Publication list

 

Photonics of vector beams
  • What are vector beams?




The optical beams with non-uniform distribution of polarization on the beam cross-section are called vector beams.



A radially polarized beam, which is a kind of vector beams, can dominantly produce a longitudinal electric field, which direction is parallel to the beam axis, around the focal spot in contrast to the focusing of a plane wave, which produce a transverse electric field. This unique feature is able to form a smaller focal spot with a completely cylindrical symmetry. 
  • Generation of vector beams directly from a laser caity

　
While the conversion of a Gaussian beam to a vector beam by a polarization convertor is possible, a directly generated vector beam from a laser cavity can be much more identical to a theoretical vector beam. Direct generation of vector beams from a conventional laser cavity can be observed, for example, by using birefringence of an optical element. 

  • Application of vector beams

Particle acceleration, laser processing and superresolution microscopy are promising applications of vector beams. We are exploring the ability of small focal spot formation of vector beams for superresolution optical microscopy, which can achieve the spatial resolution around 100 nm by simply replacing a conventional laser source to a vector beam (10.1016/bs.po.2021.01.001). Super-oscillation of vector beam is expected to be far beyond the diffraction limit of light to achieve higher spatial resolution.

In addition, we are interested in the development of laser processing utilizing unique polarization features of vector beams (10.1364/OL.405852).

 
Material processing with an intense laser field
    (metastable nano-particle, mechanical atomic bond formation)

  • Intense laser field and materials

An intense laser field beyond 1013 W/cm2 can be produced by focusing a femtosecond laser pulse. Irradiation of this intense field to materials produces high-temperature and high-density plasma because of multi-photon absorption, avalanche ionization or tunneling ionization. Light field around 1017 W/cm2 is comparable with the Coulomb field inside hydrogen atom. We are exploring this intense laser field for the synthesis of metastable alloy nano-particles and mechanical atomic bond formation by laser shock wave.

  • Solid solution alloy nanoparticles (High entropy alloy nanoparticles of Au-Pt-Rh-Pd-Ir system)


Femtosecond laser irradiation of aqueous solutions of chloroauric acid or chloroplatinic acid induces the formation of gold or platinum nanoparticles due to the reduction of metal ions by solvated electrons and hydrogen radicals generated by photo-dissociation of water molecule. While bulk alloy of gold and platinum is phase separating, nanoparticles sybthesized by the irradiation of mixed aqueous solution of gold and platinum metal ions are solid solution (10.14356/kona.2022002). Formation of the metastable solid solution nanoparticles was observed even for high entropy alloy nanoparticles of Au-Pt-Rh-Pd-Ir system.

  • Structural phase transition of materials and synthesis of organic molecules by an intense laser field

Intense laser irradiation of materials are predicted to produce an ultra-high pressure up to 1 TPa (107 atm) due to laser-induced shock wave induced by rapid expansion of plasma in the material.  Because this ultra-high pressure propagates in one direction, anisotropic phase transition of materials and simple atomic bond formation between molecules are expected (10.1002/cphc.202000563). We are challenging phase transition of carbon particles and mechanicla synthesis of hydrocarbon molecules.
Electron beam control by optical beam
  • Phase front transfer from an optical beam to an electron beam


Development of base technology to control an electron beam by an optical beam is in progress. As a first step, a phase front transfer technique from optical wave to electron wave intermediated by materials is being developed. For example, a hologram was formed by the interference processing of ultrathin film with an optical plane wave and an optical vortex beam. The processed thin film can work as a phase hologram for the generation of a vortex electron beam. In this process, a free-standing thin film with the thickness of 10 nm was processed by a single laser shot (10.1364/OE.400941). This process is high-speed and low-cost compared with focused ion beam (FIB) technique. Because light has much lower kinetic momentum, the process damage can be dramatically reduced.