Development of three-dimensional dielectric bulk metamaterials for controlling THz waves - Isotropic dispersion of fine silicon cubes-
Overview
We have developed a dielectric three-dimensional bulk metamaterial (Si
microparticle dispersion) that realizes a refractive index in the terahertz
(THz) range that does not exist in nature by dispersing fine Si cubes three-dimensionally
at random. This Si microparticle dispersion has minimal polarization dependency
and the effective refractive index can be designed according to the Si
concentration, so it is expected to be a new refractive index material
option for optical systems using THz waves.
Performance and Features
 Si microparticle dispersionl |
 Application of Si micro particle dispersion |
| The structure involves Si microparticles being uniformly dispersed in a
transparent resin with random orientations in three dimensions, resulting
in isotropic optical properties. An effective refractive index in the range of 1.53–1.96 was achieved in the frequency range of 0.1 to 0.2 THz, which includes the D-band of the 6G communication band. This effective refractive index can be freely controlled by the Si concentration in the Si microparticle dispersion. |
Future Prospects
It is expected to be applied to a wide range of fields, including 6G communication
technology, medicine, biotechnology, agriculture, food, the environment,
and security. As an isotropic optical material, it is expected to realize
optical elements with high design freedom.
Publication Details
Optics and Laser Technology, Vol. 181, Part C, (2024) 112051.
Metamaterials with Improved Refractive Index Characteristics for 6G - Isotropic dispersion of fine double-layered sprit-ring-resonators -
Overview
In March 2022, we developed a three-dimensional (3D) bulk metamaterial,
a new material that is easy to process and has a wide range of refractive
index characteristics. This time, we developed a 3D bulk metamaterial with
improved refractive index characteristics by irregularly arranging double-layered
split-ring-resonators (SRRs) in a 3D manner based on a solid amorphous
structure. This 3D bulk metamaterial can be used to create a prism that
refracts light at different angles depending on the wavelength, realizing
a spectrometer that separates terahertz (THz) waves by wavelength.
Performance and Features
 3D bulk metamaterial |
 Application examples of 3D bulk metamaterials |
| Since the metamaterial unit structures are dispersed three-dimensionally
in the transparent resin without direction dependence, polarization dependence
is eliminated and isotropic optical properties are achieved. At a frequency of around 0.34 THz, the refractive index changes by 2.314/THz. This rate of change in refractive index can be changed by the density of the metamaterial grain, making it possible to control the optical properties of the 3D bulk metamaterial. |
Future Prospects
It is expected that this material will be used for wavelength extraction
and spectrum analysis in various THz wave application fields, including
6G communications. Its isotropic optical properties are expected to lead
to the realization of highly versatile lenses and prisms.
Publication Details
Advanced Science, Vol.11, No. 34 (2024) 2405378.
Technology to calibrate the wavelength of tunable Fabry-Perot filters
Overview
The realization of an ultra-compact hyperspectral imager using a tunable
Fabry-Perot filter (FPF) with a variable air gap is expected. Conventionally,
it has been difficult to accurately measure the spacing of the variable
air gap. In this research, we developed a technology to calibrate the wavelength
of a tunable FPF by measuring the interval of a variable air gap by combining
a thickness-gradient wavelength selection filter and an image sensor.
Performance and Features
 Concept of optical principles |
 Relationship between the thickness of the transparent medium and the transmitted
wavelength |
 |
| There is a variable air gap between the SiO₂ upper substrate with an Ag
alloy film and the SiO₂ lower substrate with an Ag alloy film on the top
surface and a gradient-thickness optical filter on part of the bottom surface.
Most of the light that enters from the top passes only through the tunable
FPF to select the wavelength and reaches the image sensor, but some also
passes through the gradient optical filter. In this range, light reaches
the image sensor only at the position where the transmission bands of the
FPF and the gradient-thickness optical filter overlap. When the intensity
of the light transmitted through both filters reaches its maximum value,
the transmission peak wavelengths of both filters match, and the size of
the air gap of the FPF is detected from the pixel position of the peak
signal intensity of the image sensor. |
Future Prospects
This technology makes it easier to miniaturize spectral imagers and spectrometers,
allowing them to be used in portable devices. Other applications include
optical acceleration sensors and narrow gap sensors.
Publication Details
AIP Advances, Vol.14, Issue 7,(2024) 075006.
Frequency tunable filter for 6G communications - Expectations for use in various industrial fields that use terahertz waves -
Overview
6G communication is expected to use radio waves in a frequency band around
0.3 THz, which requires a filter that removes radio waves with unnecessary
frequencies and passes radio waves with specific frequencies. Our laboratory
has successfully developed a new tunable terahertz (THz) wave control technology
for 6G by realizing a frequency tunable filter that incorporates a mechanically
variable refractive index metamaterial inside a Fabry-Perot resonator.
Performance and Features
 Cross-sectional schematic diagram of the developed tunable filter |
 Mechanically variable refractive index metamaterial |
 Tuning of refractive index (RID) and frequency (Freq) by periodic control |
| Radio waves incident on the frequency tunable filter remove radio waves
with unnecessary frequencies and only transmit radio waves with necessary
frequencies. The transmission frequency is tuned by mechanically deforming
a mechanically variable refractive index metamaterial equipped with an
expansion and contraction mechanism. The peak frequency of the metamaterial
can be controlled in the range of 0.303 to 0.320 THz according to the periodic
change of 100 to 150 μm. |
Future Prospects
It is expected to be applied to scanning and imaging using THz waves, and
is expected to be used not only in 6G communications but also in a wide
range of fields such as medicine, biotechnology, agriculture, food, environment,
and security.
Publication Details
Optics Letters, Vol. 49, Issue 4, (2024) pp. 951-954.
Radio wave polarization control technologies for "6G" - Wide-angle control of THz wave propagation direction using novel metamaterial -
Overview
Terahertz (THz) waves, which have been specified for use in the next generation
of 5G mobile communication systems, ‘’6G’’, are easily blocked by obstacles,
and there is a need for technology to control their direction of travel
and deliver signals to shielded areas. We have developed a transmission-type
deflector that can control the propagation direction of THz waves over
a wide range of angles, and achieved the wide-angle deflection scan of
74°in the frequency band used in 6G communications (0.3 to 0.5 THz).
Performance and Features
 Image of the use of the developed transmission deflector. |
 Top view of the fabricated transmission metamaterial. |
 Deflection angle characteristics (measured values) |
| A THz wave deflector based on effective refractive index distribution control
using a transmissive metamaterial consisting of a subwavelength structure
made of low-loss dielectric silicon. We achieved deflection scanning of
34 to 74 degrees in the 0.3 to 0.5 THz frequency band, and succeeded in
realizing wide-angle scanning of terahertz waves with high power efficiency. |
Future Prospects
By using this technology in combination with the reflective deflectors
that have been developed so far, it is expected to reduce the radio wave
interference area in 6G communications.
Publication Details
Optics Express, Vol. 31, No. 17, (2023) pp. 27147-27160.
A Transparent window that has both heat shielding effect and transmission of 5G/6G radio waves
Overview
Conventional heat-shielding glass blocks radio waves in the 5G/6G communication
band, which has an impact on wireless smartphone and computer usage. In
our laboratory, we have developed an aluminum heat-shielding metamaterial
consisting of a nano-periodic structure, and by forming it on the surface
of windows such as glass, it reflects near-infrared wavelengths that generate
heat, but does not interfere with the 5G/6G communication band. We have
developed a transparent base material for heat-shielding windows that transmits
radio waves and visible wavelengths.
Performance and Features
 Usage image of the developed transparent heat-shielding window |
 Atomic force micrograph of unit structure of heat-shielding metamaterial |
 Landscape photo taken through a heat shielding metamaterial |
| ・Blocks more than 80% of near-infrared electromagnetic waves (approximately
273 THz) that increase atmospheric temperature ・Transmits visible light and radio waves in the 5G communication band (28 GHz band)/6G communication band (0.2 to 0.3 THz). |
Future Prospects
Application to building materials and heat-shielding windows for automobiles
is expected to solve social issues such as the onset of heatstroke indoors
and in cars and tight electricity supply and demand during the summer.
Publication Details
Applied Optics, Vol.62, No.28, (2023) pp. 7411-7419.
Mass production of radio wave control materials for 6G - World's first development of 3D bulk metamaterial to supply as components -
Overview
In our laboratory, we are developing a manufacturing technology providing
inexpensive and large quantities of three-dimensional bulk metamaterials
that can potentially be formed into any shape and have any refractive index
in order to control terahertz (THz) waves. Since the newly developed metamaterial
can be supplied as a solid grain material, customers can freely perform
secondary processing of the metamaterial to create THz optical elements
through precision machining such as molding and cutting.
Performance and Features
 Conceptual diagram of 3D bulk metamaterials. |
 Metamaterial-encapsulated grain |
 3D bulk metamaterials (a) External view (b) Enlarged view |
| By stirring resin grains containing metamaterial structures into liquid
resin and solidifying them using a mold, it is possible to create an 3D
bulk metamaterial with an arbitrary shape and refractive index characteristics
according to the design of the metamaterial. Since it has a structure in
which metamaterial unit structures are dispersed three-dimensionally without
direction dependence in a transparent resin, polarization dependence is
eliminated and isotropic optical properties are realized. |
Future Prospects
It is expected to be applied in a wide range of fields, including 6G communication
technology, medicine, biology, agriculture, food, environment, and security.
Publication Details
Nanophotonics,vol11,No.9, (2022) pp 2065–2074.
Advanced control technology for THz waves for 6G - A metamaterial that changes the transparency and phase of THz waves -
Overview
Many tunable filters for terahertz (THz) waves are large and expensive, and the challenge was to create a tunable filter that is small, inexpensive, and highly capable of controlling THz waves. In this research, we developed a technology to freely control the electromagnetically induced transparency phenomenon of metamaterials using micromechanical devices, and realized a tunable filter that can control the transmittance and phase of THz waves using voltage.
Performance and Features
 Schematic diagram of the developed tunable filter |
 The fabricated metamaterial unit cell |
| Operating principle When voltage is applied to the MEMS actuator, the metal rod on the movable beam moves due to electrostatic attraction and approaches the parallel metal rod on the fixed beam, causing a significant change in the transmittance and phase of the THz wave near the resonance frequency. Performance For THz waves with a frequency of 1.832 THz, transmittance can be controlled in a modulation range of 38.8% and phase can be controlled in a range of 25.3 to 47.8° depending on the applied voltage. |
Future Prospects
Combined with electronic circuits and semiconductors, advanced control of THz waves becomes possible. It is expected to be applied in a wide range of fields, including 6G, the next generation communication technology.
Publication Details
Scientific Reports,Vol.10, (2020) 20807.
Precise control of emission wavelength using nano-antennas - Possible with low-loss optical metamaterials -
Overview
Quantum dots have attracted attention as a new light-emitting substance, and there has been a strong desire to establish precise light emission control technology for practical use as a light source. In recent years, metamaterials have attracted attention as new artificial optical materials that can achieve optical properties not found in natural materials, but they suffer from a problem of high optical loss. Our laboratory has successfully developed a nanoantenna that enables precise control of the emission wavelength by combining a relatively low-loss metamaterial with quantum dots.
Performance and Features
 ADB metamaterial |
 Quantum dots placed on ADB metamaterial |
 The fabricated ADB metamaterial |
| Quantum dots are dispersed in a thin polymer film and placed on an Asymmetric
Double Bar (ADB) metamaterial, which is made up of two parallel gold bars
of different lengths. Optical properties can be controlled by the shape
of the ADB metamaterial. This time, by combining an ADB metamaterial whose
dimensions are controlled with an accuracy of 10 nm with quantum dots with
an emission center wavelength of 1366 nm, we succeeded in precisely controlling
the emission center wavelength in the range of 1350 to 1376 nm. |
Future Prospects
It is expected that this technology will become an important technology for applications such as displays, single photon sources for quantum information communication, and biochemical biomarkers, as well as for the practical application of negative refractive index, cloaking, and complete lenses in the optical region.
Publication Details
Scientific Reports, Vol.6, (2016) 33208
Nano-grating anti-reflection surface formed on the tip of optical fiber - Cheaply produced using nanoimprint technology-
Overview
An anti-reflection coating is formed on the tip of an optical fiber used in optical communications to reduce connection loss with optical components. Conventional anti-reflection coatings require high costs because they are applied using vacuum equipment. Our laboratory has developed a dedicated nanoimprinting device and has realized a technology to inexpensively fabricate an antireflection structure with many microscopic depressions at the tip of an optical fiber. This technology offers greater design freedom than traditional anti-reflection coatings, allowing the creation of a variety of anti-reflection surfaces to suit requirements.
Performance and Features
 Nano-lattice anti-reflection structure fabricated in the core of the optical
fiber tip |
| Nanoimprint technology is a technology in which a fine pattern is transferred
by pressing an original onto a polymer coated on a substrate, and because
it does not require expensive exposure equipment or vacuum equipment, nanostructures
can be manufactured at low cost. Using this technology, we succeeded in
reducing reflection loss by forming microscopic conical depressions at
270 nm intervals in the core of the optical fiber tip. The reflectance
and wavelength band are determined by the shape, spacing, and height of
the depressions. |
Future Prospects
It is expected to be applied to optical communication systems, optical sensors, optical wiring for robots, optical wiring for automobiles, optical fiber probes for biotechnology, optical fiber energy transmission, optical fiber lighting systems, laser processing, etc.
Publication Details
Optics Express, Vol. 21,Issue 1, (2013) pp. 322-328.
Biomimetics: structural colors and moth eye structures
The color of the peacock's bright feathers is due to structural color generated by nanolattices. The structural color filters enable a variety of colors. Subwavelength periodic structures are formed on Moth-eyes. By mimicking this structures, reflectance of silicon has been reduced by a factor of 100, and the light extraction efficiency of light-emitting diodes has been improved by about 60%. We have also developed inexpensive manufacturing technology using nanoimprint.
 Fig. Anti-reflection structures (Moth-eye structures). |
 Fig. Structural color filters. |
Metamaterials: toward innovative optical control
Metamaterial is a structural artificial optical material, and its unique electromagnetic mode depends on the structure. Therefore, metamaterials enable innovative optical function and electromagnetic wave control on demand. We have developed electromagnetically induced transparency metamaterials, novel metamaterials having Fano resonance, metamaterial absorbers, metamaterial sensors with integrated photodiodes, and mechanical reconfigurable metamaterials.
 Fig. Electromagnetically induced transparency metamaterials. |
Active control: movable nano-photonic devices
Photonic crystals have fine periodic structures in the order of wavelength of light and function to block and confine light. We have developed a wavelength selective filter that controls the optical coupling efficiency of photonic-crystal’s nano-resonator and a variable reflectance filter that controls the light blocking characteristics by controlling the position of photonic crystals with high accuracy using micro-actuators.
 Fig. Switching add/drop filter. |
 Fig. Variable reflectance filter. |
Development of novel surface smoothing technologies
Dry-etched silicon surfaces are rough, causing device characteristics to deteriorate and optical loss. We have developed novel surface smoothing technologies that control ultra-precise surface deformation caused by self-diffusion of silicon surface atoms in a high-temperature hydrogen atmosphere. We realize ultra-low-loss nano/micro optical devices and silicon micromechanical parts with excellent mechanical strength.
The equipment developed based on this technology has been commercialized
as a minimal laser hydrogen annealing equipment through joint development
with Kanamori Laboratory, Sakaguchi E.H VOC Corp., and AIST.
Related site: http://sakaguchi-dennetsu.co.jp/lineup/exlaser/exlaser_minimal.html