Lionel Duvillaret

Photoinduced variation of dielectric constant of silicon in the far infrared: applications to light-controllable photonic band-gap crystals in the terahertz frequency range

The far infrared behavior of doped semiconductors is dominating by the free carrier response and it is well described by the simple Drude model. As the free carriers can be photogenerated in high resistivity semiconductors, the electromagnetic response of these semiconductors can be changed, via laser excitation, from a dielectric behavior to a metallic one. Using terahertz time-domain spectroscopy, we have checked this Drude-like behavior with several silicon wafers of different doping, and with a high resistivity silicon sample photo-excited by a Ti:Sa laser beam. Then we benefited of this effect to photo-modify the transmission of a photonic band-gap crystal in the terahertz range. The photonic band-gap crystal is a woodpile structure, which exhibits a complete forbidden band around 265 GHz. Silicon- made defects are introduced in interstitial sites of the lattice of the photonic crystal in order to create a defect mode inside the photonic band-gap of the crystal. This defect mode then acts as a narrow bandpass filter at the frequency of 253 GHz. Under 400-mW laser excitation power, the resonance peak associated to the defect mode completely vanishes.

Electro-optic sampling of GaAs circuits using semiconductor lasers

Abstract Picosecond optical pulses produced by a gain-switched 1.5 μm semiconductor laser with fibre compression are used in internal electro-optic sampling of GaAs circuits. A measurement bandwidth of 60 GHz and a 20 mV voltage resolution are demonstrated.

Micromachining techniques at terahertz frequencies

In this paper we have characterized the refractive indexed and absorption coefficient of negative photoresist NANO XP SU-8 from 0.1-1.6 THz using THz time-domain spectroscopy. Over the measured frequency range it was found that the refractive index is a relatively flat function of frequency, decreasing from 1.8 to 1.7. The value of absorption coefficient is seen to increase in a line fashion over the given frequency range, being 25 cm-1 at 1 THz. From this data we have extracted functions of dielectric constant and dielectric loss tangent versus frequency, quantities which will be of use for future THz circuit design. In addition to these measurements, we have demonstrated two novel applications of SU-8. First, we have fabricated membrane-like features by using a multi-exposure photolithographic technique on a single layer of SU-8, and second we have utilized a thin layer of SU-8 as a patternable adhesion layer as part of a semiconductor epitaxial lift-off process designed to transfer III-V semiconductor epitaxial layers onto lower loss host substrates.

E-field fiber tip sensors by exploiting the electro-optic tunability of lithium niobate photonic crystals (Conference Presentation)

Biomedical engineering (BME), electrophysiology, Electromagnetic Compatibility (EMC) or aerospace and defense fields demand compact electric field sensors with very small spatial resolution, low sensitivity and large bandwidth. We show that the electro-optical property of lithium niobate coupled with the tunability of photonic crystals can answer this request through Lab-on-Fiber technology.nFirst, band diagram calculations and Finite Difference Time Domain (FDTD) simulations analysis lead to the design of the most suitable two-dimensional photonic crystal geometry. We show that light normal incidence on rectangular array of air holes in free standing X-cut thin film lithium niobate produces a very sharp and E-field sensitive Fano resonance at a wavelength of 1550nm. Then, in order to concentrate the E-Field to be detected in the photonic crystal area (20μm*20μm*0.7μm) we design a thin metallic antenna, scaled down them in such a way that it does not produce any disturbances while increasing the sensitivity.nThe LN membrane with the antenna is fabricated by standard clean room processes and Focused Ion Beam (FIB) is used to mill the photonic crystal. Then, by means of a flexible/bendable transparent membrane, we were able to align and to attach the photonic crystal onto a ferrule ending polarization maintained optical fiber.nOptical characterizations show that the Fano resonance is easily modulated (wavelength shifted) by the surrounding E-field. The novel non-intrusive E-field sensor shows linearity, low sensitivity, large bandwidth (up to 100GHz) and a very small spatial resolution (≈20μm). To the best of our knowledge, this spatial resolution has never been achieved in E-field optical sensing before.