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Self-assembly of plasmonic nanoantenna-waveguide structures for subdiffractional chiral sensing
(2021)
Spin-momentum locking is a peculiar effect in the near-field of guided optical or plasmonic modes. It can be utilized to map the spinning or handedness of electromagnetic fields onto the propagation direction. This motivates a method to probe the circular dichroism of an illuminated chiral object. In this work, we demonstrate local, subdiffraction limited chiral coupling of light and propagating surface plasmon polaritons in a self-assembled system of a gold nanoantenna and a silver nanowire. A thin silica shell around the nanowire provides precise distance control and also serves as a host for fluorescent molecules, which indicate the direction of plasmon propagation. We characterize our nanoantenna-nanowire systems comprehensively through correlated electron microscopy, energy-dispersive X-ray spectroscopy, dark-field, and fluorescence imaging. Three-dimensional numerical simulations support the experimental findings. Besides our measurement of far-field polarization, we estimate sensing capabilities and derive not only a sensitivity of 1 mdeg for the ellipticity of the light field, but also find 10(3) deg cm(2)/dmol for the circular dichroism of an analyte locally introduced in the hot spot of the antenna-wire system. Thorough modeling of a prototypical design predicts on-chip sensing of chiral analytes. This introduces our system as an ultracompact sensor for chiral response far below the diffraction limit.
Comparison of the dissociation kinetics of rapid-acting insulins lispro, aspart, glulisine and human insulin under physiologically relevant conditions. Dissociation kinetics after dilution were monitored directly in terms of the average molecular mass using combined static and dynamic light scattering. Changes in tertiary structure were detected by near-UV circular dichroism. Glulisine forms compact hexamers in formulation even in the absence of Zn2+. Upon severe dilution, these rapidly dissociate into monomers in less than 10 s. In contrast, in formulations of lispro and aspart, the presence of Zn2+ and phenolic compounds is essential for formation of compact R6 hexamers. These slowly dissociate in times ranging from seconds to one hour depending on the concentration of phenolic additives. The disadvantage of the long dissociation times of lispro and aspart can be diminished by a rapid depletion of the concentration of phenolic additives independent of the insulin dilution. This is especially important in conditions similar to those after subcutaneous injection, where only minor dilution of the insulins occurs. Knowledge of the diverging dissociation mechanisms of lispro and aspart compared to glulisine will be helpful for optimizing formulation conditions of rapid-acting insulins.
Purpose: Comparison of the dissociation kinetics of rapid-acting insulins lispro, aspart, glulisine and human insulin under physiologically relevant conditions.
Methods: Dissociation kinetics after dilution were monitored directly in terms of the average molecular mass using combined static and dynamic light scattering. Changes in tertiary structure were detected by near-UV circular dichroism.
Results: Glulisine forms compact hexamers in formulation even in the absence of Zn2+. Upon severe dilution, these rapidly dissociate into monomers in less than 10 s. In contrast, in formulations of lispro and aspart, the presence of Zn2+ and phenolic compounds is essential for formation of compact R6 hexamers. These slowly dissociate in times ranging from seconds to one hour depending on the concentration of phenolic additives. The disadvantage of the long dissociation times of lispro and aspart can be diminished by a rapid depletion of the concentration of phenolic additives independent of the insulin dilution. This is especially important in conditions similar to those after subcutaneous injection, where only minor dilution of the insulins occurs.
Conclusion: Knowledge of the diverging dissociation mechanisms of lispro and aspart compared to glulisine will be helpful for optimizing formulation conditions of rapid-acting insulins.
This thesis is focussed on the electronic properties of the new material class named topological insulators. Spin and angle resolved photoelectron spectroscopy have been applied to reveal several unique properties of the surface state of these materials. The first part of this thesis introduces the methodical background of these quite established experimental techniques.
In the following chapter, the theoretical concept of topological insulators is introduced. Starting from the prominent example of the quantum Hall effect, the application of topological invariants to classify material systems is illuminated. It is explained how, in presence of time reversal symmetry, which is broken in the quantum Hall phase, strong spin orbit coupling can drive a system into a topologically non trivial phase. The prediction of the spin quantum Hall effect in two dimensional insulators an the generalization to the three dimensional case of topological insulators is reviewed together with the first experimental realization of a three dimensional topological insulator in the Bi1-xSbx alloys given in the literature.
The experimental part starts with the introduction of the Bi2X3 (X=Se, Te) family of materials. Recent theoretical predictions and experimental findings on the bulk and surface electronic structure of these materials are introduced in close discussion to our own experimental results. Furthermore, it is revealed, that the topological surface state of Bi2Te3 shares its orbital symmetry with the bulk valence band and the observation of a temperature induced shift of the chemical potential is to a high probability unmasked as a doping effect due to residual gas adsorption.
The surface state of Bi2Te3 is found to be highly spin polarized with a polarization value of about 70% in a macroscopic area, while in Bi2Se3 the polarization appears reduced, not exceeding 50%. We, however, argue that the polarization is most likely only extrinsically limited in terms of the finite angular resolution and the lacking detectability of the out of plane component of the electron spin. A further argument is based on the reduced surface quality of the single crystals after cleavage and, for Bi2Se3 a sensitivity of the electronic structure to photon exposure.
We probe the robustness of the topological surface state in Bi2X3 against surface impurities in Chapter 5. This robustness is provided through the protection by the time reversal symmetry. Silver, deposited on the (111) surface of Bi2Se3 leads to a strong electron doping but the surface state is observed up to a deposited Ag mass equivalent to one atomic monolayer. The opposite sign of doping, i.e., hole-like, is observed by exposing oxygen to Bi2Te3. But while the n-type shift of Ag on Bi2Se3 appears to be more or less rigid, O2 is lifting the Dirac point of the topological surface state in Bi2Te3 out of the valence band minimum at $\Gamma$. After increasing the oxygen dose further, it is possible to shift the Dirac point to the Fermi level, while the valence band stays well beyond. The effect is found reversible, by warming up the samples which is interpreted in terms of physisorption of O2.
For magnetic impurities, i.e., Fe, we find a similar behavior as for the case of Ag in both Bi2Se3 and Bi2Te3. However, in that case the robustness is unexpected, since magnetic impurities are capable to break time reversal symmetry which should introduce a gap in the surface state at the Dirac point which in turn removes the protection. We argue, that the fact that the surface state shows no gap must be attributed to a missing magnetization of the Fe overlayer. In Bi2Te3 we are able to observe the surface state for deposited iron mass equivalents in the monolayer regime. Furthermore, we gain control over the sign of doping through the sample temperature during deposition.
Chapter6 is devoted to the lifetime broadening of the photoemission signal from the topological surface states of Bi2Se3 and Bi2Te3. It is revealed that the hexagonal warping of the surface state in Bi2Te3 introduces an anisotropy for electrons traveling along the two distinct high symmetry directions of the surface Brillouin zone, i.e., $\Gamma$K and $\Gamma$M. We show that the phonon coupling strength to the surface electrons in Bi2Te3 is in nice agreement with the theoretical prediction but, nevertheless, higher than one may expect. We argue that the electron-phonon coupling is one of the main contributions to the decay of photoholes but the relatively small size of the Fermi surface limits the number of phonon modes that may scatter off electrons. This effect is manifested in the energy dependence of the imaginary part of the electron self energy of the surface state which shows a decay to higher binding energies in contrast to the monotonic increase proportional to E$^2$ in the Fermi liquid theory due to electron-electron interaction.
Furthermore, the effect of the surface impurities of Chapter 5 on the quasiparticle life- times is investigated. We find that Fe impurities have a much stronger influence on the lifetimes as compared to Ag. Moreover, we find that the influence is stronger independently of the sign of the doping. We argue that this observation suggests a minor contribution of the warping on increased scattering rates in contrast to current belief. This is additionally confirmed by the observation that the scattering rates increase further with increasing silver amount while the doping stays constant and by the fact that clean Bi2Se3 and Bi2Te3 show very similar scattering rates regardless of the much stronger warping in Bi2Te3.
In the last chapter we report on a strong circular dichroism in the angle distribution of the photoemission signal of the surface state of Bi2Te3. We show that the color pattern obtained by calculating the difference between photoemission intensities measured with opposite photon helicity reflects the pattern expected for the spin polarization. However, we find a strong influence on strength and even sign of the effect when varying the photon energy. The sign change is qualitatively confirmed by means of one-step photoemission calculations conducted by our collaborators from the LMU München, while the calculated spin polarization is found to be independent of the excitation energy. Experiment and theory together unambiguously uncover the dichroism in these systems as a final state effect and the question in the title of the chapter has to be negated: Circular dichroism in the angle distribution is not a new spin sensitive technique.
In dieser Arbeit wurden die Möglichkeiten und Grenzen für Zirkulardichroismus-Messungen mit Synchrotronstrahlung untersucht. Dazu wurde ein Messaufbau für Zirkulardichroismus-Messungen an zwei Strahlrohren am Berliner Elektronenspeicherring für Synchrotronstrahlung eingesetzt, die für Messungen im Bereich des ultravioletten Lichts geeignet sind. Eigenschaften der Strahlrohre und des Messaufbau wurden in einigen wichtigen Punkten mit kommerziellen Zirkulardichroismus-Spektrometern verglichen. Der Schwerpunkt lag auf der Ausdehnung des zugänglichen Wellenlängenbereichs unterhalb von 180 nm zur Untersuchung des Zirkulardichroismus von Proteinen in diesem Bereich. In diesem Bereich ist es nicht nur die Lichtquelle sondern vor allem die Absorption des Lichts durch Wasser, die den Messbereich bei der Messung biologischer Proben in wässriger Lösung einschränkt. Es wurden Bedingungen gefunden, unter denen der Messbereich auf etwa 160 nm, in einigen Fällen bis auf 130 nm ausgedehnt werden konnte. Dazu musste die Pfadlänge deutlich reduziert werden und verschieden Probenküvetten wurden getestet. Der Einfluss der dabei auftretenden Spannungsdoppelbrechung in den Probenküvetten auf das Messsignal konnte mit einem alternativen Messaufbau deutlich reduziert werden. Systematische Fehler im Messsignal und auftretende Strahlenschäden begrenzen jedoch die Zuverlässigkeit der gemessenen Spektren. Bei Proteinfilmen schränkt die Absorption von Wasser den Messbereich kaum ein. Es wurden jedoch meist deutliche Unterschiede zwischen den Spektren von Proteinfilmen und den Spektren von Proteinen in wässriger Lösung festgestellt. Solange diese Unterschiede nicht minimiert werden können, stellen Proteinfilme keine praktikable Alternative zu Messungen in wässriger Lösung dar.