How to tune color frequencies of light using fluidic elastomer grating: an interview with Dr. Raphael Guerrero

A man standing in front of a cream building

Associate Professor Dr. Raphael Guerrero of the Department of Physics of Ateneo de Manila University attends the SPIE Photonics Asia 2014 at Beijing, China.

by Dr. Quirino Sugon Jr

Associate Professor Dr. Raphael Guerrero of the Department of Physics of Ateneo de Manila University and his student Sarah Oliva wrote a paper entitled, “Optical wavelength tuning via actuation of fluidic grating,” which was published last January 23, 2014 in SPIE’s Optical Engineering journal.

A grating is normally a polished metal with grooves similar to what happens when you grate a cheese with a fork, except that your cheese is a polished metal and your fork is much smaller than the width of your hair.  A good example of a grating is a CD (compact disc): the concentric grooves are finely spaced, so that when light strikes the CD, light beams of particular color (frequency) have a preferential option for reflecting in one direction and those of different colors are reflected in another direction.  The result is a rainbow pattern.

But instead of a metal grating, Dr. Guerrero and Sarah used Sylgard 184, an elastomer or elastic polymer, in order to form a replica of a standard metal grating through a cast and mold process.  The elastomer grating bends like rubber but looks transparent like glass.  The authors constructed a hollow cavity with a grating membrane and filled it with water using a syringe.  As the water enters the cavity, the elastomer grating expands, thereby increasing the grating’s groove spacing.  Once this happens, the various colors present in white light are transmitted through the grating differently compared to the unexpanded situation. This mechanism allows us to scan different color frequencies for a fixed light receiver, in the same way we turn the knob of an analog radio in order to tune to the correct AM or FM frequencies of our favorite radio stations.

Below is an interview with Dr. Raphael Guerrero by the Ateneo Physics News.

A man's face with the Statue of Liberty at the background

Dr. Raphael Guerrero with the Statue of Liberty in New York at the background

1. How long did it take you to finish your research?

This is the second paper I was able to publish with Sarah Oliva, one of our recent BS Physics-MSE graduates. She was member of the lab for her two undergraduate theses and we were able to produce two papers.  Her physics thesis was on the development of a fluidic actuation system for an elastomeric grating.  Overall the work took a year for the development of the theory, the collection of appropriate data, and the writing of the paper for the actuation mechanism. We demonstrated how fluid injection can increase the groove spacing in a grating replica, which allows us to change the diffraction angle of laser beams.

During her fifth year thesis for Materials Science Engineering (MSE). we looked at another set of data using an incoherent, broadband light source such as a mercury lamp. That is, instead of using the grating to deflect a laser beam,  we used the grating to select a particular wavelength of the incoherent light source.  This led to the second paper.

Basically, we used the same setup which we used for the coherent (laser) source, but replaced it with an incoherent broadband source. We used fluid injection to scan a range of wavelengths. The first paper on laser scanning was published in Applied Optics. The second on tuning a broadband light source was published in Optical Engineering. We were lucky that we got two papers out of a year of work.

Sarah worked on the theoretical framework.  It was fortunate that she was taking a programming class at that time and she was working on numerical techniques for solving polynomial equations. We needed this polynomial solution technique in order to determine how the radius of curvature changes as a function of injected water volume.

2. Who introduced you to elastomer research?

I was first introduced to elastomer research by Dr. Greg Tangonan.  In 2004 we were able to use soft lithographic techniques for making an elastomer replica of a standard commercial grating out of Sylgard 184 (polydimethylsiloxane). We had the replica sample coated in the City University of Hong Kong and presented the results in the 2004 SPP National Physics Congress in Bohol, Philippines.  That was our first elastomer paper.

After several undergraduate theses. we published our first ISI-indexed paper on elastomeric gratings in 2007.  We needed  three years to collect enough data and develop the necessary techniques.  Our paper was published in Optics Communications.  In that paper, we described the effect of the buckling of the metal layer due to strain applied on the grating.

After another batch of  undergraduate theses, we found an alternative method for actuating the elastomer grating. Previously, we just stretched it with a mechanical holder.  We became aware of how a shape memory alloy returns to its original form after applying heat.  We inserted such an alloy into our elastomer grating and heated the alloy using electric current.  This led to a concave grating.  We presented our results in Applied Optics in 2010.

The Photonics lab has been very successful with the elastomeric grating. The papers on a fluidic actuation mechanism for the elastomeric grating were published in 2012 and 2014.  We even published an education paper in 2008  where we showed how elastomeric gratings can be used for grade school experiments: students can use the grating to see the rainbow pattern of different light sources. In that paper, we marketed the grating for teaching grade school optics. There is no worry about damaging the optics because the elastomer is flexible and inexpensive. Even if you get the elastomer dirty, you could still wash it because it’s a polymer. In total, we’ve published 5 papers about the elastomeric grating.

Man standing beside Darth Vader

Dr. Raphael with Darth Vader at Legoland California

3. How many students do you have?

I have two undergraduate students who are about to defend their thesis proposals. I have three undergraduate apprentices who are incoming juniors. I have a graduate student who is also a fellow faculty member, Cindy Cease.  I have four Ph.D. students in the pipeline: we have one student working on fluorescent nanoparticles in solution, two students working on volume holography of Bessel beams, and one student studying chlorophyll -doped elastomers. These four Ph.D. students will hopefully churn out four to five papers in the near future.

4. What are your future plans?

In the short term, I hope to submit two papers. I am currently making major revisions on the theoretical framework for a paper on variable fluorescence output from an electrowetting droplet. Essentially, we have droplet of water doped with rhodamine. We placed the droplet on a dielectric and apply a voltage through a platinum wire.  When we shine UV light on the droplet, it fluoresces yellow-orange, corresponding to a wavelength of 590 nm. We noticed that when we apply the voltage while the UV light is present, the droplet’s shape changes, and we were able to tune the fluorescence to a different wavelength from yellow to green. This has interesting applications in spectroscopy  or commercial displays.

I was able to submit an original manuscript and was asked to do some revisions. I am now working on the appropriate theory to describe the phenomenon. The revised paper should be submitted before the end of the semester. I am also working on a volume holography paper, this time reporting results on the creation of Bessel beams using photorefractive volume holography instead of computer-generated holograms. Reconstructing Bessel beams is important because of their self-healing properties. Currently, I am revising manuscripts for two papers. I am also asking another PhD student to prepare a manuscript as a basis for her dissertation.  So, in a sense, I am sitting on several ISI-indexed papers.

If I survive my teaching load, I would have three papers, with one under review by the end of the semester. That’s my short term plan.

For my long term plan, I am waiting for my project proposal to be approved by DOST. The funding would allow the laboratory to purchase a variable wavelength laser source. With this I would be able to look into other topics. Right now, the immediate application of this laser would be modifying the propagation properties of Bessel beams, so that we can record them via volume holography. This proposal is part of the grant I received as one of the NAST (National Academy of Sciences) awardees last year.

5. Can you tell us more about your awards last year?

I received the NAST-OYS (Outstanding Young Scientist) award and the NAST-TWAS (The World Academy of Sciences) award for Physics. The NAST-TWAS is the World Academy of Sciences Award for Outstanding Scientists in developing countries. Last year I was nominated for the physics category of NAST-TWAS.  In other years, the award is for Chemistry or Biology or Mathematics.  I got some cash. There is also a plaque from TWAS and trophy from NAST. Each award comes with Php 500,000 research grant . To obtain this grant, you write a proposal. After a review and approval by DOST, I should be able to get half a million from each award–about a million pesos in all.

6. What is your current teaching load?

I have 6 units for my Third Summer Leave. For my first semester after sabbatical leave, I was assigned 18 units for the first semester–Ateneo is trying to get back at me for having a year off! Next semester, my load is a bit lighter.

Man in front of a giant roller coaster

Dr. Raphael Guerrero at Cedar Point, the “Roller Coaster Capital of the World”, in Sandusky, Ohio

7. What did you do in your sabbatical year.

I was able to write two papers. One paper was published in Optical engineering last January. I also supervised the graduate thesis of Rea Mero, who finished her MS Physics studies this year. Her results were the basis of the second paper I was able to write on tunable fluorescence from an electrowetting droplet. I am now in the revision stage of the second paper. I also supervised a PhD student and helped prepare a manuscript which shall serve as the main part of his dissertation. Finally, I prepared a proposal for DOST to take advantage of my OYS grants.

In addition to academic pursuits, I took a lot of time off to sleep in order to recharge for another six years without a sabbatical leave.  It was a good year.  It’s always a good year when you get published.

8. Where did you go during your Sabbatical Leave?

I spent the first two months of my vacation with my wife and her family in the US. We went everywhere along the West Coast.  We went to theme parks.  We went to Las Vegas–my first time. We then visited my brother in Tennessee. We went to the “roller coaster capital of the world” in Ohio .  I saw this roller coaster on a website and promised myself that I would ride it.  So I made sure to visit that theme park.

My wife and I also went to New York.  We went to the Natural History Museum with all the dinosaurs. We visited the Empire State Building. We visited our friends in Pittsburgh where the Andy Warhol museum can be found.

Last July I was just here, supervising my MS student and writing papers. Every now and then, I would get invited as a speaker or member of a thesis panel. I did a lot of writing and experiments with my students.

My sabbatical leave started April of 2013 and it ended with my third summer leave last May 31, 2014. That was 14 months.  And they went by just like that. It was very sad by the time May came. By June I had to go back to work with 18 units.

9. You teach tutorial classes?

I have two tutorial classes. I have graduate level Quantum Mechanics and Mathematical Methods for Physics Education. I was able to have my experimental graduate class waived. In all, I have 18 units of regular teaching load and 6 units of tutorial classes.  These filled up my schedule.  When one of my students gets results, I need time to sit down with him and think about his data.  It is hard to find time to just think about research when you are teaching or preparing your lectures.

10. Do you have any parting words

I would like to mention how critical it is to have students who turn out to be excellent researchers.  They are a big help to the Photonics lab in particular and to the the Department in general.  I am glad to find students like Sarah Oliva who as undergraduates are able to produce world class results which could be published in journals. I am riding on the shoulders of my students. Of course, in the writing of papers, you still need to help them.  But over all, with the quality of their research and hard work, I feel blessed.

A man in red standing on the Great Wall of China

Dr. Raphael Guerrero at the Great Wall of China

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About ateneophysicsnews
Physics News and Features from Ateneo de Manila University

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