■Ultra-high-density spatial light modulator

  We are engaged in research on electro-holography that shows natural three-dimensional (3D) motion images. Displaying 3D images in a wide viewing zone requires the development of a spatial light modulator (SLM) having a very small pixel pitch.
  We are researching a magneto-optical (MO) SLM driven by spin transfer switching (spin-SLM) that uses magnetic materials as pixels. The spin-SLM can modulate light by using the MO effect, in which the polarization plane of reflected light rotates according to the magnetization direction of the magnetic materials. We previously prototyped a device that can switch the magnetization direction by magnetic domain wall motion induced by pulse currents applied to the magnetic materials (a light modulation device driven by current-induced domain wall motion) and successfully verified its basic operating principle.
  In FY 2018, we optimized the composition of the light modulation layer made of a gadolinium-iron alloy, and succeeded in driving at a low-current of 0.8 mA. The current can be supplied by a cell-selection micro transistor with a size of 1 μm×2 μm, which size is required for wide viewing 3D holographic displays. Using this composition, we prototyped an element with a size of 0.5 μm×2 μm and evaluated its light modulation operation (Figure 1-13). Each end of the light modulation region has a nano magnet having an antiparallel configuration of magnetization direction (NM1, NM2), through which currents flow into the light modulation region. An initial domain is formed at an end of the light modulation region by local magnetic fields from the nano magnet. We confirmed that applying a current from right (left) to left (right) in this state expanded the area of the initial magnetic domain to the entire area of the light modulation region by domain wall motion, which turned on (white) or off (black) the reflected light. This successfully demonstrated the light modulation operation using a micro device⁽¹⁾.

■Elemental technologies for optical phased array

  For a future integral 3D display with much higher performance than current displays, we are conducting research on a new beam-steering device that can control the direction of optical beams from each pixel at a high speed without using a lens array. Focusing on an optical phased array (OPA) consisting of multiple optical waveguides (channels) as a beam-steering device, we designed, fabricated and evaluated an OPA using an electro-optic (EO) polymer that can change the refractive index at a high speed on each channel by applying an external voltage.
  The prototype OPA using an EO polymer can flexibly change the direction of output beam by applying a voltage via the channels to change the refractive index of the EO polymer and control the optical phase. We previously designed and prototyped an OPA consisting of eight channels and demonstrated an optical beam deflection of ±3.2 degrees.
  In FY 2018, we developed a technology for tightly confining light in a waveguide by using a material having a large refractive index difference for an optical waveguide, which reduced the crosstalk between channels. This technology made it possible to narrow the beam output channel waveguide pitch of an OPA to 4 μm and achieved an optical beam deflection of 22.1 degrees. We also made an optimum layout design of the device, which decreased light propagation loss at the curve and bifurcation of an optical waveguide and significantly reduced unnecessary stray light components of far-field beam patterns. This increased the peak value of beam intensity of the prototype OPA and also succeeded in optical beam scanning at a high speed of 200 kHz⁽²⁾ (Figure 1-14).

 

[References]
(1)	R. Higashida, N. Funabashi, K. Aoshima, K. Machida: "W-spin domain wall type optical modulation device for electronic holography," 2018 ITE Annual Convention Program, 32C-3 (2018) (in Japanese)
(2)	Y. Hirano, Y. Miyamoto, Y. Motoyama, K. Machida, K. Tanaka, T. Yamada, A. Otomo and H. Kikuchi: "Beam Deflection of the Optical Phased Array using Electro-Optic Polymer Waveguide Arrays of 4 μm pitch," The journal of the ITE Vol. 73, No. 2, pp. 392-396 (2019) (in Japanese)