensitized through an interference-based intensity sensing scheme. Better performance of the coupled MR sensor compared to conventional microring sensors is demonstrated using transfer-matrix theory. For a measurement system of 30 dB signal-to-noise ratio, a detection limit of 6.8 \times 10^-7 recognition indexing unit is predicted. [ABSTRACT FROM PUBLISHER]

We present simulations of midinfrared quantum-cascade lasers (QCLs) with optimized second-harmonic generation (SHG). The optimized design was obtained utilizing techniques from supersymmetric quantum mechanics with both material-dependent effective mass and band nonparabolicity. Carrier transport and power output of the structure are analyzed by self-consistently solving rate equations for the carriers and photons. Nonunity pumping efficiency from one period of the QCL to the next is taken into account by including all relevant electron-electron and electron-longitudnal (LO) phonon scattering mechanisms between the injector/collector and active regions. Two-photon absorption processes are analyzed for the resonant cascading triple levels designed for enhancing SHG. Both sequential and simultaneous two-photon absorptions are included in the rate-equation model. The current-output characteristics for both structures are analyzed and compared. Stronger resonant tunneling in the optimized structure is manifested by enhanced negative differential resistance. Current-dependent linear optical output power is derived based on the steady-state photon populations in the active region. The second-harmonic-power is derived from the Maxwell equations with the phase mismatch included. Due to stronger coupling between lasing levels, the optimized structure has both higher linear and nonlinear output powers. Phase mismatch effect is significant for both structures leading to a substantial reduction of the linear-to-nonlinear conversion efficiency. The optimized structure can be fabricated through digitally grading the submonolayer alloys by molecular beam epitaxy technique. [ABSTRACT FROM AUTHOR]

Photonic crystals (PC) are periodic dielectric structures that, if suitably designed, prohibit light propagation within a frequency band even though the constituent materials may be transparent in this range. One of the remarkable applications of such artificial photonic materials has been to provide guiding of light. Ultimately, to guide light in directions other than straight lines, PC waveguide (PCW) bends are needed, and thus are expected to be essential building blocks of photonic integrated circuits. While bending light through large angles is possible with conventional waveguides, the corner radii of such bends cannot typically be reduced to the electromagnetic wavelength, which hinders the realization of extremely compact devices. And though sharp two-dimensional (2-D) PCW bends have been proposed, the transmission is typically low and/or narrow band. Here we focus on PCWs obtained by introducing line defects in otherwise period 2-D PCs with the aims of enhancing the typical poor and low-band- width transmission through tight bends. We show how PCW bends occurring at heterojunctions between different PCs may enable unprecedented flexibility in meeting these aims. The deformation introduced to the usual PC lattice lifted off the angle constraint and resulted in the power transmission greater than 90% over in the 95-nm bandwidth. [ABSTRACT FROM AUTHOR]

Discusses the theoretical investigation on coherent control of excitons in quantum wells at high repetition rates to determine the practical constraints for application in high-bit-rate optical switching. How the dynamics of coherent control of the excitons population was examined; How the constraint was implied for high-speed optical switching.

The article reports on the role of phase in measuring the intensity and wave amplitude of laser light. It is stated that based on observation, phase particularly the phase of the waves which is often ignored encodes further details of the internal workings of the laser which is composed of numerous layers of two or more semiconductors. Phase also contains subtle but important information about the wave and the physical mechanisms that produced it. In addition, it also involves following dynamics which may be too fast to track directly the ocean waves.

Recently, terahertz pulse imaging has been used extensively to characterize internal defects in various electrically insulating materials. In this work, terahertz pulse imaging is used to identify the damage induced by low-velocity impact on woven glass-fiber reinforced polyamide laminates. Several impact energies are considered to study the damage initiation and propagation. The permanent indentation, known to be a relevant damage indicator, is extracted from the terahertz results and validated through comparison with profilometry. Further, the criticality of the damage, in terms of the number of composite plies in which the cracks propagate, can be clearly determined from terahertz imaging. These observations are validated through X-ray tomographic observations and analysis. Finally, a strong connection is established between the evolution of the permanent indentation and the appearance, and criticality, of the low-velocity impact-induced damage. These observations suggest that terahertz imaging is a reliable technique for the nondestructive assessment of impact damage in glass fiber composites. [ABSTRACT FROM AUTHOR]

Measuring mill‐scale thickness in a nondestructive and noncontact fashion in a production environment remains a challenge, as knowledge of the thickness is central to processing of the steel strip. Terahertz time‐of‐flight tomography is demonstrated here to enable nondestructive and noncontact measurements of tertiary mill‐scale thickness down to ≈10 μm on production hot‐rolled steel, building on our recent work on thickness evaluation of laboratory‐produced scale on steel coupons. X‐ray diffraction (XRD) results show that the compositions of the mill‐scale films manufactured under production conditions are slightly different from the laboratory‐prepared steel coupons, resulting from the different compositions of phases present in the two types of samples. The mill‐scale thickness is obtained after advanced signal processing of the raw data and shows good agreement with values obtained from destructive measurements. Compared to conventional techniques, such as scanning electron microscopy which is destructive, eddy current testing, which requires direct contact with the surface of samples investigated and is not suitable for large areas, or elevated temperatures, and acoustic inspection, whose spatial resolution is largely limited by its relatively long‐wavelength, terahertz time‐of‐flight tomography may be implemented for in‐line nondestructive thickness characterization and quality control of optically thin tertiary mill scale on steel products, even in a production environment. [ABSTRACT FROM AUTHOR]

The article presents a study which proposed a locating and imaging technique for hidden-information extraction from layered structures in the case of ultralow signal-to-noise ratio (SNR). Topics discussed include measurement setup and processing framework, signal analysis and physical explanation, and applications in cultural heritages.

Investigates the effects of conductive apertures on spatiotemporal terahertz pulse shaping in a three-dimensional problem space with three-dimensional simulation using the finite-difference time domain method in tandem with a transformation equation and a symmetry technique. Confirmation of the accuracy of the described technique; Simulation of apertures of various shapes and sizes.

Despite predictions of the paperless office, global demand for printing and writing paper remains strong, and paper appears to be here to stay for some time. Not only firms, but also governments, libraries, and archives are in possession of large collections of legacy documents that still must be sorted and scanned. In this study, terahertz-based techniques are demonstrated to address several routine tasks related to the automated paper handling of unsorted legacy documents. Specifically, we demonstrate terahertz-based counting of the number of sheets in unconsolidated paper stacks, as well as locating stapled documents buried in paper stacks. [ABSTRACT FROM AUTHOR]

We study experimentally and theoretically the permutation entropy (PE) of the optical intensity $I(t)$ of an external-cavity semiconductor distributed feedback laser in the coherence collapse regime. Our PE analysis allows us to uncover the intrinsic dynamical complexity at multiple timescales of the delayed-feedback system, as well as to investigate how the experimental observations can be determined by modeling. An overall good agreement between experiment and theory corroborates the effectiveness of the Lang–Kobayashi model, though the model underestimates the entropy on the timescale of the relaxation oscillations and can lead to a time-delay signature that is less evident than in experiment, indicating a potential vulnerability of chaos encryption. This provides a critical test of the standard theoretical framework in which chaotic external-cavity semiconductor lasers are understood. [ABSTRACT FROM AUTHOR]

A critical issue in optical chaos-based communications is the possibility to identify the parameters of the chaotic emitter and, hence, to break the security. In this paper, we study theoretically the identification of a chaotic emitter that consists of a semiconductor laser with an optical feedback. The identification of a critical security parameter, the external-cavity round-trip time (the time delay in the laser dynamics), is performed using both the auto-correlation function and delayed mutual information methods applied to the chaotic time-series. The influence on the time-delay identification of the experimentally tunable parameters, i.e., the feedback rate, the pumping current, and the time-delay value, is carefully studied We show that difficult time-delay-identification scenarios strongly depend on the time-scales of the system dynamics as it undergoes a route to chaos, in particular on how close the relaxation oscillation period is from the external-cavity round-trip time. [ABSTRACT FROM AUTHOR]