**Author details**

ments and considering only the typical behaviour. The heterostructures realized with com‐ posite materials show a current with 3 orders lower than the heterostructures realized only with monomers. The charge carrier transport is mainly affected by the insulating character of the polymeric matrix. The highest current (3x10-8 A) has been obtained in heterostructure Si/polycarbonate:MM5/Si for an applied voltage of 1 V and for voltages between 0.1 V and 1 V the characteristic is weakly superlinear [61]. For the heterostructures realized only with monomers the I-V characteristics are linear at low voltages and become strongly superlinear

The films obtained from the polycarbonate containing the monomer with two nitrous sub‐ stituents (MM5) to the aromatic nucleus have shown good transparency, and photolumines‐ cence in the green region and promising electrical properties at voltages >0.6 V (I=10-8 A)

Also this organic/organic composite material seems to be promising for optoelectronic appli‐ cations, the spin coated composite layers are characterised by a specific morphology and a high degree of disorder which affect the optical and electrical properties and make difficult

In this chapter we summarize some of the most important results of our work in the field of new materials for applications in the field of optoelectronics. Our interest was focused on organic molecules containing electrons, occupying non-localised molecular orbitals in strongly conjugated systems, such as aromatic derivatives compounds (benzil, m-DNB, PTCDA, ZnPc, Alq3, TPyP) for which we have evidenced large transparency domain and

We have realised a comparative investigation on the properties of the same aromatic deriva‐ tive compound as bulk and thin film material showing both good optical, including lumi‐ nescent, properties. The interest in studying bulk organic crystals is justified by the perspective to use these materials as a crystalline host matrix both for organic and inorganic guests/inclusions. The organic matrix assures an efficient fluorescence mechanism and from the guest component it is expected an improvement in stability, emission properties of the matrix and electrical mobility. A special attention was paid to the preparation methods both for bulk crystals (emphasising the correlation between the growth interface stability and quality of the organic crystal) and thin films (emphasising the effect of the thin film deposi‐ tion method -directional solidification, vacuum evaporation, MAPLE- on the properties of

Thin films from the above mentioned aromatic derivatives are preferred as organic matrix for host/guest systems because of the major problems associated with bulk crystalline matri‐ ces determined by the difficulties of their growing, processing and doping to assure homo‐

with a close to linear characteristic at voltages between 1 V and 10 V [64].

for voltages >0.2 V.

330 Optoelectronics - Advanced Materials and Devices

their control.

**5. Conclusions**

good fluorescence emission.

the organic film and heterostructures).

geneous distribution of the guest atoms.

Florin Stanculescu1\* and Anca Stanculescu2

\*Address all correspondence to: fstanculescu@fpce1.fizica.unibuc.ro

1 University of Bucharest, Bucharest-Magurele,, Romania

2 National Institute of Materials Physics, Bucharest-Magurele,, Romania

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**Chapter 13**

**Optoelectronic Oscillators Phase Noise and**

A classical method to characterize the spectral density of phase noise of microwave oscilla‐ tors is to compare the device under test (DUT) to another one, we call it the reference, with the same frequency if noise is expected to be better for the reference. In most of case it is possible to then characterize oscillators or synthesizers. But it is only possible if we can as‐ sign the same frequency to the DUT and the reference signal. However for some applica‐ tions, we see that the DUT delivers a frequency hard to predict during the fabrication. We here focus on characterizing a special class of oscillators, called optoelectronic oscillators (OEO) [1]. An OEO is generally an oscillator based on an optical delay line and delivering a microwave signal [2]. Purity of microwave signal is achieved thanks to a delay line inserted into the loop. For example, a 4 km delay corresponds to a 20 μs-long storage of the optical energy in the line. The continuous optical energy coming from a laser is converted to the mi‐ crowave signal. This kind of oscillators were investigated [3]. By the way, such OEO based on delay line are still sensitive to the temperature due to the use of optical fiber. Recently, progress were made to set compact OEO thanks to optical mini-resonators or spheres [4 - 7]. Replacing the optical delay line with an ultra-high Q whispering gallery-mode optical reso‐ nator allows for a more compact setup and an easier temperature stabilization. In order to introduce into the loop the fabricated resonator in MgF2 [8], CaF2 or fused silica, it has to be coupled to the optical light coming from a fiber. Best way to couple is certainly to use a cut optical fiber through a prism. But a good reproducible way in a laboratory is to use a ta‐ pered fibber glued on a holder. Then appears a problem for determining the phase noise of such compact OEO, because the frequency is rarely predictable. It is impossible to choose the frequency of the modulation (for instance exactly 10 GHz) and that's why it is necessary to develop new instruments and systems to determine phase noise for any delivered signal

> © 2013 Salzenstein; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

© 2013 Salzenstein; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

distribution, and reproduction in any medium, provided the original work is properly cited.

**Stability Measurements**

Additional information is available at the end of the chapter

Patrice Salzenstein

**1. Introduction**

http://dx.doi.org/10.5772/3463

