Random nanostructured silver surface illuminated with infrared ultrashort laser pulses.
Bright, sub-diffraction plasmonically enhanced continuum emitters can be observed.
The first thing to note about the research field of plasmonics is that it actually does not discuss plasmons, the quasiparticle describing the collective oscillation of conduction electrons. It rather is concerned with surface plasmon polaritons, which are plasmons coupled to light propagating at the surface of a material. Localized surface plasmon polaritons can be found confined in nanostructures, able to create hotspots of large electric field enhancement, used techniques for biosensing, sub-diffraction limit light focussing and surface enhanced raman spectroscopy (SERS).
Most research in plasmonics is done on lithographically manufactured substrates. My current research in Regensburg is performed on random structures, for example silver mirrors made by a century old wet chemistry procedure which are reported to yield higher enhancement factors - the measure of effectiveness for SERS - than regular structures. Also when illuminated by high densities of low energy photons these nanostructured films are known to emit continuum white light emission. I use these in a all optical time resolved far-field measurement as a method to unravel the origin of unexpected narrow line widths, indicating extraordinary long lifetimes of surface plasmon polaritons on a rough film.
Showcasing a organic light emitting diode, with an active layer of MEH-PPV
(Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]).
Light emitting diodes (LEDs) are commercially available for over 40 years now and abundant in todays technology as status indicator or energy saving lighting element. In the last years a small modification to the LED caught a lot of attention by introducing organic materials. This new class of electroluminescent devices - which emit light when supplied with electric current - is called simply called organic light emitting diode (OLED). Known for their use in display applications in today's electronics and demonstrations of flexible prototype displays, industrial effort is mainly put in low cost fabrication and performance research.
The basic component of most OLEDs, polymers, are often compared to coiled Spaghetti, given that these long and thin molecules consisting of many repeating units are able to align in almost any way. These unordered systems provide a research challenge, as well as an opportunity since they consist primarily of carbon, oxygen and hydrogen. The spin-orbit coupling - describing the interaction of the spin and the quantized angular momentum - is low for these elements in contrast to anorganic semiconductors, resulting in long spin relaxation times. To understand the physical mechanisms underlying the transport of charge and emitting properties of organics, as well as to probe their applicability in spin manipulation, OLEDs provide a unique tool: The possibility to observe electroluminescence from singlets and triplets simultaneously, allowing it to measure spin correlations optically. This work was published in Angewandte Chemie International Edition. Having proven to be able to engineer such devices, I participate in measurements concerning an optical access to magnetic field effects in OLEDs.
(a) Absorption of microwave radiation in a ferromagnet as function of magnetic field. (b) Corresponding voltage signal
generated by spin precession damping induced thermal gradients.
A recently emerging new field of physics is spin caloritronics, examining the interaction of spins - the quantum mechanical angular momentum characteristic of an electron - and heat currents. As most devices in spintronics have ferromagnets incorporated, an obvious connection of spins and heat is given by the curie temperature. It describes the critical point where the macroscopic magnetization due to parallel alignment of spins in a ferromagnet is lost due to the thermal motion of involved electrons. Less evident effects were recently discovered, for example the spin Seebeck effect - where a temperature gradient in a ferromagnet leads to an injection of spins into an adjacent non-ferromagnetic material - in 2008 by Uchida et al., or thermal spin transfer torque, a switching of magnetization direction induced by angular momentum carrying heat currents.
On the other hand magnetization dynamics are also influencing heat currents. As new concepts for computing and information storage on the basis of spin manipulation are introduced, it is important to account for the dissipated energy of damped spin precession. In my diploma thesis - performed in 2009 in Munich - I showed that even temperature changes as small as 10 mK at room temperature due to dampening of spin precession amounts to a measurable voltage across a semiconductor - ferromagnet - nonmagnetic metal hybrid structure, which may lower the performance of voltage based readout schemes.
Measurement setup for all electrical measurement of
spin pumping and spin caloric effects. Taken from http://dx.doi.org/10.1016/j.ssc.2009.11.020.
Magnetization dynamics - the temporal evolution of magnetic dipole moments - in hybrid structures and spintronics - the utilization of the quantum mechanical angular momentum characteristic of an electron, forming an magnetic moment - are not only of physical interest but also due to the demand of electronic devices capable of maintaining their information without current. One of the most known applications of magnetization dynamics is the 2007 physics nobel prize winning giant magnetoresistance (GMR) effect, used in hard disk drives. It describes spin-dependent scattering of spin-polarized electrons across a hybrid structure, yielding a change of resistance depending on the magnetic environment.
One goal of spintronics, however, is to eliminate the need for charge transport altogether. An approach is the spin pumping or spin battery effect, predicted by Tserkovnyak et al., where magnetization dynamics in a ferromagnetic material induce a spin current into an adjacent non magnetic material. It was shown that this spin current is able to induce a voltage which was found for non magnetic metal - ferromagnetic material - non magnetic metal heterostructures by Costache et al. in 2008. Based on the work of A. Wittmann et al. I performed measurements which prove this voltage to be present in semiconductor two dimensional electron gas (2DEG) - ferromagnetic material - non magnetic metal heterostructures, indicating the possibility of pure spin current injection into an anorganic semiconductor. That work was published in part 2009 in Solid State Communications.