Browsing by Author "Tammela, Juhani"

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  • Enhanced Nonlinear Optical Responses in MoS2 via Femtosecond Laser-Induced Defect-Engineering
    (2024-11-12) Akkanen, Suvi-Tuuli; Arias Muñoz, Juan Camilo; Emelianov, Aleksei V.; Mentel, Kamila K.; Tammela, Juhani; Partanen, Mikko; Das, Susobhan; Pettersson, Mika; Sun, Zhipei
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    2D materials are a promising platform for applications in many fields as they possess a plethora of useful properties that can be further optimized by careful engineering, for example, by defect introduction. While reliable high-yield defect engineering methods are in demand, most current technologies are expensive, harsh, or non-deterministic. Optical modification methods offer a cost-effective and fast mechanism to engineer the properties of 2D materials at any step of the device fabrication process. In this paper, the nonlinear optical responses of mono-, bi-, and trilayer molybdenum disulfide (MoS2) flakes are enhanced by deterministic defect-engineering with a femtosecond laser. A 50-fold enhancement in the third harmonic generation (THG) and a 3.3-fold increase in the second harmonic generation (SHG) in the optically modified areas is observed. The enhancement is attributed to resonant SHG and THG processes arising from optically introduced mid-band gap defect states. These results demonstrate a highly controllable, sub-micrometer resolution tool for enhancing the nonlinear optical responses in 2D materials, paving the way for prospective future applications in optoelectronics, quantum technologies, and energy solutions.
  • Optical second-harmonic generation from niobium oxide dihalides
    (2023-10-20) Tammela, Juhani
     Sähkötekniikan korkeakoulu | Master's thesis
    Two-dimensional (2D) materials show extraordinary nonlinear optical properties with higher nonlinear susceptibilities than those found in bulk materials. To fully utilize the nonlinear optical properties of 2D materials, further characterization and understanding of the phenomena are needed, in addition to new 2D materials with high optical nonlinear susceptibilities. Recently, niobium oxide dihalides (NbOX2) are an emerging group of 2D materials with unique physical properties, such as high second-order optical nonlinearities and ferroelectricity at high curie temperatures. The materials provide a possible platform for novel applications, such as non-volatile optically active memory devices and light sources for quantum technologies. This thesis examines the nonlinear optical properties of NbOX2 materials via second harmonic generation (SHG) by investigating power and polarization dependencies together with anisotropic properties at different wavelength regions. In addition, strain effects on the SHG are investigated by introducing variable uniaxial tensile strain to the samples. To examine the SHG properties, samples of NbOCl2 and NbOI2 are fabricated via mechanical exfoliation and transferred to silicon and flexible polydimethylsiloxane substrates. The SHG is investigated using a home-built multiphoton microscope that uses a wavelength-tunable femtosecond excitation laser together with a galvanometer to create micrographs of the samples. The results obtained in this thesis show a quadratic power dependency of SHG with a high degree of anisotropy that has a unique dependency on the incident wavelength. In addition, the results indicate crystal orientation-dependent strain modulation of SHG from NbOCl2. This thesis suggests NbOX2 materials provide high optical second-order nonlinearities with a high degree of tunability, allowing for more extensive studies in strain tuning of nonlinear optical properties.
  • Vuotovirran mittauslaitteiston suunnittelu ja toteutus
    (2020-05-25) Tammela, Juhani
     Sähkötekniikan korkeakoulu | Bachelor's thesis