Are Better than One

Developments in multi-photon processes
By Bill Weaver

As a spectroscopist, I'm predisposed to recognizing the importance of light in our modern economy. Nonetheless, the recent flurry of articles, white papers and news releases on the topics of multi-photon microscopy, imaging and lithography warrants a closer look. (Pun intended.) Perhaps it is merely a few incremental discoveries that are getting great press, but it appears these developments are catalyzing advancement in several disparate communities simultaneously, not unlike the coincident photon processes themselves.

Several different models can be used to describe the interaction of light and matter, ranging from classical wave motion and particle kinematics to the quantum mechanics of particles behaving like waves and waves having particle characteristics. Models are used to describe processes in a general sense and typically require refinements and extra terms when applied to individual cases. Although the term "photon" elicits an association with other particles such as electrons and protons, it is the wave model and its parameters of wavelength and frequency that are most often used to explain traditional processes involving light.When propagating through a vacuum, a pulse of electromagnetic radiation keeps to itself and continues on its way indefinitely. However, when it runs into matter, several things can happen and each process can be used to tell us something about the matter or the photon. Absorbing the energy carried by the photon, causing the photon to wink out of existence and the matter to behave differently at a new level of internal energy is one possible interaction. The useful aspect of spectroscopy is that different collections of matter, such as unique groups of atoms and molecules, only absorb energies corresponding to available energy levels within the group; resulting in a pattern or spectrum of individual energies that can be used for identification.A second interaction process is the emission of a low-energy secondary photon after absorption of a high-energy primary photon — a process known as fluorescence. The emitted fluorescence can be collected by a microscope and used to map out the structure of biological materials. Problems arise when the primary high-energy photon is absorbed by surrounding tissue that is not of interest, possibly resulting in tissue damage. A solution first described theoretically in the early 1930s involves the absorption of two low-energy primary photons simultaneously; aptly-named "two-photon absorption" (TPA). The surrounding tissue is often transparent to these low-energy photons, is no longer damaged, and permits the light to travel deep into the sample. To increase the probability of simultaneous absorption, the TPA process requires a large number of photons and only occurs in the focal point region of a focused beam or the crossing point of two focused beams, resulting in a vast improvement in resolution and image quality over traditional confocal fluorescence microscopy.

The absorption of light also is useful in the manufacture of integrated circuits and micro-electro-mechanical systems (MEMS). In a process known as projection lithography, small structures and connections can be created by exposing an absorbing material to light projected through a mask. The absorbed energy drives chemical reactions that cause the material to harden and remain while unexposed material is subsequently washed away with solvent. As a line-of-sight process, three-dimensional objects must be exposed in layers and there is a fundamental limit to how small a slit in the mask can be made for a given wavelength of light. In general, light only can be focused and passed through a slit greater than one-half its wavelength. Any smaller and the stream of light undergoes the process of diffraction wherein the slit acts as a point source of light that sends photons in all directions and results in a "blurry" image.

Using TPA, the focal point of the low-energy primary beam can be scanned in three-dimensions without absorption of the beam itself. This permits the production of MEMS devices without the use of masks and layers. Secondly, physicists at Osaka University have demonstrated the use of a TPA monomer requiring a threshold light intensity value, such that polymerization only occurs at the very center of the diffraction-limited focal point of the TPA beam. By controlling the intensity, solidified voxels (3-D volume elements) were created with a spatial resolution of 120 nm using a primary beam of 780-nm pulses from a mode-locked Ti:Sapphire laser having a TPA diffraction limit of 195 nm.

More recently the non-classical, quantum model of light has been leveraged to enhance two-photon processes. Rather than concentrating a large number of similar-wavelength photons into a small focal spot to increase the chance of simultaneous absorption, non-linear crystals can be used to split high-energy photons into two or more lower-energy photons that are exact copies of each other. This process, known as "quantum entanglement," does not split the intensity of a collection of photons; it actually splits the individual photons themselves. These low-energy entangled photons can be transmitted through media that is transparent at their energy level and then focused together at the desired location of TPA. Quantum entanglement has a host of other special properties that show promise in lithography, MEMS, computing and communication. Be on the lookout for news of its uses. This isn't the first giant story that began with the phrase "Let there be Light."

Bill Weaver is an assistant professor in the Integrated Science, Business and Technology Program at LaSalle University. He may be contacted at