Frantz Stabo-Eeg

Well-conditioned multiple laser Mueller matrix ellipsometer

We report on the design and performance test of a multiple laser Mueller matrix ellipsometer (MME). The MME is well conditioned due to the integration of the recently reported achromatic 132-deg compensators based on biprisms, in combination with high-quality Glan-Thompson polarizers. The system currently operates between 300 and 2700 nm, without the need to change any optical components except for the detector. Four lasers are employed as light sources (405, 532, 633, and 1570 nm) to test the performance in both reflection and transmission modes. Thus, the system is used to determine the Mueller matrices and associated optical constants of known optical systems: 1. optical rotatory power of D-glucose in solution, 2. reflection of a native oxide c-Si wafer, and 3. the properties of a liquid crystal spatial light modulator. The results show that the system matrices of the MME have condition numbers between the optimal and 2 during operation, resulting in small experimental errors. To the best of our knowledge, there is no other MME reported with such good conditioning over a comparably wide spectral range.

Design and characterization of achromatic 132° retarders in CaF2 and fused silica

Mueller matrix ellipsometers (MMEs) are known to be less affected by noise if the compensators have a 132° phase shift between the slow and fast axis. This paper reports on the design and characterization of two custom-made achromatic compensators with minimal deviation from 132° retardation as a function of wavelength. The compensators are based on the dual Fresnel rhomb geometry. One pair was made of CaF2 and the other of fused silica. These materials were selected due to their extended transparency range and optical quality. The CaF2 compensators were experimentally characterized in the 450–8000 nm range, whereas the less IR-transparent fused silica prisms were characterized between 450 and 1900 nm. Both kinds of retarders possessed the necessary properties needed for a broad-band achromatic Mueller matrix ellipsometer, since they were both shown to give well-conditioned system matrices within the measured wavelength regions. Specifically, the condition number for the dual prisms used as a Polarization State Generator (PSG) or Analyzer (PSA) was determined to be in the range 1.73–1.96, close to the optimal 31/2 value. Simulations of the optimal retarder configuration in terms of incidence angles and rotation angles are presented and discussed. From literature values of the optical constants it is shown that the compensators perform well at ultraviolet wavelengths down to approximately 150 nm.

Characterisation of Liquid Crystals for broadband optimal design of Mueller matrix ellipsometers

The optimal design over a broad spectral range of Liquid Crystal (LC) based Polarisation State Generators (PSG) and Polarisation State Analyzers (PSA), requires detailed knowledge of the spectral behaviour of the LCs. The full Mueller matrix measurement formalism based on the Eigenvalue Calibration Method, does in principle not require an exact modelling of the polarizing components, however, it is required that the condition number for both the PSG and the PSA is close to optimum over a wide spectral range. Two LC technologies are investigated here, Ferroelectric LC (FLC), and Liquid Crystal Variable Retarders (LCVR). In the case of a FLC based PSG, additional components, such as fixed retarders also need to be properly characterised in order for a proper broad spectral optimal design. These issues are here studied by characterizing the FLCs, the LCVRs and the fixed retarders with the help of a commercial visible Mueller matrix ellipsometer, and a polarizer-rotating sample-rotating analyzer near infra red set-up. The results are analyzed in the framework of the optimal condition number over a broad spectral range.

Determination of the phase- and polarization-changing properties of reflective spatial light modulators in one set-up

The use of liquid crystal spatial light modulators in applications, require good characterization of phase, polarization and amplitude shifting properties. This report presents a new approach for simultaneously characterizing the depolarization and controlling the polarization properties of a reflective twisted nematic liquid crystal spatial light modulator (LC SLM). The SLM was set up as a part of a Michelson interferometer. The phase response was determined by using a piezo-electric actuator for phase stepping in the reference arm. During the polarization measurement the reference beam was removed and the polarization state of the input and output was determined by a polarization state generator (PSG) and a polarization state analyzer (PSA), each consisting of a polarizer and a quarter-wave plate. Hereby, both phase response and polarization control properties could be determined independently in the same measurement configuration simply by changing static polarization components. The systematic rotation of the quarter wave plates of the PSG and the PSA using stepper motors gives out-put data whose Fourier transform in terms of angular frequency components can be used to determine all the elements of the Mueller matrix. The Mueller matrix of a commercial SLM (Holoeye LC-2500) was determined for 17 evenly spaced voltage levels addressed to the SLM.

Discriminating land mines from natural backgrounds by depolarization

Results from an experimental polarimetric investigation of 7 different types of land mines and 3 types of plants with the aim to explore the possibility in discriminating surface land mines from natural backgrounds are presented. The samples Mueller matrices at both specular and non-specular angles during 405 nm 1570 nm laser illumination were collected. Also included in this study is reflection spectra from the mines taken from 400 to 2500 nm as well as actual images of surface land mines hidden in a natural environment during different weather conditions. The mines had a reflection coefficient between 5-15 % with peak values around 510 nm due to the embedded green pigment. The mines were found to be less reflective in wet compared to dry conditions. The polarimetric study revealed that the samples had similar retardance and diattenuation values for small incident angles, but that discrimination between the samples could be made by monitoring the depolarization of the incident light for several incident angles, as a function of the angular distribution of scattered light. The land mines generally experience less depolarization than the investigated plants, specifically for specular angles around 1570 nm where the mines act as a non-depolarizing sample with depolarization index close to 1.0. The depolarization index is significantly smaller for specular angles from the plants, becoming 0.4 or below. Both plants and mines experience more depolarization for non-specular angles. A non-specular angular scan with a constant bi-static angle resulted in a Lorentzian shaped depolarization index curve, with characteristic differences in the fitted line-shape parameters. Remote laser based polarimetry might thus be a promising supplementary technique in recognizing surface mines or other related man-made objects from a natural background. Conclusively, the depolarization index as a function of angular distribution of scattered light along with its wavelength dependence is a metric that produce significant differences in the polarimetric signatures.

Biosensing and -imaging with enantiomeric luminescent conjugated polythiophenes using multiphoton excitation

We report on the initial time-resolved luminescense and nonlinear absorption properties of two polythiophenes 3-substituted with chiral charged amino acid-derivatized substituents, POWT and POMT. The photo-physical characterization yielded quantum efficiency typically in the range 0.01 - 0.1, however, with two-photon absorption cross-section better than or similar to a typical two-photon reference chromophore, such as fluorescein. They were tested as conformational sensitive optical probes for the recording of pH-induced conformational changes of synthetic peptides, proteins and samples of protein amyloid fibrils characteristic of amyloid related diseases. Particularly, the POMT polyelectrolyte with the L-enantiomeric side chains is shown to favor this induction of well defined structure as judged by the circular dichroic signal as well as a stronger enhancement of luminescense for the L-form over the D-form complex. Furthermore, time-resolved fluorescense and two-photon induced fluorescence both also showed a difference in the complexation with the D and L form. This shows that the multi-photon excitation path can be an efficient means for chiral recognition of biomolecular complexes. It is demonstrated how the conjugated polyelectrolyte L-POMT can be used to spectrally image the formation of amyloid fibrils of insulin using both one- and two-photon absorption based fluorescence imaging.

Biosensing and -imaging with enantiomeric luminescent conjugated polythiophenes using single- and multiphoton excitation

Summary form only given. Proteins can alter their conformation and aggregate leading to pathogenic states. Beta sheet-rich fibrillar assemblies are known as amyloid fibrils and these can lead to many devastating diseases known as the amyloidoses, including Alzheimers disease and the systemic amyloidoses. Thioflavin T or S (ThT or ThS) and Congo red are commonly used as detection agents for amyloid fibrils. ANS, Bis-ANS and DCVJ are known to prove valuable in order to study the conformational state and evolution of the aggregation process into amyloid fibrils. Simple, sensitive, and versatile tools that detect the fibrillar conformation of amyloidogenic proteins are of great importance. In this regard, fluorescent conjugated polyelectrolytes (CPs) have been studied for being used as amyloid specific dyes for staining of amyloid-containing tissue and as conformational sensitive probes for the detection of conformational changes in biomolecules. These are capable of selectively detecting biological processes and have large potential for being used as biosensors and in biomolecular devices. In particular, biospecific interactions can be detected through the impact on photoinduced charge or excitation transfer or through the conformational alterations of the polyelectrolyte chains. We present results of time-resolved luminescense and nonlinear absorption properties of polythiophenes 3-substituted with chiral charged amino acid-derivatized substituents. The photo-physical characterization yielded quantum efficiency typically in the range 0.01 - 0.1, however, with two-photon absorption cross-section more than 10 times better than a typical two-photon reference chromophore, such as fluorescein.