Maarten Vanbel

Synthesis and Characterization of Holmium-Doped Iron Oxide Nanoparticles

Rare earth atoms exhibit several interesting properties, for example, large magnetic moments and luminescence. Introducing these atoms into a different matrix can lead to a material that shows multiple interesting effects. Holmium atoms were incorporated into an iron oxide nanoparticle and the concentration of the dopant atom was changed in order to determine its influence on the host crystal. Its magnetic and magneto-optical properties were investigated by vibrating sample magnetometry and Faraday rotation measurements. The luminescent characteristics of the material, in solution and incorporated in a polymer thin film, were probed by fluorescence experiments.

Resolving enantiomers using the optical angular momentum of twisted light

Helical dichroism to resolve enantiomers using twisted light carrying only optical orbital angular momentum. Circular dichroism and optical rotation are crucial for the characterization of chiral molecules and are of importance to the study of pharmaceutical drugs, proteins, DNA, and many others. These techniques are based on the different interactions of enantiomers with circularly polarized components of plane wave light that carries spin angular momentum (SAM). For light carrying orbital angular momentum (OAM), for example, twisted or helical light, the consensus is that it cannot engage with the chirality of a molecular system as previous studies failed to demonstrate an interaction between optical OAM and chiral molecules. Using unique nanoparticle aggregates, we prove that optical OAM can engage with materials’ chirality and discriminate between enantiomers. Further, theoretical results show that compared to circular dichroism, mainly based on magnetic dipole contributions, the OAM analog helical dichroism (HD) is critically dependent on fundamentally different chiral electric quadrupole contributions. Our work opens new venues to study chirality and can find application in sensing and chiral spectroscopy.

Second Harmonic Generation Indicates a Better Si/Ge Interface Quality for Higher Temperature and With

In order for germanium (Ge) to replace silicon in advanced MOSFET channels, proper passivation of Ge is required. For this purpose, an ultrathin epitaxial Si cap was grown on Ge(001), and we applied second harmonic generation (SHG) in order to probe the Si/Ge interface quality. SHG indicates a better interface quality for a growth temperature of 500°C rather than 450°C. Similarly, a better quality of the interface is observed upon replacing the conventional H2 carrier gas with N2. Additionally, from the SHG signal, we were able to extract both the thickness of the native SiO2 layer (~4 monolayers (MLs)] and the thickness of the strained Si layer (relaxation at ~12 MLs). These results are important for building Ge-based electronic components.

\hbox{N}_{2}

Monitoring oxidation steps is an important factor during the fabrication of semiconductor devices, because transistor performance can be greatly affected by defects in the passivation layer. As an example, we discuss the formation of a gate stack in metal oxide semiconductor (MOS) devices using Ge as an alternative channel material. Building an MOS gate stack on Ge requires passivation of the interface between the dielectric (typically a high-k material such as Al2O3 or HfO2, grown by means of atomic layer deposition (ALD)) and the Ge channel. Such passivation can be obtained from a very thin Si layer, epitaxially grown on Ge. The Si surface receives an oxidizing clean (O3 or wet chemical clean) before the ALD step. In this work, second-harmonic generation (SHG) data are presented for silicon layers with varying thickness, grown with either trisilane (Si3H8) or silane (SiH4) and with various cleaning steps. The trend in second-harmonic response upon azimuthal rotation of the samples was comparable for bot...

Rather Than With

In this paper, we describe for the first time the synthesis of new chiral nanosized metal oxide surfaces based on chiral self-assembled monolayers (SAMs) coated with metal oxide (TiO2) nanolayers. In this new type of nanosize chiral surface, the metal oxide nanolayers enable the protection of the chiral self-assembled monolayers while preserving their enantioselective nature. The chiral nature of the SAM/TiO2 films was characterized by variety of unique techniques, such as second-harmonic generation circular dichroism (SHG-CD), quartz crystal microbalance, and chiral adsorption measurements with circular dichroism spectroscopy. The chiral resolution abilities of the SAMs coated with metal oxide (TiO2) nanolayers were investigated in the crystallization of a racemic mixture of threonine and glutamic acid. Our proposed methodology for the preparation of nanoscale chiral surfaces described in this article could open up opportunities in other fields of chemistry, such as chiral catalysis.

\hbox{H}_{2}

We describe a nonlinear optical study of gold triangles that exploits a higher order plasmonic resonance. A comprehensive nonlinear optical characterisation was performed both by second harmonic generation (SHG) and two photon fluorescence spectroscopy (2PF). We demonstrate and explain the enhancement of the coherent and incoherent nonlinear optical emission by a higher order multipolar mode of the plasmonic structure. The peculiarities of the mode shape and its influence on intensity and polarisation of the nonlinear signal are experimentally and numerically confirmed.

as the Carrier Gas

Al2O3 passivated n-doped Si substrates are investigated by second-harmonic generation (SHG) upon applying an external electric field by a corona wire. The observed change in the SHG response upon applying an external electric field is attributed to charge transfer in the semiconductor. Capacitance-voltage measurements are performed to affirm this conclusion. Upon applying a large negative electric field over the structure, a clear alteration in SHG signal is observed, which corresponds to tunneling of holes from the n-doped silicon into the aluminum oxide layer.

Second-harmonic generation reveals the oxidation steps in semiconductor processing

Because the germanium native oxide constitutes a poor dielectric, building metal oxide semiconductors (MOS) gate stacks on Ge requires passivation of the interface between the dielectric and the Ge channel. Different approaches to perform this passivation are available: GeO2 growth prior to high-k depositing, sulphur passivation, etc. The interface properties of these MOS stacks are important, because they determine the electrical properties of the whole structure. Dangling bonds introduce extra energy levels within the band gap, which results in a loss of efficiency in switching a MOS - field effect transistor on and off. Fixed charges near the interface enlarge the voltage needed for switching between on and off state as well. Hence, characterizing these interfaces is a key challenge in semiconductor fabrication. This can for example be achieved using Second Harmonic Generation (SHG) to probe the interface, because SHG is an inherent surface and interface sensitive technique. In this work, we present SHG as an promising surface and interface characterization tool for semiconductors for passivated germanium samples. Different SHG responses are shown for germanium samples with a sulphur passivated Ge or high-k dielectric on top of Si. We show that the oxide layer as such is not probed by SHG and that different bonds over the Ge/oxide interface result in a difference SHG response.

Chiral Thin Films of Metal Oxide

Second harmonic generation microscopy has recently become an important tool for studying materials. In this article, we use a recently developed analytical method, for second-harmonic generation microscopy, to determine the point group symmetry of micro crystals of enantiomerically pure 1,1’-bi-2-naphtol.

Nonlinear optical enhancement caused by a higher order multipole mode of metallic triangles

Since the discovery of graphene in 2004 by Novoselov and Geim, a lot of research emphasis has been directed towards its characterization. Most of the important scientific breakthroughs have been obtained on exfoliated graphene (produced via the well known ‘scotch tape’ method), nowadays, different synthetic routes have been developed to obtain largescale graphene. Among several optical techniques, Raman spectroscopy is the one most often employed to characterize the defects, number of graphene layers and other properties of the graphitic films regardless of their fabrication method. In this work, we will report on the microscopic imaging of the two-photon fluorescence (2PF) properties and the second harmonic generation (SHG) in both single layer and few layer graphene.