Frank Schrempel

Structural processing of enstatite by ion bombardment

During their lifetime, cosmic dust silicates suffer from a continuous processing by annealing, cosmic ray and UV irradiation, destruction and possibly also interstellar recondensation. Since the discovery that a significant proportion of star- dust silicates leaves their star in crystalline form, the question arose as to why the interstellar silicate dust component does not show any indication of crystallinity. Amorphization due to ion irradiation is one possible explanation for the effect. In this paper, the results of irradiation experiments of submicrometre-sized clinoenstatite (MgSiO3) particles with 400 keV Ar + and 50 keV He + ions are presented. The irradiation doses have been varied between 1×10 16 and 1×10 18 ions/cm 2 for He + ions and 1×10 14 up to 5×10 14 ions/cm 2 for Ar + ions. These doses are comparable to those values that an interstellar silicate grain should be exposed to during its average life-time of 4×10 8 years. Threshold values for amorphization have been amounted to 1×10 17 and 3×10 14 ions/cm 2 for 50 keV He + and 400 keV Ar + ions. Besides the structural changes in the microcrystallites morphological modifications in the grains, but no change of the chemical composition are found. Conclusions of potential astrophysical relevance have been drawn.

Fabrication of nanoscale lithium niobate waveguides for second-harmonic generation

Nanoscale waveguides are basic building blocks of integrated optical devices. Especially, waveguides made from nonlinear optical materials, such as lithium niobate, allow access to a broad range of applications using second-order nonlinear frequency conversion processes. Based on a lithium niobate on insulator substrate, millimeter-long nanoscale waveguides were fabricated with widths as small as 200 nm. The fabrication was done by means of potassium hydroxide-assisted ion-beam-enhanced etching. The waveguides were optically characterized in the near infrared wavelength range showing phase-matched second-harmonic generation.

Second harmonic generation in free-standing lithium niobate photonic crystal L3 cavity

We report on second harmonic generation in a photonic crystal L3 cavity drilled in a thin self-suspended lithium niobate membrane. The cavity, resonant for the pump beam in the telecom wavelength range, exhibits a quality factor of around 500. Second harmonic generation has been measured with a low power continuous laser. A conversion efficiency of 6.4×10−9 has been estimated with an input coupled power of 53 μW.

Light propagation in a free-standing lithium niobate photonic crystal waveguide

We report on the light propagation in a one-line-defect photonic crystal waveguide (W1 PhC WG) patterned into a 450 nm thick free-standing lithium niobate membrane by ion-beam enhanced etching. The Bloch wave vectors and transmission spectrum of this PhC WG were retrieved from optical near-field images. The experimental data show good agreement with simulations performed with the three-dimensional (3D) finite-element method and the 3D finite-difference time-domain method. Those results are promising for the development of integrated optics devices operating at telecom wavelengths and based on free-standing lithium niobate PhC membranes.

Ultrathin membranes in x -cut lithium niobate

Ion-beam enhanced etching is used to pattern a bulk lithium niobate crystal with ultrathin membranes. By the implementation of an air gap beneath the membrane, high index contrast is achieved. A buried amorphous layer, created by irradiation with He ions, is removed by means of wet chemical etching in hydro-fluoric acid. Membranes having thicknesses down to 200 nm are fabricated. The etched air gaps and the membranes exhibit a uniform thickness over the entire etched area, and their widths can be purposefully adjusted over a wide range by choosing appropriate ion energies and fluences as well as annealing conditions.

In situ observation of surface oxide layers on medical grade Ni-Ti alloy during straining

Medical grade Ni-Ti alloys with shape memory or pseudo-elastic behavior exhibit good biocompatibility because of an electrochemically passive oxide layer on the surface. In this work, the mechanical stability of surface oxide layers is investigated during reversible pseudo-elastic deformation of commonly applied medical grade Ni-Ti wires. Surface oxide layers with varying thickness were generated by varying annealing times under air atmosphere. The thicknesses of the surface oxide layers were determined by means of Rutherford backscattering spectrometry. In situ scanning electron microscopy investigations reveal a damage mechanism, which is assumed to have a significant influence on the biocompatibility of the material. The conditions that lead to the appearance of cracks in the surface oxide layer or to the flaking of surface oxide layer particles are identified. The influence of the thickness of the surface oxide layer on the damage mode is characterized. The possible impact of the damaged surface oxide layer on the materials biocompatibility and the potentials to reduce or avoid the damage are discussed.

Fabrication of ridge waveguides in zinc-substituted lithium niobate by means of ion-beam enhanced etching

We present results on the fabrication and characterization of ridge waveguides in zinc-substituted lithium niobate. High-quality waveguides were fabricated by a combination of liquid-phase epitaxy and multiple applications of ion-beam enhanced etching. The two major demands on ridge waveguides, a very low side-wall roughness and a rectangle shape with side-wall angles close to 90 degrees , were realized simultaneously by using this technique. Single-mode waveguiding at a wavelength of 1064 nm was demonstrated with attenuation values of 0.9 dB/cm.

Depth dependence of radiation damage in Li+-implanted KTiOPO4

KTiOPO4 single crystals were irradiated at 100 and 295 K with 1 MeV Li þ -ions at an ion fluence of 3 � 10 15 cm � 2 and subsequently annealed at 620 and 720 K. The irradiated layers were analyzed by means of various complementary methods (Rutherford backscattering spectrometry, cross-sectional transmission electron microscopy, electron-energy loss spectroscopy, m-line spectroscopy). The complex damage structure produced consists of a slightly damaged cov- ering layer, a transition layer with amorphous clusters and a buried amorphous layer the thicknesses of which depends on the irradiation temperature. The depth distribution of the damage is in accordance with the distribution of the nuclear energy deposition. The buried amorphous layer possesses a significantly reduced refractive index thus acting as a refractive index barrier for waveguiding. Upon thermal treatment, defects anneal mainly in the covering layer, the barrier height is preserved and the slope of the barrier becomes steeper. The attenuation of light in the waveguide after thermal treatment is as low as 1.5 dB cm � 1 . 2002 Published by Elsevier Science B.V.

Deep light ion lithography in PMMA — A parameter study

Abstract Three-dimensional microstructures have been produced in Polymethylmethacrylate (PMMA) by proton irradiations at 1.8 and 6.0 MeV and by irradiation with He+ ions at 1.0 and 1.8 MeV. The irradiated volumes were removed chemically by a properly adjusted etching process. The resulting structures are characterized by a root mean square roughness in the order of a few nanometers. The influence of the dose, the dose rate, the particle energy and the developer temperature is discussed. The parameter range for optimal irradiation and developing conditions is deduced.

Etching of Ion Irradiated LiNbO3 in Aqueous Hydrofluoric Solutions

Homogeneously damaged surface layers of a thickness of 400 nm were generated in x-cut LiNbO 3 single crystals using multiple-energy Ar + -irradiation to study the etching behavior in aqueous HF solutions. Rutherford backscattering/channeling analysis was applied to investigate the damage formation. Different acid temperatures and concentrations were used, showing that the etching rate can be increased by increasing the temperature from 24 to 55°C maintaining the high contrast of the technique. The dissociation of HF in aqueous solution is discussed, and reaction kinetics well established for the HF etching of SiO 2 are then applied to obtain information on the etching mechanism. Therefore, the concentration dependence of the etching rate is analyzed and it is found that the etching process can be described as an attack of either HF or HF 2 - , which is catalytically supported by the presence of H + ions.