Browsing by Author "Kujala, Sami, Dr., Aalto University, Department of Electronics and Nanoengineering, Finland"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Fabrication of Semiconductor Nanowires for Optical Studies
    (Aalto University, 2017) Kakko, Joona-Pekko; Kujala, Sami, Dr., Aalto University, Department of Electronics and Nanoengineering, Finland; Elektroniikan ja nanotekniikan laitos; Department of Electronics and Nanoengineering; Nanotechnology; Sähkötekniikan korkeakoulu; School of Electrical Engineering; Lipsanen, Harri, Prof., Aalto University, Department of Electronics and Nanoengineering, Finland
    Nanowires (NWs) are wire-like nanoscale structures that hold promise for next generation electronic and optoelectronic applications, such as light emitting diodes and solar cells. The small dimensions of the NWs cause optical phenomena that can be utilized in these applications. This thesis presents the results of the optical studies of NWs with controlled dimensions and positions and how to engineer the response of light in NWs to increase the efficiency of optoelectronical applications. Selective-area epitaxy was used to fabricate NWs with controlled dimensions and positions for the optical studies. Reflectance and cross-polarization spectroscopy were used to study the response of light from NW arrays. Reflectance revealed that the light trapping can be improved with Parylene-C coating of NWs and with dual-type NW arrays. Cross-polarization was used to characterize the optical modes in NWs. This is crucial, for example, in the optimization of the wavelength selective absorption in NWs. Position-control of the NWs also enabled to draw conclusions how certain optical phenomena and lattice vibrations can couple between neighbouring NWs. When the spacing was sufficient, the second harmonic generation from NWs had a non-overlapping emission pattern, centered on individual NWs. The large enough spacing allowed to see them clearly. When the NWs were used as THz radiation sources, the generated intensity was highest when the wavelength of the excitation light matched to the NW spacing. Additionally, quantized lattice vibrations in NWs were not affected by the NW spacing. Only the change in diameter had an effect on the lattice vibrations, thus no elastic coupling existed between neighbouring NWs.
  • Thumbnail Image
    Item
    Molecular materials for photonics
    (Aalto University, 2017) Pale, Ville; Kujala, Sami, Dr., Aalto University, Department of Electronics and Nanoengineering, Finland; Elektroniikan ja nanotekniikan laitos; Department of Electronics and Nanoengineering; Micro and Quantum Systems; Sähkötekniikan korkeakoulu; School of Electrical Engineering; Tittonen, Ilkka, Prof., Aalto University, Department of Electronics and Nanoengineering, Finland
    Billions of years of evolution has produced a variety of functional components that enable life on Earth. The notable examples are chlorophylls that have a vital role in converting the energy of the Sun into chemical energy, and nucleobases that act as the building blocks in DNA/RNA, which store and convey the genetic information in living organisms.  Thematically, this thesis is divided into two parts. The first part studies how the natural components, chlorophyll and uracil molecules, could be utilized in modern photonic applications. Publication I investigates the properties of supramolecular Zn chlorin-poly(4-vinylpyridine) assemblies that mimic the biological antenna structure. Moreover, this approach was used to create assemblies with macroscopically homogeneous pigment distribution. Publication II studies how FRET-mediated energy-transfer could be used to enhance the performance of the materials studied in Pub. I . In Publications III and IV, atomic/molecular layer deposition is established as a method to create fundamentally new three-dimensional sodium networked uracil assemblies with novel optical properties.  The second part investigates how silver nanoparticle assemblies could be utilized in surface enhanced spectroscopic techniques. Generally, Publications V and VI investigate two economical nanofabrication methods to create large-scale plasmonic substrates. In Publication V , azopolymer lithography was utilized to create periodic plasmonic nanoparticle arrays for fluorescence enhancement. In Publication VI , a method utilizing cryogenic deep reactive ion etching with inductively coupled plasma was used to create a plasmonic substrate for surface enhanced Raman scattering applications.  The results obtained in this thesis could pave the way for new biomimetic photonic materials and enable the utilization of economic and large-scale nanofabrication methods in creating new plasmonic materials. Especially, molecular layer deposition was established as a promising and scalable method to create materials with novel structural and optical properties.