Trying to get a job? Try the online job fair at CLEO!

There are many things you do not want to miss in CLEO:2012 – A conference full of high quality technical sessions spiced by cutting edge presentations from invited speakers, not to mention the inspirational talks of renowned plenary speakers. For young graduate students, these are stimuli they want to boost their research. On the other hand, in the mind of senior graduate students, there is one more mission besides getting loaded with technical knowledge – Landing on a job after graduation. The good news is that you can get two birds with one stone since CLEO provides a nice channel for you to get connected with your potential future employers.

If you are interested in staying in the academia, your advisor(s) and the department may be the best resources for you. However, if you consider changing the tracks and exploring the industrial career, CLEO: 2012 is something you cannot miss. It brings employers from the entire US under one roof, and you get to meet them all. This year, you can try the online job fair by CLEO and WORKinOPTICS by OSA to get a head start. Unfortunately, not all the employers are actively involved in the online job fair. As a result, walking throughout the exhibition hall will be your next move.

Trying to get exposed in the exhibition hall is a must. To get you exposed in a right way is not that straightforward. For the past few years, I feel lucky to have the opportunity to look into these job-hunting games from both sides (as a senior graduate student trying to impress future employers in the conference, and a employee actively working in the tradeshow). Here are some tips I hope that help:

1.      Get to know the companies you want to visit before hand – Even though your lab has the instruments from the company you want to drop by, it does not mean you know the company, not at all. Try to do the homework to learn the histories of the companies, including their competitors and their niche technologies. This is the appetizer (topic) for you and the people who work on the exhibition hall in the first encounter. You intrigue them with the right motive, and it will be the impression that lasts in their minds. Besides, by studying the companies, you will find out the photonics industry is a big intricate web and companies are related to each other in a very intimate way.

2.      Set the right goal – Your goal is not to give the resume away. Instead, your goal should be building up a new connection/strengthen the existent ones with the companies through the representatives. Making a good impression, staying in touch with them, and updating them with your research progress are means to achieve those objectives. You will never know when there will be a vacancy in the company. And believe me, when there is a vacancy, the first thing they do is to request their colleagues to see if they know anyone who is qualified. You want to be the one that comes cross their minds.

3.      Who will you encounter? – Most of the time, you will bump into a sales representative, but not always. There is a chance you will meet technical sales support people, product managers, directors of divisions in the company, CTOs, and marketing personals. If you are a Ph. D. student, try to talk to people who have strong technical backgrounds such that they appreciate your effort. If you are a master candidate with a minor/major in marketing, you may find yourself more comfortable to talk to the product managers.

Of course you cannot tell one person’s job title by face. What happens if you pick the wrong one at the first place? Don’t’ worry; just ask politely after a nice and warmed up conversation. People who work on the floors are nice, and their duty is to help, in all possible ways. They will not say no to you. That harms them in a bigger way.

4.      Never just hand in your resume right away, and do not walk away immediately after you do so – when standing on the carpet of the company’s territory, do not just look for the representatives and hand in your resume. Spreading as many as you can does not guarantee you a job. In addition, by doing so, your resumes won’t reach the places they are supposed to.

Wrap your purpose in a delicate way! For example, start the conversation with your interest in the new lasers that are released by the company in 2012. Ask technical details to show your knowledge. Then, slowly express your expertise in this field, and ask if there is any opening in the company. If yes, trying to learn more, if not, stay motivated and talk about the instruments in further depth. Simply walk-in, drop the resume, and walk away, you basically leave no indentation at all.

5.      Avoid rush hours when planning your visit – In almost all the conferences, there is a time period where no technical sessions are happening. I call it the rush hour on the floor. It is true that at this time window, there will be more representatives working on the floor, but there will be ten times more visitors. So do the math.

6.      Remember to get the contact information from the people you talk to – Trust me, when you start job hunting, you will need it. And you will regret you did not get it before. Asking for their business cards should be a habit for you if you want to start your career in the industry. You should also stay in touch with them. Ask them if they are visiting your area, if they plan to release new products and so on. One day, they might be your future colleagues.

A few more things about job hunting you might want to think about. I find there are many people who are not aware of this, or neglect this.

1.      Job-hunting requires a warming up time – It is rare to start getting a phone interview or response from the companies right away when you start to post your resumes on the websites. It takes about 1-3 months. Do not get frustrated. Keep polishing your resumes. Make this as a habit.

2.      Resumes have to be tailored to the jobs – There is no such thing as a universal resume that you can use for all the jobs posted. You have to spend time on each and every single one when submitting your applications (at least for those jobs you feel your skills match seamlessly). Writing a cover letter for your dream job will help for sure.

3.      Do not abuse online job-hunting websites – Choose no more than three job posting websites for yourself (for the optical science people, WORKinOPTICS is definitely a good one to have). All the websites nowadays have most of the jobs posted by other websites. Three job-hunting sites are enough to cover them all. You do not want to submit three copies of the resumes to a company, do you? In addition, try to apply for the jobs at the company’s website
(s). When a job hunting website gives you a job posting of a specific company, you go to that company to apply directly. Do not rely on the “middle man” of the Internet!

Finally, I want to use one of my mottos to encourage all of us – “When you started your graduate study, you have already started your career. Jobs are just methods to achieve your life long career.” Indeed, a job may seem to be the only thing that matters for you when the time is pressing. You might even feel desperate when you are in the process. But this is just a short period of time in your life. Once you really get one, spend some time to figure out what you want to achieve in your career and lay it out in brevity. This is what matters the most! Having an idea of your career path is a very proud thing to possess. It is the rudder of your professional life and it keeps you from being lost in the ocean of the diverse jobs.

Best of luck in job hunting!

DISCLAIMER

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

Recording the data at the “ultrasfast” rate for your digital device.

The principle of magnetic storage used by most hard drives is an important pillar in the evolution of modern digital world. Before the advent of flash memory, it dominated the way we saved our data. Simply speaking, binary information (0 or 1) is presented by small magnets pointing forward (0) or backward (1); let’s say the north is the head. Writing the data is by changing the pointing directions of these magnets, usually fulfilled by an electric coiled wrapped head (by applying the current into the head, you create a strong external magnetic field that realign the directions of the small magnets in the hard drive, one at a time). In addition, packing in as many magnets as possible in a limited volume will define the capacity of a hard drive, and this is improving ever since the first device available. Imaging the first computer I had came with a hard drive of 400 MB, and now a decent one has a few TB storage capacities. By comparing the number, you can realize how much effort and advances in the business of data storage. For a very nice introduction, you can find at hard drive 101: magnetic storage.

Figure 1.  A generic ferrimagnet, composed of Fe and Gd, shows the alignment of magnetic moment. Courtesy of I.Radu et al., Nature 472 205 (2011).

A nice paper where ultrafast laser pulses (sub 100 fs) instead of external magnetic field are used to write data intrigued my curiosity. I know immediately that it is the heating effect that causes the change of the magnetization of the small magnets in the hard drive. But for me, the heat has no directionality, how it can tell the magnet to point forward or backward. It should just erase the information since an ultrafast laser pulse can easily create a hot environment above Curie temperature where the magnetization is destroyed. So in my mind, an ultrafast laser is a hard drive terminator, not a hard drive writer. Driven by this curiosity, I dug in to find out, and this is how:

Strong magnets are either ferromagnetic of ferrimagnetic (that is right, only one letter difference). Iron is ferromagnetic, since when it gains magnetization, all the molecules have the magnetic moments (or the moment generator – the atoms’ spins) aligned in one direction. On the other hand, ferrimagnetic materials contain different atoms or same atoms in different chemical forms. For example, the alloy of GdFeCo or magnetite is ferrimagnetic.  When these materials gain magnetic power, the magnetic moments of different atoms (or same atoms with different chemical forms) are pointing in the opposite direction. However the magnitudes are different. As a result, the cancellation is not complete, and some magnetic power retains. This lengthy discussion has a purpose, since in order to use ultrafast laser to write on the device, we need ferrimagnetic materials as our small magnets. Figure 1 is an example of a ferrimagnetic material made of the alloy of GdFeCo. The small arrows are the direction of the spins or magnetic moments if you prefer.


The working principle is like this: When an ultrafast laser pulse hits the ferrimagnetic material (in this paper, a alloy where the active components are Gadolinium (Gd) and Iron (Fe)), the temperature shoots up and all spins are free to flip (or randomize) due to the energy shot. As a result, magnetic moments are decreasing toward zero. In other words, they start to demagnetize (I got this part right). Here is the catch: Fe demagnetize faster than Gd. Like figure 2 shows, when the overall magnet moment of Fe reaches 0 before 0.4 ps, Gd is still decreasing toward zero. At this instant, another principle plays a pivotal role – exchange interaction between Gd and Fe. This exchange interaction says that the flips of the spins of the Gd and Fe can undergo collaboratively, with the spin of a Gd atom flips from up to down, that of a Fe atom goes from down to up. Mother Nature likes the exchange interaction, since it is more energy efficient. Now incorporate this to our discussion. Right after the 0.4 ps, the overall magnetic moment of Gd is still decreasing; it means the spins are flipping down. This gives the spins of Fe atoms a direction, which favors to flip up against Gd’s to please Mother Nature. So the magnetic moment of Fe starts to build up, instead of staying randomized. After the overall magnetic moment of Gd hits zero, the overall magnetic moment of Fe has built up quite a bit. This built-up will guide the entire alloy to relax into an overall ferrimagnetic form. If you are still with me, you know the secret of this technology. With this principle at hands, you can use an ultrafast laser to write the data in an unprecedented speed -- for a few hundreds of picoseconds you can write a bit. And maybe scientists can push this limit even further. 

Figure 2. The evolution of the overall magnetic moment after the sample is hit by an ultrafast laser pulse. As can be seen from the diagram (inset of (b)), Fe demagnetizes faster than Gd. The difference in demagnetization rate makes the reversal of the magnetization possible by an ultrafast laser pulse. Courtesy of I. Radu, et al., Nature 472 205 (2011).

Researchers have demonstrated this phenomenon theoretically and experimentally (this article). As can be seen from figure 3, the magnetic moment flips back and forth after the ultrafast pulses hit the sample.

Figure 3. The magneto-optical image of GdFeCo alloy obtained after the action of a sequence of ultrafast laser pulses. (a) and (b) shows the images of the film with magnetic moment pointing down and up, respectively. (c) and (d) show the magnetization reversal after interacting with ultrafast pulses. The boundary of the circles shows the spot size of the light, and the scale bar is 20 um. Courtesy of T.A Ostler and et al., Nature Communication DOI:10.1038/ncomms1666.

Amazingly, ultrafast lasers are finding their applications at the frontiers in so many fields. They shine and rise in the fundamental chemical physics, advanced spectroscopy, astronomy, machining (Jim has a nice article about it), and now, computer science. Who knows what their next stop will be?

DISCLAIMER

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

The world’s smallest Stirling engine is powered by laser!

When people mention the word “laser” to you, what is the first thing coming to your mind? Most of us associate lasers to their scary and destructive power, just like how we are educated in the Star Wars movie series. In reality, lasers can be quite gentle and perform very accurate and precise assignments, like micro-machining (Jim has a nice article about it). In fact, laser can be so gentle that researchers have used it to power the world’s smallest Stirling engine, which is composed of single tiny melamine bead (~ 3 um in diameter) in the water bath.

To realize how this ingenious microscopic engine works, we have to step into the phenomenon of optical trapping/tweezers first. Thanks to the detailed illustration on wiki, I can just summarize it in a few sentences -- When the laser is tightly focused, or when it has the Gaussian beam intensity distribution, the tiny particle will be trapped in the focus or the center of the Gaussian beam, just like being trapped in a potential well. This is a result of momentum conservation. When the refracted light rays exit the particle, they exert momentum kicks to the particle, and the net result of these kicks is a force that traps the particle at the center of the focus. If the particle is in the focus, this force is zero. If the particle drifts away from the center, the kicks will be imbalanced and a net force will pull it back to the center. This particle behaves exactly like it is in a potential well. The steepness of the well depends on the laser intensity as you might guess it already. And our talented researchers use this technique to power the microscopic engine.

Here is how it goes. Figure 1 shows the comparison of a microscopic Stirling engine with a macroscopic one. As shown in step (1), the bead is trapped in a potential well by a focused laser beam. From step (1) to (2), the laser intensity is increased such that the bead would be confined in a smaller volume due to the steeper potential well. This is similar to moving a piston to squeeze the volume in the chamber. From (2) to (3), the water bath is heated by another NIR laser, and this step is similar to heating a macroscopic chamber. From step (3) to (4), the potential well is relaxed and the work is exerted from the bead to the surrounding, just like in macroscopic world, the gas is pushing the piston to exert work for useful application. From (4) to (1), the NIR laser is turned off, and the bead is cooled down, just like in the traditional Stirling engine, the gas is cooled back to the ambient temperature. Smart and elegant design, isn’t it?

Figure 1. The realization of the microscopic Stirling engine. Courtesy of V. Blickle and C. Bechinger in Nature Physics doi:10.1038/nphys2163 (2011).

Not only the realization of microscopic machines is presented, but also its power and efficiency are characterized.  They have shown that if the entire cycle is working at a rate of 7.2 s, the power is at its maximum. So a micro-machine has a working ethic of macroscopic time scale. They also calculate the average work output, which is in the range of 10^-21 J! Since the machine is tiny, it is prone to the stochastic fluctuation. As a result, some of the cycles are exerting negative works. But fortunately, when average over many cycles, the machine works reliably.

Optical tweezers technique has been used for various applications. Beside the one we just present, Block’s group in Stanford is the true master in applying it to biophysical study. They have used this technique to investigating the transcription of the DNA and the motions of kinesin motors inside the cell, just to name a few. The basic principle is to attach the molecule of interest to the bead or beads, exert the force to the bead(s) through the laser, just like figure 2. By carefully controlling the laser intensity, the force can be finely tuned in a delicate way. So the step motion or the transcription of the DNA can undergo in a subtle and controllable way. In addition, by monitoring the location of the bead, we can extract the size of the step motion of the molecules (the molecules themselves are too small to see). This sophisticated setup has been perfected by Block’s group, and they are able to extract the step motion of kinesin and DNA transcription with a resolution of a few nanometers. This is another master piece of scientific work, because we are talking about a motion in a microscopic world through macroscopic technique. I strongly encourage people who are interested in this topic to take a look of their research website, since it contains lots of information, even some insightful literature!

Figure 2. Using the optical tweezers to probe microscopic world. The force, when tuned properly, can switch the reaction on and off since the reaction involves the change of the molecular length. Also by monitoring the bead location, we can know the step size of the motion. Courtesy of Block's research group website.

I keep wondering when the Nobel Prize was granted on the optical tweezers, did they envision its cool applications we described here already? Maybe...

DISCLAIMER

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

“Metamaterial tiles” are hot in many applications – including invisibility cloak!

Various forms of metamaterial have generated a lot of scientific attention in the past few decades. Some exciting “potential” applications include the well-publicized invisibility cloak (Thanks to Harry Potter). As you may know already, metamaterial gains its bizarre optical property (such as negative index of refraction) by its internal composition or structure, rather than its original physical property. Most metamaterial has its magic only in specific wavelength region and this wavelength region is correlated to how small you can make the internal structures of the metamaterial. This is exactly why almost all the research on metamaterial focuses on THz region since THz has very long wavelength and we do not need to make the structures awfully small to concoct the magic (I did read some articles about “universal metamaterials”, but it seems a long way to go. Let’s dream of that coming in CLEO 2012).

Digging into more details, you can have 2D or 3D metamaterial depending on your applications. 2D metamaterial – or so called metamaterial tiles (m-tiles) – seems to make a huge leap in guiding the advance in the invisibility cloak and sensing platform. And they are easier to make (through the help of photo-lithography, or micro-machining on the surface). With this powerful combination, a booming in this field seems inevitable. Let us take a peek of its potential application in invisibility cloak first:

It is realized that for a TE plane wave, it is possible to have a perfect invisibility cloak providing that the metamaterial has the right permittivity and permeability. However this cloak has to have circular inner and outer boundaries. As far as we know, it is very difficult to make exact circular cloak even with nowadays technology. To get around it, certain compromise has to be made. Instead of using hollow sphere or cylinder, we can use hollow polyhedral, each facet of which is made by m-tiles. Having this idea in mind, research group in Germany carried out a simulation study, and the result is really promising. Polyhedral made by m-tiles will actually give quite satisfactory results, and it can hide the structure within it very well (figure 1). In some circumstance, you can even rotate the polyhedral without losing its cloaking magic.

Figure 1. An invisibility cloak made by a faceted dodecahedral. This simulation shows that the plane wave can propagate through it without too much distortion and objects can be hidden inside the dodecahedral. Courtesy of Oliver Paul, Yaroslav Urzhumov, Christoffer Elsen, David Smith, and Marco Rahm.

The powerful units of m-tiles have actually simple internal structures. Described in great details in this article, you can easily change (tune) its optical property by changing its size and shape. As shown as an example on figure 2, three hexagonal m-tiles of slightly different structures have different absorption peaks in THz region. This “easy to fabricate and tune ability” makes m-tiles more and more popular in the research world. Considering making these tiles on a flexible film, you can actually fold them into a functional shape with even more interesting applications. And maybe one day we will have some advanced mosaics made of various m-tiles.

Figure 2. Different shapes of Hexagonal m-tiles. Each side of the structure is ranging from a few um to tens of um. By slightly modifying the structures, each of them absorbs different THz frequency. This flexibility makes m-tiles very versatile. Courtesy of Christopher M. Bingham, Hu Tao, Xianliang Liu, Richard D. Averitt, Xin Zhang, and Willie J. Padilla in Optics Express 16 23 18565 (2008).

How about getting a step further -- making these m-tiles on the paper and transforming them to biosensor platforms? In a nutshell, researchers from Tufts University and Boston University use micromachining to fabricate micro-stencils on silicon nitride film. These micro-stencils have many of m-tiles on it. With the help of micro-stencils, they then imprint the pattern of the m-tiles on the paper by spraying on the paper substrates using electron beam evaporation (figure 3). Using this way, you can make as many m-tiles as you want! Now, this sensor is ready to be radiated by THz radiation. Since paper is relatively transparent in THz region, it is a very good substrate (a disposal one). Once the molecules have attached to the m-tiles, they will change the electric capacity of each m-tile. This change of capacity will reflect on the absorption peak of the m-tiles. And this makes it a good sensor for various molecules. In fact, this is quite a new way to sense the molecules. It can achieve the sensitivity of ~ mmole/L concentration. Not bad as a paper-based sensor!

Figure  3. Using micro-stencils to imprint as many m-tiles as you want on the paper! The inset shows how the absorption spectra of m-tiles are modified when different amounts of the molecules (in this case, urea) are attached to them. Courtesy of Hu Tao , Logan R. Chieffo , Mark A. Brenckle , Sean M. Siebert , Mengkun Liu , Andrew C. Strikwerda , Kebin Fan , David L. Kaplan , Xin Zhang , Richard D. Averitt , and Fiorenzo G. Omenetto in Adv. Mater., 23, 3197–3201 (2011).

In the near future, more applications of m-tiles can be seen, indeed.

Trip note:

Luckily, I attended several conferences in China at the end of October. I spent two weeks visiting several cities (Beijing, Wuxi, and Wuhan). In Wuhan – Optics Valley of China, I met student chapter of OSA in POEM 2011. I felt awesome! OSA had a booth, an eye catching poster, and a bunch of energetic students in this conference. It made me feel like home away from home. Way to go, OSA!

Figure 4. The poster behind OSA booth in POEM 2011, Wuhan, China. Energetic student chapter of China promoted OSA nicely. Felt so warm when I saw this.

DISCLAIMER

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

Zoo of super resolution microscopy.

Microscope, one of the most popular optical instruments, has been paving the way of biological science for the past three hundred years. With the aid of the microscope, detailed observations of sub-cell size resolution were made possible. This, in turn, accelerated our understanding of the biology in an unprecedented way. Three hundred years have passed; we now arrived at a new cross road -- While triumphing on the universe of biology, a desire to develop microscopes with specificities and better resolutions is creating another revolution.

Specificities problems are less optical relevant. It is like painting different organelles of the cell with different colors. To do so, scientists use fluorescent dyes to attach to different organelles or encode them directly into the genetic codes of the proteins. So we can differentiate what they are and where they are. Scientists are quite good in doing so.

Resolution is another story. It is a barrier imposed by fundamental physics. In other words, the enemy of a microscope is diffraction, which prevents how well you can resolve two points on the focal plane. Same principle also applies to how tight you can focus a collimated beam. Using the traditional microscope, you cannot have resolution better than hundreds of nanometers if visible light is used. The axial resolution is not much better. As a result, no matter how small the particle in the focal plane is (in this case, the fluorescent dye), you would always observe a blob with some sizable volume. How do achieve better resolution? What kind of tricks scientists can play to break the diffraction limit?

For me, the first milestone in super resolution is called FIONA (Fluorescence Imaging with One Nanometer Accuracy). What a lovely name! In a nutshell, it fits the fluorescent signal with a Gaussian function. By doing so, it finds the center of the dye theoretically. Just like finding a center of the blob in the example we gave above. This method is generally adopted in modern microscopy since it localizes the location of the dye in the lateral plane quite well. There is a caveat though -- you cannot have too many dyes in focal point. This is just going to screw up your fitting.

Same mathematical manipulation does not work satisfactory in axial direction. In addition to multi-photon microscopy which aims on attacking this problem, there are other neat techniques existent. The way to get around it is modifying and mixing the experimental setup with other optical phenomena. The most eye-catching technique to me is the research led by professor H. Hess in HHMI. By putting a three-way beam splitter, the florescent signal from the dye in the focal plane would interfere with itself and generate different interference pattern depending on how far the dye is offset from the true focal point. This method achieved tens of nm of axial resolution. What impresses me the most is the feeling I have when trying to understand the diagram of the experimental layout. Suddenly, you realize, the imagination to advance optical science is unlimited.

Figure 1. The optical layout for interference microscopy. Courtesy of G. Shtengel, et al. in PNAS 106 9 3125 (2009). 

Other neat ways emanate from bright minds also. One way to do so is to create bizarre spatial beam profiles at the focal plane. By putting an annular apodization mask, the work from the research group led by professor E. Betzig in HHMI created the Bessel beam profile at the focal plane. Combined with structural illumination, they created a focal spot, which has z resolution of less than 300 nm.  Another research group led by professor W. Moerner in Stanford used spatial liquid modulator (SLM) to create helix beam profile at the focal plane. With the help of de-convolution algorithm, they could localize a fluorescent dye with ~ 20 nm z resolution in a total of 2 micron depth. This is truly amazing.

Focusing the light into a tiny spot is not the only solution. In the branch of developmental and embryonic biology, the speed of taking the image with decent resolution is of prime concern. To solve this problem, scientists used a century old technique (using a tube lens to focus the light into a light sheet) with the modern spice of fluorescent labeling in the genetic level. Generated light sheet at the focal point excites a plane of fluorescent proteins in one shot. Research led by professor P. Keller (now in HHMI also) in European Molecular Biology Laboratory (EMBL) used this technique to elucidate the developmental process of a zebra fish embryo in the first 24 hours. A full digital documentation of the embryo is resulted. Optical science is delving very deep to search the origin of nature, isn’t it! A good website of light sheet microscopy can be found here!

Figure 2. Light scanned microscopy. The laser beam illuminated the sample from the side and excited the fluorescent proteins in the plane. Courtesy of  P. Keller et al. in Science 322 1065 (2008).

Coincidentally I found a very intuitive and interactive website for super resolution microscopy. It is like a power shot to everyone who is interested in knowing more in modern microscopy.

DISCLAIMER

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

Evolution, the master of optical science!

Do not get me wrong; evolution is an expert of all physical science. But it intimately links nature to optical science without doubt -- from cyanobacteria that have been converting solar energy to chemical energy for 3 billion years to human beings who rely on vision for surviving.

Neuroscience indicates that about 25%~ 50% of the brainpower and as many as 30 different areas of the brain are devoted to vision processing. This simply means that each human being is hard wired as an optical scientist, although we hardly recognize this. Over the past millions of years, evolution has perfected our imaging device in a subtle way. Recently, a report on Biomedical Optics Express shows for the first time the eyes’ imaging sensors -- cones and rods by using adaptive optics to minimize the aberration caused by the eye structure. As shown in the figure 1, cones, the round structures, create red, green, and blue perception of colors. There are about 6-7 millions of them, concentrated at the center of the retina -- forvea. A friendly and easy to digest article about this topic can be found here.

Figure 1. The cones in the fovea region of the eye. The retina is illuminated by 796 nm and 680 nm, respectively. The scale bar is 10 micron. Photo is courtesy of A. Dubra and Y. Sulai in Biomedical Optics Express Vol. 2 No. 6. 1757 (2011).

The other component of eye that always surprises me is the crystalline lens. It is 9 mm in diameter and 4 mm thick – a tiny optics. The structure of it is more or less like a transparent onion, formed by ~ 20000 very fine layers. Each layer is composed of cells of elongated shape (about a few micron thick and ~ 10 mm long). What makes it special is the variation of the index of refraction. In the inner core of the lens, the index of refraction is about 1.406, while it changes to 1.386 at the less dense cortex. This kind of design combined with the change of the lens shape makes us see things clearly whether they are far or close. In fact, evolution designs gradient index optics way before we even learned about it.

Changing the focus to animal kingdom, you can find more examples that not only make your eyes wide open, but also give us ideas to advance our knowledge of optical science. For example, Lobster uses reflective unit in the eye to focus much more light onto retina (figure 2a). This is definitely one of nature’s demonstrations on micro lens system. Mantis shrimp can detect circular polarized light thanks to intrinsic quarter wave plates made of cells in their eyes (figure 2b, for details about this work, here it is). You might wonder why we need to have polarized vision except 3D movie utilizing this principle to create stereo perception. Next time, when you buy sunglasses, get polarized ones. Wear them and observe the world! Asphalt road reflects light differently depending on light’s polarization. You can see that windshields are not that homogeneous in transmitting light any more. Due to its dichroism caused by tension when molding, it transmits one polarization better. Press a piece of thick plastic, and observe its change in transmission of light (you will induce dichroism by stress). LCD screens can only been seen clearly when you tilt your head in one direction. Turn your head in a different way, the screen become completely dark since most screens emit polarized light. After these daily life experiments, you might regret that we lose this feature during evolution.

Figure 2. (a) The reflective unit of a lobster's eye. (b) A cross section of the cells that work as quarter wave plate in the eye of Mantis shrimp. The scale bar is 10 micron. (c) The phase retardation introduced by the cells. They work nicely in the entire visible region of the light spectrum. The photo is courtesy of T.-H Chiou, S. Kleinlogel, T Cronin, and et al. in Current Biology 18 429 (2008).

We actually lose even more. Homo sapiens can only see three different colors, while birds, some mammals, and insects see the forth kind – Ultraviolet. For instance, Reindeer has UV eyesight. This special ability has evolution advantage. Lichen, on which the animal feeds, absorbs UV light, so it would appear black to reindeer eyes. The animal's traditional predator, wolves, would also appear darker against the snow, as their fur absorbs UV light. Apparently, under UV illumination, things are very different in polar area. I will suspect most of the animals living in Arctic or Antarctic area are endowed with this ability. How birds acquire UV vision is another fun story. Thanks to a beautiful article by Scientific American, bird’s UV vision is illustrated in details. A quick summary from this article: I realize our vertebrate ancestors had 4 types of cones in the eyes.  While birds inherit this feature, our mammal ancestors actually lost two of them! Fortunately, nature is kind to us. Through mutation, we regained a third variation of cones when walking down the evolution road. Without that mutation, we will all be color blinded!

Figure 3. How our perception of the colorful world improved, or degraded down the evolution road? Courtesy of Scientific American Magazine.

It is time for some wild experiment and conjecture. An article published on Nature in 2004 seems to tell us that the migratory avian creatures use the interplay of light, electrons, and earth magnetic field to guide their navigation. In a nutshell, it is called “radical-pair mechanism”. The light creates a coupling between an unpaired electron spin and nuclear spin through a light induced electron transfer. The earth’s magnetic field alters the dynamics of transitions between spin states. These transitions in turn affect reaction rates and products. Such effects can be amplified and used by the creatures. What does this tell us? Quantum mechanics and light matter interaction are working in a fine and elegant form. For a quick read, follow this link.

Story like this can be found at every corner on earth. After all, we are all offspring of nature, and nature relies on sun (the ultimate light source) in countless and ingenious ways. 

DISCLAIMER

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

Interested in optical rulers? Well, which kind!?

Recently, three-dimensional plasmon rulers based on nano-rods are reported on Science. Hopefully, it will be cool weaponry for measuring the structures of the molecules in the near future. Over the past few decades, optical rulers based on different principles emerged from various branches of optical science. At the same time, researchers are trying hard to push each methodology to the limit. Optical Rulers, as a result, attract an army of researchers and spin off fruitful results. A quick summary of them seems to be a fair amount of content for everyone.

First thing first, what does an optical ruler do? Quite straightforward, it measures the dimensions of the molecular structures. For example, what is the distance between two subunits of a hemoglobin protein? What is the height of a membrane protein when measured from the membrane surface? Put into one sentence – optical rulers are aimed to map out the 3D structures of the molecules such that we can use this information to figure out the functionality of the molecules.

First ruler comes to your mind, I guess, will be the technique of X-ray diffraction. It is a true powerful optical ruler. After all, the DNA structure is solved by it, and Nobel Prize acclaims this technique for more than once. It has great resolution ~ 1Å, and you do not have put anything attached to the molecules. However, the pitfall is that, you have to crystallize the molecules which are merely impossible for some molecules, and the crystal forms of the molecules are in general, not the in vivo forms of the molecules. A report on C&EN and JACS beautifully illustrate the structure changes dramatically depending on the environment.

What is the king of in vivo optical ruler? I would say so far it is NMR. It has the resolutions of a few Å, and the algorithm is advanced so much that complex proteins are revealing their true forms (through more advanced multi-dimensional NMR). The principle is very similar to the trick we play with tuning forks. If you hit on one of the forks and bring the other replica close in, you feel the vibration on the second one and actually both will make the same tone without two touching each other. The energy (in terms of sound wave) resonates in these two forks. In NMR, intrinsic atomic spin plays the role of the tuning fork. By incorporate the isotopes (such as ¹H ¹³C and ¹⁵N, these atoms have nonzero spins) into the amino acids of the proteins, the spins of these atoms behave like tuning forks with different tones. Imaging if there are two or more isotope atoms close to each other, the energy (in this situation, the microwave qunta) will be transferred between the isotopes (let’s say between ¹H and ¹⁵N) assuming one of them is excited by microwave. NMR is specialized in measuring this energy transfer. The closer they are the more efficient energy transfer is. This efficiency drops proportional to 1/r^-6, where r is the distance between two spins. Now, if you have many of these isotope atoms located at different amino acids of a protein, you can figure out which isotopes (or more interestingly, which amino acids) are closer to each other. With the help of computer, you can infer the structures of the proteins, much like a complex trigonometry based on the relative positions of the spins.

Figure 1. A typical 2D NMR spectrum. Each blob can be thought as a sign of energy transferring between the spins of some specific hydrogen and nitrogen. By doing this kind of cross mapping, we can figure out the 3D structure of the molecules.

Two other techniques, Förster resonance energy transfer (FRET) & multi-dimensional IR spectroscopy, utilize similar principles we just described. In FRET, fluorescent dyes are attached the molecules of interest and lasers with optical frequencies are often used. If you excite one of the dye with the laser, and if the second dye is very close to the first dye, you can actually observe the light emitted from the second dye, much like the tuning fork instance we mentioned. It has the resolution of ~ nm and is widely used in biological society. On the other hand, multidimensional IR spectroscopy uses intrinsic vibrational modes of the molecules, such as the stretching mode of C=O. C=O is abundant in a protein which makes it very attractive. Through this technique, you can follow the energy is transferring from one C=O to another, and figure out the distances between two amino acids. The resolution is also in the Å scale. Another neat thing about it is that, multi-dimensional IR is able to monitor the structural change in fs to ps time scale, and this makes it very unique.

Last but not least, let’s touch the topic that initiates this short article – an optical ruler based on plasmon. As you may already learn, the plasmon is some electrons oscillating on the surface of a nano-rod. Lasers with optical frequencies are very effective in exciting these plasmon modes. Another thing you also need to know is that, plasmon modes are sensitive to the surrounding, especially when there are other nano-rods around. Again, just like the tuning fork, energy (in terms of plasmons) that locates on one nano-rod can hop onto another nano-rod. The efficiency of this hopping, and the resonant frequency of the plasmon mode are highly dependent on the overall geometry of the nano-rods and the distance among them. This is the principle we are applying. The ruler is composed of 5 nano-rods (figure 2), carefully spaced to each other. Depending on the overall geometry, the transmission spectra of the ruler is changing dramatically. This change serves as a “legend” for 3D mapping (figure 3). 

Figure 2. 3D layout of the plasmon ruler. The nano-rod in the middle has a dimension of  40*80*260 nm and is directly excited by the light source. Courtesy of N. Liu, M. Hentschel, T. Weiss, A. Alivisatos, and H. Giessen in Science 332 1407 (2011).

Figure 3. Depending on the relative position of the middle rod with respect to the other four, the transmission at certain wavelengths (as pointed out as resonance I and II) changes dramatically. This is the optical legend that can be used to infer the relative positions of the rods. Courtesy of N. Liu, M. Hentschel, T. Weiss, A. Alivisatos, and H. Giessen in Science 332 1407 (2011).

The future goal will be attaching this genre of nano-rods onto different domains of the molecules and monitoring the spectra of them. By doing so, you can figure out the distance among the rods and then map out the distances among different domains of the molecules (figure 4).

Figure 4. By attaching the nano rods (shown in yellow) onto different domains of the molecule, we can infer the 3D structure of it by interpreting the spectra of the nano rods. Courtesy of A. Mastroianni, S. Claridge, and A. Alivisatos in JACS 131 8455 (2009).

Well, next time when people are interested in optical ruler, maybe you should just say: “which kind of them are you interested!?”

DISCLAIMER

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.