Random Laser

You may ask, "I know what a laser is, but what is a random laser?" A normal laser consists of a total-reflection mirror and a translucent mirror facing each other, with a light-amplifying medium (gas, liquid, or solid) between them. When energy is applied to this amplifying medium, light is generated, and as it travels back and forth between the mirrors, the light is amplified and only light of a specific wavelength is selected and emitted from the translucent mirror. As a result, the laser emits a nicely corrugated beam of light.

In contrast, a random laser does not use mirrors, but instead uses a medium with an irregularly distributed refractive index. For example, a fine powder or a spongy medium containing many small bubbles. To these are added dyes or other light-amplifying substances, and energy is applied externally. The generated light is then scattered irregularly and amplified by repeatedly hitting the powder or bubbles inside the medium. Unlike a normal laser, various optical paths are created, some of which are highly amplified, and the laser begins to oscillate. Thus, instead of obtaining large optical gain by moving back and forth between mirrors numerous times, a random laser obtains large optical amplification by zigzagging through an irregular medium.

This laser does not emit light with as well-defined a waveform as a normal laser, but it emits a "cleaner" light than a fluorescent lamp or LED. On the other hand, the emitted light is not a beam like a laser, but goes out in various directions like normal lighting.

Although research on random lasers has only just begun and applications are few, their narrow spectrum, ability to emit high-brightness light at a wide angle, and the fact that they do not emit noise like lasers when used as illumination light are all potential applications for high-brightness lighting, displays, optical measurement, and other applications.

We are exploring structures to control the path length distribution of light by designing the spatial distribution of scattering particles to achieve laser oscillation at a lower threshold. Super-diffusive and fractal structures are candidates. We are also investigating the use of optical tweezers to dynamically control the distribution of particles in order to achieve the desired properties.

Light Scattering from Skin and Particulate Media

In recent years, much research has been devoted to studying the properties of light scattered by the skin. Skin has a complex surface topography and internal structure, and it is not well understood which elements affect the behavior of light and how. Therefore, we are conducting research to analyze the reflection and transmission characteristics of skin in detail using computational methods such as Monte Carlo simulations. In addition, various powders such as nanoparticles are increasingly being used in cosmetics and other applications that are applied to the skin. However, the structure of newly developed fine particles is complex, and their optical properties cannot be determined using conventional theoretical formulas. Therefore, we are conducting research to determine their light scattering properties using the finite-difference time-domain (FDTD) method. The results will be used to develop new powder particles.

Object Identification Using Laser Speckle (Past study)

When a laser beam is irradiated onto a credit card or paper, the light is scattered on the surface and inside the card or paper before being emitted. As a result, the light irregularly interferes with each other, producing a random pattern called a speckle pattern. This speckle pattern is unique to the irradiated object and can be called a "fingerprint of light." To produce the same speckle pattern, the microscopic surface shape and refractive index distribution inside the object must be exactly the same, making counterfeiting virtually impossible. Therefore, speckles can be used as an authentication key to provide strong security for confidential documents. We are conducting research on object identification using the temporal intensity fluctuation of speckles as a key.

Light Amplification in Quasi-Periodically Layered Media (Past study)

A type of mirror that reflects light is called a dielectric multilayer mirror. It consists of alternating layers of transparent films with sub-micron thicknesses and different refractive indices, and has a higher reflectivity than a conventional aluminum vapor deposited mirror. However, this high reflectance only occurs for light of a specific wavelength determined by the angle of incidence. In the past, the basic structure was a periodic structure with two different film thickness distributions, but in this laboratory, we are analyzing the optical properties of multilayers with fractal structures and other structures that differ from this structure. We are investigating whether mirrors with high reflectivity can be obtained over a wider range of angles and wavelengths. Conversely, we are investigating structures that can produce optical filters and amplifiers with very narrow reflectance and transmittance spectra. This structure could be applied to lasers that emit a "cleaner" light with a better waveform.

Optical Measurement System for Multiple Scattering Materials (Past study)

When a laser beam is irradiated onto paper, clothing, a mass of fine powder particles, such as potato starch, a highly concentrated emulsion or suspension, such as milk, or living tissue, the light is scattered many times inside the media before it is emitted. This phenomenon is called multiple scattering of light. Laser light is coherent light, so even objects with such random structures should be able to be analyzed while retaining their internal information. By successfully extracting this information, we can gain insight into the properties of complex objects.

Micro Optical Elements Using Photopolymer (Past study)

Photo-curable resins are resins that change from liquid to solid when exposed to ultraviolet or visible light, and have recently been used for 3D modeling and adhesives. These resins have an interesting property that the surface of the solid after curing becomes uneven when the irradiating light is given a two-dimensional intensity distribution. We have studied the fabrication of microlens arrays (two-dimensional arrays of microlenses), diffraction gratings, and light diffusers using this property. Visible-light curable adhesives, which are safer and more readily available, are used as resins.

Optical Measurement System for Multiple Scattering Materials (Past study)

When a laser beam is irradiated onto a mass of fine powder particles, such as potato starch, a highly concentrated emulsion or suspension, such as milk, paper, clothing, or living tissue, the light is scattered multiple times inside the particles before being emitted. This phenomenon is called multiple scattering of light. Laser light is coherent light, so even objects with such random structures should come out while retaining their internal information. By successfully extracting this information, we can learn about the properties of complex objects.