There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

The synthesis of massive arrays of monodispersed carbon nanotubes that are self-oriented on patterned porous silicon and plain silicon substrates is reported. The approach involves chemical vapor deposition, catalytic particle size control by substrate design, nanotube positioning by patterning, and nanotube self-assembly for orientation. The mechanisms of nanotube growth and self-orientation are elucidated. The well-ordered nanotubes can be used as electron field emission arrays. Scaling up of the synthesis process should be entirely compatible with the existing semiconductor processes, and should allow the development of nanotube devices integrated into silicon technology.

https://science.sciencemag.org/content/sci/283/5401/512.full.pdf

Self-Oriented Regular Arrays of Carbon Nanotubes and Their Field Emission Properties

The term photonic crystals appears because of the analogy between electron waves in crystals and the light waves in artificial periodic dielectric structures. During the recent years the investigation of one-, two-and three-dimensional periodic structures has attracted a widespread attention of the world optics community because of great potentiality of such structures in advanced applied optical fields. The interest in periodic structures has been stimulated by the fast development of semiconductor technology that now allows the fabrication of artificial structures, whose period is comparable with the wavelength of light in the visible and infrared ranges.

http://ieeexplore.ieee.org/iel5/6354/16969/00781867.pdf

Photonic crystals

We demonstrate that a monolayer graphene membrane is impermeable to standard gases including helium. By applying a pressure difference across the membrane, we measure both the elastic constants and the mass of a single layer of graphene. This pressurized graphene membrane is the world's thinnest balloon and provides a unique separation barrier between 2 distinct regions that is only one atom thick.

/pdf/impermeable-atomic-membranes-from-graphene-sheets-xtis5huy2d.pdf

Impermeable atomic membranes from graphene sheets.

A large amount of work world-wide has been directed towards obtaining an understanding of the fundamental characteristics of porous Si. Much progress has been made following the demonstration in 1990 that highly porous material could emit very efficient visible photoluminescence at room temperature. Since that time, all features of the structural, optical and electronic properties of the material have been subjected to in-depth scrutiny. It is the purpose of the present review to survey the work which has been carried out and to detail the level of understanding which has been attained. The key importance of crystalline Si nanostructures in determining the behaviour of porous Si is highlighted. The fabrication of solid-state electroluminescent devices is a prominent goal of many studies and the impressive progress in this area is described.

The structural and luminescence properties of porous silicon

The highly sensitive surface of porous silicon (PSi) and the possibility to measure changes in its electrical properties can be used for electrical biosensor applications. The sensing scenario can be extended from the well-characterized organic solvents and pH sensors to detection of biological processes involving the presence of charged molecules. We have investigated the change in electrical properties (capacitance and conductance) of macroporous silicon layers upon exposure to organic solvents and water. Evaluation of our sensor to detect a biological event was done by addressing DNA hybridization. As opposed to the previous work published in this field, in our devices the electrical contact is made on the crystalline silicon (c-Si) substrate. This allows a complete exposure of the surface to the sensing species and reduces the generation of ionic currents through the porous silicon matrix. We will report results of a complete characterization of our device including response speed, selectivity and sensitivity.

Porous Silicon Electrical Biosensors

Femtosecond bleaching measurements are performed in n-doped GaAs at both 300 K and 14 K using low densities of carriers injected at 2 eV. The dwell time of electrons excited far above the Fermi sea is reduced because of hot-electron/cold-electron interactions. The response at 14 K is much stronger and slower than that at 300 K. This is attributed to: (i) the proximity to the Fermi level of the conduction band states probed via split-off valence band coupling; (ii) slow Landau damping of non-equilibrium plasmons, primarily by the minority holes which then heat the majority electrons.

https://iopscience.iop.org/article/10.1088/0268-1242/9/5S/015/pdf

Hot carrier dynamics near the Fermi edge of n-doped GaAs

Free carrier absorption (FCA) in silicon is the major obstacle toward achieving optical gain in Si nanostructure systems. In this Letter, we present experimental results of pump-induced loss for TE and TM polarization in multislot SiO2/nc-Si waveguides. Continuous wavelength and ultrafast studies of carriers excited in the nc-Si multilayers reveal strong suppression of transmission loss related to FCA in Si nanostructures for TM-polarized probe light. We demonstrate theoretically and experimentally that FCA may be reduced under TM polarization as much as 9 times compared to TE polarization.

Suppression of free carrier absorption in silicon using multislot SiO2/nc-Si waveguides

Micron-size channels have been made inside silicon by femtosecond laser-assisted HF etching. The etching is guided by localized carriers generated via two-photon absorption. Its potential of 3-D drilling has been demonstrated.

Three-dimensional femtosecond photochemical etching in silicon

The presence of point defects is expected to influence the properties of free carriers in semiconductors. Low-temperature-grown (LT) GaAs is a material with a high density of point defects and a unique combination of properties: sub(picosecond) mobile carrier lifetime, good carrier mobility and very high dark resistivity.1 The ultrashort mobile carrier lifetime in LT GaAs and other III-Vs has been extensively investigated by many groups and found to be due to trapping.2–6 The purpose of the present paper is to report results on the ultrafast thermalization dynamics of hot carriers in LT grown III-Vs.

Philippe M. Fauchet

Papers

Porous Silicon Electrical Biosensors

Hot carrier dynamics near the Fermi edge of n-doped GaAs

Suppression of free carrier absorption in silicon using multislot SiO2/nc-Si waveguides

Three-dimensional femtosecond photochemical etching in silicon

Hot Carrier Thermalization Dynamics in Low-Temperature-Grown III-V Semiconductors