MIT Researchers Explore New Mechanism to Modify Diffusivity of Optical Devices

A team of researchers from the Media Lab of MIT and Harvard University are looking at a novel mechanism that can alter the diffusivity of an optical device or the extent to which it disperses light by immersing particles of glass in a fluid.

A mild temperature change radically alters the degree to which a solid-fluid mixture bends light. (Photo Credit - Courtesy of the researchers)

The novel diffuser in its present form can be used for calibrating a number of imaging systems. The scientists feel that their mechanism could pave the way for tunable optical devices with applications in sensing, imaging, and photography, or holographic video screens.

During the experimental phase, the solid-liquid mixture illustrated plenty of striking changes in diffusivity than current theory would have estimated, so the researchers also created a new computer model to illustrate it. That model could assist them to devise highly complex applications for the basic technology.

The details of the new study have been published in the latest issue of the American Chemical Society’s ACS Photonics journal.

The glass and the fluid in the prototype were selected as they have very similar refractive indices, which mean that light travels via them at similar speeds. When light travels from a material with a high refractive index to one with a lesser refractive index, it alters direction; this is the incidence behind the well-known illusion of a straw appearing to bend when it is placed in a glass of water.

The prototype created by the team exploits the fact that alterations in temperature change the refractive indices of the materials.

It’s hard to find a solid and liquid that have exactly the same refractive index at room temperature. But if the speed at which the refractive index changes for solid and liquid is different — which is the case for most solids and liquids — then at a certain temperature they will exactly match, to the last digit. That’s why you see this giant jump in transparency.

Barmak Heshmat, a postdoc in the Media Lab’s Camera Culture group and corresponding author on the paper

Heshmat is assisted on the paper by Ramesh Raskar, the NEC Career Development Associate Professor of Media Arts and Sciences and head of the Camera Culture group, and Benedikt Groever, a graduate student in engineering and applied science at Harvard.

Study in contrast

The experiments revealed to the researchers that a temperature change of 10° would enhance the diffusivity of their device tenfold, and an alteration of 42° modified it a thousandfold.

Heshmat feels that a temperature-modulated adaptation of the filter built by his team could be utilized to calibrate sensors applied in medical imaging, research of material flows, and analysis of cells. For example, medical-imaging systems are mostly calibrated using devices termed as “tissue phantoms,” which copy the optical properties of various types of biological tissues. Tissue phantoms can be costly, and several of them may be needed for calibration of a single imaging device. Heshmat believes that a cheap version of the team’s filter could imitate a number of tissues.

However, the basic principle shown by the prototype created by the researchers could have wider impact.

The heat effect on the refractive index of the solid or the fluid, handled in isolation, is very mild. However, when the two are blended together, the diffusivity effect is remarkable.

Heshmat debates that the same would be accurate of other types of trial materials whose refractive indices alter as a reaction to either an electric field or light. Also, electrical or optical activation would extend the application range for tunable optical devices.

If you have photorefractive changes in a solid material in a solid phase, the amount of change you can get between the solid and itself is very small. You need a very strong field to see that change in your refractive index. But if you have two types of media, the refractive index of the solid is going to change much faster compared to the liquid. So you get this deep contrast that can help a lot.

Barmak Heshmat

Application

In holographic displays, cells containing a mixture of fluid and electrically responsive solid materials could transform their diffusivity when they are charged by an electrode, which is very much similar to those cells containing ionized gas and being capable of changing their color in plasma TVs. Light can thus be steered by the adjacent cells in directions that are slightly different, and these cells also imitate the reflection of light off of a contoured surface and develop the illusion of three-dimensionality.

Tunable diffraction gratings can be developed by using liquid-solid mixtures. These gratings are used in a few sensing applications in order to filter out light or various other electromagnetic radiations of specific frequencies. The gratings can also be used in tunable light diffusers belonging to the sort photographers developing the strongly directional light obtained from a flash into a more ambient light.

The diffusivity of a liquid-solid mixture is predicted by the computer model that is described in the paper presented by the researchers. This prediction is made based on the physical characteristics of the solid particles, (how spiky or jagged they are), and also on their level of concentration in the liquid. Heshmat feels that this model can be used to produce solid particles customized for particular applications.

It is possible that this method may even cross the realm of engineering and science.

I understand the obvious potential scientific applications listed in the abstract. But I think this kind of approach could potentially be useful for designing new artwork — for interior design, for example. You can design furniture parts or artwork that will change the light-matter interaction and visual perception on demand or through a programmed interface, which would bring dynamic light effects indoors. Similarly, it can be used in architectural designs to replace curtains by structured interfaces.

Aydogan Ozcan, a professor of electrical engineering at the University of California at Los Angeles

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