|Photonics research line|
Current Researchers: F. Agulló López, J. Olivares Villegas, M. Crespillo Almenara, J. Manzano Santamaría.
The research activity is based on the study of the mechanisms of the electronic damage generated in crystal with photonic interest by means of irradiation with high energy heavy ions (swift heavy ions) , with particular interest on the study of the modified optical properties of the irradiated materials, with the aim of demonstrating new photonic applications (based on optical waveguides). Materials studied so far are: LiNbO3, TiO2, SiO2, KGW, BaMgF4. The activity in optical applications is carried out in collaboration with the group of Nonlinear optics of the "Depto. de Física de Materiales" of the UAM (leaded by Mercedes Carrascosa).
Moreover, the electronic damage is being studied in optical materials like SiO2, Al2O3 which are also interesting in the field of nuclear fusion (reactor wall materials, i.e. for windows). This is made in cooperation with the CIEMAT.
When ions with energy of several MeV´s enter in a crystal they first slow down along the ion track due to the electronic interaction or the so called electronic stopping power (Se). They travel a few microns in this stopping regime and at the end of the ion range the main slowing down mechanism is due to elastic nuclear collisions or so called nuclear stopping power. For a given material the electronic stopping power increases with the projectile atomic number or mass and is in the range of (0.1 – 40) keV/nm.
Above a certain (amorphization) threshold in their electronic stopping power (Se ≈ 5 keV/nm, for the LiNbO3 crystal) ions generate latent amorphous tracks of nanometer diameter and several microns long. These unique nanostructures are being proposed for several applications giving rise to the so called field of Ion Track Technology.
Below the amorphization threshold damage (defects, etc) can also be generated and accumulated with increasing fluence until homogeneous amorphous layer are formed.
The main research progress developed at CMAM in this area can be classified in the following concepts:
Modelling of the damage generation and accumulation:
The thermal-spike model has been revised. Although it is a reasonable model to explain the phenomena occuring above threshold invoking the material melting, it does not explain the sub-threshold damage generation. First a model extension was proposed. A significant improvement has been achieved in the development of a model based on the mechanism of the non radiative decay of the self-trapped excitons. A concept of double spike (excitation and thermal) has been proposed.
The following titles of our publications related to modelling summarize the activities:
The fabrication and systematic characterization of nanopores produced by means of chemical etching of the latent amorphous nanotracks has been developed. Interesting and unique forms related to the crystal structure are obtained. These pores can be refilled with metals to produce nanocomposite of novel optical properties. See the “nanopore” references.
Optical aplications: Optical waveguides:
The method of high energy ion irradiation has been succesfully applied to the fabrication of novel optical waveguides in LiNbO3 and other crystals (KGW, c-SiO2, BaFM4).
By means of damage accumulation homogeneous amorphous layers can be generated with irradiations of the type sub-threshold (i.e. using ions whose electronic stopping power is lower than the one require to generate amorphous nanotracks in the first ion impacts). These layers of sharp boundaries can be placed a few microns inside the crystal by using high energies (for example O, F ions of 10-30 MeV ), defining a step-like surface waveguide that provides high optical confinement.
It has also been demonstrated at CMAM the fabrication of optical waveguides by irradiation with ultralow fluences (1012 at/cm2) in the regime above threshold by choosing the ions energy so as to place its maximum stopping power inside the crystal. A nanostructure effective medium is generated in this way, where by choosing an amorphous fraction of just a 10% a functional optical waveguides are obtained.
We have also demonstrated the feseability of fabricating very thick optical waveguides, of several 10´s of microns, using ultralow fluences of very high energy ions (> 400 MeV, at GANIL, France and GSI, Germany). These waveguides are required for example for infrared applications. One special application is within the novel field of Astrophotonics. Collaboration with Laboratory Astrophysics Grenoble.
 Thanks to the high energy available at CMAM we can place the maximum of the stopping (i.e. the Bragg peak) buried a few microns. This is a condition required to produce an optical waveguide.
The threshold for amorphous latent track formation is about 5-6 keV/nm for LiNbO3. Br 45 MeV ions create such tracks.