O. Crégut

Quasi-solitonic behavior of self-written waveguides created by photopolymerization

We investigated the condition of unique self-written channel and multichannel propagation inside bulk photopolymerizable materials. Light was introduced in the medium by a single-mode optical fiber. At a very low beam power of 5 muW , a unique uniform-channel waveguide without any broadening was obtained by polymerization. When the input power is increased to 100 muW , the guide becomes chaotic and multichannel. We connected two fibers separated by a 1-cm distance. The results open the door to studies of the optical and electro-optical properties of photopolymerized guides doped by nonlinear optical chromophores and to possible applications in integrated optical devices.

Optical gain in porous silicon grains embedded in sol-gel derived SiO2 matrix under femtosecond excitation

Porous silicon grains embedded in the phosphorus doped SiO2 matrix exhibit improved photoluminesce properties and better stability in comparison with native porous silicon samples. We have tested this material for the presence of room temperature optical amplification under femtosecond (100 fs, 395 nm) excitation. Combined variable stripe length and shifted excitation spot experiments reveal positive optical gain, the net modal gain coefficient reaching 25 cm−1 at a pump intensity of 1.1 W/cm2 (mean power). The gain spectrum is broad (full width at half maximum ∼130 nm), peaked at ∼650 nm, and is slightly blueshifted with regard to the standard photoluminescence emission.

Photonic jet driven non-linear optics: example of two-photon fluorescence enhancement by dielectric microspheres.

The two-photon excited fluorescence from a dye solution is enhanced when a small amount of micro-meter sized silica beads are added. This observation is made in the simple scattering regime (inter-sphere distance four times larger than their radius) and is shown to depend on the concentration of the silica spheres. For a solution of rhodamine B, the enhancement can reach more than 30 %. As complementary experiments show that the fluorescence efficiency is unchanged, we argue that the non-linear absorption is enhanced due to focussing of the incident beam in the near-field of the spheres, a situation previously referred to as photonic (nano-)jets [3]. Our calculations indeed show that for the parameters of the spheres studied near-field focussing leads to an intensity concentration close to the sphere surface. We suggest that these photonic jets could be used to enhance other non-linear optical effects.

Frequency-stabilized laser-diode-pumped Nd:YAG laser

We describe a frequency-stabilized diode-pumped Nd:YAG laser that is actively frequency stabilized relative to a reference Fabry-Perot cavity using the Pound-Drever technique. We describe the servo loop and the measurement of its noise and gain performance and demonstrate its ability to reduce the laser frequency noise close to the shotnoise limit of 12.5 mHz/ radicalHz. This corresponds to a linewidth of approximately 1 mHz, well below the Schawlow-Townes limit of 0.13 Hz that applies for a free-running laser.

Structural and photoluminescence properties of ZnO thin films prepared by sol-gel process

The present study focuses on the structural and optical properties of ZnO thin films fabricated by sol-gel process and spin coated onto Si (100) and quartz substrates. The ZnO films have a hexagonal wurtzite structure with a grain size of about 50 nm. The x-ray photoelectron spectroscopy measurements reveal the presence of Zn2+ and of zinc hydroxyl groups at the film. Optical properties were studied by photoluminescence (PL) and absorption spectroscopy at low and room temperatures. The absorption spectrum is dominated by a sharp excitonic peak at room and low temperatures. At room temperature, PL observations show two transitions: one near the absorption edge in the ultraviolet (UV) region and the second centered at 640 nm, characteristic of the deep electronic levels in the bandgap. The spectrum at 6 K is dominated by donor bound exciton lines and donor-acceptor pair transitions. LO-phonon replica and two-electron satellite transitions are also observed. These optical characteristics are a signature of g...

Optical properties of ZnO thin films prepared by sol-gel process

The present study focused on ZnO thin films fabricated by sol-gel process and spin coated onto Si (100) and quartz substrates. ZnO thin films have a hexagonal wurtzite structure with a grain diameter about 50nm. Optical properties were determined by photoluminescence (PL) and absorption spectroscopy. The absorption spectrum is dominated by a sharp excitonic peak at room and low temperatures. At room temperature, two transitions were observed by PL. One near to the prohibited energy band in ultraviolet (UV) region and the other centered at 640nm, characteristic of the electronic defects in the band-gap. The spectrum at 6K is dominated by donor-bound exciton lines and donor-acceptor pair transition. LO-phonon replica and two-electron satellite transitions are also observed. These optical characteristics are a signature of high-quality thin films.

Closely packed luminescent silicon nanocrystals in a distributed-feedback laser cavity

Silicon nanocrystals (Si-ncs) of sufficiently small size, emitting luminescence at short wavelengths (which implies the occurrence of quasi- direct radiative recombination) and being densely packed in a planar thin film (which ensures short stimulated emission (StE) lifetime) can become a suitable active material for the observation of StE in the visible region. In this paper, we describe a fabrication method of nanostructures of this type, based on enhanced electrochemical etching of silicon wafers followed by embedding porous silicon grains into an SiO2 matrix. Further, we report on time-resolved photoluminescence spectra and optical gain measurements performed via the variable-stripe-length and the shifting-excitation-spot methods. Finally, we realize a transient wavelength-tunable distributed-feedback-laser (DFL) cavity with inserted densely packed Si-ncs as an active medium. We demonstrate an increase in emission intensity on the blue emission wing (below 600nm), which is spectrally shifting in accordance with the cavity tuning. We also present a mathematical model of the DFL cavity enabling us to simulate the experimental

Epitaxial thin films of multiferroic GaFeO3 on conducting indium tin oxide (001) buffered yttrium-stabilized zirconia (001) by pulsed laser deposition

Epitaxial films of an alternative multiferroic material, GaFeO3 (GFO), were grown by pulsed laser deposition on yttrium-stabilized zirconia (001) and on conducting buffer layers of indium tin oxide (001). They present a perfect epitaxial growth along the GFO [010] axis and six crystallographic variants in the film’s plane. Their magnetic properties are close to those of the bulk with an out-of-plane [010] hard direction and a Curie temperature of ∼200K. The films did exhibit ferroelectric properties when characterized by electrostatic force microscopy.

Time-resolved measurement of the refractive index for photopolymerization processes

A double-interferometer technique is employed to examine the dynamics of a photopolymerization process. The dye molecule is eosine Y. The refractive index and the thickness of the photopolymerizable film are measured as a function of time. During the photopolymerization process, the first quantity increases by 2%, while the second quantity decreases by more than 4%. Therefore, the refractive index cannot be measured by means of single-interferometer techniques. By fitting our experimental curves to a rate equation, the quantum yield and the absorption coefficient of the sample can be determined with good accuracy.

Optical gain observation on silicon nanocrystals embedded in silicon nitride under femtosecond pumping

We report the observation of positive optical gain in silicon nanocrystals (Si-nc) embedded in silicon nitride measured by the variable stripe length technique. We evidence the onset of stimulated emission and report gain coefficients up to 52 cm−1 at the highest excitation power (6.5 W/cm2). Photoluminescence dynamics presents two distinct recombination lifetimes in the nanosecond and the microsecond ranges. This was interpreted in terms of fast carrier trapping in nitrogen-induced localized states in the Si-nc surface and subsequent slow radiative recombination, suggesting that carrier trapping in radiative surface states plays a crucial role in the optical gain mechanism of Si-nc.