RESUMEN
Adiabatic three-wave mixing processes enable broadband, efficient, and robust frequency conversion by slowly varying the phase mismatch between the interacting waves along the interaction region. Up until now, this method was mainly used in the case in which one of the waves was undepleted. Here we experimentally study fully nonlinear adiabatic processes by implementation in type I and type II second-harmonic generation processes, where the undepleted pump approximation does not hold. Using quasi-phase-matched interaction in chirped gratings, we obtain conversion efficiency approaching 60% and 80%, with corresponding wide thermal acceptance bandwidths of >100°C and 30°C, respectively. The transition between the depleted and undepleted pump regimes is also studied by varying the input polarization angle in the type II process; thus we also test current theory with arbitrary initial conditions. The results are in excellent agreement with analytic predictions for the fully nonlinear adiabatic process.
Asunto(s)
Oro/química , Nanoestructuras/química , Nanotecnología/instrumentación , Nanotecnología/métodos , Tolueno/análogos & derivados , Amoníaco/química , Cristalización , Ensayo de Materiales , Microscopía de Fuerza Atómica , Nucleótidos/química , Espectroscopía Infrarroja por Transformada de Fourier/métodos , Azufre/química , Tolueno/químicaRESUMEN
A new form of biomineralization has been studied in the pineal gland of the human brain. It consists of small crystals that are less than 20 microm in length and that are completely distinct from the often observed mulberry-type hydroxyapatite concretions. A special procedure was developed for isolation of the crystals from the organic matter in the pineal gland. Cubic, hexagonal, and cylindrical morphologies have been identified using scanning electron microscopy. The crystal edges were sharp whereas their surfaces were very rough. Energy dispersive spectroscopy showed that the crystals contained only the elements calcium, carbon, and oxygen. Selected area electron diffraction and near infrared Raman spectroscopy established that the crystals were calcite. With the exception of the otoconia structure of the inner ear, this is the only known nonpathological occurrence of calcite in the human body. The calcite microcrystals are probably responsible for the previously observed second harmonic generation in pineal tissue sections. The complex texture structure of the microcrystals may lead to crystallographic symmetry breaking and possible piezoelectricity, as is the case with otoconia. It is believed that the presence of two different crystalline compounds in the pineal gland is biologically significant, suggesting two entirely different mechanisms of formation and biological functions. Studies directed toward the elucidation of the formation and functions, and possible nonthermal interaction with external electromagnetic fields are currently in progress.