Hidden-information extraction from layered structures through terahertz imaging down to ultralow SNR.

Noninvasive inspection of layered structures has remained a long-standing challenge for time-resolved imaging techniques, where both resolution and contrast are compromised by prominent signal attenuation, interlayer reflections, and dispersion. Our method based on terahertz (THz) time-domain spectroscopy overcomes these limitations by offering fine resolution and a broadband spectrum to efficiently extract hidden structural and content information from layered structures. We exploit local symmetrical characteristics of reflected THz pulses to determine the location of each layer, and apply a statistical process in the spatiotemporal domain to enhance the image contrast. Its superior performance is evidenced by the extraction of alphabetic characters in 26-layer subwavelength papers as well as layer reconstruction and debonding inspection in the conservation of Terra-Cotta Warriors. Our method enables accurate structure reconstruction and high-contrast imaging of layered structures at ultralow signal-to-noise ratio, which holds great potential for internal inspection of cultural artifacts, electronic components, coatings, and composites with dozens of submillimeter layers.

Terahertz Imaging for Paper Handling of Legacy Documents.

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.

Reconfigurable multimode photonic-crystal waveguides.

We present the design and analysis of a novel reconfigurable photonic-crystal waveguide (PCW). The predefined waveguide is the result of the refractive-index variation of three rows of holes that can be obtained by the infiltration of liquids within what are otherwise air holes in a two-dimensional triangular-lattice photonic crystal. We compute the power transmission through the reconfigurable PCWs as well as through arbitrary waveguide bends. The advantages of writing reconfigurable PCW of a multimode nature are highlighted. We demonstrate the necessity to infiltrate high-refractive-index substances to obtain efficient power transfer via reconfigurable manner.

Self-collimated waveguide bends and partial bandgap reflection of photonic crystals with parallelogram lattice.

The photonic crystal structure with parallelogram lattice, capable of bending a self-collimated wave with free angles and partial bandgap reflection, is presented. The equifrequency contours show that the direction of the collimation wave can be turned by tuning the angle between the two basic vectors of the lattice. Acute, right, and obtuse angles of collimating waveguide bends have been realized by arc lattices of parallelogram photonic crystals. Moreover, partial bandgap reflection of the parallelogram lattice photonic crystals is validated from the equifrequency contours and the projected band structures. A waveguide taper based on this partial bandgap reflection is also designed and proved to have above 85% transmittance over a very wide operating bandwidth of 180 nm.

Graded index photonic crystals.

We explore two-dimensional triangular lattice photonic crystals composed of air holes in a dielectric background which are subject to a graded-index distribution along the direction transverse to the propagation. The proper choice of the parameters such as the input beam width, gradient coefficient, and the operating frequency allow the realizations of the focusing (lens) and guiding (waveguide) effects upon which more complex optical devices such as couplers can be designed. Numerical results obtained by the finite-difference time-domain and planewave expansion methods validate the application of Gaussian optics within a range of parameters where close agreement between them are observed.

Supersymmetric optimization of second-harmonic generation in mid-infrared quantum cascade lasers.

We describe a procedure to optimize mid-infrared quantum cascade lasers with respect to the resonant second-order optical susceptibility, based on supersymmetric quantum mechanics. Realization of the optimized potential can be obtained by composition grading of ternary alloys, such as in InGaAs/AlInAs-based structures. The approach yields designs not only with strongly enhanced predicted nonlinear conversion efficiencies, but also with significantly improved modal gain compared with demonstrated devices.