Heidrun Schmitzer

Phase matched second harmonic generation using thin film ZnTe optical waveguides

Wide gap II‐VI semiconductors have strong second order susceptibilities χ(2) and are therefore promising materials for efficient second harmonic generation. We have grown high quality single crystalline ZnSe/ZnTe/ZnSe/GaAs (001) waveguides by metalorganic‐vapor‐phase‐epitaxy. Using end‐fire coupling we observe a phase matched signal of the 1170.5 nm fundamental wave. The fundamental beam is generated by a tuneable KTP optical‐parametric‐oscillator pumped by a ps‐Ti:sapphire laser system. Phase matching is achieved by coupling the TE0 fundamental mode with the TM2 second harmonic mode within the symmetric ZnTe waveguide.

Eliminating thermal effects in z-scan measurements of thin PTCDA films

We investigate the two-photon absorption (TPA) and nonlinear refraction of a micrometer thick 3,4,9,10-perylentetracarboxyl-dianhydride (PTCDA) film using z-scans with tightly focused 100 fs laser pulses. The PTCDA film was grown by organic molecular beam deposition on a Pyrex substrate. To study the influence of sample heating, the pulse repetition rate was varied between 4 MHz and 50 kHz with an acousto-optic pulse selector. We find that thermal effects diminish for pulse repetition times longer than 5 and 0.75 µs when using a 10x or 20x microscope lens, respectively, resulting in a TPA coefficient of 6 cm/GW and a nonlinear refractive index of 1.2 x 10⁻¹³ cm²/W at a wavelength of 820 nm.

Effects of uniaxial pressure on polar whispering gallery modes in microspheres

We investigate the detuning of whispering gallery modes (WGMs) in solid polystyrene microspheres (PM) as a function of axisymmetric stress applied to two antipodal points of the microsphere we call poles. We specifically investigate WGMs passing close to these poles, so-called polar WGMs. The applied uniaxial pressure reduces the geometrical circumference of the PM but also increases locally the refractive index at the flattened poles. Our experiments show that the WGMs shift to higher frequencies with increasing pressure and that the magnitude of the strain-induced shift depends on the radial mode number n. Furthermore an energy splitting between azimuthal modes linearly increasing with the pressure is observed. A theoretical model based on a classical ray optics approach is presented which reproduces the main results of our experimental observations.

Nonlinearity in the rotational dynamics of Haidinger's brushes

Haidingers brushes are an entoptic effect of the human visual system that enables us to detect polarized light. However, individual perceptions of Haidingers brushes can vary significantly. We find that the birefringence of the cornea influences the rotational motion and the contrast of Haidingers brushes and may offer an explanation for individual differences. We have devised an experimental setup to simulate various phase shifts of the cornea and found a switching effect in the rotational dynamics of Haidingers brushes. In addition, age related macular degeneration reduces the polarization effect of the macula and thus also leads to changes in the brush pattern.

How Rosalind Franklin Discovered the Helical Structure of DNA: Experiments in Diffraction

Rosalind Franklin, a chemical physicist (1920–1958), used x‐ray diffraction to determine the structure of DNA. What exactly could she read out from her x‐ray pattern, shown in Fig. 1?1 In lecture notes dated November 1951, R. Franklin wrote the following: “The results suggest a helical structure (which must be very closely packed) containing 2, 3 or 4 co‐axial nucleic acid chains per helical unit, and having the phosphate groups near the outside.”2 This was 16 months before J. D. Watson and F. Crick published their description of DNA, which was based on R. Franklins x‐ray photos. How they gained access to her x‐ray photos is a fascinating tale of clashing personalities and male chauvinism.2,3

Phase-matched third-harmonic generation in mercury-(I)-chloride.

Using phase-matched third-harmonic generation we determine the effective nonlinear susceptibilities in Hg2Cl2 (Calomel) to /chi(3)eff,I/ = 4.5 x 10(-22) m2V(-2) and /chi(3)eff,II/ = 9.7 x 10(-22) m2V(-2) for type I and type II phase matching, respectively. The type III phase matching uses the same tensor components as type I and is deduced to be /chi(3)eff,III/ approximately equal to 1.5 x 10(-22) m2V(-2). The effective third-order susceptibilities of Hg2Cl2 are two orders of magnitude higher than those of CaCO3, and the tensor components chi11 - 3chi18 exceed the components of ADP by a factor of 5. These measurements demonstrate that Calomel might be a promising material to be used for nonlinear optical devices.

Optically driven bacterial screw of Archimedes

The linear momentum transfer from photons to asymmetrically shaped structures generates an optomechanical rotation, but these micron-sized structures require costly, high-precision fabrication. Nature, however, provides a great diversity of engineered forms and dead, but rigid, asymmetrically shaped bacteria can provide a low cost alternative. In this work, we show that helical bacteria rotate very quickly in optical tweezers. Their rotation is fast enough to create a strong whirl, such that they may act as micropumps in thin capillaries.

Controlling guided modes in plasmonic metal/dielectric multilayer waveguides

We investigate the mode properties of planar dielectric aluminum-quinoline (Alq3) multilayer waveguides comprising one single or three equally spaced embedded nanometer-thin (∼10 nm thick) Alq3-Mg0.9:Ag0.1 composite metal-island layers. The plasmonic waveguides were fabricated by organic molecular beam deposition. Transverse magnetic (TM) and transverse electric (TE) modes were selectively excited using the m-line method. The symmetric plasmonic TM0 mode was launched in all waveguides and—in addition—two higher order plasmonic TM1 and TM2 modes were generated in waveguides comprising three metal layers. Other TM modes have hybrid dielectric-plasmonic characters, showing an increased effective refractive index when one electric field antinode is close to a metallic layer. TM modes which have all their antinode(s) in the dielectric layers propagate essentially like dielectric modes. TE modes with antinode(s) at the position of the metal layer(s) are strongly damped while the losses are low for TE modes comp...

Interferometric optical path measurement of a glass wedge with single photons and biphotons.

We have measured the phase structure of a glass wedge with single photons and biphotons in a Mach-Zehnder interferometer using parametric downconverted light from a Hong-Ou-Mandel particle interferometer as the source. By scanning the wedge through the focus of a microscope objective we find a doubling of the period of the interference pattern in the coincidence counts for biphotons compared to the single-photon experiment. We compare our measurement setup with classical ones and discuss some of the problems of superresolution in quantum lithography.

Optically Controlled Rotation Of PTCDA Crystals In Optical Tweezers

Small birefringent particles held in optical tweezers can rotate under the spin angular momentum transfer from polarized light. We tweezed and rotated organic semiconductor 3,4,9,10‐perylenetetracarboxylic dianhydride (PTCDA) platelets with a HeNe Laser beam. The dynamics of this spin angular momentum transfer from photons to matter is discussed. For the first time the wobbling rotation is measured as a function of the ellipticity of the incident laser polarization.