Jan 27

Why a Temporal-Cloak is so Great: Uncovering the Hype

By JamesVanHowe Uncategorized No Comments »

From R. Boyd and Z. Shi,"News and Views" Nature, Jan 5, 2012, explaining temporal-cloaking

This post originally appeared on Jim’s CLEO Blog and is reproduced with permission from the author.

At Frontiers in Optics 2011 just this last October, Moti Fridman from Alex Gaeta’s group presented work on a the first experimental demonstration of temporal-cloaking using a time-lens system. The work was based upon a theoretical paper from Martin McCall et al in the February issue of the Journal of Optics, and at the beginning of this month, appeared in an in-depth treatment in the January 5, issue of Nature. Besides the usual barrage of bloggers latching onto science-fictionesque results of new research, time-cloaking was also written up in traditional news media such as the Christian Science Monitor.

Temporal-cloaking certainly sounds like something out of Star Trek, but what is it and why is it so great? What makes a temporal cloak truly exciting, and what a majority of the recent articles and posts fail to highlight, is that the temporal-cloak allows cloaking over an infinite section of space albeit for a finite duration of time.

Let’s imagine Harry Potter and his invisibility cloak. If the invisibility cloak is a temporal-cloak, Harry can move as far as he wants to the left-and-right and up-and-down without being seen for duration of the cloaking window. Harry can also move a little bit forward and backward without being seen, but not much or else he will walk out of the cloaking time-window (which is 50 ps for the Gaeta group’s work or about 1.0 cm in fiber). It is crucial that he is in the right place in the axial dimension (forward/backward) since the window occurs at a specific place in space, but he has total freedom in the transverse dimension for the duration of the cloak. Conceivably Harry could pull-off a bank robbery as long as the bank and the vault are inside that particular infinite pancake of cloaking window and within the duration of the window.

Contrast that to a spatial cloak which gives cloaking for an infinite amount of time, but only a finite section of space. If Harry has a spatial invisibility cloak, then he can stand in one spot for as long as he wants without being seen.

Finally, if Harry has a spatio-temporal cloak, conceivably he can maintain invisibility for any duration of time and throughout any volume of space.

The temporal-cloak shown by the Gaeta group is not a practical cloak. If you scrutinize the setup you’ll find that the way that they detect a cloaked event is through lack of nonlinear mixing. A nonlinear signal tells them the event is detected, and no signal tells them that the event is cloaked. You could just turn the power down to get the same result. They also couple into and out of the cloaking window with fiber-couplers between the cloaking apparatus. You can’t send both the signal and the event to be cloaked down the same fiber because if the “event” goes through the same time-lens system as the “signal” the event will appear superposed instead of cloaked. Basically they had to sneak it into the right spot at the right time along a different path of propagation.

However, the point of the work was not to show practical temporal cloaking for masking or encryption, but to show the very odd, very fundamental, and very cool phenomena of creating and tailoring gaps in time. So even if the temporal-cloak won’t be used anytime in the near future for cracking safes, it does bring the optics community closer to a true spatio-temporal invisibility cloak. It might be time to start brushing up on the rules of Quidditch.

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Jan 09

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

By Frank Kuo CLEO Conference, CLEO: Applications, Uncategorized No Comments »

This post originally appeared on CLEO BLOG by Frank Kuo and is reproduced with permission from its author.

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

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?

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Sep 10

Zoo of super resolution microscopy.

By Frank Kuo Uncategorized No Comments »

This post originally appeared on CLEO BLOG by Frank Kuo and is reproduced with permission from its author.

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)

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Jul 30

Evolution, the master of optical science!

By Frank Kuo Uncategorized No Comments »

This post originally appeared on CLEO BLOG by Frank Kuo and is reproduced with permission from its author.

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.

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Jul 14

Are we Entering a Solar Boom?

By JamesVanHowe Uncategorized 1 Comment »

First Solar employees working on the 21 MW solar power station in Blythe, CA in the Mojave Desert. The project was completed in December 2009. Photo from cnet News; originally from First Solar. First Solar just received $4.5 billion in DOE loans to build three new stations in the Mojave Desert whose total output will be 1.33 GW

This post originally appeared on Jim’s CLEO Blog and is reproduced with permission from the author.

This summer seems to be marked by a frenzy of solar energy initiatives and development. The Business News section in the May issue of Nature Photonics reported on four recent major investments in solar technology manufacturing: JA Solar of Shanghai plans to build a 3 GW capacity plant in Hefei, China for the manufacturing of monocrystalline silicon solar cells. Investors have pledged $2.05 billion over the next four years, and production is slated to begin in 2012. Polysilicon Technology Company, a joint venture between Mutajadedah Energy of Saudi Arabia, and KCC Corporation of Seoul will build a $1.5 billion facility to produce solar-grade polysilicon in Jubail, Saudi Arabia by 2017. The Indian government is discussing a joint venture with nanotech company, Rusanano, of Moscow to obtain a consistent supply of silicon for Indian photovoltaic manufacturers with hopes of obtaining 2,000 tons of silicon ingots for solar cell production. And SoloPower of San Jose was guaranteed $197 million from the U.S. Department of Energy (DOE) to build a plant in Oregon for the manufacturing of flexible copper-indium-gallium diselenide (CIGS) for light-weight solar panels.

The DOE made even bigger news for solar energy investment, however, at the end of June when it promised $4.5 billion for the construction of three different California photovoltaic power plants: Antelope Valley Solar Ranch 1, the Desert Sunlight Project, and the Topaz Solar Project. Arizona-based company First Solar, Inc will sponsor all three projects, constructing each solar array with cadmium telluride (CdTe), thin-film photovoltaic modules. Together, the new power plants will provide 1.33 GW (powering the equivalent 275,000 U.S. homes) and offset the generation of 1.8 megatons of carbon dioxide. As described by Alexis Madrigal, author of “Powering the Dream: The History and Promise of Green Technology,” (Da Capo Press, 2011), in a June 17, interview on NPR’s Science Friday, the Mojave Desert solar plants will prove to be particularly effective when compared to other green initiatives. One reason for their effectiveness is their location- the solar plants will be simultaneously near large population centers, L.A. and Las Vegas, with ideal conditions for sunshine- the desert. This is in contrast to wind energy where ideal locations for wind farms often correspond to areas with low population densities (like the plains of North Dakota) and so power distribution becomes an issue. Additionally, the sunlight in the desert suits itself to matching peak output of the solar grid with peak usage- as everyone cranks up the air conditioning at the hottest time of the day, the PV modules are cranking out the most amps.

Time-line of photovoltaic efficiencies for various cell types; from the National Renewable Energy Lab

The choice of thin-film CdTe for the solar cells is once again due to balancing cost and efficiency. First Solar claims that its CdTe modules have the smallest carbon footprint (this includes fabrication and recycling of the module over its lifetime) compared to any photovoltaic on the market, as well as the fastest energy payback time (EPBT). They also note that the high temperature coefficient of CdTe allows their modules to perform better than silicon at higher temperatures, which will obviously be crucial given the heat conditions of the Mojave.

Other summer solar news include McGraw-Hill’s June 13, announcement to build the world’s largest private solar plant at its East Windsor, New Jersey campus. Though New Jersey is not as sunny as the Mojave Desert, the plant is slated to generate an impressive14 MW.

A detailed solar map was released by the City University of New York on June 16, which shows the solar energy production potential of New York City’s rooftops. The New York Times reported that the solar map, made by making LIDAR sweeps the previous year, shows that two-thirds of the New York’s rooftops have great potential for solar harvesting. If these rooftops were covered with solar panels, the city could use them to meet half of its electrical power consumption needs, even at peak use.

NYC roofs were not the only ones in the solar lime-light recently. Google announced on June 14, a partnership with SolarCity in which they will provide a $280 million fund to help finance SolarCity’s solar panel leasing program for rooftops across the U.S.   …For the full original post, click here.

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May 06

The Real-Life Tony Stark

By JamesVanHowe Uncategorized 1 Comment »

Robert Downey Jr. plays Tony Stark, defense contractor, billionaire playboy, scientific genius, and alter ego Iron Man. Image from www.comicbookmovie.com, still from Iron Man 2

This post originally appeared on Jim’s Cleo Blog and is reproduced with permission from its author.

On Tuesday, May 3, I sat in on part of the Market Focus talks at the CLEO expo on defense. The Market Focus sessions cover various business and commercial applications of optics research. Last year was the first time I attended a Market Focus session, and I knew I had to go back. It is a little expo in itself that requires no walking- you to sit down and find out trends and problems that need solving in particular commercial areas. Great fodder for new research ideas!

Although the sessions are broken up into specific talks, what makes these sessions unique is that they turn into a round-table discussion at the end (and even during) the session. They have a more intimate and informal feel than the technical talks and have been organized with a specific agenda of bringing the attendee to a common understanding of the particular market being addressed.

The first speaker, Craig Hoffman, from the Naval Research Laboratory, described science and technology development of infrared imaging systems. He broke up NRLs work in this area by spectral region:

-Visible: 0.4-0.7 microns (high photon energy makes devices tolerant to noise, but scattering makes it bad for imaging through dust or fog)

-Near IR: 0.7-3.0 microns (better for imaging through climate, but resolution gets worse because the wavelength is getting longer; becoming less tolerant to noise)

-Mid-IR: 3.0-5.0 microns (getting very good at seeing through climate, but getting even worse with resolution and noise tolerance; detectors may need to be cryogenically cooled to circumvent thermal noise)

-Longwave: 5.0-14.0 microns (least prone to scattering, worst for resolution and noise)

NRL is looking to piggy back imaging systems in these regions for applications in target acquisition, surveillance, and reconnaissance. The shorter wavelength systems use reflected light to gain information about detail of a target whereas the longer wavelength systems make use of emissive properties of a target to gain bulk properties like thermal imaging.

For example, new thermal imaging systems use mid-infrared light detection to gain detailed information about a target, but also use longwave detection in order to gain a wider field of view. You need both since by themselves the former sacrifices field for resolution and the latter sacrifices resolution for field of view.

Hoffman went on to describe military imaging problems that need better answers 1) Detailed target identification. You don’t want to just know if a target is a tractor or a tank, but exactly whose tank it is (friend or foe?) and with enough time to either make evasive maneuvers or decide how to engage. 2) Fast data acquisition for reconnaissance. For this application, you want to collect data from an aircraft that is flying high and fast. You don’t have the burden of real-time analysis like target acquisition (you can spend weeks later to analyze data), but you do need to collect enough information, with enough quality during the short acquisition time. 3) Surveilling a small area for weeks on end to look at changes in patterns like traffic flow, building construction, etc.

Hoffman spoke briefly about things like SWaP- size, weight, and power. This acronym represents all the things that should be as small as possible for a viable military product. Pete Vallianos from N2 Imaging systems followed up Hoffman’s talk with more of the parameters, tests, and requirements related to SWaP. Vallianos underscored the importance of practicality and robust requirements when it comes to making products for the military. He repeatedly reminded the attendees that the military is not interested in your research per se (definitely not technology for its own sake), but rather interested in how technology might solve problems. While being developed, it needs to go through a variety of rigorous tests- one of the stress tests from the Marines is dropping your product from a height of six feet onto a piece of plywood . If it doesn’t survive, it’s back to the drawing board.

Vallianos described some specific product development interests of the military in imaging:

-microbolometers
-small eye safe lasers
-CMOS, low level light detection
-lightweight visible optics
-high transmission in optics across the visible spectrum through the longwave IR
-moldable aspheric lenses
-robust broadband optical coatings
-OLEDs
-LCDs
-Lightweight optical “network” on a soldiers back
-Any decrease in power for powered optics to get grid of as many of the batteries as possible a soldier needs to carry in his or her pack.

Though the speakers did not describe any iron suits with flying capability, or magic cold-fusion-like power supplies, I think Tony Stark still would have been proud of this session…for the full post, click here.

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May 03

Lasers can Mold and Manipulate Metal as easy as Play-doh

By JamesVanHowe CLEO: Applications, Uncategorized No Comments »

Dr. Marshall Jones from GE Global Research; Photo from GE

This post originally appeared on Jim’s Cleo Blog and is reproduced with permission from its author.

The head of the student machine shop at Cornell, Bob Snedeker ( Sned), liked to remind us in a sarcastic fashion that its easier to take material away from a workpiece than to put it back on- warning: be careful about how much you take off as you cut. Or as the old saying goes in carpentry, “measure twice, cut once.” This is not necessarily true for laser machining of metals. Laser cladding, which was one of the topics discussed in the tutorial, AMB1 “Industrial Applications of Laser Materials Processing,” by Dr. Marshall Jones from GE Global Research, is a technique in which material can be added to a workpiece where too much was accidentally cut off. Like Play-doh, you can just put back on what you need. Wow, if only I could have laser-cladded my tool bits, and special nut and bolt we were required to make in order to graduate from machine-shop training! Sned had high standards and we spent many hours to make a piece to find out we needed to start over with fresh stock. It was back to the grindstone (literally!) until those bits had a perfect angle and facet.

GE uses laser cladding to clean up mistakes that may have been made for particularly expensive pieces such as airfoils for aviation. You don’t want to throw these out and start over. Laser cladding is also used for coat metals with another protective metal surface- hardfacing.

Another laser processing technique explained by Marshall was laser-shock peening. Peening (as in a ball peen hammer-a remnant tool from days of blacksmithing) is a technique that reduces the fatigue of a metal (like preventing cracks from spreading) by applying a compression force to the surface. In the old days, this was done with a hammer, Marshall uses a “laser hammer.” To create a shock wave powerful enough to peen, you need a laser beam with an a power density of 1010 W/cm2 and an interaction time with the surface of no more than 10 ns. Using an interface like water, through which the compression force propagates to reach the metal surface, can make peening more effective. GE also uses shock peening for aviation pieces in order to extend the life of a particular part.

Besides other applications, Marshall briefly discussed the laser systems themselves. The conventional lasers used for processing are CO2 lasers and Nd:YAG systems. Unfortunately they have 10% and 3% wall-power efficiency respectively, and CO2 lasers require expensive specialty fiber for coupling due to the long emission wavelength. Ytterbium-doped fiber lasers and ampliers are beginning to replace these current workhorses due to high wall-power efficiency, 30%, and all-fiber configurations (zero optical alignment and high flexibility in footprint and beam delivery). The main disadvantage to high-power fiber lasers is expense. However, as fiber-systems continue to be developed, they may very well replace their bulk system competitors in the near-future…For the full original post, click here.

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Sep 27

FiO/LS 2010: Everything and the Kitchen Sink

By JamesVanHowe Uncategorized 1 Comment »

This post originally appeared on Jim’s Cleo Blog and is reproduced with permission from its author.

Karl Koch of Corning, Inc., and this year’s General Program Chair for Frontiers in Optics 2010 Annual Meeting, is not fibbing in the slightest when he says FiO “..covers almost all topic areas that the Optical Society concerns itself with.” Where else could you find sessions like Astrophotonics, Vison and Color, Optical Design with Unconventional Polarization, Laser-based Particle Acceleration, Lasers for Fusion and Fast Ignition, and Sensing in Higher Dimensions all in the same program?

This year’s conference is being held in Rochester, NY during October 24-28. Whether attending or not, be sure to catch up with FiO bloggers Laura Waller and Stephen Roberson for daily conference updates. Besides the breadth of topics, what makes FiO/LS special is a smaller, more intimate atmosphere than the typical conference fare. Quality has not been sacrificed for breadth. This years conference includes presentations by heavy-hitters such as Emil Wolf, James Gordon, Stephen Block, Jim Fujimoto, Sunney Xie, Alain Aspect, and many more.

To help build your specific conference itinerary, I recommend watching the you-tube shorts of the different subcommittee chairs describing work in their particular topics. I found Alfred U’Ren’s, of Universidad Nacional Autonoma de Mexico, descriptions of Quantum Electronics abstracts (above) particularly helpful. Among other talks, Dr. U’Ren highlighted work by Stephen Barnett, FTuZ1, “The Enigma of Optical Momentum,” in which Barnett seems to have solved a longstanding paradox between conflicting descriptions of optical momentum in materials. Through a combination of browsing the shorts and looking at the online planner, I also became interested in FTuS7, “Tensile Strength Analysis of Laser Skin Welding with Thulium Laser System,” whose authors are seeking a photonic replacement to suturing wounds using Mid-IR light.

Though the plenary is typically a staple, I still feel compelled to urge you to attend Joseph Eberly’s Ives Medal Address. Dr. Eberly will be receiving OSA’s highest honor. His talk, “When Malus Tangles with Euclid, who Wins?” stands to be creative and enlightening.

Finally, another unique quality of FiO/LS is an emphasis on education in science and optics. Science Educators day, Wednesday, October 27, 4:30-8:00 pm, is a chance for middle and high school educators, professional or volunteer outreach, to learn new hands-on demos and experiments in optics for the classroom. Additionally, FiO/LS is one of the few professional optics conferences that has an undergraduate symposium, Monday, October 25, 12:00-6:30 pm. FiO/LS is leading the way to mentor and encourage the future generation of optical scientists and engineers. Mark your calendars and support these budding, young scientists…for the full original post, click here.

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Sep 08

Oh what an entangled web we weave…

By JamesVanHowe Uncategorized 3 Comments »

The nerdy license plate of fellow Augustana College physics professor Cecilia Vogel, referencing the famous 1935 Phys. Rev. paper by Einstein, Podolsky, and Rosen which introduced the idea of entanglement and questioned the completeness of a quantum mechanical description of reality

This post originally appeared on Jim’s Cleo Blog and is reproduced with permission from its author.

Looking back through some of the literature in photonics and optics published this summer, I was most fascinated by three experiments concerning reliable generation of entangled photons. Two groups, one from China1 and another from Vienna2, showed independent reports of heralded generation of entangled photon pairs. Another, from Toshiba of Europe , demonstrated ‘on-demand’ entangled photons from a quantum dot embedded inside an LED, making an entangled-LED or more simply, ELED3. These works have been nicely summarized in the News and Views section in the August issue of Nature Photonics: “Entangled photons report for duty,” by Pieter Kok4 and “A spooky light-emitting diode” by Val Zwiller5 .

The recent News and Views articles by Kok and Zwiller stirred the inner physicist within me, and sent me down a path of literature searching too detailed, too mathy , and too long for a blog post. However, I’ve attempted to give my own understanding of how the exciting work of heralded and on-demand entangled photon sources can be put into a broader context. If you’re a beginner like me and want more information on the fundamentals of quantum information, I recommend Kok’s website and review article6, Gisin’s review7, and Dehlinger’s article geared for setting up entanglement experiments for the advanced undergraduate laboratory curriculum8.

Whether or not you’re a practicing quantum mechanic, you likely know that entanglement is a crucial ingredient for quantum information processing. Entanglement using photons may be the best method for practically achieving quantum computing and for quantum communication due to their coherence, low transmission loss, and ease of manipulation. The most widely used method for generating entangled photon-pairs is through spontaneous parametric down-conversion in a nonlinear crystal (SPDC). This technique is so robust that it has recently been employed in a number of undergraduate teaching labs to help physics students understand the photonic nature of light and the non-intuitive implications of quantum mechanics8,9,10.

Beating the Odds

Figure 1. reproduced and adapted from ref. 4. Creating entangled photon pairs. (a) In normal operation, a parametric down-converter (PDC) produces an unknown number of entangled photon pairs in each pulse. Detectors must then ‘post-select’ the correct events that contain exactly one pair. (b) The basic setup used by Pan and Walther's lab; a particular four-photon detection event can occur only when three pairs are present, with the remaining two entangled photons propagating freely. This creates precisely one ‘heralded’ entangled photon pair.

The problem with SPDC is that the process of pair generation is probabilistic. More often than not, zero or multiple pairs are generated, see Fig 1 (a). Like a game of black-jack at a Vegas card table, more often than not you don’t get the cards want, and so more often than not, the house wins. However, if you are clever and can count cards, you can guarantee a win even though the odds are against you. You aren’t changing the probability distribution of what is being dealt, you are just predicting what will be played. You can select which cards you want and pass on others without having to see their face values. In the language of quantum entanglement, you would be ‘heralding’ or announcing the cards you want before they are dealt.

The heralded entangled photon source produced by Jian-Wie Pan’s lab in China1 and Philip Walther’s lab in Vienna2 was done through clever counting. Both groups used a setup similar to Fig 1. (b) such that when three photon pairs were generated simultaneously by SPDC, two of the pairs would be ‘peeled off’4 by the beam splitters, and remaining would be guaranteed to be a single entangled pair. Essentially, the simultaneous firing of four detectors at the outputs of the ‘peel off’- beam splitter herald a single remaining entangled photon pair. You have to wait for a three-photon pair event, but you can be guaranteed an entangled output.

Fig. 2 (a) Reproduced and adapted from from ref. 3. Schematic of the active region of the ELED, showing the emission of a polarization entangled photon pair through the biexciton cascade. (b) reproduced from ref. 5. Optical microscope image of the from Toshiba Europe.

Another way to beat the house is to change the probability of cards drawn- use your own deck. Salter et al.3 from the Shileds group of Toshiba Europe essentially took this approach to creating entangled photon pairs by using an entirely different physical mechanism than SPDC. Using the radiative decay of the biexciton state in a quantum dot Fig. 2 (a) , the Toshiba group created an ‘on-demand’ entangled source. The biexciton state is created by the capture of two electrons and two holes. So long as the two excitons are degenerate in energy (no fine structure splitting) the output will be entangled. In fact, one of the experimental hurdles overcome by the Tohsiba group was to grow quantum dots emitting photons of the right energy, near 1.4 eV (887 nm), in order to have very small fine-structure splitting. Because the source is not probabilistic and no clever counting setups are needed, it is referred to as ‘sub-Poissonian’5. What makes Salter’s work so hot is that the on-demand source is driven electrically. No bulky pump lasers are needed like they are for an SPDC source. The entangled-LED, or ELED, could possibly be scaled down to submicron sizes for on-chip integration5, see the microscope image in Fig. 2 (b)…For references and to read the full post, click here.

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May 17

Why I’m Green with envy

By David Nugent Uncategorized 2 Comments »

I’m not usually susceptible to pangs of jealousy, but some papers presented at CLEO really make me think I chose the wrong career. I am referring to the veritable banquet of talks on anti-reflection and light trapping methods for solar cell enhancement.

Don’t get me wrong, as an investment analyst I am as much interested in the greenback as the green movement. By my calculations a 1% increase in the efficiency of a polysilicon solar panel (from 16% to 17%), or a 3% increase in the output of a multi-junction cell (from 35% to 38%), will be worth around $1 billion across Europe in additional PV revenues by 2013. Little wonder that companies such as Du Pont and GE have announced major investment programs into photovoltaic enhancement.

Table: Economic value of PV efficiency increases in European solar power generation

Source: Elucidare Limited

CLEO Symposium on Photonics for Advanced Energy Technology: Photonics for Power Generation and Delivery

Monday, May 17. 10.15 am to 12.00 pm

CLEO kicks off its photovoltaic programme with a morning session on advanced solar cell design and performance.

First up, Sarah Kurtz (Natl. Renewable Energy Lab, USA) will examine high-efficiency photovoltaic technology, in particular the latest breakthroughs in multi-junction techniques where near-perfect materials have achieved over 40% in efficiency under concentrated sunlight.

Next, Weiquan Yang (Univ. of Texas at Arlington, USA) et al will describe flexible solar cells based on stacked crystal-line semiconductor nanomembranes on plastic substrates. The reported efficiency of only 1.5% must be understood in the context that these cells are only for 1 um thick. Remember, the important factor is US$/Watt, not efficiency alone.

Keeping with the nano theme, Zongfu Yu (Stanford Univ., USA) et al will discuss the fundamental limits of nanophotonic light-trapping in solar cells. Light trapping is an efficiency enhancement technique whereby solar light can be captured within sub-wavelength thick absorption layers using periodic surface gratings. Via a rigorous electromagnetic approach, Yu reveals that the standard bulk optic limit developed by Yablonovitch can be exceeded in the nanophotonic regime, opening new avenues for highly efficient solar cells.

Finally, Jan-Gustav Werthen (JDS Uniphase Corp., USA) will give an overview of Power-over-Fiber, a technique for powering remote devices such as sensors and FFTH transceivers. I am slight puzzled that Werthen should describe this as an “emerging technology” because five years have passed since JDSU paid US$9.7 million to acquire Photonics Power Systems, the company that already had spent many years developing power-over-fiber technology. I guess he means this in relation to its application to FTTH networking, which truly is a more recent introduction, at least here in Europe.

Advanced Concepts in Photovoltaics

Monday, 17 May. 1.30 pm to 3.15 pm

In a session otherwise dedicated to the topic of plasmon-enhanced photon absorption, this programme commences with a talk by Chang-Hong Shen (Natl. Nano Device Labs, Taiwan) on thin-film flexible silicon solar cells fabricated at 100°C by high density plasmas.

Next up, Wenli Bai (Lehigh Univ., USA) introduces plasmonic back structures designed to enhance the efficiency of thin film solar cells. Metallic structures with one-dimensional periodic nanoridges attached to thin-film amorphous silicon (a-Si) solar cell are proposed to enhance the cell efficiency in a wide near-IR range.

Niraj N. Lal (Univ.  of Cambridge, UK) et al discusses nanovoid plasmonic-enhanced low-cost photovoltaics. Gold and silver nanovoid structures generate localised plasmon modes which are harnessed to enhance organic and amorphous silicon solar cell performance. According to Lal, higher absorption at plasmonic resonant wavelengths indicates significant potential for enhanced photocurrent and efficiency. Bravo, though with gold hitting a record high last week, I hope Lal will provide some insight into the cost effectiveness of this technique.

Fu-Ji Tsai (Natl. Taiwan Univ., Taiwan) et al presents work on absorption enhancement using localized surface plasmon-induced scattering with metal nanoparticles, while Yifen Liu (Iowa State Univ., USA) discusses grating-induced surface plasmon-polaritons for enhancing photon absorption  in organic photovoltaic devices. Using a grating electrode to launch laterally propagating surface plasmon-polaritons along the active layers of the organic photovoltaic device, Lui creates a standing wave to increase the device thickness and thereby enhance photon absorption.

Finally, Jing-Shun Huang (Natl. Taiwan Univ., Taiwan) discusses inverted polymer solar cells with paired metal oxide modifications through solution processing. The 5% conversion efficiency is respectable, in my opinion, for a polymer solar cell.

Photonic Crystals I

Tuesday, May 18, 8.00 am

For a wake-up introduction to the subject of photonic band gap materials and their use as light trapping crystals in solar energy harvesters, I recommend the tutorial session by Sajeev John (Univ. of Toronto, Canada). John will review the physics, microfabrication, and applications of these structures. Solar buffs like me should then excuse ourselves and trot along to the parallel session on novel materials for enhanced solar cell performance. See below.

Novel Materials for Enhanced Solar Cell Performance

Tuesday, May 18. 8.00 am to 9.45 am

In an invited speech, Jeff Nause (Cermet, Inc., USA) will discuss multijunction solar cells based on tunable bandgap ternary InGaN p-n junctions. Cermet is perhaps best known for its work on lattice matched, high efficiency solid state emitters grown on ZnO substrates.

Next up, Broderick (MIT, USA) et al will present work on Si-on-insulator thin film solar cells featuring textured back reflectors. Reportedly cells at all thicknesses demonstrated significant external quantum efficiency (EQE) enhancement due to the back reflector.

In the first of a series of papers on light trapping techniques, Simone Zanotto (Univ. degli Studi di Pavia, Italy) discusses absorption enhancement in thin-film silicon solar cells with a periodic etching in addition to an AR-coating. The authors claim an increased short-circuit current of up to 36.5%.

Here follows a series of anti-reflection and light-trapping papers from Taiwanese universities. Ping-chen Tseng (Chiao-Tung Univ., Taiwan) et al discussed antireflection and light trapping of periodic subwavelength surface structures formed by colloidal monolayer spheres deposited lithography on thin film solar cells; Shu-Chia Shiu (Natl. Taiwan Univ.) et al talks about reducing Si reflectance using high-density Si nanowires fabricated by metal-assisted etching; Min-An Tsai (Natl. Chiao-Tung Univ., Taiwan) et al presents results on efficient enhancement in a InGaP/GaAs dual-junction solar cell by broadband  and omnidirectional antireflection nanorod arrays. Finally, Shu-Chia  Shiu (Natl. Taiwan Univ., Taiwan.) introduces a new type of hybrid solar  cells  based  on  a heterojunction  between PEDOT:PSS and vertically-aligned n-type GaAs nanowire  arrays  is  investigated. Such solar cells exhibit a power conversion efficiency of 3.46 %.

Joint CLEO/QELS Post Session

Tuesday, May 18.

Don’t forget the poster session. Ragip Pala (Stanford Univ., USA) et al present a paper entitled “Plasmonic  Solar Cells with Broadband Absorption Enhancements”.  This describes a computational-experimental  study optimizing plasmon-enhanced absorption in thin film solar cells. Pala investigates the effect of different geometries where 2-dimensional periodic-aperiodic arrays of metal nanostructures sit above or below the active material. It will be interesting to compare and combine this work with the Yablonovitch paper by Zongfu Yu (see above).

Dr David Nugent is Founder and CEO of Elucidare Limited, a boutique technology development and investment advisory business.

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