The shiniest berry in the world – what biophotonics in nature has taught us.

It is an understatement when we describe nature as the most talented painter. In fact, she is not only the greatest artist, but also the most renowned scientist, in essentially all aspects. Her scientific achievements are found everywhere. For example, today, much of our knowledge in the field of bio-photonics is just a re-discovery of what she has done (Another interesting topic which relates the evolution to optical science can be found here).

Many of the astonishing color patterns we found in the insect kingdoms are manifestations of nanometre-scale architectures. These architectures are collaborative works of cells. Those cells cooperate together to create optical effects we widely apply in modern photonic science. For instance, butterflies have cells structures that look like multi-layer reflective coatings on their wings. Depending on the thickness of each layer, different colors present vividly. Same tricks have been adopted and perfected by many shiny beetles. As shown in figure 1, enchanting colors on the surface of the insects are precisely the magic of multilayer structures. The layers are mostly composed of thin parallel sheets of chitin (secreted by the epidermis and often interspersed with other organic components). These layers differ in refractive index. And again, depending on the spacing between these layers and their indices of refractions, different colors can be reflected. Furthermore, some insects have arrays of very fine elements, known as nipple arrays, which look like micro lenses with subtle variation of index of refraction, to reduce reflectivity in their compound eyes and enhance collecting the light from the environment. Nature did create optical science way before mankind stole fire from Prometheus!




Figure 1. (a) A presentation of simple cuticular multilayer reflector. (b) The cross section of a cuticular reflector. (c) A colorful buprestid. (d)-(f) Different structures of cuticular multilayer reflectors commonly seen in insects. Courtesy of A. E. Seago, P. Brady, J-P. Vignerson, and T. D. Schultz in J. R. Soc. Interface 6(supp2) S165–S184 (2008).

Recently, researchers have discovered another beautiful example. A marble berry (Pollia condensata) originates from Africa produces colors like jewels (Figure 2). Looking at the photos of it, I actually feel it looks more like a star sapphire. Maybe we could nickname it “star-sapphire-berry”? Or “jewel berry”? Each berry is about 5 mm in diameter, and according to the authors, there are several wonders of these berries: (1) its charming metallic color is rare in nature. (2) If you look carefully, it has a brilliant pixelated appearance with green, purple, and red speckles (The authors call it “pointillism”, apparently they also think nature is an artist). (3) The color, if well preserved, can last more than 40 years, and counting. (4) It reflects about 30% of the sunlight, which makes it the shiniest organism we have found in nature so far. Colorful beetles can only take the 2^nd place. So how do all these happen?
 

Figure 2. (a) The charming marble berry (Pollia condensata). (b) A single bunch of the fruit. Courtesy of S. Vignolini, P. J. Rudall, A. V. Rowland, A. Reed, E. Moyroud, R. B. Faden, J. J. Baumberg, B. J. Glover, and U. Steinera in PNAS 109 39 15712 (2012).  

The secret of this appealing demonstration lies on the structures of the cell walls of the cells that populate the surface of the fruit. Each cell has the cell wall composed of multi-layer cellulose micro-fibrils. The layers of micro-fibrils form an assembly called “helicoidal assembly” in which each layer has an orientation slightly shifted compared to the previous layer (Figure 3). The periodicity (p) of this helicoidal assembly defines the variation of the index of refraction in the cell wall, and this feature further defines what color the cell wall can reflect. The maximum reflectivity wavelength (λ) is found to be λ = p *2*n, where n is the average index of refraction of cellulose (n = 1.53). In the case of the marble berry, most of the cells have periodicity of 145 nm, which makes its blue coloration in general. Since each cell is a pixel on the surface of the berry and the value of p varies from cell to cell (from 125 to 200 nm), and, you see the fine pixilation of different colors (ranging from red, green, to purple). In addition, due to the handedness of the hellioidal assembly, the reflected light from each cell is either right-handed (RH) circular polarized, or left-handed (LH) circular polarized (Each cell has its own handedness, and on average, the total number of cells having RH equals to that having LH).

Figure 3. Top left: The layers of the cellulose micro-fibrils form the helicoidal assembly. Top right: The simulation shows the structure of each layer and how cirlular polarized light is reflected. Bottom: How light is reflected when it hits different cells on the surface of the berry. There are 6 cells shown in the figure, each one has a different periodicity, so each one reflects different colors of light. Courtesy of the same authors in figure 2.

The surface of the berry is scrutinized under a powerful microscope and the result is shown in figure 4. In A/B/C of figure 4, only LH/RH/cross polarized light is selected for detection. In A and B (especially the insets) of figure 4, the reflection from each cell is either LH or RH circular polarized light (The cell reflects a shiny strip of light in either LH or RH detection, but not both). Overall, these microscopic images clearly depict that each cell is a building block of the alluring coloration on the surface of the berry. Amazingly, another organic bio-photonics is deciphered by the scientists and presented to us.



Figure 4. (a)-(c) The images of the berry under microscope. The microscope is set up in a way that it picks up only the desired polarization as specified in the text. (d) The experimental setup. (e) The reflective spectra of two different cells. Courtesy of the same authors in figure 2.

If you look carefully enough, wonders are hidden in nature everywhere. We as human are picking up very fast in adopting the gift given by nature into technology applications. I hope by now, you are intrigued by bio-photonics. If that is the case, mark your calendar and join us in bio-photonics section in CLEO 2013. It will be a great conference for you to immerse in this astounding field.

Already doing research in this direction? Submit your abstract before January 30^th. Share with us!

DISCLAIMER

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

A big step forward in lens design -- aberration-free ultrathin flat Lenses made of metamaterials of gold antennas.

If you think metamaterial has only “invisible cloak” and/or “negative index of refraction” in her hat, think again. Researchers from the school of engineering and applied science at Harvard utilize a powerful feature of the metamaterial to create ultra-thin and flat lens that is diffraction limited. They also create a flat axicon as another example when pioneering in this field (A quick glimpse on the Axicon: A lens with a flat surface on one side, and a conical shape on the other, has the ability to focus a Gaussian beam into a Bessel beam at the focal region, and create hollow ring beam shape in the far field).

To focus light, we need to create a converging spherical wavefront, or at least, a wavefront that is converging. To do so, we need to introduce different phase retardation on different portion of the incident light. A spherical lens does so by letting the light pass through different amount of material. For example, when light is passing through the center of the lens, it lags behind compared with that passes through the edge of the lens. As a result, there is a phase difference between them. This phase difference, or phase retardation between them, produces a converging wavefront. However, when a plane wave like light passes through a spherical lens, it suffers from spherical aberration. That is to say the spherical lens does not produce a perfectly converging spherical wavefront which is required for the light to focus tightly. The light exiting from the edge of the lens suffers stronger deviation. The best way to solve this so far is to use an aspherical lens to correct this imperfection.

A puzzle like this can be solved beautifully by the use of the metamaterial. If we are able to create metameterials which can introduce different phase retardation, arrange them in a way such that they produce a converging spherical wavefront, we can have a flat, thin and aberration-free lens. This idea is realized and performed nicely finally. Researchers create gold antennas with different shapes. When impinged by the light, each shape is able to create different amount of phase retardation while the scattering intensity remains similar. From figure 1, we can see clearly that only the phase of the impinging light is shifted while the scattering amplitude remains close to each other. The lens is then realized by patterning silicon wafer with these gold nano-antennas using electron beam lithography. The antennas with different shapes are arranged in a ring pattern which would give the desired phase retardation. The spacing between the antenna arrays is covered with silver and titanium which completely reflects the fraction of the incident beam that is not impinging on the antennas. The wavelength of the incident light is 1.55 um, and the diameter of the lens is just merely 0.45 mm.

 

Figure 1. (a) The small antennas at work: different shapes of gold antennas are able to produce phase retardations. The sizes of the antennas are ranging from 85-180 nm with the width of 50 nm. (b) The experimental layout. (c) The arrangements of the antennas on a lens made of silicon wafer. The antennas are spaced ~ 500 nm to each other. Courtesy of F. Aieta, P. Genevet, M. Kats, N. Yu, R. Blanchard, Z. Gaburro, and F. Capasso in NANO letters.

The lens has one more special feature, it takes linear polarized light and focuses the light in a cross polarization way. In other words, if you put in x-polarized light, the resultant converging wavefront is y-polarized, and vice versa. In addition, to create a converging spherical wavefront, the phase retardation has to be a hyperbolic function vs. the radius of the lens. And to create an axicon, the phase retardation has to be conical. A contour plot of the phase retardation vs. the radius of the lens is shown in figure 2. The results compared to the calculation are very promising, as shown in figure 2, lower part.

Figure 2. The phase retardation introduced by the gold nano antennas. On the left is the phase retardation for a focus lens; on the right is that for an axicon. The result, shown in the bottom of the figure fit well with the simulation! Courtesy of F. Aieta, P. Genevet, M. Kats, N. Yu, R. Blanchard, Z. Gaburro, and F. Capasso in NANO letters.
 

By arranging the antennas differently, we can create virtually all kinds of lens. For example, if the array of the antennas has translational symmetry, we can have an elliptical lens. By tailoring the phase front to have great curvature, we can have a high NA objective. In addition, they are flat, thin and easy to assemble. There is a caveat to be aware of. At the moment, only 1% of light is transmitted, since the antennas only cover small amount of the area on the wafer. In order for the real application, the lens efficiency has to be decent, maybe around 80%. The researchers are trying diligently to increase the efficiency, presumably by increasing the density of the antennas.

There are quite a few other interesting flat lens designs using nanotechnology. A few examples are nano-holes and nano-slit lens. For the reflective optics, flat dielectric grating reflector with nano-scale patterns is also attractive. Feel excited? Just click on the links.

DISCLAIMER

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

Applications of photonics in photochemistry, green energy, and more!

I am pretty sure you have loaded up your crazy mind with a big chunk of knowledge on Wednesday. Plenary session, a whole day of exhibition, and enthusiastic poster session guarantee everyone finding its own corner. What excites me the most is to see the interplay between different research fields. Like all of us today, I am happy to learn that photonics also finds its applications at each corner of the science. Let’s encapsulate a couple of them:

! Harvesting green energy with the help of photonics fibers !

Converting the solar energy into chemical energy is not a new idea. One interesting way of doing so is to grow algae, such as cyanobacteria. After you grow tons of them, you essentially squeeze them to get Algae oil, which is used to fuel the world (I sincerely hope it smells like olive oil). However, just like everything in the practical world, it faces some challenges, especially in terms of efficiency. It turns out that cyanobacteria are very picky about where they live. The amount of sunlight has to be just right for them to prosper. Like figure 1 shows, the optimal condition is only about 10 cm thick somewhere below the surface of the pond (or pond reactor). Same situation applies for the tube reactor. As you can see from the figure, most of the space is wasted.

Figure 1. The optimal zone where the algae grown. Courtesy of  D. Erickson at http://www.cctec.cornell.edu/events/ctvf11/Jung.pdf.

Thanks to the researchers from Cornell University, we seem to have a solution now (a PowerPoint presentation about this topic is also available online). By pumping the light into a photonic crystal fiber (PCF) or a waveguide, you create total internal reflection on the inner surface. However, some very small portion of the E field is leaking through, which is known as evanescent field. Interesting enough, the intensity of evanescent field is very sensitive to the input angle of the light, and it dies out about 1 um away from the surface of the PCF or the waveguide (1 um is just about the size of a single cyanobacteria, figure 2, upper left). Utilizing the phenomenon of evanescent field, you can control the light intensity very precisely. As a result, the 1 um layer outside the PCF or the waveguide can be adjusted to a sweet home for the bacteria. On the other hand, you can pack a lot of the PCFs or waveguides into a container of the media where the bacteria grow.  This design also saves space (figure 2, right)!

Figure 2. (a) A cartoon representation of the waveguide, a bacterium, and the decay evanescent field. (b) An experimental setup for the growth of the bacteria. On the right, an idea proposed by the authors shows how to couple the sunlight into 4 waveguides. Courtesy of M. Ooms et al. in PCCP 14 4817 (2012).

! PCF as a nano chemical reactor !

Measuring the absorption of a solution tells us a lot about the microscopic world. Some time in the journey of our academic life, we all had the chance to measure the absorption of an unknown solution, and tried to figure out what is in it. Normally we used a standard 1cm cuvette to fulfill our mission. We learned from the conference that PCF can play a better role in this old task. Professor Russell’s group in Max Planck Institute utilized a hollow core kagome PCF to replace the traditional sample cuvette (kagome is a pattern constructed by interlaced triangles; kagome PCF’s cross section has this pattern, figure 3). There are a couple of reasons to support doing so: 1. The sample volume per optical path length is very small since the hollow core diameter is very small (2.8 nL cm^-1 in the fiber the researchers used). 2. You can have a very long optical path length that is extremely useful for very dilute sample (you need to have a long optical path in order to have enough absorption by the sample -- Beer’s Law). 3. Light travels in a diffractionless single mode within the fiber, which reduces a lot of complexity.

Using these strengths, they are able to monitor a real time reaction undergoing in the PCF. What you have to do is to pump the solution into the PCF and monitor the absorption over the course of time (figure 3). The photo-chemical conversion of vitamin B12 to hydroxoco-balamin [H2OCbl]⁺ in aqueous solution was measured for several pH values from 2.5 to 7.5 by this way.

Figure 3. (a) The cross section of the kagome PCF. (b) The light distribution in the fiber. Middle: the photochemical reaction in the experiment. Bottom: The experimental setup where a single-syringe infusion pump is used to pump the solution into the fiber. Courtesy of J. Chen et al. in Chem. Eur. J. 16 5607 (2010).

Some other applications such as using 3D inverse-oval photonic crystal to detect different solvents are out there waiting for your exploration. Don’t stop here; stay thirsty as Jobs like to say.

Time flies by pretty fast. Enjoy the rest of the conference, and check out what you can do in San Jose (maybe drive to San Francisco after the conference, you deserve a nice trip after a full week of intellectual challenges)!



DISCLAIMER

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

Nanolasers are the rising stars

If you attend “QM1H • Spasers and Nanoemitters” today, you know exactly what I am talking about. Exciting new materials, including metamaterials, quantum wells, and quantum rods, are used for the realizations of the nanolasers. If you missed it (which is quite possible since there are many other outstanding technical sessions packed today), this short article is your second chance. Three examples of nanolasers are presented here (summarized from today’s speakers) to give you a taste of the flavors.

Nanoscale coaxial lasers:

This is a piece of artwork of nanofabrication. The researchers from UCSD are able to fabricate a nanoscale coaxial laser cavity on an InP substrate (figure 1). It is composed of a metallic rod with different coaxial disks. One of the disks, the gain medium shown in red, is made of 6 quantum wells (each one is made of Inx=0.734Ga1–xAsy=0.57P1–y / Inx=0.56Ga1–xAsy=0.938P1–y, with an overall height of 200 nm). They are sandwiched between SiO2 and air plugs. With the help of these two plugs, the entire device behaves like a cavity which supports a few sparse EM like modes (figure 2). If you pump the device in a right way, you can excite these modes and build them up. The lower air plug also allows pump energy into the cavity and couples out the light generated in the coaxial resonator. So once you build up the modes, you can couple the light out. In other words, you can make this device lase.



Figure 1. The structure of a coaxial laser cavity. The enitre thing is ~ 500 nm in all dimensions. (b) and (c) shows the SEM images of two different structures. Courtesy of M. Khajavikhan et al. in Nature 482 204 (2012).

Figure 2. The EM like plasmonic modes that can be supported by the cavity. Two different structures support different modes. Some of the modes can be pump and excited by 1064 nm laser. Courtesy of M. Khajavikhan et al. in Nature 482 204 (2012).

The researchers pump this nanolaser with a 1064 nm laser and it will lase at 1.26 and 1.59 micron at room temperature depending on the overall structures of the nanodevice. I would like to have one of these as souvenir. 

Lasing spasers:

SPASER stands for Surface Plasmons by Stimulated Emission of Radiation -- an idea proposed by Bergman and Stockman in 2003. They suggested that it is possible to construct a nanodevice in which a strong coherent field is built up in a spatial region much smaller than the wavelength. Simply speaking with the help of figure 3, you can induce and build up surface plasmons – the oscillation of electrons on the surface of a nanostructure – by providing it energy. In the example of figure 3, the energy is coming from the excited nanocrystal quantum dots (NQD). Once this oscillation starts, it further drains energy from NQDs into it and builds up strongly. As an analogy, the nano-structure (nano silver shell, for example) confines the plasmons onto its surface, which behaves like a laser cavity confining photons. And the quanta of the plasmons are like photons in the cavity.

Figure 3. (a) A theoretical spaser made of a nano silver sphere coated with NQDs. (b) the energy diagram shows how the energy is transferred from the NQDs to the plamonic modes of the nano silver sphere. (c) and (d) show two different plasmonic modes of it. Courtesy of M. Stockman in Nature Photonics 2 327 (2008).

However, the ideal spaser does not emit light; it simply converts more energy into its in-phase plasmonic modes. One way to make it emit light is to create an array of nanostructures – a principle proposed by professor N. Zheludev et al. and shared at CLEO 2012:

In their “lasing spaser”, the nanostructures are a two dimensional array of metallic nanowires (figure 4). They are situated on the surface of the amplifying medium (gain medium). One of the possible amplifying media can be a substrate packed with quantum dots. A working principle is like this: a pump source is used to excite the amplifying medium (in figure 4, a pump laser is used to excite the quantum dots in the substrate), the nano wires drain the energy from the excited quantum dots into its plasmonic modes, and the currents start to oscillate back and forth within each nanowire (figure 4). Most of the currents cancel each other if viewing the device from the far field. Only the currents on the edge of the array survive the cancellation and behave like an oscillating dipole that emits light (figure 4, bottom). The light would be more intense if the plasmonic modes have more quanta. The entire device can be a few tens of microns while it emits near or mid-IR light (tunable by adjusting the structure of the nanowires).

Figure 4. The device is made of an array of nanowires, gain medium (shown in green), and pumped normally with a laser. At the bottom, it shows how the nanowires behave when viewing them from the far field: the currents in the center of the array look like cancelling each other out, while the currents at the edge can emit light. Courtesy of N. Sheludev in Nature Photonics 2 351 (2008). 

Lasing in self-assembled microcavities of CdSe/CdS core/shell colloidal quantum rods:

This laser is a bit bigger. It is about a few hundred microns but it is something you might be able to do if you have the quantum rods provided by the researchers of Italy. What you have to do is very straightforward. Dissolve the quantum rods in toluene, put a droplet of it on the glass substrate, and wait until it dries. Then you get a microcavity that will lase. Pretty amazing, and this is how:

The building block of this microcavity is a CdS quantum rod (about a few tens of nanometers) with an embedded CdSe nanocrystal (figure 5). What special about this rod is that it is a strong fluorescent little guy with a QE of ~ 70%. In other words, if you pump it with light, it tends to give back its energy through fluorescence. Put a droplet of toluene containing many of the rods on the surface, it will dry out in a special way: The convection created inside the droplet due to the evaporation of it pushes the rods condensate at the border of the droplet. Not only so, the rods will pile up in a regular pattern. Microscopically, you have a wall (tens of microns wide and tens of nanometers thick) of rods piled regularly on the border of the droplet. This wall is very fluorescent (since they are made of rods), and can behave like a cavity (since the wall has very different refraction index compared to the surrounding, the fluorescent light can be bounced back and forth between the wall). If you pump the wall with laser (532 nm) normal to the glass surface, you are able to create a lasing phenomenon at ~ 610 nm (figure 5 bottom). You can proudly say: everyone is able to make a microlaser if the rods are available in the market.

Cheers! And look for more at CLEO 2012!

Figure 5. The building block of the microlaser: a CdSe embeds in a CdS quantuam rods. The lower plot shows how the device is lasing: You pump the dried droplet at its border normally and the device can lase at ~ 610 nm. Courtesy of M. Zavelani-Rossi in Nanoscale 2 931 (2010).

DISCLAIMER

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

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.