CLEO/Europe-EQEC 2017 Mika Fuchida

CD-7.4 Piezo-Tunable Second-Harmonic-Generation in a Whispering-Gallery Resonator (Christoph S. Werner, Freiburg Uni.

In this research, a hybrid resonator of WGM with a piezo element on its axis was fabricated, and the resonance wavelength can be tuned by controlling the radius. The resonator was fabricated by first drilling a hole of several mm in a LiNbO3 wafer by laser processing, bonding it to a piezo post, then mounting it on a spindle and using the same laser to shape the rim, followed by grinding and polishing to obtain the final shape and surface. The laser is a 150 fs femtosecond laser with a power of 1 W and a center wavelength of 388 nm, which is aligned in 1,2 nm increments (if the wavelength is too short, the surface will melt too much and become uneven, which is not good). The width of the resonator is only 100 µm for a thickness of 250 µm and a diameter of several mm, and a Q value of about 108 is obtained. Applying a voltage to the piezo element physically changes the geometry of the resonator, enabling mode-hop-free tuning of SHG light around 520 nm with a span of 28 GHz (span exceeding that of FSR). This method is superior to temperature-based tuning in that the tuning speed is faster.

CD-8.3 Laser-Active Whispering-Gallery Resonators as Versatile Platform for Optical ThreeWave Mixing (Simon J. Herr, Freiburg Uni.)

This work, presented in the oral session of Nonlinearities in resonant structures (CD-7), is the first time that laser oscillation and second-order nonlinear processes have been observed simultaneously in a single whispering gallery resonator (WGR). WGM micro optical resonators have attracted attention as a broadband light source because of their high efficiency and frequency conversion. However, conventional methods of generating nonlinear effects require the external coupling of a laser with a narrow linewidth and tunable wavelength, which is a significant technical requirement for real-world applications. In the method demonstrated in this study, an inexpensive CW laser (wavelength of approximately 820 nm) with a linewidth of several GHz and a non-narrow linewidth is first coupled to the WGR as a pump light. Since the FSR of the resonator is also several GHz, the laser light always couples to a mode with a Q-value of about 105 (several GHz linewidth). This light oscillates as an Nd-doped laser in a narrow linewidth mode (wavelength of about 1080 nm), and second harmonic generation occurs in the Nd doped LiNbO3 resonator (wavelength of about 540 nm). Thus, the pump light necessary to generate second-order nonlinear effects can be provided by a laser oscillating in the same resonator, which greatly simplifies the system and eliminates the need for expensive lasers. Furthermore, an optical parametric process has been confirmed, although no causal relationship has been shown as to whether it is caused by the laser light generated in the resonator, and the realization of a self-pumping broadband light source is expected.

CE-8.6 Second-Harmonic Generation Imaging for Crystal Structure Characterization in IIIV Nanowires (Maria Timofeeva, ETH Zurich.)

This is the last presentation in the session titled "Nonlinear optics," and although it is not directly related to the research theme of Tanabe Lab, it is of interest to me. In a smaller room behind Honda's presentation, Ingo's group presented their research on practical integrated devices at room temperature by growing a layer of KTN, a material that induces higher EO and Kerr effects than LN and other materials, on MgO. The research presented here demonstrates a method for imaging the crystal structure of materials in which SHG occurs, without the need to irreversibly thin the material as in TEM. The sample is a GaAs nanowire, and a pulsed light source of 3.5 mW at 820 nm is applied to the entire sample to generate second harmonic generation, and the intensity distribution of light at 410 nm is captured by an electron image-multiplying CCD camera. (2) tensor is known for each crystal structure. Therefore, by rotating the polarization of the pulsed light source and obtaining the intensity distribution in polar coordinates, it is possible to determine the type of crystal structure at the observation point, and even if it is a mixture, it is possible to determine the ratio of the two. Since this method does not require a vacuum or ultra-low temperature, does not require scanning, and is not destructive, it is expected to be applied to inspect whether an optical element has a pure crystal structure or a periodic heterostructure.