Theses 2014

Abstract: In this master thesis, we present a 3D modelling of thermal lensing effect in an edge-pumped trapezoid Yb:YAG/YAG thin disk laser. At first, a Monte-Carlo ray tracing method and finite element analysis (FEA) is used to calculate the absorbed pump power from our sides in such a way, that the pump light are totally trapped inside the crystal after total reflections. To study this configuration we performed a detailed simulation of our delivery system in our laser simulation code software ASLD. Secondly, we optimized the absorption efficiency of our laser which is obtained 66.7% due to the optimum Yb:YAG dopant of 10% and efficient thickness of 0.2 mm. Then, using the result of absorbed pump light distribution as a heat source for the crystal to compute the temperature distribution inside the crystal and perform the consequences of opto-mechanical properties including Von Mises stress and deformation components distribution inside the crystal via Finite Element Analysis (FEA). Furthermore, the thermal lens power is considered in our modelling as an important consequence of opto-mechanical effect which is compared in different output powers. Finally, we applied the Dynamic Multimode Analysis (DMA) method to solve the rate equation and calculate the output powers respect to different length of cavities and different output coupler curvatures respect to the dealing beam quality in multimode operation. 

About this project: The advances made in laser technology must address vast and continuously evolving needs in multiple application areas [1]. Among different types of lasers, the all-solid-state lasers, ideally diode pumped (DPSSL) are best placed to address majority of these needs because of a robust and compact setup, a relatively high efficiency with corre-sponding low thermal cooling requirements, and a long lifetime [2]. Research and de-velopment on nonlinear optics and laser physics have been growing fast over the past twenty years. Behind such an impressive growth is the need for excellent thermal prop-erties and high damage threshold for high power and high energy applications, a large emission spectrum for ultra-short pulse generation, a long lifetime of the excited state for high-energy extraction and the access to new wavelength ranges [1]. Depending on the type of problem to be addressed, these issues demand the development of materials which have a host and a dopant specific in each case. Thus, research on and charac-terization of new laser materials is inevitable to answer to the above mentioned chal-lenges or societal needs of today and tomorrow in general [2].

Efficient use of laser materials requires full characterization of their absorption and emission properties. For most classes of crystals, measurements of these properties can be done along dielectric frame axes which are best described with 3-by-3 second rank linear permittivity tensor. However, in the case of monoclinic crystals, the maximum values of absorption and fluorescence are not along the principal axes of the dielectric frame, but tilted at an angle with respect to one of the axes of the dielectric frame [1]. In general, monoclinic crystals are biaxial crystals such that the imaginary part of refrac-tive index is different than principal refractive indices in the tri-rectangular dielectric frame (x, y, z) [3].

In this master thesis project, we have studied Yb-doped borate type-Yb:Li6(Y/Gd)(BO3)3-monoclinic crystals. The use of these new amplifier media was explored for the devel-opment of new solid-state lasers in different spectral domains: the near-infrared with Ytterbium-doped crystals for short-pulse generation. The project was done in the Short lasers: applications & materials (SLAM) research group which is mainly focusing on pho-tonics at the University of Bordeaux, France. Characterization and tests of new laser materials is one of the themes of the group.  Based on previous experiments, some of the anisotropic properties of highly concentrated Ytterbium-doped monoclinic gain me-dia with hosts from borate family, Yb-doped borate type crystals were characterized. As a continuation of these studies on borate-type crystals [2, 4-6], the relationship between linearly polarized light, absorption and emission of these crystals with the real and imaginary refractive index surfaces was studied with a new and relatively simple approach.

Conclusions: With a consideration of the so called dielectric frame, the relative dielectric permittivity tensor of monoclinic crystals is described by a 3×3 diagonal real matrix plus imaginary matrix with diagonal and two similar off-diagonal elements. The off-diagonal elements depict the relative rotation of the imaginary plane with respect to the principal dielectric plane that are perpendicular to one of the dielectric axes which is parallel to the 2 fold axis or perpendicular to the mirror m. With this concept, from each host matrix, two types of Yb-doped monoclinic crystal samples were prepared and the dependence of the absorption coefficient on the polarization angle for a linearly polarized source at the maximum absorption wavelength of the samples was studied.

From the result of these experiments, the orientation of the dielectric axes were identi-fied with respect to the crystallographic orientations. Consequently, this leads to the full description of the imaginary index surfaces of the LGB:Yb and LYB:Yb monoclinic sam-ples.

However, for laser operations, the “good” absorption axis is not necessarily the “best” emission axis. Therefore, further investigations should be made on the lasing and gain experiments which was already tried during the course of this project [Appendix B, Ap-pendix C]. The absorption experiments should also be improved with a better mechanics for the crystal mounts to minimize angular misalignment of the samples.

Generally, in this master thesis project, we were able to develop rather simple and complete theoretical and experimental approach to characterize the absorption of Yb-doped borate type monoclinic crystals or monoclinic crystals in general. Given the po-tentiality of Yb-doped crystals for future laser performances, it was an amazing journey.

The overall work of this thesis is organized in three main parts. The first part of the work reviews Raman spectroscopy, its challenges and the available methods developed to tackle them. The second part is the implementation of a reconstruction algorithm of Raman spectra from a fluorescence corrupted spectra. A shifted excitation principle is applied to get rid of the fluorescence followed by baseline correction method to completely remove the fluorescence. The reconstruction method was able to get rid of the fluorescence without losing significant Raman information. The final part of the thesis is devoted to the results and discussions. Effect of excitation wavelength shift and strength of fluorescence are analyzed. Nine different tissues from four different pigs are differentiated with a misclassification error of 1.74%. Tissues of soft bone and hard bone showed the highest misclassifications. Significant differences, which are attributed to the spectral feature of lipids and proteins, among the Raman spectra of normal tissue and tumor tissue of the oral cavity of human are also observed..

Abstract: Chiral metamaterial are a class of metamaterials that show optical activity, ellipticity and negative refraction. Optical activity and ellipticity generally occurs together. The aim of this thesis is to obtain a high optical activity with low ellipticity. A bi-layered structure comprising of both a negative and a positive structure was used for this purpose. These structures were fabricted by using focussed ion beam (FIB). The transmission spectrum for the structures is measured by a light microscope equipped with a spectrometer. The bi-layered structures were attempted to be fabricated by using FIB and by micromanipulators. Also, different arrangements of the bi-layered structures were modelled by the finite difference time domain (FDTD) method. From the modelled structures, an optical activity of 61° was obtained with zero ellipticity.

Conclusion: In this thesis, the high potential of Raman spectroscopy (RS) to differentiate tumour from normal tissue was studied. Because of its capability to provide an objective „chemical fingerprint“ of the examined region, RS can be used for the identification of molecular changes through specific spectral patterns in oral cancer tissue. I showed the potential of identifying oral tumour with high sensitivity and specificity in an ex vivo setting.

The hardware and software of the former set up was improved to reach high spectral resolution and intensity with lower excitation power (100 mW) and very short integration time (100 ms) giving rise to high diagnostic speed of the clinical Raman sensor, too. Hardware development was carried out through utilizing the Ventana spectrometer with twice higher spectral resolution than the QE65000 spectrometer (that was calculated theoretically and was confirmed by the experiment) and high sensitivity, wide numerical aperture hand-held Raman probe which has 2.5 times larger solid angle than the former Raman probe.

Development of software was done with the aim of having highly automated spectral data acquisition and analysis. Thus, a robust and user-friendly software, which can synchronize all process of shifting wavelength, spectra acquisition and spectral analysis for physicians was established which allows for a time-saving and cost-reducing application.

The suppression of the fluorescence background was done with the SERDS (shifted-excitation Raman difference spectroscopy) method followed by a polynomial fit. And, the reconstructed Raman spectrum of normal tissue represented intense peaks at 1258, 1308, 1458 and 1667 cm^-1. In contrast, the Raman spectrum of oral tumour had lower peak intensities at 1291, 1342, 1458 and 1667cm^-1. Hence, the Raman spectral difference between normal and tumour oral tissue of human was used for tumour diagnosis. There are obvious shifts from 1258cm^-1 of normal tissue to 1291cm^-1 peak of tumour tissue and from 1308cm^-1 peak of normal tissue to 1342cm^-1 peak of tumour tissue that were applied as criterion for differentiation. Besides, there is a clear intensity difference between normal and tumour tissue in 1458 cm^-1 peak that was also used for discrimination. Accordingly, a statistical method will be implemented for the classification of the reconstructed Raman signal to convert the Raman spectrum into a diagnosis that would be carried out by my colleague in the other study.

Abstract: The electromagnetic properties of synthetic materials at terahertz frequencies offer great opportunities for non-destructive testing in quality assurance or production process control. Terahertz tomography systems providing three-dimensional images of specimens have several advantages over conventional X-ray or ultrasonic systems including the opportunity of a spectral analysis of the material properties. However, terahertz tomography systems are still a current field of research and the demand for applicable improvements in both, system design and reconstruction algorithms is still highly topical.

The here developed tomographic imaging system is one of the first hybrid systems that combines transmission and reflection system architecture. Transmission terahertz tomography using reconstruction algorithms similar to X-ray tomography allows to analyse material properties but resolution in the propagation direction of the beam and use algorithms based on the synthetic aperture radar principle adopted from ultrasound tomography. The increased amount of collected information in a hybrid system combining both architectures enables the development of more efficient algorithms providing significantly higher resolution.

The hybrid system consists of an electronic terahertz source and four heterodyne detectors which simultaneously measure the transmitted and reflected radiation. The system works similar to a frequency-modulated continuous wave radar. It sweeps the emitted signal frequency between 70 and 110 GHz, detecting both, amplitude and phase of every single frequency component. As advancement the development of a system using optically generated terahertz pulses allowing spectral analysis of specimens is already planned. The time of flight is determined by detecting the frequency difference between the emitted and received signal. Telescopes consisting of two lenses provide facile alignment of optics as well as the opportunity to modify beam parameters by exchanging lenses easily. One rotational and two translational stages are used for scanning the specimen. First results show reconstructed three-dimensional images of samples with dimensions up to 140 millimetres where the development of hybrid reconstruction algorithms promises significant improvement in resolution.

Conclusion: In the present work, the instrumental line broadening of an existing SLS experiment was investigated by the reference fluid toluene and the results were applied to a semitransparent IL [BMIM][TCM].

Conclusion: In the present work, the instrumental line broadening of an existing SLS experiment was investigated by the reference fluid toluene and the results were applied to a semitransparent IL [BMIM][TCM].

Abstract: One of the most important tools for biomedical research is fluorescence microcsopy. Selective labelling of molecules enables the observation of process kinetics within living cells but is limited due to diffraction to a maximum resolution of half the wavelength in use. To further increase resolution it is possible e.g. to use structured instead of homogeneous illumination light. The spatial frequencies of illumination and flurophore distribution will mix, thus encoding fine details of the structure, which would usually be lost, in observable spatial frequency bands. The so obtained additional information is computationally decoded afterwards to offer super-resolution. Depending on the particular structure of the illumination light the respective reconstruction algorithms vary a lot. In addition, the set-ups and acquisition procedures need to be adapted to generate the needed patterns and record the appropriate amount of different raw image data. Therefore, it is useful to compare different illumination geometries as they all have their strength, weaknesses and optimal fields of application. For example, sinusoidal illumination in a total internal reflection configuration (TIRF-SIM) allows extremely fast, high-contrast imaging with the theoretical possibility of unlimited resolution improvement via the use of non-linear effects (SSIM) but is restrained to two-dimensional imaging. In contrast, three-dimensional sinusoidal illumination (3D-SIM) requires more reconstruction and hardware efforts but is capable of removing out-of-focus light computationally and enables volumetric data at even higher imaging rates than 3d-SIM but lacks resolution improvement in the axial direction. Diffraction limited multi-focal spot pattern illumination (MSIM), similarly to SROS-SIM, does not offer super-resolution in the axial direction but enhances penetration depth and contrast and therefore is useful in thick sample imaging. To optimally compare these techniques, the standard reconstruction algorithms were implemented in Matlab and a SIM microscope was built that is switchable between TIRF-SIM, SROS-SIM and MSIM. It is therefore ideal to comparatively evaluate the respective performances regarding resolution, imaging speed, and reduction of out-of-focus-light. Additionally, a new kind of reconstruction algorithm was developed, joint Richardson-Lucy deconvolution for widefield MSIM, that improves reconstruction results from noise corrupted raw data.

Conclusion and Future Work: In this thesis, a beam profile imaging routine and a sample inspection imaging routine were implemented into a continuous-wave terahertz photomixing system. The beam profile measurement was performed to recognize and characterize the emitter (photomixer) properties and to examine the alignment of the terahertz beam with an opto-mechanic setup. The emitter was whown to form a beam focus in front of its facet, with frequency dependent properties. A similar frequency dependent behaviour was found for the intermediate focus, which is formed by the opto-mechanic setup for sample inspection purposes.

A sample inspection routine was developed based on two scan methods: as step-by-step scan and a continuous scan. Both methods were throughly tested for various parameters.

Finally, both scan methods were used to inspect an envelope, where the teraherzt images of the envelope clearly revealed its content. But, one of the objects could not be identified with terahertz imgaing. Based on its spectral fingerpringts the unknown object’s material was identified with an additional spectroscopic analysis.

There are some works left for the future:

• An automatic data analysis is desired for beam profile imaging routine.
• So far there is only one emitter characterized. For statistiscal validations, the beam profile routine should be applied for different emitters.
• The implemented sample inspection routine is compatible with TOPTICA’s incoherent photomixing setup. In future, the routine should be extended to a coherent system for phase-sensitive sample inspection.

Conclusion and Outlook: The focus of this thesis was to investigate LEC architectures that can be fabricated by solution processing. In particular, the aim wa to replace the ITO electrode with silver nanowires.

It was shown that AZO and additional buffer layers as ETLs couldn’t be successful to inject electrons into the organic semiconductor and therefore the inefficient injection cannot provide electrons and holes for radiative recombination and the light output from the device is low.

In the normal structure of LECs, we applied a new structure design which didn’t utilize any ETLs. Equally interesting are the results of the performance optimization of the different HTL and active layer thicknesses and it was demonstrated how to utilize thick active material layers.

In the last step, both ITO and the evaporated silver top electrode were successfully replaced with silver nanowire as anode and cathode, respectively.

In conclusion, this thesis has demonstrated LEC fabrication and operation under ambient conditions, and fully solution-processed devices that feature silver nanowire electrodes.

Successful implementation of solution processing methods could allow LECs to become an ultra-low-cost lighting alternative for various large area fabrications, using roll to roll coating and slot die casting.

The gap between LECs and currently available lighting technology has become smaller indicating the potentials of LECs in future’s technology. Nevertheless, further development could primarily address the limited lifetime of LECs by increasing ionic conductivity of active layer ink, and also improve efficiency using solution processed reflector bottom electrode like silver ink in order to increase the light emission.