Martin Čopič

Ferromagnetism in suspensions of magnetic platelets in liquid crystal

More than four decades ago, Brochard and de Gennes proposed that colloidal suspensions of ferromagnetic particles in nematic (directionally ordered) liquid crystals could form macroscopic ferromagnetic phases at room temperature. The experimental realization of these predicted phases has hitherto proved elusive, with such systems showing enhanced paramagnetism but no spontaneous magnetization in the absence of an external magnetic field. Here we show that nanometre-sized ferromagnetic platelets suspended in a nematic liquid crystal can order ferromagnetically on quenching from the isotropic phase. Cooling in the absence of a magnetic field produces a polydomain sample exhibiting the two opposing states of magnetization, oriented parallel to the direction of nematic ordering. Cooling in the presence of a magnetic field yields a monodomain sample; magnetization can be switched by domain wall movement on reversal of the applied magnetic field. The ferromagnetic properties of this dipolar fluid are due to the interplay of the nematic elastic interaction (which depends critically on the shape of the particles) and the magnetic dipolar interaction. This ferromagnetic phase responds to very small magnetic fields and may find use in magneto-optic devices.

High-power pulsed diode-pumped Er:ZBLAN fiber laser

We report on the operation and performance of a gain-switched Er:ZBLAN fiber laser based on an active pulsed diode pump system. The produced laser pulses offer high peak powers while retaining the high average powers and efficiency of the cw regime. The measured pulse duration was about 300 ns and nearly independent of the pump repetition frequency. The maximum obtained 68 W of peak power is the highest reported, to our knowledge, for diode-pumped Er:ZBLAN fiber lasers, and the 2 W of average power at the repetition frequency of 100 kHz is 2 orders of magnitude higher than previously reported average power in a pulsed regime. The obtained slope efficiency was 34%.

Spontaneous liquid crystal and ferromagnetic ordering of colloidal magnetic nanoplates.

Ferrofluids are familiar as colloidal suspensions of ferromagnetic nanoparticles in aqueous or organic solvents. The dispersed particles are randomly oriented but their moments become aligned if a magnetic field is applied, producing a variety of exotic and useful magnetomechanical effects. A longstanding interest and challenge has been to make such suspensions macroscopically ferromagnetic, that is having uniform magnetic alignment in the absence of a field. Here we report a fluid suspension of magnetic nanoplates that spontaneously aligns into an equilibrium nematic liquid crystal phase that is also macroscopically ferromagnetic. Its zero-field magnetization produces distinctive magnetic self-interaction effects, including liquid crystal textures of fluid block domains arranged in closed flux loops, and makes this phase highly sensitive, with it dramatically changing shape even in the Earths magnetic field.

Role of Interionic Processes in the Efficiency and Operation of Erbium-Doped Fluoride Fiber Lasers

Great increase in the output power of erbium-doped fluoride fiber lasers has recently been achieved, but the predicted high slope efficiency was not observed. We conduct an investigation into the possible underlying reasons, based on the newly available spectroscopic data, and our measurements on the running fiber laser are compared against computer simulation. The results indicate that interionic processes in the present fibers are much weaker than previously expected and only allow for an operation with 15-30% efficiency where the excited-state absorption is shown to have an important contribution.

Brillouin and Raman study of the ferroelectric phase transition in PbHPO4

Abstract In order to investigate the dynamics of members of a novel series of ferroelectrics we have measured the Raman spectra of PbHPO4, as well as the Brillouin spectra in the paraelectric and ferroelectric phases. Mode assignment has been made. A weak underdamped soft mode has been observed below 100 cm-1. The Brillouin spectra revealed low frequency dispersion below Tc which could be fitted very well to the simple relaxation type formula. By comparison with far infrared reflectivity data we conclude that a low frequency relaxing mode and not the mode which is observed in Raman is responsible for the transition.

Enhancement of second harmonic generation in helical Sc liquid crystals

Abstract In a ferroelectric liquid crystal, a special type of phase-matching for optical second harmonic generation (SHG) is possible, where two counter-propagating fundamental waves create second harmonic waves at the edge of the selective reflection band. We compute the SHG intensity in such a situation and show that, at slight detuning from exact phase-matching, useful resonance enhancement can be obtained. A considerable amount of SHG also appears when the second harmonic frequency is in the reflection band, where the SHG wave is non-propagating.

Pump absorption and temperature distribution in erbium-doped double-clad fluoride-glass fibers

We investigate diode pump absorption and temperature distribution in three erbium-doped double-clad fluoride fibers. Absorption is measured via fluorescence intensity and temperature distribution is measured with thermal imaging. Ray-tracing calculations of absorption and heat-equation modeling of temperature distribution are also conducted. We found excellent agreement between measurements and calculations for all fibers. Results indicate that erbium-doped fluoride fiber lasers have already reached maximum output powers allowed under natural convection cooling, with fiber end being the most critical. We propose cooling and fiber design optimizations that may allow an order-of-magnitude further power-scaling.

Polarization modulation instability in liquid crystals with spontaneous chiral symmetry breaking.

We present a theoretical model which describes the polarization-modulated and layer-undulated structure of the B7 phase and gives the phase transition from the synclinic ferroelectric smectic-C(S)P(F) phase to the B7 phase as observed experimentally. The system is driven into the modulated phase due to the coupling between the polarization splay and the tilt of the molecules with respect to the smectic layer normal. The modulation wavelength and the width of the wall between two domains of opposite chirality are estimated.

Photodynamics of stress in clamped nematic elastomers

We describe the complex time dependence of the buildup of force exerted by a clamped photoelastomer under illumination. Nonlinear (non-Beer) absorption leads to a bleaching wave of a significant cis isomer dye concentration deeply penetrating the solid with a highly characteristic dynamics. We fit our experimental response at one temperature to obtain material parameters. Force-time data can be matched at all other temperatures with no fitting required; our model provides a universal description of this unusual dynamics. The description is unambiguous since these are clamped systems where gross polymer motion is suppressed as a possible source of anomalous dynamics. Future experiments are suggested.

Population dynamics in Yb:Er:phosphate glass under neodymium laser pumping

Characteristics of ytterbium to erbium energy transfer in Yb:Er:phosphate glasses have been studied for a case when Nd phosphate glass laser is used as a pumping source. A theoretical model of the energy transfer was developed to reproduce the experimentally observed population dynamics of the Er/sup 3+/ upper and lower laser levels. The results show that saturation and frequency hole burning of the inhomogeneously broadened Yb/sup 3+/ absorption line are of primary importance in determining the dynamics and efficiency of the Yb/sup 3+/-Er/sup 3+/ energy transfer under Nd laser pumping. The influence of other loss channels that have been identified in previous studies is found to be relatively small. The model also yields the lifetime of the frequency hole, as well as the effective rates of energy transfer between groups of Yb/sup 3+/ and Er/sup 3+/ ions in phosphate glasses. >