Fakultät für Physik - Digitale Hochschulschriften der LMU - Teil 01/05

The early universe consisted of a dense, hot and ionised gas of electrons, protons, neutrons, some light atomic nuclei and photons. The universe at that time was optically thick as the photons could not travel very far before being scattered on electrons in the dense gas. The temperature was too hot for the electrons to combine with the nuclei and form atoms. But as the universe expanded it cooled. About 300 000 years after the Big Bang, the temperature of the dense gas filling the universe was about 3000 degree Kelvin. This temperature allowed the formation of the first atoms in the universe. The electrons combined with the protons to form atoms. The probability for a photon to scatter on a neutral atom is much less than the probability to scatter on free electrons and protons. For this reason it is said that the universe got transparent when the electrons were bound to the protons. The photons continued travelling in a straight line without being scattered. About 12 billion years later some of these photons hit a detector on the planet called 'the earth'. And it provided scientists with valuable information about the origin of the universe. This radiation which has travelled more or less unchanged from the earliest times until today is called the cosmic microwave background radiation, and is the topic of this Ph.D. thesis.

In dieser Arbeit werden die Rauschquellen in Sperrschicht-Feldeffekttransistoren (JFETs) eingehend auf ihre

Ursachen und Wirkungsweise untersucht. Das Rauschverhalten von JFETs wirkt sich beispielsweise in

Halbleiter-Detektor-Verstärker-Systemen auf die Energieauflösung solcher Systeme aus, in denen JFETs aufgrund

ihres geringen Rauschens oft als erste Verstärkungsstufe eingesetzt werden. Diesbezüglich wird eine Methode

entwickelt, mit Hilfe derer aus Rauschmessungen an JFETs die Energieauflösung eines einfachen

spektroskopischen Systems berechnet werden kann, in das der vermessene Transistor als erste

Verstärkungsstufe eingebaut ist. Außerdem wird gezeigt wie aus temperaturabhängigen Rauschmessungen auf

die Eigenschaften von Kristalldefekten in Halbleitern geschlossen werden kann.

Im theoretischen Teil der Arbeit werden zuerst grundlegende Rauschmechanismen in Halbleitern beschrieben wie

sie auch in JFETs auftreten. Auf die Herleitungen der Rauschspektren des thermischen Rauschens, des

Diffusionsstrom-Rauschens, des Generations-Rekombinations-Rauschens und des „Random-Telegraph-Signal“-Rauschens

(RTS-Rauschen) wird ausführlich eingegangen.

Das RTS-Rauschen kommt durch den Einfang und die Emission von freien Ladungsträgern in/aus

Kristalldefekte(n) hinein/heraus. Die Abhängigkeiten des RTS-Rauschens von der Lage des Kristalldefekts im

Bauelement und den Eigenschaften des Kristalldefekts selbst werden detailliert analysiert. An den Beispielen

eines Widerstandes und eines JFETs wurden Simulationen durchgeführt, mit Hilfe derer der Einfluß einzelner

Kristalldefekte auf das Rauschverhalten des jeweils betrachteten Bauelements bestimmt werden kann.

Im experimentellen Teil der Arbeit werden Messungen an verschiedenen JFETs vorgestellt, in denen das

Rauschen in Abhängigkeit von der Frequenz und der Temperatur aufgenommen wurde. Auf die angeführten

Rauschmessungen wird die oben erwähnte Methode angewendet, mit Hilfe derer man die Energieauflösung eines

einfachen spektroskopischen Systems berechnet werden kann, in das der vermessene Transistor als erste

Verstärkungsstufe eingebaut ist. Dadurch gewinnt man ein Bild vom Verhalten des betrachteten

spektroskopischen Systems in Abhängigkeit von der Temperatur und der Filterzeit eines in das System

integrierten Filters, der zur Optimierung des Signal-zu-Rausch-Verhältnisses dienen sollte. Daraufhin wird

exemplarisch am Beispiel eines rauscharmen JFETs gezeigt, wie man anhand von Rauschmessungen die

Eigenschaften und die Lage von Kristalldefekten bestimmen kann. Zum Abschluß der Arbeit werden noch

Wiederholungsmessungen an einem Bauelement widergegeben, bei denen sich das Rauschverhalten von

sogenannten multistabilen Kristalldefekten manifestierte. Multistabile Defekte sind Kristalldefekte, die nicht nur

eine stabile sondern mehrere mögliche Konfigurationen im Kristallgitter besitzen. Übergänge zwischen den

verschiedenen Zuständen können durch verschiedene Einflüsse wie z.B. durch die Temperaturbehandlung

während einer Rauschmessung zustande kommen.

Coherent control of molecular dynamics deals with the steering of quantum mechanical systems

with suitably shaped ultrashort laser fields. The coherence properties of the laser field are

exploited to achieve constructive interference for a predefined target wave function via a phasecorrect

superposition of wave functions. The goal of coherent control, the selective preparation

of a target state, is an important prerequisite for mode-selective chemistry.

The laser pulse tailored to drive the system from the initial to the target state as a perturbation

can in general not be determined by a quantum-mechanical calculation, since usually even the

Hamiltonian of the system is unknown. A practical alternative is to determine the required

shape of the laser field in a feedback-controlled regulation loop which uses a signal derived from

the experiment as feedback. The loop is repeated until a pulse that suits the requirements is

obtained. Experiments in this area have until recently mostly been limited to the wavelength

regime of Ti:Sa lasers and their fundamentals.

This work deals with the fundamentals of feedback-controlled shaping of ultrashort laser pulses

with respect to both establishment of its technical prerequisites and its application to suitable

model systems. The feedback loop has been tested using a simple optimization experiment with

known outcome; then it was applied to experiments of progressively increasing complexity. From

the optimized pulses, physical insight into the optimization process has been gained.

In the first part of this work, the required technology has been implemented and standardized

such that control experiments might employ it as a standard tool. One of the technical prerequisites

was the frequency conversion of the 800 nm Ti:Sa laser pulses to a wavelength range

suited to the particular systems. To this end, non-collinear optical parametric amplifiers have

been built in different designs that routinely produce tunable sub-20 fs pulses in the visible. The

characterization techniques for ultrashort pulses have been implemented as well.

Pulse shapers with cylindrical instead of spherical mirrors have been implemented for the modulation

of broadband pulses, and their functionality has been explained both theoretically and

experimentally. A new liquid crystal device, the core of our pulse shapers, has been developed in

cooperation with the group of Thomas Feurer at the Universit¨at Jena and the Jenoptik GmbH

which allows for the generation of more complex pulse shapes than with other commercially

available devices to date.

Using a pulse shaper to modulate the white light continuum that serves as the seed for the

non-collinear optical parametric amplifier, generation of phase-locked two-color double pulses

has been achieved, with tunable wavelengths, delays, and relative carrier phases between the

single pulses. The basic principle, phase conservation during optical parametric amplification,

has been demonstrated. With this setup, control experiments which require pulses with the

above described attributes in electronically controllable form are possible for the first time.

An evolutionary strategy used as the optimization algorithm in the feedback loop has been

programmed and characterized both in simulation and experiment using a simple optimization

experiment, namely pulse recompression by phase compensation.

In the second part of this work, pulse recompression of ultra-broadband spectra in the sub-20fs

regime serves as an example of utility of feedback-controlled optimization. This experiment simultaneously

served as a further test of the feedback loop in the limit of a physically unreachable

optimization goal.

It has been demonstrated that a suitable parameterization of the electric field, implemented by

a mapping of the optimization parameters adjusted by the algorithm to the physical parameterscontrolling the liquid crystal mask affords a means of acquiring physical knowledge from the

retrieved optimal electr

The work at hand presents a comprehensive investigation of solid-state lasers based on thin

films of a disordered organic semiconductors (OS). The high luminescence yield of OS

together with their conductivity and the ease of processing them into sub-micron thick films

on substrates of virtually any shape and size open vast opportunities both for the realization of

novel devices and for the exploration of innovative resonator geometries. Within the course of

this work optically pumped lasers with spatially distributed feedback were fabricated by

deposition of a thin film of an OS on a nanopatterned substrate. Upon optical excitation with

femtosecond laser pulses laser operation is observed. It is characterized using time-integrated

and time-resolved spectroscopy.

As active medium two of the most promising electroluminescent organic materials were

employed, namely the conjugated polymer MeLPPP and the composite molecular system

Alq3:DCM. Owing to their large gain coefficient low-threshold light-amplification by

stimulated emission can be observed from slab waveguides formed by a thin layer of the OS

on a planar substrate. In periodically modulated waveguides Bragg scattering gives rise to

distributed feedback. Utilizing UV-embossed plastic substrates with an appropriate onedimensional

surface corrugation, edge- and surface-emitting lasers were fabricated. Excellent

agreement is found between the performance of these 1D-DFB lasers and the predictions of a

quantitative numerical modeling. Unless further precautions are taken these lasers operate on

a continuum of lateral modes, characterized by a broad emission spectrum and a large beam

divergence.

A significantly improved performance is achieved by application of a two-dimensional

corrugation. The specific properties of light-propagation in the resulting 2D photonic crystal

give rise to monomode laser operation, accompanied by a reduction of the laser threshold, an

increased differential efficiency, and by the emission of a circularly shaped, diffractionlimited

laser beam. It is demonstrated that the emission linewidth is (time-bandwidth)

transform limited by the duration of the laser pulse. The mechanism leading to monomode

operation at low excitation density as well as the photonic band structure at high excitation

density is revealed by a Laue model for the feedback in such a 2D photonic crystal laser.

By use of a self-assembly technique waveguide structures with stochastically distributed

scatter centers were fabricated. Lasing with resonant feedback is observed and attributed to

the formation of random closed loop cavities (random lasing). The underlying process of

recurrent light-scattering is made possible by the strong scattering, the reduced dimensionality

of the waveguide geometry, and the large gain of the organic semiconductor material.

In order to explore the technological potential of organic semiconductor lasers two alternative

concepts for electrical operation were examined. In a hybrid concept an OS DFB laser is

merely used as a tunable solid-state laser source. Thanks to the low laser threshold a very

compact setup can be realized utilizing a microchip pump laser with a repetition rate of

several kilohertz. In the approach of making an organic diode laser the low mobility of

disordered organic semiconductors imposes severe constraints on the device architecture.

Several structures were developed and fabricated that concurrently permit large current

densities and possess a low laser threshold. As a result, optically pumped lasing is for the first

time demonstrated in structures that are suitable for electrical excitation.

In the first part of this thesis,we examine the preparation of a single-

mode radiation field in arbitrary pure quantum states via resonant inter-

action with a sequence of two-level atoms.The preparation is achieved

by choosing an appropriate (in general entangled)initial state of the

atomic sequence,and does neither require a final state measurement of

the atoms,nor a control of the atom-field interaction.Furthermore,the

method is applicable also when starting from mixed initial field states.

We show how to determine the optimal initial atomic state which pre-

pares the desired field state with the maximum fidelity,and prove the

feasibility of our state preparation method by numerical calculations.

In the second part,we demonstrate the noise-induced control of quantum

jumps in a fundamental open quantum system.Here,in addition to the

subsequent interaction with a flux of two-level atoms,the quantized field

is also coupled to a thermal environment.Under certain experimental

conditions,the photon field exhibits a bistable behavior,with quantum

jumps between two metastable states.In the presence of a small peri-

odic signal (i.e.,a modulation of the initial state of the two-level atoms

crossing the single-mode resonator),the best synchronization of these

quantum jumps with the signal is achieved at an optimal,nonvanishing

temperature of the environment.This stochastic resonance e ffect can

be observed in di fferent components of the atomic Bloch vector on exit

from the cavity.

The third part treats a speci fic problem concerning the characterization

of entanglement between two quantum mechanical two-level systems.

We consider the optimal decomposition of a two-qubit state into an en-

tangled and a separable part,with maximal weight of the latter,and

derive necessary and su fficient conditions for the optimality of the de-

composition.