Torben Skettrup

Ultraviolet detectors in thin sputtered ZnO films

Ultraviolet-sensitive photodiodes have been made using the Schottky barrier formed in the contact layer between a thin sputtered layer of ZnO and Au. The sputtering parameters for the ZnO layer were optimized. The IV characteristics and the sensitivity spectra of the ZnO–Au photodiodes were investigated.

Second-harmonic imaging of ferroelectric domain walls

Domain walls in periodically poled ferroelectric KTiOPO4 and LiNbO3 crystals are observed by making use of second-harmonic (SH) generation enhancement in the transition regions between neighboring domains. SH images of domain walls obtained with various samples for different polarization configurations are presented. The SH generation enhancement is found especially pronounced for the polarization of the SH radiation being perpendicular to the domain walls. The origin and selection rules for the contrast in SH images of domain walls are discussed. The results obtained suggest that the domain walls produce a deteriorating effect on SH generation by quasiphase matching.

Diode-pumped single-frequency Nd:YVO 4 laser with a set of coupled resonators

350 mW of single-frequency power from a diode-pumped solid-state Nd:YVO(4) laser has been obtained from a coupled resonator design without any intracavity elements. Single-frequency operation was obtained by use of a very short laser rod and a coupled resonator design. The two coupled resonators were formed by the two faces of a very short Nd:YVO(4) laser crystal and an output coupling mirror. The interaction of the two coupled cavities caused a modification of the eigenmodes supporting laser action in a single longitudinal mode. This design, which is extremely simple, represents a cost-eff icient way of obtaining single-frequency output.

Far- and Near-Field Second-Harmonic Imaging of Ferroelectric Domain Walls

Abstract Domain walls in periodically poled ferroelectric LiNbO3 crystals are observed with both far- and near-field imaging techniques that make use of second harmonic generation in the transition regions between neighbouring domains. Second harmonic images of domain walls represent bright lines of ∼0.5 μm in width (as measured with a near-field microscope) for the polarization of the second harmonic radiation perpendicular to the domain walls. Origin and selection rules for the contrast in second harmonic images of domain walls are discussed.

Laser resonators with several mirrors and lenses with the bow-tie laser resonator with compensation for astigmatism and thermal lens effects as an example

Laser resonators with several mirrors (lenses) have been investigated in a systematic fashion. They have been grouped into classes according to their number n of mirrors/lenses. Stability polynomials, beam waist radii and locations have been obtained for each group up to n = 4. The bow-tie laser resonator is discussed as an example and ways to correct for astigmatism and thermal lens effects in this resonator are presented.

Laser modes and threshold conditions in N-mirror resonators

Two formal methods for finding laser modes and threshold conditions in laser resonators containing as many as N mirrors are presented. The first method is based on an analysis determining the reflectivity and the transmittivity of an N-mirror system with gain. This is an extension of the classical 2 × 2 matrix method. The second method is based on self-consistency equations for the system and directly yields the circulating fields of the individual resonators. A set of rules has been proved to allow these fields to be calculated directly by means of inspection. The laser oscillation condition for an N-mirror system is found. Examples are given for systems with as many as five mirrors.

Triangular laser resonators with astigmatic compensation

The magnitudes and locations of the beam waists in both the sagittal and the tangential planes have been found by means of the ABCD matrix method for a triangular resonator. Equilateral and isosceles resonators are discussed, and curves are given from which resonators with astigmatism-free beams can be designed. A frequency-doubled triangular Nd ring laser has been constructed after this design, and it is demonstrated that this laser emits a single longitudinal mode with a circular TEM(00) Gaussian beam.

Achromatic phase retarders constructed from right-angle prisms: design

The behavior of achromatic 90 and 180° phase retarder devices has been computed. These devices are made of three right-angle prisms so that the transmitted beam is not deflected or deviated from its incoming direction although the beam has suffered four internal reflections. Conventional BK-7 prisms and quartz prisms can be used as achromatic 180 and 90° phase retarders if suitable dielectric coatings are applied to the reflecting surfaces. With other glass types extremely achromatic 180° retarders can be obtained with <0.2% deviation from 180° throughout the visible wavelength range. This is five times better than the so-called superachromatic retarders made of several birefringent plates of different materials.

Second‐order optical nonlinearities in dilute melt proton exchange waveguides in z‐cut LiNbO3

Planar optical waveguides with different refractive indices are made in z‐cut LiNbO3 with a dilute proton exchange method using a system of glycerol containing KHSO4 and lithium benzoate. The optical second‐order susceptibilities of these waveguides are measured by detecting the 266 nm reflected second‐harmonic signal generated by a 532 nm beam directed onto the waveguide surface. It is found for this kind of waveguides that in the waveguide region all the second‐order susceptibilities take values of at least 90% of the original LiNbO3 values for refractive index changes less than ∼0.013 at the 632.8 nm wavelength, whereas the susceptibilities are strongly reduced for larger index changes.

Frequency tuning and stability of Nd:YVO4 in a dual coupled cavity

Abstract Frequency tuning and stability properties of single- and multi-cavity designs have been investigated theoretically and experimentally. Special attention is paid to a dual coupled cavity single-frequency diode-pumped solid-state Nd:YVO4 laser crystal. 350 mW single frequency output power has previously been demonstrated using dual coupled linear cavity. Frequency tuning versus temperature and cavity length of a coupled cavity Nd:YVO4 have been investigated and compared to those of a non-planar monolithic, unidirectional Nd:YAG ring laser. A way to extend the tuning range by design of the air to crystal length ratio is proposed. Finally the long term frequency stability of both lasers has been measured. The stability of the lasers are presented through the Allan variance of the frequency fluctuations.