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as plasmonic hot-spots. [2] The resulting spectroscopic technique, known as surface-enhanced Raman spectroscopy (SERS), [3] surpasses the inherently low sensitivity of Raman by counterbalancing its low scattering efficiency [4] through spatial localization of the sample molecules proximal to the hot-spots on SERS substrates. So far colloidal nanoparticles of various shapes and sizes have been extensively used in SERS; [5][6][7][8][9] however, the lack of control over their relative orientation and separation limits efficient plasmonic coupling therebetween. [10] While this issue is addressed in SERS substrates made of immobilized nanoparticles, [11][12][13][14] interparticle gaps in these substrates are typically optimized to generate maximum field confinement for single wavelengths. Broadband SERS substrates can be made by immobilizing a mixture of nanoparticles resonant over a range of laser wavelengths on surface plasmon polariton (SPP)-supporting thin films. [15,16] However, the magnitude of SERS signal enhancement and the broadband nature of such substrates remain highly dependent on the nanoparticle-film separation as well as the interparticle distances which are difficult to control in practice. More robust broadband SERS substrates have been recently fabricated by sputtering randomly sized silver nanoparticles on glass-silver-glass multilayered substrates, [17] yielding plasmonic resonances over the 400-1100 nm

Layered-structure materials are currently relevant given their quasi-2D nature. Knowledge of their physical properties is currently of major interest. Niobium ditelluride possesses a monoclinic layered-structure with a distortion in the tellurium planes. This structural complexity has hindered the determination of its fundamental physical properties. In this work, NbTe2 crystals were used to elucidate its structural, compositional, electronic and vibrational properties. These findings have been compared with calculations based on density functional theory. The chemical composition and elemental distribution at the nanoscale were obtained through atom probe tomography. Ultraviolet photoelectron spectroscopy allowed the first determination of the work function of NbTe2. Its high value, 5.32 eV, and chemical stability allow foreseeing applications such as contact in optoelectronics. Raman spectra were obtained using different excitation laser lines: 488, 633, and 785 nm. The vibrational frequencies were in agreement with those determined through density functional theory. It was possible to detect a theoretically-predicted, low-frequency, low-intensity Raman active mode not previously observed. The dispersion curves and electronic band structure were calculated, along with their corresponding density of states. The electrical properties, as well as a pseudo-gap in the density of states around the Fermi energy are characteristics proper of a semi metal.

The usage of ultrathin flexible silicon foil can further extend the functionality of silicon and emerging silicon-based tandem solar cells particularly in building and vehicle-integrated photovoltaics where high-efficiency, lightweight, and flexible solar panels are highly desired. However, silicon's relatively weak optical absorption coefficient especially in the near infrared (NIR) region limits its optoelectronic applications with a reduced wafer thickness. Herein, we seek to overcome this limitation by exploring the wave interference phenomenon for effective absorption of NIR light in ultrathin silicon. Particularly, inverted pyramid photonic crystals (PhCs) with nano−micrometer-scale feature sizes are carved directly on silicon. Detailed experimental and theoretical studies are presented by systematically examining the optical properties of PhC-integrated thin silicon substrates (down to a 10 μm thickness). The corresponding maximum photocurrent density for a thin absorber is projected and compared with that predicted by Lambertian's limit. In contrast to traditionally configured microscale inverse pyramids, we show that a small mesa width is critical to achieving high optical performance for a wave-interference-based absorption enhancement. Mesa widths as small as 35 nm are realized over a large wafer-scale fabrication using facile techniques. The optical performance of 10 μm silicon indicates that an ideal photocurrent density approaching 40 mA/cm 2 is feasible. This study indicates that photonic crystals provide strong wave interference in ultrathin silicon, and in particular, we observe high optical absorption even after removing more than 90% of the silicon from conventional "thick" Si wafers.

We delineate the four principal surface plasmon polariton coupling and interaction mechanisms in subwavelength gratings, and demonstrate their significant roles in shaping the optical response of plasmonic gratings. Within the framework of width-graded metal–insulator-metal nano-gratings, electromagnetic field confinement and wave guiding result in multiwavelength light localization provided conditions of adiabatic mode transformation are satisfied. The field is enhanced further through fine tuning of the groove-width (w), groove-depth (L) and groove-to-groove-separation (d). By juxtaposing the resonance modes of width-graded and non-graded gratings and defining the adiabaticity condition, we demonstrate the criticality of w and d in achieving adiabatic mode transformation among the grooves. We observe that the resonant wavelength of a graded grating corresponds to the properties of a single groove when the grooves are adiabatically coupled. We show that L plays an important function in defining the span of localized wavelengths. Specifically, we show that multiwavelength resonant modes with intensity enhancement exceeding three orders of magnitude are possible with w < 30 nm and 300 nm < d < 900 nm for a range of fixed values of L. This study presents a novel paradigm of deep-subwavelength adiabatically-coupled width-graded gratings—illustrating its versatility in design, hence its viability for applications ranging from surface enhanced Raman spectroscopy to multispectral imaging.

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High performance in crystalline thin (-10 IJm) film silicon solar cells requires complete optical absorption over its spectral range. Geometrical schemes are ineffective due to their large feature dimensions. Enhanced optical absorption can be achieved through two mechanisms based on diffractive and physical optics. In diffractive approach, light confinement is achieved through obliquely propagating transmission orders that effectively fill the frequency space. In the physical optics approach, rigorous coupled wave analysis is used to calculate optical absorption in subwavelength grating structures based on wave guiding mechanism, A 10-lJm thick Si film in SOl configuration was chosen to perform a comparative evaluation of these two approaches. Optical transmission of planar Si films was compared with randomly textured and deeply etched two-dimensional gratings structures. Transmission from random structures was diffuse and translucent, while that from gratings, was weak and wavelength-dependent.Although SOl substrates are not practical for large-scale manufacturing, they have been determined to highly effective for understanding and optimizing optical transmission and device performance.

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