Looking good on paper

17 Jun 2002

Using paper as a substrate for optical structures could provide us with disposable optics and flexible displays. Rob van den Berg reports.

From Opto & Laser Europe May 2001

Applying vacuum coating

One of the largest conferences on optics is not an obvious place for someone from a firm that develops processing equipment for the paper industry. However, physicist Raimo Korhonen from Metso Corporation in Tampere, Finland, gave an invited talk earlier this year to the Photonics West Conference in San Jose, California.

For a few years now, Korhonen has been involved in the search for what he calls "intelligent paper" or "intelligent packaging", finding ways of integrating diffractive optics into products that are paper based. He wants to make use of the processing capabilities that are common in the paper industry for novel applications: sensors in packaging to check the freshness of food, remote product tracking devices to help logistics and even paper-based displays.

Korhonen said: "The use of paper as a substrate has many advantages: it is cheap, disposable and much is known about the optical properties of this wonderful material, or rather a wide variety of materials."

A network of fibres forms the basic structure of paper - typically a few millimetres long and a few tens of micrometres thick - which gives the material its basic strength. The spaces between the fibres can be filled with small particles - clay, talc or chalk - which smooth the surface and cut manufacturing costs. It is these filler or pigment particles that determine to a large extent the paper's optical properties, such as colour, opacity, brightness and gloss.

"The surface structure, however, is also very important, for instance for printing," said Korhonen. "It can be changed by calendering - this involves pressing it between two or more rolls. But it can also be pressed by a roll that has an engraved surface, to create a desired surface topography."

Embossed microtext

At San Jose, Korhonen showed an example of an embossed microtext on paper. He claims that the same process can be applied to make a machine-readable information storage medium - a paper CD-ROM.

"Depending on the information density required, one can use a different quality paper. But for structures of a few tens of micrometers the quality need not even be very high," said Korhonen.

Much finer structures can also be embossed, such as diffractive gratings with periods of 2 to 5 µm and a depth of only 1.8 µm. These patterns were produced with the help of an electron beam and then transferred onto a nickel plate using an electrolytic method. The paper samples were pressed with the plates at pressures that are similar to those applied during calendering and at speeds that can reach several hundreds of meters per minute. That makes all of these paper-based techniques ideal for large-volume applications.

Embossed grating

"A German manufacturer of medical diagnostics is interested in this kind of optical element. It uses these gratings as components in optical biosensors, for instance for the measurement of glucose levels in blood." The exact method of operation of such a sensor is a closely guarded secret, but the grating could be used to detect changes in the optical path length in the presence or absence of bound glucose.

The most obvious paper-processing technique with a large potential for optical applications is vacuum coating. This is already widely applied to making complex optical structures for low-emissivity windows and antireflection coatings.

For Metso Corporation, however, the driving force is in a different area - the needs of product packaging (i.e. making barrier layers for food packages to block the penetration of moisture, oxygen or odours, or for keeping a gas inside). Several metals, like aluminium, and many dielectric materials, such as the white pigment titanium dioxide, can be coated in large-scale, reel-to-reel machines on a plastic or paper substrate.

Vacuum-coated holograms

Korhonen said: "Coating and embossing units may also work together. For instance, a hologram can be embossed on a plastic foil, which is metallized by a high-refractive index dielectric, to produce the hologram effect. These can be used to foil counterfeiters or simply prove that a certain product comes from the original manufacturer."

Another way is to use tiny pigment beads that have a grating pattern on their outer surface. "When applied in a coating paste," said Korhonen, "they give the end-product a characteristic glittering appearance, which is hard to copy."

Metso has also looked into the possibility of processing the recently developed electronic inks, which are now marketed in flexible displays. "A sheet of paper is the ideal flat and flexible display. It would be fantastic if we could use our experience in paper making for the 'intelligent paper' of the future."