Sunday, November 25, 2012

Infinity and Beyond

by Angela Calchera

By my estimation, scientists are the modern-day equivalent of the explorers in the days of Magellan and Columbus.  Back in the third grade, I read a book about Christopher Columbus and promptly changed my career goal from fire fighter to explorer.  As I continued through my school days, however, I became more and more convinced that all of the lands left to discover were outside of earth, and  I dropped that career goal for something more practical, like being a doctor.  Then, after "exploring" the world of medicine by shadowing an ophthalmologist for a day, I realized that I didn't like needles and sick people very much and decided to pursue the hard sciences instead.  Now that I am nearing a terminal degree in Physical Chemistry, I realize that I have come full circle in a way, and that, not only do many "lands" remain to be discovered without ever leaving the Earth, but I am one of the explorers seeing them for the first time.

Today in  my lab, Chad, the originator of this blog, stopped by my office to complain about the irritating sounds that his lab equipment makes.  He uses a pulsed laser that flashes five times or so per second, making a really grating clicking noise as it does so.1  During the conversation he made a remark about our laser not making that noise because it is continuous.  In reality, the laser we use in our lab is also pulsed, but instead of getting five pulses per second, we get 1000 pulses per second, which is faster than human eyes can detect.  It makes noise when it pulses on and off as well, except instead of hearing clicks, we hear a hum that has a frequency of exactly 1 kHz.  Since Hertz is a unit of "per second" and the "k" stands for "kilo," meaning 1000, that is a frequency of "1000 per second," so essentially it is the sound of our laser clicking on and off 1000 times per second.

The amazing thing is that these one thousand pulses each only last for 120 femtoseconds.  Why is this so amazing?  "Femto" is a prefix that means 10-15, which is one millionth of a billionth of a second.  Another way to think about it is 0.000000000000120 seconds.  So, it starts and ends pretty darn fast.2  How fast?  Well, we get 1000 of these a second, which seems really fast already - so fast that your ears cannot hear the individual "clicks."

Even though we can appreciate that a femtosecond is a very short period of time, it is difficult to truly appreciate just how very short it is, so let's stretch our 120 femtosecond pulse out to one second.  That means that on this new scale, one normal second would now equal 264,000 years.  Since there would be just a thousand pulses during this period, the laser would only actually be "on" for one second every 264 years.  So, the laser that to human eyes and ears appears to be continuously "on" is actually "off" most of the time.  This should disturb you, and maybe even hint that there's a lot going on in the world that your senses are not showing you.3

Just about every physical and chemical phenomenon has a "lifetime," which is to say, that very rarely does anything actually happen instantly, even if it sometimes seems that way.  For example, shining infrared (pronounced: infra-red) light on some molecules causes the atoms in bonds to vibrate like balls on a spring.  In this way, we can "see" (or understand) something happening on a molecular scale.  This motion lasts for a finite amount of time, and we can use a second quick pulse of light to see how long this motion lasts, as well as how it changes over time.4  The shorter the laser pulses, the faster the phenomena we can study and measure.  This technology opens up a new world of the ultrafast and ultra-small that before these lasers were invented were impossible to study on this level.

Every day when I fire up the laser and begin an experiment, I travel into the unknown world of the very small and the very fast.  I am exploring the world around me, and seeing things that, although they've been right under our noses the whole time, we have just never seen them before.  Although I am not facing the possibility of bloodthirsty sea creatures or sailing off the edge of the earth, I am still pushing boundaries and learning to see new "lands" never seen before. And like the discoveries of the explorers of history, the discoveries made by scientists like me open up whole new worlds to human the human mind.

That's not the only irritating noise in his lab.  He'd want me to mention that.
"Ultrafast" is the term used by people who work with lasers to describe that regime of pulse duration.
I once met a Mapuche man in Argentina who told me that his religion taught him to believe in all things seen and unseen.  He said that one of his most spiritual experiences in his life came when he read about an electron microscope and saw some images recorded using the instrument because he felt that that particular technology was allowing the seen and the unseen to become one.  I think that is a beautiful way of describing that sense of wonder I feel when I ponder the infinity right here around me.
In my lab we do something called Vibrationally-Resonant Sum Frequency Generation Spectroscopy, which takes advantage of this lifetime.  The first pulse is in the infrared and causes molecules to vibrate, and the second pulse comes in and converts the vibrational energy into a generated pulse of light that has a frequency equal to the sum of the frequency of light that started the vibration and the frequency of light that hit second.  We use optics and a detector that are only capable of steering and detecting the higher frequencies, so without both pulses interacting with each other and the sample, we can't pick anything up.