Sculptured thin films (STFs) are a class of nanoengineered materials comprised of parallel columns whose shapes are twisted and bent during fabrication. These films have many potential applications, some of which have been realized in experiments. I studied the properties of STFs at Penn State with my advisor.
Groups at several universities have fabricated STFs; they are led by
Here is an essay on STFs that is geared to a general audience. It is based on the nontechnical abstract (a supplement to the traditional abstract) from my doctoral thesis.
Imagine the kind of trees that might grow at the poles of a warm alien planet. If the planet's axis were tilted with respect to the plane of its orbit, parts of the planet would be bathed in sunlight for long periods of time; in those areas its sun would wheel about the sky and not set. (The same thing happens in Earth's polar regions, but those regions are too cold for large trees to grow.) Suppose, as on earth, that the trees tend to grow towards the sun. If they grew fast enough, or the rotation rate of the planet was slow enough, the trees would grow in helical, corkscrew-like shapes.
If the orbital path of our hypothetical planet were complex enough, perhaps skirting about a binary star system or being regularly perturbed by a large gas giant, the trees might grow in other bizarre shapes. What a strange world for a xenobiologist to explore!
This fanciful idea captures the basic concept behind the growth of sculptured thin films (STFs), which are now made routinely on our own planet, at linear scales some hundred million times smaller than a California redwood. The fabrication process for STFs is simple enough that an experienced tinkerer could make some basic ones in her basement, with an investment of perhaps $1000 in used parts.
Here, in a nutshell, is how to do it. Prepare a sturdy, sealable, metal or glass bell jar, and place at its bottom a crucible filled with the material for the film. STFs can be made from many different materials, including both metals and oxides. Make some provision for heating up the crucible, perhaps by wrapping the heating element from a toaster around it. Place a substrate--glass or silicon will do--at the top of the chamber. Affix the substrate to a motorized mount so that it can be tilted and/or rotated. Then pump the air from the chamber until you have a high vacuum. Heating the crucible will cause some of the material inside it to evaporate and land on the substrate. If the substrate is cool enough, tiny columns--nanowires--of the evaporated material will begin to form. Those nanowires will be bent and twisted depending on how the substrate tilts and rotates, respectively, during the deposition.
In this process, the crucible is analogous to the sun of our alien planet. The substrate corresponds to the surface of that planet, and the sequence of rotations and tilts mimics the the effects of the planet's motions. The difference is that in our plant analogy, only the energy and not the material for growth comes to the trees via the sunlight. Also, the nanowires that make up STFs form spontaneously. There is no blueprint, per se, that influences their formation like DNA does for plants on earth.
Why should we bother making such films? In three words: to manipulate light. The films have other applications, to be sure, including as traps and sieves for biomolecules, viruses, and bacteriums, and perhaps for cooling of microelectronic circuitry. But the most advances in STF technology have been in the area of optics, the science of producing, focussing, measuring, and in general manipulating, light.
People have been studying the optical properties of thin films for a long time. Sir Isaac Newton did it. But until now people have not had the extraordinary control over the microstructure of the films they made and used. With STF technology, the length, shape, size, and orientation of the nanowires that comprise the film can be controlled. That control, along with the ability to make the films from a large variety of materials, means that their optical properties can be tailored for a wide variety of applications. STFs can be designed to reflect, transmit, and absorb different wavelengths (corresponding, in the visible spectrum, to the colors that we see). They can be designed so that the infiltration of chemicals between the columns will change their reflectivity and transmissivity in a controlled way, or even produce light by fluorescence.
One of the most exciting potential applications of STFs in optics is for the manipulation of ultrashort optical pulses. Such pulses, which are of only a few femtoseconds duration (a femtosecond is a millionth of a billionth of a second) have potential applications for communications, controlling chemical reactions, surgery, and remote sensing, to name just a few areas of investigation.
By solving the equations that describe light propagation, we have been able to predict what will happen when an ultrashort light pulse hits one of these films. As expected, parts of the pulse will be reflected from and parts will be transmitted through the film. We can predict the shapes and energies of those reflected and transmitted pulses. The microstructure of STFs could be designed to shape the pulses in useful ways.
To process information in the optical realm, a device must be able to affect optical pulses differently depending on their characteristics. For instance, an STF can be designed to reflect light in a certain wavelength band. They can be designed to reflect one polarization of light. The polarization of light refers to the direction or series of directions that the electric field--which, along with the magnetic field, comprises the light--point as it propagates. And they can do both simultaneously. So an STF-based device could be used to separate incoming pulses based on their spectral content and polarization state.
They can also be designed to discriminate between pulses having different carrier phases. The ability to stay in phase is one distinction between a good marching band and a poor one. If the marchers keep perfect time (in sync with what is known as some kind of beat or carrier signal) but take their steps out of phase, their feet will not hit the ground at the same time. While we can easily see variations of phase in a marching band, with light the electromagnetic fields change so fast that not even our fastest electronic devices can measure optical phase directly. In optics, what we measure directly is the power or intensity of the light.
However, two pulses can have the same variations with time in their intensity, but different carrier phases. It turns out that if you reflect two such pulses from an STF, the differences in carrier phase will show up as variations in intensity in the reflected pulses. This phenomenon hopefully will make it easier to measure the carrier phases of ultrashort pulses. That, in turn, might make it feasible to transmit more bits of information per pulse down optical fibers, gain more control over chemical reactions, or measure more accurately the properties of our planet's atmosphere. After all, we must take care of this planet, if we ever hope to visit one with trees shaped like corkscrews.
This file was last modified at 01:44 on 03 Nov 2008.