Research

This is an introduction to the research conducted by the Iwami Laboratory.

The Iwami Laboratory is developing next-generation devices using light control via metasurfaces.

A metasurface is a two-dimensional metamaterial that controls the phase of transmitted or reflected light by arranging subwavelength structures in a planar array. As it can reproduce refractive index distributions not found in natural materials, it enables the creation of devices that are thinner and more multifunctional than conventional optical components. The Iwami Laboratory is engaged in the development of new applications for metasurfaces, such as metalenses and holography, as well as efforts to improve their performance.

Electron microscope images of metasurface produced by the Iwami Laboratory

Development of full-color
metasurface holograms using wavelength multiplexing

Holography (Note 1), a technology that records and reproduces the wavefront of light, is attracting attention as the ultimate three-dimensional display, as it allows three-dimensional images to be viewed with the naked eye. Holography projects images by utilising the interference (Note 2) of light passing through each pixel, and the resolution of the projected image and the viewing angle are largely determined by the pixel density of the hologram. Consequently, to project high-resolution images viewable from a wide range of angles, ultra-high-density display devices with pixel spacing of 1 μm or less are required. Consequently, extensive research is being conducted into ‘metasurface holograms’, which aim to project high-quality moving images using metasurfaces—structures in which meta-atoms, unit structures smaller than the wavelength of light, are arranged at extremely high densities (of the order of several hundred nanometres).

One of the major challenges facing metasurface holograms is the difficulty of achieving full-colour imaging. To produce a full-colour projected image, it is necessary to control at least three colours of light (blue, green and red). Previous research has employed methods such as ‘fabricating separate holograms for each colour and superimposing the projected images’ or ‘setting different polarisation states (Note 3) for each colour of light and projecting them’. However, these methods suffered from issues such as structural bloat and reduced projection efficiency. Therefore, in this study, we aimed to achieve ‘full-colour projection using a single metasurface without restricting the polarisation state of the incident light’ (Fig. 1).

To achieve full-colour projection as shown in Figure 1, it was necessary to ‘identify a structure capable of independently controlling light of each colour’ and to ‘eliminate the unwanted projected images (crosstalk images) caused by structural errors’. In this study, we achieved these objectives through the analysis of over 20,000 types of meta-atoms and the examination of equations governing light propagation, and we successfully projected full-colour video (Fig. 2).

Fig. 1: Left: Overview of the projection method used in this study; Right: A section of the resulting projected image (※)

The actual footage has been released(Supplementary Material Visualization 1, Visualization 2)。

Note 1) Holography
A technology capable of recording and reproducing the wavefront of light. It allows three-dimensional images to be viewed without the need for special glasses and is often referred to as the ultimate three-dimensional display. A medium on which the wavefront of light is recorded is called a hologram.

Note 2) Interference
A phenomenon in which multiple light waves influence one another, generating new light. Holography intentionally induces this interference to generate light that allows three-dimensional images to be observed.

Note 3) Polarisation state
One of the three fundamental properties of light, referring to the direction of oscillation of the light wave. States such as linear polarisation and circular polarisation exist, and they possess the important property that ‘orthogonal polarisation states can be controlled independently of one another’.

Multifunctional metasurfaces
for the miniaturisation of gas cells for atomic clocks

An atomic clock is an oscillator stabilised using the natural transition frequency of an atom, producing a stable and accurate frequency. For devices to communicate with one another, they must share the same time, and atomic clocks serve as the standard for generating this reference time. In recent years, efforts have been underway to miniaturise atomic clocks and integrate them into devices such as smartphones, with the aim of establishing a stable time synchronisation system.An atomic clock is an oscillator stabilised using the natural transition frequency of an atom, producing a stable and accurate frequency. For devices to communicate with one another, they must share the same time, and atomic clocks serve as the standard for generating this reference time. In recent years, efforts have been underway to miniaturise atomic clocks and integrate them into devices such as smartphones, with the aim of establishing a stable time synchronisation system.

A major obstacle to the miniaturisation of atomic clocks lies in the size of the gas cell, the component that contains the gas. In this study, we propose a reflective gas cell incorporating a metasurface (Fig. 2, left). The metasurface combines the functions of a prism (which bends light), a lens (which collimates the beam) and a quarter-wave plate (which converts the polarisation state), thereby reducing the thickness of the optical elements compared to conventional designs. The fabricated gas cell has a height of just 1.05 mm (Fig. 2, right), making this a technology that will contribute significantly to the miniaturisation of atomic clock packages.

Fig. 2: Left: Overview of the metasurface and gas cell; Right: The fabricated gas cell

Top-hat beam shaper
Metasurface

Indocyanine Green (ICG) fluorescence imaging is used to detect tumours within the abdominal cavity. ICG is a near-infrared fluorescent dye; this technique utilises its property whereby, when retained in the abdominal cavity and irradiated with infrared light of a certain intensity or higher, it emits stronger fluorescence at tumour sites than in healthy tissue. Generally, it is considered difficult to diagnose the presence of residual tumours in the vicinity of areas that have undergone radiotherapy due to tissue inflammation and other factors. Furthermore, coaxial endoscopic light sources capable of ICG fluorescence imaging in confined areas typically have a Gaussian intensity distribution centred on the centre, making it difficult to clearly distinguish significant differences in fluorescence intensity emitted from the tumour site. Consequently, the top-hat beam has attracted attention. This is a beam with a wide and uniform intensity distribution, enabling the high-precision detection of tumours at any position within the irradiation area (Figure 4, left). Therefore, in this study, we will fabricate a metasurface that converts a Gaussian beam into a top-hat beam (Fig. 4, right).

Fig. 4: Left: Gaussian beam and top-hat beam; Right: Overview of the metasurface developed in this study

Fig 5 shows the evaluation results for the metasurface. The left-hand side of Fig 5 shows the light intensity distribution after shaping, as captured by a beam profiler. We succeeded in obtaining a highly efficient and uniform top-hat-shaped light intensity distribution. Furthermore, we confirmed that characters could be distinguished using the fluorescent dye ICG, which is used in applications such as pathological diagnosis (right-hand side of Fig 5).

図5　左：ビーム整形後の光強度分布、右：ICG蛍光観察像

Fabrication of Polarisation-Splitting Metalenes and the Influence of Their Shape

In recent years, there has been a growing demand for the acquisition of sophisticated optical data in fields such as autonomous driving and facial recognition systems. One example of such technology utilises the differences in reflectance between different polarised light states. Polarisation refers to the state in which light vibrates regularly in a specific direction. There are various types of polarisation; for example, there is linear polarisation parallel to the x-axis (x-polarisation) and, similarly, y-polarisation. Polarised imaging, an imaging technique that utilises this polarisation, relies on the differences between images captured in multiple polarised components and has been used to visualise information that is invisible to the naked eye. Sensing technologies, such as vehicle detection, are examples of this.

In this study, we therefore fabricated and evaluated a metamaterial lens for use in polarised imaging (Fig. 6, left). A metamaterial lens is a type of metasurface that possesses light-focusing capabilities. Metasurfaces can control the polarisation state of emitted light by utilising the birefringence resulting from the structure of the metamaterial atoms. Consequently, a metamaterial lens can focus and form an image of the polarisation state at a different location. Unlike conventional polarising plates, they offer the advantage of improved light utilisation efficiency, as they separate rather than absorb orthogonal polarisation components. Furthermore, high-performance image sensors manufactured using advanced processes can be used as they are by replacing existing lenses with metamagnetic lenses. We actually fabricated several samples and evaluated their optical performance. We also successfully investigated the influence of the shape of the fabricated polarisation-separating metamagnetic lenses on optical performance (Fig. 6, right).

Fig. 5: Left: Schematic diagram of a polarisation-splitting metamaterial lens; Right: Microscopic photograph of the fabricated metamaterial lens

A polarisation-splitting metamaterial lens suitable for use in the far-infrared region

Although far-infrared radiation is used for applications such as temperature detection, the use of germanium lenses in refractive optics results in high costs. Furthermore, in the case of microbolometer sensors, their MEMS structure presents challenges such as the difficulty of polarisation separation.
In this study, we fabricated a linear polarisation-separating metamaterial lens using silicon for the far-infrared region. When images were captured using a commercially available camera equipped with a microbolometer sensor, the image disappeared for orthogonal polarisation, indicating that we have successfully fabricated the polarisation-separating metamaterial lens.