P.D. Townsend

Optical effects of ion implantation

Publisher Summary A refractive index can be increased by ion implantation by changes in density and structure, by the addition of high-polarizability impurity ions, by a reduction of the plasma effect that increases the index and that is most important in the wavelength region far from the energy gap, and by absorption changing in the index in the region of the gap, that is, via the Kramers–Kronig relation. This chapter discusses the optical properties of implanted semiconductors, electrooptic components formed by ion implantation, general principles that determine most luminescent systems, effects of implantation temperature, luminescence centers in LiF-Mg-Ti radiation dosimeters, and use of line spectra in defect studies. In the LiF system, the luminescence bands are broad, and if alternative versions of the same complex exist, they cannot be resolved from the spectra. The addition, by implantation, of ions with incomplete inner electron shells opens up new possibilities as the lattice distortions of the free-ion energy levels are strongly perturbed by the defects in the neighborhood of the ion.

Controlled production of aligned-nanotube bundles

Carbon nanotubes might be usefully employed in nanometre-scale engineering and electronics. Electrical conductivity measurements on the bulk material, on individual multi-walled and single-walled nanotubes and on bundles of single-walled nanotubes have revealed that they may behave as metallic, insulating or semiconducting nanowires, depending on the method of production—which controls the degree of graphitization, the helicity and the diameter. Measurements of Youngs modulus show that single nanotubes are stiffer than commercial carbon fibres. Methods commonly used to generate nanotubes—carbon-arc discharge techniques, catalytic pyrolysis of hydrocarbons and condensed-phase electrolysis—generally suffer from the drawbacks that polyhedral particles are also formed and that the dimensions of the nanotubes are highly variable. Here we describe a method for generating aligned carbon nanotubes by pyrolysis of 2-amino-4,6-dichloro-s-triazine over thin films of a cobalt catalyst patterned on a silica substrate by laser etching. The use of a patterned catalyst apparently encourages the formation of aligned nanotubes. The method offers control over length (up to about 50 μm) and fairly uniform diameters (30–50 nm), as well as producing nanotubes in high yield, uncontaminated by polyhedral particles.

Optical properties of silicon nanoclusters fabricated by ion implantation

A method for the fabrication of luminescent Si nanoclusters in an amorphous SiO2 matrix by ion implantation is reported. We have measured the dose (concentration of excess Si atoms) and annealing time dependence of the photoluminescence of Si nanoclusters in SiO2 layers at room temperature. The samples were fabricated by ion implantation and subsequent annealing. After annealing, a photoluminescence band below 1.7 eV has been observed. The peak energy of the photoluminescence is found to be almost independent of annealing time, while the intensity of the luminescence increases as the annealing time increases. Moreover, we found that the peak energy of the luminescence is strongly affected by the dose of implanted Si ions, especially in the high-dose range. We also show direct evidence of widening of the band-gap energy of Si particles of a few nanometers in size by employing photoacoustic spectroscopy. These results indicate that the photons are absorbed by Si nanoclusters, for which the band-gap energy i...

The formation of waveguides and modulators in LiNbO3 by ion implantation

Ion implantation is an attractive method for writing optical circuits for use in integrated optics. In LiNbO3 it is shown that there are large changes produced in both refractive indices n0 and ne by the energy deposited in nuclear collisions between the implanted ions and the lattice. The process is insensitive to ion species and at 300 K a universal curve exists for the decrease of n0 as a function of deposited energy by nuclear collisions. Saturation changes of −6% occur at 300 °K, and larger values are noted for 77 K implants. The saturation condition is reached after the deposition of ∼1022 keV cm−3 from the ion beams. Because the indices are reduced, ion beams have been used to write low index boundaries to define waveguiding regions. By using energetic light ions (e.g., 2‐MeV He+) negligible change is produced in the surface layer where the energy loss is primarily electronic and thus the low index region is formed deep within the solid. The computed and measured mode characteristics are in good ag...

An introduction to methods of periodic poling for second-harmonic generation

Second-harmonic generation (SHG) can be produced by phase matching using the birefringence of nonlinear crystals via the modal dispersion in the case of optical waveguides. Such an approach limits the range of frequencies which can be doubled and also the choice of the nonlinear coefficients. One solution to both problems is to modify the crystal so as to have regions of periodic domain polarity. Whilst this approach does not allow a perfect phase match between the fundamental and harmonic, it nevertheless can be entirely constructive throughout the interaction length of the material and is termed quasi-phase matching (QPM). Periodic modulation of the nonlinear coefficient along the direction of propagation can achieve conversion efficiencies up to 20 times greater than with previous methods. Candidates of interest for quasi-phase-matching are wide band gap inorganic crystals such as LiNbO3, LiTaO3 and KTP, and also organic materials if they are transparent, stable against optical damage and have large nonlinear coefficients. To achieve QPM a variety of methods are being tried in order to invert domains periodically, either during the crystal growth phase, or subsequently by altering the lattice of the crystal. For inorganic ferroelectrics most effort has been concentrated on domain inversion in LiNbO3 and LiTaO3. Techniques have included application of pulsed electric fields, fields generated during electron bombardment, thermal pulsing or chemically driven movement of lithium. Many of the methods are semi-empirical in that the mechanisms by which the lattice re-structures are poorly understood. This review will therefore not only list the methods that are currently being used, but also comment on the underlying physical processes which allow, or prevent, the re-structuring of the lattice and the domain walls, whilst preserving the non-centrosymmetric characteristics of the lattice. An understanding of mechanisms is valuable for related poling applications in other crystals and it is further noted that many amorphous systems, including glasses used for optical fibre communication, may be stimulated to show periodic structural changes although the usage precedes the knowledge of the mechanisms. The commercial applications and research possibilities for efficient SHG guarantee that this topic area will continue to be central to photonics for a considerable time.

An overview of ion-implanted optical waveguide profiles

Abstract Ion implantation produces a variety of optical changes in insulators including major alterations of the refractive indices. Consequently ion beam implantation has been used to form optical waveguides in a very diverse range of materials. These include key optoelectronic crystals, such as LiNbO 3 , Quartz, YAG. PLZT or BGO, amorphous glasses such as silica, and organics such as PMMA. Despite the generality of the technique there are major differences in the form of the index profiles. In particular the extraordinary and ordinary indices often behave differently. Electronic excitation and nuclear collision regions of the ion range can differ not only in the form. but also in the sign. of the index change. Further differences are noted in the subsequent thermal stability of these regions. Alternative mechanisms of defect formation, diffusion, stress and preferential surface decomposition are discussed and related to various examples of ion-implanted optical waveguides. The effects of these features are of major importance in the formation of low loss waveguide devices.

Sputtering patterns and defect formation in alkali halides

Abstract We have separated the sputtering patterns of NaCl irradiated with low energy electrons into component features from the directional ejection of halogen atoms and a random evaporation of the metal atoms. The sputtering yields anti-correlate with exciton luminescence. Lifetime studies reveal the presence of several exciton states with different diffusion energies. The sputtering yield shows a resonance with the production of surface excitons. The experiments confirm the view that defect formation in the halogen sub-lattice proceeds by a non-radiative exciton decay which initiates a replacement collision sequence along a chain of halogen ions.

A SIMPLE ROUTE TO SILICON-BASED NANOSTRUCTURES

.Powders of the nanoparticles were obtained from the colloidal solutions by removing the water with a rotary evaporator (bath temperature 50 C). Transmission electron micrographs of the samples were taken using a Philips CM 300 UT electron microscope, working at 300 kV acceleration voltage. A Philips Xpert system was used to measure the X-ray diffraction pattern of powder samples. UV-vis absorption spectra of the colloidal solutions were recorded with a Lambda 40 spectrometer (Perkin‐Elmer). Photoluminescence spectra were recorded with a Spex Fluoromax 2 spectrometer having a spectral resolution of 0.5 nm.

Optical waveguides in LiNbO3 formed by ion implantation of helium

The paper reports the formation of optical waveguides in LiNbO3 by the implantation of helium ions. The ion beam damage defines the low‐index regions which surround the waveguide. The computed index profile and the observed modes are in agreement. Changes in n0 of up to 7% are recorded as a saturation index change.

Formation of silver nanoparticles in soda–lime silicate glass by ion implantation near room temperature

The synthesis of silver nanoparticles in soda–lime silicate glass near room temperature is reported. Nanoparticles were prepared during 60 keV Ag-implantation with doses from 2 to 4×1016 ions/cm2 at a current density of 10 μA/cm2. Detailed evaluations were made of the dose contribution at the bulk-glass temperatures of 20°C, 35°C, 50°C and 60°C. The particle size distribution was assessed by monitoring optical reflectance from both the implanted and rear face of the samples. Depth data were provided by Rutherford backscattering analysis. Samples prepared with a high dose at 60°C were characterised by more complex reflectance spectra, with overlapping peaks, compared to reflectance data for room temperature implants. Additionally, comparisons between implants of the same dose and beam conditions for glass samples of different-thickness show the influence of surface heating and its influence on Ag nanoparticle formation. The factors, which influence the growth of metal nanoparticles and the differences in the observed optical properties are discussed.