These are just a few of the breakthrough applications that already are possible as a result of a new generation of organic, photonic materials developed recently by scientists at the Photonics Research Laboratory (PRL) in the UB Department of Chemistry and the Polymer Branch of the Materials Directorate at the U.S. Air Force Wright Laboratory in Dayton. The work is funded by the U.S. Air Force.

Photonics-often called the information technology of the 21st century-is the counterpart of electronics, using light instead of electrons to acquire, transmit, store and process data.

Whereas most university laboratories tend to focus either on the fundamental science behind new materials or on industrial applications for them, the research team headed by Paras N. Prasad, director of the Photonics Research Laboratory, emphasizes both.

The work is a collaborative effort with the Polymer Branch of the U.S. Air Force Wright Laboratory, UB's Advanced Microscopy and Imaging Laboratories (AMIL), UB's Department of Biological Sciences and the Photodynamic Therapy Center at Roswell Park Cancer Institute in Buffalo.

Three patents have been filed by the PRL group on the new materials, including one that covers a new approach to photodynamic cancer therapy.

The new materials are novel chromophores or dyes. Representative compounds are ASPT (trans-4-[P-(N-ethyl-N-hydroxylethyl- amino)styryl]-N-methylpyridinium tetraphenylborate) and APSS ((4-[N-(2-hydroxyethyl)-N-methyl) amino phenyl]-4'-(6-hydroxyhexyl sulfonyl)stilbene).

They demonstrate a phenomenon called multiphoton absorption, which was predicted as early as 1931.

"Usually, when you excite a fluorescent material with light, you produce an emission that is shifted toward the red-or lower energy-end of the optical spectrum, which means it has a lower photon energy than that of the pump source," explained Jayant D. Bhawalkar, research assistant professor in the Photonics Research Laboratory.

"The emitted photon has a lower energy because part of it is lost internally."

He noted, however, that it is well-known that molecules of certain fluorescent materials can absorb two or more photons simultaneously (multiphoton absorption) if pumped with a light of sufficiently high intensity, such as light from a pulsed laser. This produces fluorescent emissions with higher energy than the pumping photon, a process called upconversion.

Until now, most materials have been capable only of a very weak absorption of two photons, making them inadequate for most applications.

In contrast, the materials developed at UB and the Polymer Branch of the Wright Laboratory exhibit strong multiphoton absorption, as well as strong fluorescence emission.

Some of these new materials also have excellent two-photon pumped lasing properties, which allows them to be used for producing frequency upconverted laser light.

This makes possible the development of solid state dye lasers and fiber lasers that convert infrared laser emission to visible blue light.

"Such lasers are required to increase the number of channels (bandwidth) in optical communications, to increase the density of optical data storage and for undersea communications," said Guang S. He, senior research scientist at the Photonics Research Laboratory.

A scientific paper on an upconversion laser authored by Prasad and his colleagues was published in Applied Physics Letters (Dec. 18, 1995).

Reaching Deep Tumors

One of the most exciting applications for the new materials is in enhancing photodynamic cancer therapy.

In collaboration with Thomas J. Dougherty, one of the founders of photodynamic therapy, the UB scientists have shown that their dyes can help to extend this new cancer therapy to treat deep tumors.

Dougherty is chief of the Department of Radiation Biology and the Photodynamic Therapy Center at Roswell Park and a UB associate research professor of biophysics and research professor of radiation oncology.

"Photodynamic therapy works with a chemical photosensitizer, which is injected into the patient and which selectively accumulates in a tumor," said Bhawalkar. "When the laser light illuminates it, the photosensitizer reaches an excited state and converts the oxygen in the tissues to a highly reactive form, which proves deadly for the tumor."

The current technology utilizes visible light to shine on the tumor.

But, Bhawalkar said, visible light has poor tissue penetration, so the promise of photodynamic therapy has been largely unfulfilled for deep tumors.

Now, the scientists in the Photonics Research Laboratory have found that when the photosensitizer is injected into the patient with one of their new dyes, infrared light can be used on the tumor, allowing for deeper tissue penetration.

"The dye absorbs the infrared laser light through two-photon absorption," Bhawalkar said. "You pump with two photons of low energy (infrared) in order to get enough energy to excite the photosensitizer. Now the dye works like a photon harvester, capturing two low-energy photons and transferring higher energy to the photosensitizer."

Working with Dougherty, the UB team has demonstrated that in animal tests, the new treatment destroyed deep tumors and produced no side effects.

"Although these are preliminary studies, they are very exciting," said Prasad.

Layers of data

The strong multiphoton absorption of the new chromophores also has dramatic applications for more efficient information storage.

"These new materials could revolutionize data storage," said Prasad, "because they allow data to be stored in the depth of a disk."

Currently, compact disks store information only on their surfaces.

The UB scientists, together with the scientists at the Polymer Branch of the Wright Laboratory, have demonstrated volume or 3D storage in polymers doped with one of the new dyes.

"The presence of the dye enables the polymer to strongly absorb infrared light due to multiphoton absorption and this absorption can be confined to a very precise area by tightly focusing the laser beam," said Bhawalkar.

The absorbed light causes the material to change properties, such as the color or the fluorescence emission, and corresponds to the "writing" process.

The "writing" of the data will occur only at the focal point, where multiphoton absorption is possible because the laser intensity is at a maximum.

"Now it is possible to optically penetrate the depth of the disk and access any given spot in it precisely, because the multiphoton absorption occurs only at that spot," said P.C. Cheng, professor of electrical and computer engineering and the director of the Advanced Microscopy and Imaging Laboratories at UB.

"The spot where absorption occurs gets bleached, thus reducing the fluorescence emission from that spot," he explained.

The presence or absence of bleached spots then becomes the data that is "read."

The UB team demonstrated this using a confocal microscope to "write" several stacks of data inside a polymer disk.

They also were able to record several seconds of an animated Bugs Bunny movie in a cubic volume, with each side of the cube about the thickness of a human hair.

"These efficient new materials are making these applications possible for the first time," said Prasad.

Nondestructive probe and other applications

By using the new chromophores, materials scientists employing confocal microscopy now will be able to view thick samples in three dimensions.

"Conventional confocal microscopy using visible green-laser light generally limits the sample thickness to about 20 microns because the light does not penetrate any deeper," said Cheng.

"With these new chromophores and multiphoton excitation, it is possible to use infrared light, which allows us to nondestructively probe deeper into polymers, coatings and other samples, up to a depth of more than 250 microns."

Cheng, Prasad and co-workers have a paper on this application in press in the Journal of Scanning Microscopies.

The strong multiphoton absorption of these compounds also makes it possible to construct an optical power limiter. This device passes low-intensity light, but cuts off high-intensity laser light, which would be useful for eye and sensor protection.

A paper on this kind of device was published by the UB researchers in Applied Physics Letters (Oct. 23, 1995).

"We are trying to understand the relationship between the molecular structure and both the strength of two-photon absorption and the intensity of the fluorescence emmission," said Bruce A. Reinhardt, project leader at the Polymer Branch of Wright Laborabory.

"Once that relationship is better understood, we should be able to modify the molecular structure to optimize the two-photon absorption, the fluorescence emission and the color of the emission."