Ateneo Physics faculty Dr. Raphael Guerrero presents his work on volume holography at the SPIE Optics and Photonics 2015 Conference

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Dr. Raphael Guerrero, Associate Professor of the Department of Physics, attended the SPIE Optics and Photonics Conference at San Diego, California last August 9-14, 2015

by Quirino Sugon Jr

Dr. Raphael Guerrero, Associate Professor of the Department of Physics, attended the SPIE Optics + Photonics Symposium held in San Diego, California last August 9-14, 2015. Dr Guerrero presented a talk entitled, “Volume holography with Bessel-like reference beams,” with his PhD student, Jonathan Manigo as his co-author:

We report volume holographic recording and reconstruction of plane waves using Bessel-like reference beams.  A photorefractive lithium niobate crystal (0.05% Fe:LiNbO3) is employed as the holographic medium in a two-wave mixing set-up.  The reconstructed plane wave has the same appearance as a Bessel beam, displaying a central maximum and concentric rings.  Over a propagation range of 10 to 50 cm, the central intensity is observed to oscillate between maximum and zero intensity.  The holographic reconstruction is capable of self-healing and propagation properties are preserved even with the use of a partially blocked readout beam.  A theoretical framework based on the interference of a plane wave and a Bessel beam simultaneously reconstructed from a volume hologram is able to describe our experimental results.

The Optics + Photonics 2015 Symposium was attended by 4,500 scientists, exhibitors from 183 companies and 250 student chapter leaders. The conference was organized by SPIE, an international society for optics and photonics. SPIE is also a founding partner of the United Nations program,  International Year of Light 2015.

Below is a short discussion on the physics of Volume Holography and Bessel Beams, followed by an interview with Dr. Raphael Guerrero by the Ateneo Physics News.

A. PHYSICS OF HOLOGRAMS AND BESSEL BEAMS

1. Holograms

Holography is the science of generating 3D images using the interference of light waves. These images don’t need special glasses as when you watch 3D movies. In Star Wars, for example, a hologram of Princess Leia’s message to Obi-Wan Kenobi was projected by R2-D2. This hologram is not the same as security holograms used in credit cards and driver’s licenses which are technically not holograms at all but simply stacks of images that can be viewed from different angles.  Nevertheless some of these holograms are indeed true holograms in that they use rainbow holography techniques.

Holograms are normally constructed by passing laser light through a beam splitter. One part of the beam goes to the object which reflects the light onto a photosensitive plate. The other beam goes to a mirror and is reflected back to the plate. The two beams are made to meet at the photographic plate where they form an interference pattern.  This pattern recorded on the plate is the hologram of the object.  Once the hologram is recorded, laser light similar to that used to make the hologram is used to illuminate the photographic plate and voila! a virtual image forms out of thin air. To create a moving 3D image of Princess Leia, a simple photographic plate would not be enough since many images to capture motion should be recorded. That is why you need multiplexing techniques for holographically storing several images. One way is to use encode one of the recording beams using an elastomer phase mask: as you stretch the elastomer, different holograms may be created, which allows you to create a short movie similar to the short gif videos you see in Buzzfeed.

Holography can also be performed using a 3D crystal instead of a flat plate. This is called volume holography, where the thickness of the recording material is much larger than the wavelength of light used in recording. Dr. Guerrero and his student used a lithium niobate crystal for their volume holography experiments.

2. Bessel Beams

Bessel beams are named after Friedrich Bessel who studied the differential equation that bore his name. The solution to this differential equation. is called a Bessel function. This function arises when you try to solve wave propagation problems involving systems with cylindrical symmetry, such as the circular membrane of drums, i.e. rock concerts. In Optics, Bessel functions also arise because of the cylindrical symmetry of lenses, i.e. the output of the laser can be fashioned to behave approximately as a Bessel beam whose cross-sectional intensity distribution follows the Bessel function distribution.

Bessel beams have many interesting properties. One is nondiffracting propagation, i.e. the beams don’t spread out over long distances, which may be useful if you wish to design blaster rifles for Imperial Stormtroopers. Another property of the beams is that they are self-healing so they can be partially blocked but their waves regroup themselves after the obstacle, just like ghosts passing through walls.

Guerrero and Manigo used a  laser beam with a Bessel profile to record information in a photorefractive crystal.  From their experiments, they found that using a Bessel beam to read a hologram leads to interesting properties of the reconstructed output such as self-healing and a built-in oscillation of the beam intensity.

B. INTERVIEW WITH DR. RAPHAEL GUERRERO

1. Where have you been?

I was in the US last month to attend the SPIE Optics + Photonics 2015 Symposium San Diego, California. One part of the symposium was a conference dedicated to optical data storage. I presented a paper on volume holography with Bessel-like reference beams. I co-authored this paper with my PhD student, Jonathan Mañigo. We are hoping that the paper will be part of his dissertation. I was able to meet many scientists from China and the US working in holographic data storage. Although the optical data storage conference was just on Aug 9, the entire SPIE symposium was from Aug 9 to 14.

At the symposium, you can attend the plenary talks. All sessions are open. I listened to an interesting plenary presentation about how they managed to land a probe on Rosetta, a comet. The project team leader described how they went about designing the mission to land a probe on the comet and how they used optical techniques. There were also talks on the importance of nanotechnology in modern optics and on new types of materials with interesting applications. Overall it was a very nerdy event.

2. Any side trips after the conference?

This time I visited Grand Canyon during some personal time. After I visited my brother in Tennessee to attend my nephew’s wedding, I flew to Phoenix in Arizona. I rented a car and went on a roadtrip adventure. I drove to Flagstaff, which is a city close to Grand Canyon, about 2 hours away. I drove all the way from Phoenix to Flagstaff and Grand Canyon. It was a desert environment and my airconditioning was always turned way up. My wife joined me in Arizona to fulfill our dream of getting a scenic picture together at the South Rim of Grand Canyon.

Grand Canyon is truly very grand. It’s a sensory overload. There’s so much to see in these vast and beautiful geologic structures that took billions of years to create, especially the different rock layers of the canyon. I checked off my target photos one by one. I have a nice photo at the edge of Grand Canyon. What’s weird is that there are no safety precautions. At the South Rim the edge is a mile high. Accidents happen and some tourists become permanent residents.

3. How many students do you have right now?

I have four undergraduates who are working to defend their theses at the end of the school year. They are working on diverse topics: holography, nanocellulose, liquid waveguides, and photovoltaic cells. I accepted three new Juniors, but I have yet to determine what their specific areas will be. This gives me a total of seven undergraduate students this semester.

In terms of graduate students, I have four active PhD in Physics students. Jonathan Manigo is in line to hopefully graduate by the end of this school year. We have submitted a paper to a journal and are hoping for a positive review before his defense in the Second Semester. Alvie Asuncion is working on volume holography and Flora Renovalles is using oblique deposition of aluminum to enhance the diffraction output of an elastomeric grating. Crismar Patacsil is working on fluorescence properties of nanoparticle solutions. These are the four PhD students at the dissertation level. If we get very lucky, they should be publishing within the year to revitalize our PhD program. I have an optimistic assessment of their status. I don’t have any MS Physics students yet. But I am happy that we have new MS Physics students in the department and hopefully some of them will be working here at the Photonics Lab.

4. Do you have any research grants?

I was able to obtain a grant from the National Academy of Science and Technology (NAST) as part of my Outstanding Young  Scientist Award. The grant package is half a million for research. I allotted most of it for a new laser, while the remaining amount I budgeted for a few supplies and conference assistance  The lab now has a new tunable He-Ne laser  with five output wavelengths. We bought it for Php 350,000 pesos from a supplier in Singapore. The laser was manufactured in the US and is worth USD 8,500. The wavelengths are 633 nm, 612 nm, 604 nm, 594 nm, and 543 nm. For our paper in San Diego, we used the yellow line at 594 nm and the green line at 543 nm. It is really convenient to switch wavelengths with a tunable laser. Before when we do holography, we have separate lasers for each color and which involved realigning optics. Now, we can have the optics stay aligned and simply select the appropriate color to record and read out the holograms.

5. What are you teaching now?

I have my usual undergraduate Introductory Physics courses. I have two sections of Physics 11 for Bio and Health Science majors who are taking up Physics for their future medical careers. I have a section of Ps 1 Conceptual Physics. I’m starting Ps 1 with Optics, teaching freshmen about lenses and how light has changed our perception of the universe. I have a class of undergraduate Physics majors with Ps 197 Quantum Mechanics. It’s a large class of 20 students. The graduate students have me as their teacher in a few electives. I am teaching Ps 259 Quantum Electronics, as well as Ps 260 Geometric Optics, and Ps 201 Theoretical Mechanics. Aside from these, I also teach Sci 10 which discusses technology and how it benefited society. I advise thesis students,  coordinate the Photonics laboratory, and advise graduate students during enrollment. I am definitely earning my pay!

6. What are your plans for this year and the next?

I am looking forward to having several PhD students graduate, that’s my wish for this year. If all goes well with the paper we are writing, I’ll find time to start writing another paper which shall form the basis for another student’s dissertation. Next in line is Flora Renovalles’ paper on elastomeric gratings coated using oblique angle deposition.

7. Any parting words?

I wish I was still on vacation! But while I’m at work, it’s always nice to have productive days when I can still do some research while teaching.

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Dr. Raphael Guerrero at the edge of one of the cliffs of Grand Canyon

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About ateneophysicsnews
Physics News and Features from Ateneo de Manila University

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