Multiparametric Porous Silicon Sensors

We investigated the possibility of using several sensing parameters from porous silicon in order to improve gas selectivity. By fabricating porous silicon optical microcavities, three independent quantities can be measured, i.e. the electrical conductance, the photoluminescence intensity, and the wavelength of the optical resonance. We monitored the change of these three parameters as a function of NO2 (0.5-5 ppm), ethanol (300-15000 ppm) and relative humidity (0-100%). Preliminary results confirm that the examined species affect the parameters in a different way, both as a relative change and as dynamic

Giant birefringence in optical antenna arrays with widely tailorable optical anisotropy

The manipulation of light by conventional optical components such as a lenses, prisms and wave plates involves engineering of the wavefront as it propagates through an optically-thick medium. A new class of ultra-flat optical components with high functionality can be designed by introducing abrupt phase shifts into the optical path, utilizing the resonant response of arrays of scatters with deeply-subwavelength thickness. As an application of this concept, we report a theoretical and experimental study of birefringent arrays of two-dimensional (V- and Y-shaped) optical antennas which support two orthogonal charge-oscillation modes and serve as broadband, anisotropic optical elements that can be used to locally tailor the amplitude, phase, and polarization of light. The degree of optical anisotropy can be designed by controlling the interference between the light scattered by the antenna modes; in particular, we observe a striking effect in which the anisotropy disappears as a result of destructive interference. These properties are captured by a simple, physical model in which the antenna modes are treated as independent, orthogonally-oriented harmonic oscillators

Aberration-free ultra-thin flat lenses and axicons at telecom wavelengths based on plasmonic metasurfaces

The concept of optical phase discontinuities is applied to the design and demonstration of aberration-free planar lenses and axicons, comprising a phased array of ultrathin subwavelength spaced optical antennas. The lenses and axicons consist of radial distributions of V-shaped nanoantennas that generate respectively spherical wavefronts and non-diffracting Bessel beams at telecom wavelengths. Simulations are also presented to show that our aberration-free designs are applicable to high numerical aperture lenses such as flat microscope objectives

All-linear time reversal by a dynamic artificial crystal

The time reversal of pulsed signals or propagating wave packets has long been recognized to have profound scientific and technological significance. Until now, all experimentally verified time-reversal mechanisms have been reliant upon nonlinear phenomena such as four-wave mixing. In this paper, we report the experimental realization of all-linear time reversal. The time-reversal mechanism we propose is based on the dynamic control of an artificial crystal structure, and is demonstrated in a spin-wave system using a dynamic magnonic crystal. The crystal is switched from an homogeneous state to one in which its properties vary with spatial period a, while a propagating wave packet is inside. As a result, a linear coupling between wave components with wave vectors k≈π/a and k′=k−2ππ/a≈−π/a is produced, which leads to spectral inversion, and thus to the formation of a time-reversed wave packet. The reversal mechanism is entirely general and so applicable to artificial crystal systems of any physical nature

Porous Silicon

This article deals with generalities and definitions of porous silicon (PSi): fabrication techniques, structural properties, chemical properties, electronic properties, electrical properties, optical properties, and actual or potential applications of PSi. Optical properties include light transport, photoluminescence, and electroluminescenc

CMOS fabrication of a light emitting diode based on silicon/porous silicon heterojunction

The fabrication in a standard CMO line of a light emitting diode (LED) based on silicon/porous silicon heterojunction is discussed. To fabricate the LED in a CMOS line, the porous silicon formation must be performed either as the last or as an intermediate step. The former option requires a masking layer to protect the metallization level of the CMOS devices from the electrochemical solution for the porous silicon formation, whereas the latter forces an interruption in the process. Experimental test on several materials, routinely used in CMOS processes, show that no standard mask is suitable to fully protect the CMOS from the electrochemical etch. Hence porous silicon formation should be performed as an intermediate ste

Growth and luminescence of porous silicon: X-ray and photoluminescence measurements

The authors have shown in their previous studies that applications of x-ray scattering techniques in a transmission geometry open an avenue for in situ structural investigation of silicon/solution, in general, semiconductor/solution interfaces. In these studies, the surface morphological evolution was deduced from x-ray reflectivity measurements and the pore-pore pair-distribution function was obtained from diffuse scattering measurements. The combination of these two techniques provided a complete phenomenological description of the pore formation and growth based on the most fundamental Langevin equation of interface-evolution phenomena, known as Edward-Wilkinson model. In essence, the high-current-density condition, where the surface etching occurs via surface oxidation of silicon and dissolution of the surface oxide, inhibits short-wave length fluctuations and the low-current-density condition, where the surface etching occurs via direct dissolution of H{sub 2}SiF{sub 6} formed at the interface, amplifies fluctuations with length scales of space-charge width. These two conditions are found to be consistent with the sign inversion of coefficient for the curvature dependent term of the Langevin equation. In this study extending the previous studies, the authors will show that the luminescence and the two conditions under which the porous silicon (PS) produced are directly related, based on x-ray and photoluminescence (PL) measurements

CMOS compatible Si/SiO2 multilayers for Light Emitting Diodes

We report photoluminescence and electroluminescence at room temperature in diodes based on Si/SiO2 multilayers. The multilayers are fabricated by alternating Si and SiO2 layers, whose thickness is, respectively, 3.5 and 5 nanometers. In photoluminescence, a single band is observed, centered at 800 nm, which is due to electron-hole pair recombination under quantuum confinement. on the other hand, in electroluminescence, two bands are reported. The first band is in the infrared spectrum, and is blackbody radiation. The second band is visible, and is originated by relaxation of a single type of electrical carrier (electrons), as suggested by a fast decay time (less than 0.1 s). Possible mechanisms can be hot-electron relaxation or coupling with surface plasmon-polariton

Silicon nanostructures for photonics

SPIE Nanotechnology E-Bulletin, November 2004 [invited

Silicon nanostructures for Photonics

Using light to convey signals around electronic chips could solve several current problems in microelectronic evolution: these include power dissipation, interconnect bottleneck, input/output of the chip to optical communication channels, signal bandwidth