Thursday, December 30, 2010
Like usual, this Christmas holiday my family and I spent some time at my mother-in-law's rural home in Spring Valley, Wisconsin enjoying good food, fresh air, and the escape from the confines and bustle of city-living. However, what was different about this year's visit was her home's new Terabit/s capacity for current and future digital communication and entertainment needs. My mother-in-law lives in one of six million American homes that currently have Fiber-to-the-home (FTTH) connectivity.
Her service provider, West Wisconsin Telecom Cooperative, is part of a growing number of American rural telecoms and municipalities surpassing their urban counterparts in the future of lightwave communications. Much of this rural technological growth has been made possible through grants and low-interest loans from the United States Department of Agriculture (USDA) Rural Utility Service (RUS) Telecommunications Program whose aim is to improve education, health, and economic opportunities of rural families and businesses through broadband internet access.
(Above: The neighbor's farm across the street .)
Seeing and using FTTH in person at my mother-in-law's got me excited about its implications on photonics research and applications, and science in general. Though CLEO is not a telecom- centered conference like OFC (optical fiber communications conference), there is much that fundamental science and optics research can leverage from telecom technology and visa-versa. The growth in one field breeds growth in the other. For a nice paper about the symbiotic relationship between great physics discoveries and advances in telecommunications technology, see Brinkman and Lang's 1999 Reviews of Modern Physics paper "Physics and the Communications Industry."
Though I may be biased towards fiber-based technologies, to me the links between work presented at CLEO and the growth of telecom is pervasive. For starters, the first CLEO plenary speaker, Donald Keck, will share stories of how his team at Corning pioneered the first usable low-loss fiber in 1970 and discuss its place in the broader context of the ensuing optical technology and information revolution. The exciting history of bringing down the loss of optical fiber (which in its infancy was an opaque 1000 dB/km) involves a Nobel prize (Charles Kao correclty hypothesized the bottleneck to transparency) and the development of the groundbreaking fabrication technique of modified chemical vapor deposition (MCVD) by John MacChesney's team at Bell Labs, which brought loss down to what it is today.
As Keck will likely discuss, making fiber transparent paved the way not only for a host of other telecom and information technologies but new fields of photonics. For communications, once you have an acceptable waveguide, you still need sources, modulators, detectors, amplifiers, routers, multiplexers, switches. For the most recent breakthoughs in telecom be sure to attend CLEO: Science and Innovation 12: Lightwave Communication and Optical Networking.
Naturally, the field of quantum communications is tied up in current telecom technology. CLEO Symposium on Quantum Communications will host papers in both fundamental science research in quantum information and actual quantum communication testbeds- fiber-based and free-space.
You can find telecom-derived technology in CLEO: Science and Innovation 11: Fiber Amplifiers Lasers and Devices. This session includes topics such as CW and pulsed mode-locked fiber oscillators, amplification in doped fibers, ultra-wideband fiber amplifiers, Raman amplifiers, coherent and incoherent combination of fiber lasers and amplifiers, fiber-based nonlinear effects, fiber-grating and microstructured fiber devices. The applications for these technologies range from spectroscopy, biomedical imaging, and materials processing.
(Above: Sign marking the location of buried fiber)
My excitement at my mother-in-law's FTTH led to some disappointment about the speed of her computer (any computer for that matter). She has ~100 Tb/s capacity, but only ~Gb/s capability. To get the capability you need to move to all-optical signal processing and optical computing. CLEO will host a variety of sessions regarding or related to this important technological goal. See CLEO-QELS Fundamental Science 6: Nano-optics and plasmonics; CLEO: Science and Innovation 7: Nano-optics Micro- and Nano-photonic devices; and CLEO: Science and Innovation 9: Components, Integration, Interconnects and Signal Processing. Among other ground-breaking research, I anticipate contributed papers from the Cornell Nanophotonic's Group whose P.I., Michal Lipson, recently received a MacArthur Genius Award , ($500k no strings attached) for her pioneering work in silicon-based circuits for practical optical computing.
The list of sessions at CLEO inspired by optical fiber and telecom technology goes on. One of my hopes for 2011 is to witness FTTH to more of the country, rural and urban. This isn't just job security for us all, but may prove pivotal to the growth and expansion of innovation in optics and science.
Tuesday, November 23, 2010
Today, the day before Thanksgiving, is one of the busiest holidays for air-travel in the U.S. The latest hubbub in U.S. airport security is the use of x-ray body scanners to detect for potential explosives or weapons carried by passengers. The scanners spray the traveler with soft x-rays and then detect the back-scattered radiation to produce an image of the passenger, minus his or her clothing. Many travelers have found this new level of security too invasive and have opted out for the traditional pat-down. Others have used their experience as the "butt" of jokes; Humorist Dave Barry recently described his ordeal of going through airport security of having both the scan and a pat-down due to a "blurry groin area" from the image. The threat of body-scanner boycotts from various websites and blogs prompted transportation security administration (TSA) chief John Pistole to plead to holiday-travelers to put security needs above personal modesty since the pat-down takes much longer and will lead to travel delays, inconvenience, and economic hardship for the travel industry.
The first thing that came to my mind in the flurry of stories I've been hearing regarding body-scanners and privacy issues was Terahertz (THz) radiation. Though I don't remember the particular conference, I still have the images in my head from a THz radiation talk not long after the Columbia Space Shuttle disaster in 2003 of space-shuttle foam and a body-scan. The presenters were using these images to motivate the applications of THz radiation for imaging- the THz image of the foam as a nondestructive technique to look at the structural integrity of shuttle foam, and the THz image of a body-scan as a better control of threat-detection in airports. Thz radiation was a budding field at this time and has since exploded. Last CLEO conference there were nine different sessions involving THz research and applications. Sessions ranged from new THz sources and detectors to THz waveguides and metamaterials. At the 2010 postdealine session, Xi-Cheng Zhang's group of Rensselaer Polytechnic Institute (likely the same group whose space shuttle foam and body-scan I remember from 2003) presented a paper on remote sensing using broadband THz sources.
One of the reasons the 2003 images stuck in my head was that the body-scan model was wearing what I assumed to be some kind of metallic underwear. What makes THz radiation useful for a body-scan is that it is readily transmitted through non-metallic and non-polar materials like clothing. Obviously the model for this research shared the same concerns as many current U.S. travelers. However, there is more to THz imaging than just seeing through clothing which could make THz scanners both a more effective and less invasive tool than x-ray scanners.
Unlike x-rays, there are a number of explosive, chemical, and biological agents of interest for threat-detection that have characteristic THz spectra1, including PETN which was found in the "underwear bomber's" briefs after his foiled attempt to blow-up a plane near Detroit last Christmas. Rather than build up an image of the body and rely on the scanner operator to judge the potential threat from visual inspection, a THz scanner could look at the reflected spectra point-by-point to compile a molecular fingerprint by comparing to a database of absorbance spectra. Effectively, this technique is "THz spectroscopy through clothing". You build up a chemical map across the body instead of a body image. TeraView of Cambridge announced work on such a scanner last January when the U.K. ran into similar passenger discontent over x-ray body-scans. Besides giving travelers back their modesty, this technique likely could give better threat detection as well as less false-positives since threats are identified chemically instead of visually.
To find the latest breakthroughs on THz scanners, be sure to attend sessions under CLEO: Science & Innovation: Terahertz Technologies and Applications, or CLEO: Applications & Technology: Government & National Science, Security & Standards Applications this May. Until then, have happy, safe, and hopefully noninvasive travels.
References1. J. F. Federici et al., "THz imaging and sensing for security applications-explosives, weapons, and drugs," Semicond. Sci. Technol., 20, S266-S280, (2005).
Sunday, October 31, 2010
Above: reproduced from Ref. 1. Magnetomotion OCT (MM-OCT) of chicken breast tissue.
CLEO conference organizers recently posted the categories for the Special Symposia which are to include five areas: 1) Nano-bio-photonics, 2) Broadband Spectroscopy: New Techniques and Sources, 3) Quantum Communications, 4) Fiber Parametric Devices and Applications, and 5) Light-emitting Nano-plasmonic Devices.Among other invited speakers in the Nano-bio-photonics Symposium, Stephen Boppart from University of Illinois, Urbana-Champaign, will be discussing "Endogenous Molecules or Exogenous Molecularly Targeted Contrast Agents for OCT." OCT, or Optical Coherence Tomography, was pioneered in the early 1990's by James Fujimoto of MIT, who will also be giving an invited talk in this Symposium. OCT uses the cleverness of interfering low-coherence light to scan through tissue to depths up to a few millimeters and with an axial resolution equivalent to the coherence length of the source, typically less than 10 microns. Complete OCT setups can be purchased commercially, like Thorlab's OCS1300SS. In fact, during my last eye-checkup I asked my optometrist if she had heard of OCT since they are becoming more widely used by opthamologists to probe retinal health and conditions. She did. In fact she had participated in a clinic to understand how to use OCT and read images.
Though roubst, OCT has been limited to a small number of medical applications because it is insensitive to standard molecular probes used by biomedical researchers such as fluorophores or bioluminescent markers. This is because fluorescence and bioluminescence arise from inelastically scattered light, and OCT is sensitive only to elastically scattered light (light coherent with the source). Therefore, in order to use molecular probes with OCT to diagnose a wider variety of diseases or to answer fundamental questions of cellular function, one must use a different set of probes that are sensitive to elastic scattering properties of light like changes in scattering itself, absorption, polarization, phase, or frequency.1From the title of the talk, I imagine that Professor Boppart will be discussing some of the exogenous probes (probes introduced into the tissue) he and his group have created and used for OCT like metallic nanoshells and microspheres engineered from a variety of materials to enhance scattering.1 My favorite of the ingenuous probes are particles with high magnetic susceptibilities such that when modulated with a magnetic field, their subsequent motion changes the local scattering properties of the tissue, see the figure above.1 I also suspect he will be talking about endogenous probes (where the tissue of interest naturally contains helpful probes) like collagen that can be stimulated by a coherent process like second harmonic generation, or Hb/HbO2 whose scattering and absorbing properties are frequency-dependent.1 I'm excited to hear advances in this work as well as attend the other symposia.
The latter four symposia are open for submission of contributed papers and you can find the CLEO or QELS subcategory to which to submit beneath the particular symposium description. I'm particularly interested in the Broadband Spectroscopy Symposium since one of the emphases is Mid IR sources, a new interest of mine. I also am excited that the Quantum Communication Symposium, which like the Nano-bio-photonic Symposium, has been placed under the designations of QELS: Fundamental Science, and CLEO: Science and Innovation, and CLEO: Applications and Technology. These symposia will attempt to show their fields from the idea-stage all the way to commercialization.So if you think you're special (which if you're planning on submitting work to CLEO you most certainly are), take a moment to see if your work falls into the Special Symposia categories.
References1. S. A. Boppart et al., "Optical probes and techniques for molecular contrast enhancement in coherence imaging ," Journal of Biomedical Optics, 10, 041208, (2005).
Monday, September 27, 2010
This Thursday marks the official Call for Papers for CLEO 2011 in Baltimore. The deadline for submissions (with the exception of post-deadline papers) is December 2, 12:00 pm EST. For your convenience, and hopefully not to stress you out, I've embedded a countdown timer at the top of the blog.
CLEO 2011 will prove to be even more exciting this coming May due to the addition of a new conference, CLEO: Applications and Technology. The classic CLEO conference, CLEO: Science and Innovation and CLEO/QELS: Fundamental Science, will still remain intact. The new conference will "explore the intersection of academic research with product commercialization." Papers submitted to the Applications and Technology conference will demonstrate the transition of fundamental and applied research toward product commercialization. The conference programmers emphasize work should be pre-commercial. Topic categories include Biomedical, Environment/Energy, Government and National Science, Security and Standards Applications, and Industrial.
The addition of CLEO: Applications and Technology, to classic CLEO, QELS, Market Focus, and the Expo gives continuity to the spectrum (no pun intended) of innovation at the conference. Under one roof conference-goers will now be able to learn about break-throughs in fundamental science (QELS), applied science (CLEO), the transition of applied research to commercial products (Applications and Technology), commercial developments and research ready-for-commercialization (Market Focus), as well as see and purchase commercial products first-hand (Expo).
In my opinion, the move to foster more collaboration and communication between fundamental research and product development is wholly positive. I recently showed my electronics class the NOVA special Transistorized which recounts the development of the transistor. The history of this device that revolutionized all of our lives shows how synergy between commercial enterprise and fundamental science can produce both Nobel laureates and corporate behemoths like Sony and Intel. Let's hope history repeats itself in Baltimore.
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.
Tuesday, September 7, 2010
(Above: 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)
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 .
For me, quantum entanglement may be one of the coolest and weirdest properties of light. Entanglement ‘spooked’ Einstein and led to a paradigm shift in our fundamental understanding of the nature of measurement and reality. Particles of matter or light don’t live in well defined states until we force them to by a measurement. This is already weird, but now if they are entangled, the measuring process is shown to be nonlocal–a measurement on one particle simultaneously determines the state of the other.
As a young graduate student, I found myself filling up conference itineraries with talks on quantum computing, cryptography, and key distribution. My head spun with trying to understand all the bras and kets that accompanied explanations of quantum information protocols. Admittedly, I should have paid better attention in modern physics and quantum mechanics when I was a student (sorry professors Vogel, Coppersmith, and Drell!). However, frustrated with Alice, Bob, and that pesky Eve, my reaction was to give up, and since my research dealt with large numbers of photons, I decided to happily and naïvely live in a classical world.
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
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.
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 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).
Though each technique is a groundbreaking achievement, there are a number of practical limitations to be overcome. The rate at which either gives entangled pairs is quite low. For heralded pair generation using a typical SPDC setup, the rate of three-photon events, from which you cleverly select a single pair, ranges from 0.001 to 0.1 Hz2. Cranking up the pump laser power can give you more three-pair events, but then it will start to give you four-pair events as well, which will trigger your detectors and give a false-positive.
For the ELED, the Toshiba group showed an entangled pair rate of 3.0 Hz. This can be improved by increasing the coupling efficiency as well as increasing the injection current. Intrinsically, this device shows better promise as an ‘on-demand’ source since the probability of generating and entangled pair per voltage pulse is 3%3 - about 10,000 times more probable than generating a three-photon pair in SPDC4.
Both techniques show an entanglement fidelity of more than 80%. The primary reason that heralded generation using SPDC comes up short of perfect entanglement is due to the nonzero probability of the creation of simultaneous four-photon pairs. As mentioned above, four-photon events will also simultaneously trigger your detectors and give you a two-pair output instead of the single pair you expect. The imperfect entanglement in the ELED made by Toshiba primarily comes from unentangled background light from the surrounding diode structure. Eighty percent fidelity may not be useful for all quantum information protocols but is high enough for important components of quantum computing like teleportation and entanglement swapping3.
Finally, both techniques are not quite ready to be integrated on-chip. Walther et al.2 made their pump source with a frequency doubled Ti:Sapphire laser Though Ti:Sapphs are the workhorses of ultrafast optics and are very robust (See earlier post for more details), they currently require a good amount of space on an optics table(roughly four feet long and 1 foot wide), expert knowledge of ultrafast optics to troubleshoot, and a good amount of money (~$100k). The ELED beats SPDC in this regard since it is compact and requires a simple low-voltage electrical pulse. Unfortunately, successful operation requires near-liquid helium temperatures. Salter et al.3 showed operation at a chilly 5 K.
Despite such hang-ups, one has to keep in mind the practicality of early computing and information processing. Could Mauchly and Eckert, the inventors of the ENIAC (which consumed 150 kW of power, used 18,000 vacuum tubes, and was so big that it needed to be housed in a 30 x 50 foot room) ever envisioned an iPod Touch? And even if the quantum version of the iPod Touch never materializes, perhaps this cool physics is just worth doing for its own sake.
1. C. Wagenknecht et al., "Experimental demonstration of a heralded entanglement source," Nature Photonics, 4, 549-552, (2010).
2. S. Barz et al., "Heralded generation of entangled photon pairs," Nature Photonics, 4, 553-556, (2010).
3. C. L. Salter et al., “An entangled-light-emitting diode,” Nature, 465, 594-597, (2010).
4. P. Kok, “Entangled photons report for duty,” Nature Photonics, 4, 504-505, (2010).
5. V. Zwiller, “A spooky light-emitting diode,” Nature Photonics, 4, 508-509, (2010).
6. P. Kok et al., “Linear optical quantum computing with photonic qubits,” Rev. Mod. Phys., 79, 135-174, (2007).
7. N. Gisin et al., “Quantum Cryptography,” Rev. Mod. Phys., 74, 145-195, (2002).
8. D. Dehlinger and W. M. Mitchell, “Entangled photons, nonlocality, and Bell inequalities in the undergraduate laboratory,” Am. J. Phys, 70, 903-910, (2002).
9. E. J. Galvez et al., “Interference with correlated photons: five quantum mechanics experiments for undergraduates,” Am. J. Phys, 73, 127-140, (2005).
10. J.J. Thorn et al., “Observing the quantum behavior of light in an undergraduate laboratory,” Am. J. Phys, 72, 1210-1219, (2004).
Wednesday, May 26, 2010
Saturday, May 22, 2010
I hope to see everyone at CLEO/QELS in Baltimore, May 1-6, 2011.
I would like to give special thanks to April Zack, head of OSA student chapters and the young professionals organization of the OSA, Dominique Smith, from OSA marketing communications, and Juliann Grant, OSA social media consultant, without whom this blog would not have happened.
Also, special thanks to Sam Rubin of Thorlabs for sending me home with enough Thorlabs T-shirts for everyone in my lab at Augustana, as well as the doggy bag of lab snacks which saved me one night at the hotel when I came down with a bad case of the munchies!
Most of all, thanks to all of you who took time to read the posts. I certainly benefited from writing them. Never would I have looked so carefully at what was available to me at the conference. I hope you came away with something as well.
Thursday, May 20, 2010
Many of us at CLEO are interested in building or using light sources, particularly lasers. After all, that's what the "L" stands for in CLEO. However, two papers in tonight's postdeadline session, CPDA5, "Coherent Perfect Absorbers: Time-reversed Laser," and CPDA6, "Darker than Black: radiation-absorbing metamaterials," address either destroying it or fully converting it into another form of energy. The former may be helpful for controlled optical energy transfer, and the latter for optimized energy harvesting in solar cells. Though physically different, both techniques utilize negative contributions of dielectric permittivity (one relies on a negative imaginary part and the other a negative real part).
If you are interested in light absorption, or cool optical analogs to astrophysics, you may also want to check out related work by Evgenii Narimanov, author of CPDA6, using metamaterials to create a propagation medium for EM fields analogous to the curved space-time near a black hole, in this case, an "optical black hole."
Kieu and his collaborators at Arizona have been doing just that by developing SAs using carbon nanotubes. Though not the primary motivation, Amer Nevet, from Technion in Haifa, and his collaborators have also been developing effective SAs by showing the first example of two-photon gain in semiconductors, CKK1, "Direct Observation of Two-Photon Gain in Semiconductors."
From CLEO abstract CTuII2
Kieu's novel SA made from carbon nanotubes, packaged in a fused, fiber-coupler, has allowed him to make robust, few-cycle pulses in an all-fiber, ultra-compact footprint(see the Figure above from his Abstract). In fact, I have had some first-hand experience recently with building a similar oscillator. Using a fused carbon-nanotube SA from Kphotonics, a spin-off company started by Kieu, and approximately $500 of fiber components, it took only a couple hours of splicing for me to generate a self-starting 400 fs oscillator. It mode-locked the very first time I turned it on and requires no adjustment other turning up the pump power (truly turn-key). This is the easiest laser that I have ever built. Besides applications of this source for fiber-based frequency combs (CLEO talk CMX4) and super-continuum sources, Kieu is trying to market this laser for use in classroom demonstrations and teaching labs at the college level.
From CLEO abstract CTuKK1
The picture above shows the waveguide structure used by Amir Nevet and his collaborators to demonstrate two-photon gain. Two-photon gain is generated when a photon pair stimulates the emission of another identical photon pair in a second-order process (see schematic below). This means that gain is nonlinear, and now proportional to intensity. A pulse amplified in this way will experience larger gain at the peak and lower gain elsewhere. This has the same effect as a saturable absorber- even better, however, since the pulse experiences net gain rather than net loss. This technology holds great promise of changing typical means of pulse compression and mode-locking. The other great things about two-photon gain is it could be used for bi-stability and generating squeezed states of light.
Electron transition diagram of fully (doubly) stimulated two-photon emission
Wednesday, May 19, 2010
Image from Webb Lab, Cornell University, Adapted by J. van Howe. Left: One photon fluorescence in a fluoroscein solution. Right: two-photon fluorescence in the same solution.
At the very first conference I attended as a new graduate student, I asked my advisor, "So what talks are you going to?" To my surprise he said, "Talks? I don't go to any talks, I catch up with my friends and colleagues and go to the expo to get ideas." My graduate advisor was exaggerating. He did, and still does, go to talks, but his advice then, which I still take to heart, made me realize how important it is to talk to colleagues after and in between talks, as well as the importance of the expo. From an academic point of view, the expo typically is a place to buy new tools to further on-going research. From from my graduate advisor's point-of-view it is also a place to find strengths and weakness in developing technology, the latter typically being more helpful for beginning new research.
As I was scouting companies to find out more information on Mid-IR laser sources (by the way IPG photonics and Daylight Solutions seem to be leading the market in sources in this spectral range), I learned about the impact multiphoton microscopy (MPM) has had on the sales of Ti:Sapph and OPO systems from Stephen Knapp of Coherent, Inc. In fact, there will be Market Focus talk, in the Biophotonics session on Thursday on the showroom floor from Arnd Krueger of Spectra Physics regarding this very subject. Also, be sure to check out an entire session devoted to MPM work, today at 1:30 pm in room A4, or check out the abstracts on the conference CD if you miss it.
A Ti:Sapph plus an OPO is not cheap. You're talking around $200k or more depending on options. These bulk solid state systems used to be the purview of only laser jocks. However, companies like Coherent and Spectra Physics are making them more into turn-key systems in order to put them into the hands (or onto the optical tables anyway) of biologists and biomedical researchers. Though companies like Coherent and Spectra Physics are leading the way of making these sources turn-key, the fact that they are still very expensive and fairly large makes us fiber laser specialists excited. We think the next generation of MPM sources will be all fiber-based and therefore truly compact, turn-key, and a fraction of the price.
To my surprise, Stephen told me that Coherent will sell up to 50 a quarter, mainly for use in MPM! So what is so great about MPM? Well, it is arguably the leading technique for deep-tissue imaging. Being in Chris Xu's group at Cornell University when I was a graduate student, who helped pioneer two-photon microscopy with Winfred Denk, and Watt Webb when he himself was a Cornell grad, MPM buzz has rubbed off on me and I can't miss an opportunity to say something about it. Particularly, I wanted to share one of my favorite photos taken in the Webb lab (above) that gets right to the essence of how MPM works.
The goal of MPM is to get rid of the scattering background outside of the focal plane, the photo on the left. These photons are noise and do not contribute to meaningful information about the sample. Using two-photon fluorescence, however, one gets photons mainly near the focal plane and nowhere else, thereby significantly reducing the background. This allows the biomedical researcher, or surgeon, to scan deep (~1 mm) through tissue. Reduction in scattering occurs because in the two-photon configuration, the probability of a fluorescence event is much greater at the focus of the objective than anywhere else.
The two-photon cross section of typical fluorophores is around 30 orders of magnitude smaller than the one-photon cross section. In order to get a two-photon event, photons must arrive "simultaneously." "Simultaneously" can be figured out from the Heisenberg uncertainty principle to be about 0.5 fs - just use the energy of the transition and solve for time. In order to get a two-photon event, one needs to bunch photons together to stack probability. The microscope objective does this in space and a pulsed excitation source does it in time. The focus is the most probable spot for excitation since photons are bunched in both space and time at this point and no where else. This is why you need a pulsed excitation source for MPM. For more details on MPM see the Dr. Bio webpage at Cornell.
Image from Webb lab and Nikitin lab at Cornell University. Left: H&E stained histology of sliced ovary. Right:Multiphoton image of a follicle within an unstained, intact ovary from mouse. Autofluorescence (green) derives from NAD(P)H and retinol within the tissue. SHG (red) delineates the bursa.
Above are some more pretty pictures of nonlinear microscopy in action. Note that on the left is a typical histology of a mouse ovary. The tissue has been excised, and then stained. On the right shows two-photon microscopy combined with second harmonic generation microscopy of an intact mouse ovary. The moral of the story is the one on the right required no cutting and therefore is much less invasive.
Tuesday, May 18, 2010
The line-up of the Lasers Rock concert is:
Phat Photonics, Oregon Health & Science Univ., USA
Eric Hansotte, Maskless Lithography, USA
Free Lunch Band, Lawrence Livermore Nat'l Lab, USA
Brian Kolner, Univ. of California Davis, USA
Bob Fisher, RA Fisher Associates, LLC., USA and Steven Block, Stanford Univ., USA
Yoshiaki Nakajima, Fukui Univ., Japan
Rumor has it that Steven Block is a capable bluegrass musician. I'm also particularly interested in hearing Brian Kolner who's fantastic 1994 review paper in J. Quant. Electronics, "Space-time duality and the Theory of Temporal Imaging" was the basis for much of my thesis work. Time-lens work appears in this conference in presentations CThBB7, JTuD57, and CThN5.
As I am trying to think of songs with references to lasers to request at the concert, I can only think of one, Killer from Queen, "Gunpowder, Gelatine, dynamite with a laser beam..." There must be more. Any help blogosphere?
Monday, May 17, 2010
From Arjun G. Yodh, Biomedical Optics Group, University of Pennsylvania MRI axial slice, DOT axial slices of relative total hemoglobin concentration (rTHC), relative blood oxygen saturation (rStO2), relative tissue scattering (rSc), Optical Index, and a 3D image of region of interest are shown for malignant (left-side) and benign lesions (right-side). The black line indicates the tumor region.
I arrived at CLEO this afternoon bleary-eyed from a long plane ride and lack of sleep. However, three talks in CLEO Applications: Spectroscopy and Imaging held my attention firmly. What impressed me the most was how much information the particular researchers extracted from tissue or a tumor using what seemed like a small amount of data or rudimentary tools.
In presentation AMD4, Arjun Yodh, from University of Pennsylvania demonstrated the power of using highly scattered light from a tissue to not only reconstruct an image deep beneath the tissue surface (~ 1 cm), but to also gather functional information such as blood flow to and from a tumor. This technique, called diffuse optical tomography, relies on a diffusion model of photons through tissue, analogous to the diffusion of heat. In the figure above, Yodh and his collaborators could distinguish between malignant and benign breast tumors based on the functional information from diffuse scattering and absorption.
In presentation AMD1, Urs Utzinger from University of Arizona, showed fairly high specificity and selectivity in diagnosing ovarian cancer in post-menopausal patients by fluorescence signals, using UV-A to Near-IR excitation. Selectivity was accomplished by compiling and comparing excitation-emission matrices for malignant and benign tumors. Each value of an excitation-emission matrix is simply the intensity of the emission signal, where the rows of the matrix corresponds to the excitation wavelength and the column corresponds to the emission wavelength.
From SPIE,Response of cellular motility to the drug nocodazole. The initial state shows strong motility in the outer healthy shell, decreasing over approximately 1h as the microtubules are disassembled inside the cells. The bar is 100μm.
Finally, in presentation AMD3, David Nolte, of Purdue University showed that he could diagnose the effects of drugs on a tumor by how much it wiggled and shook- its motility, see the picture above. My favorite figure in his talk showed the frequency and strength of cell oscillations as a function of time after a drug or another kind of stimulus, such as heat, had been introduced. What part of the cell wiggled depended additionally on its health indicating motility can be used to label a cell's state.
Thursday, May 13, 2010
Schematic of SERS Technique from Kneipp et al Chem. Soc. Rev., 37, 1052–1060, (2008).
There has been a lag in my blog posting lately as I was recently performing my civic duty as juror in a narcotics case in my county for the last three days. Of course as a laser scientist my mind wandered from the case from time-to-time to the subject of how lasers could have been used to aid the investigation. After some browsing through databases, I found some studies using Raman Spectroscopy and surface-enhanced Raman spectroscopy (SERS) for identification of illegal drugs.
Though no CLEO papers directly address the spectroscopy of controlled substances, there are 65 CLEO papers devoted to applications of Raman scattering, 113 on laser spectroscopy, and an entire session devoted to SERS and applications of Raman scattering, CFA. Surface-Enhanced and Fiber Raman Technologies, on Friday May 21.
SERS and SERS-related techniques have particularly been exploited in biomedical spectroscopy in recent years. Three papers in Friday's session use SERS specifically for biomedical applications. CAF2 exploits SERS to analyze DNA, CAF4 demonstrates SERS through fiber and with a more standard biological imaging wavelength of 800 nm, and CAF6 uses SERS to perform spectroscopy on human skin.
For those who don't know or need refreshing, a Raman spectrum gives information of the characteristic vibration of molecule, a "vibrational fingerprint". Raman cross sections are typically orders of magnitudes weaker than those from fluorescence spectroscopy. By introducing metal nanostructures into a solution of molecules to be probed, a cell, or tissue, one can greatly enhance the Raman signal due to interactions of the opitcal field with surface plasmons. Check out a nice review paper from the Kneipp's for more background, Chem. Soc. Rev., 37, 1052–1060, (2008).
Tuesday, May 4, 2010
Falafel's Drive-in, Photo from Gena S. from Yelp.com
This will just be my second visit to San Jose. Last time was for CLEO 2007 and I pretty much didn't leave the downtown area (just ping-ponged back-and-forth from the convention center to my hotel). For this visit, I'm interested in taking some time to explore what the city has to offer when there is some conference down-time. My hope is that any blog followers with more experience in Northern California, or who are native to the area, will help point us tourists in the right direction. So please comment!
My first assumption is that most of us will not have a car. So my recommendations from surface web-browsing of area attractions and restaurants will be contingent upon reasonable trip-time using public transportation or walking.
The Winchester Mystery House and Restaurants along the way:
Is it a freak of architecture, a wonder, or just plain bizarre? The last time I went to San Jose, a colleague recommended that I visit The Winchester Mansion . I never did, but will be going this time. One legend has it that after the premature deaths of both her daughter and husband, Sarah Winchester, heiress to the Winchester rifle fortune, sought advice from a medium in Boston, MA. The medium told her that her misfortunes were due to the spirits of American Indians, Civil War soldiers and others killed by Winchester rifles. To appease the spirits she was to move West and build a home for them and never cease construction. The mansion, whose rooms were continually added and remodeled until Sarah's death is a labyrinth of winding corridors, stairs that descend and then ascend before they reach their destination, doors that open to blank walls, and other oddities. A tour of the mansion costs $28. From the convention center take Bus 23 to the Winchester Shopping Center from which you can walk the rest of the way. Travel time is about 45 minutes.
Some searching on tripadvisor highlighted some restaurants along or near Bus Route 23. The most popular on tripadvisor was Falafel's Drive-in at 2301 Steven's Creek Blvd, which offers African, Mediterranean, Middle-Eastern cuisine. And hamburgers of course! Take Bus 23 to San Carlos and Topeka and then walk west rest of the way- San Carlos Blvd. becomes Steven's Creek Blvd. Total travel-time 24 minutes. Another restaurant in this area slated for good Indian cuisine is Amber India in Santana Row. This time, take Bus 23 from the convention center to Valley Fair Shopping Center, then walk south the rest of the way to Santana Row Shopping Center. Total trip-time 40 minutes. Another recommended restaurant in Santana Row is Thea, which serves Turkish and Greek food.
Other Recommended Restaurants:
If you want to see belly dancing while enjoying Moroccan cuisine, take the light rail north to the Gish stop. The Moroccan Restaurant Menara is located at 41 E. Gish. Travel-time is a short 11 minutes.
La Victoria, immediately next to the convention center on San Carlos, was ranked third on tripadvisor. The cuisine is Californian/Mexican. In the comments, many customers raved about the orange sauce which they sell in bottles if you want to take some home.
Green Space and Exercising:
Though I don't run as often as I should, this is one of the ways I try to stay in shape. It is also a nice way to explore a city. Unfortunately, San Jose seems to be a lot of highway and concrete, but it looks like are at least a couple of parks fairly easy to get to from the convention center. What looked most promising to me was to take the light rail to the Penitencia Creek stop and then follow Penitencia Creek Rd northeast toward Alum Rock Park. It is just 2.5 miles along Penitencia Creek Rd to the base of the Park. Along the way you can leave the road and run through Penitencia Creek County Park which connects to Penitencia Creek Trail. If you are a runner you may already know about mapmyrun.com, which gives you suggested routes and mileage from local runners. Just make sure you give your Falafel plenty of time to digest before running up Alum Rock.
Sunday, April 25, 2010
Though maybe a bit nerdy, or perhaps just naive, this has been an epiphany for me. For any conference-goers reading these posts, if you don't already, I urge you to take a look at the conference program well before you get to San Jose. Do a little planning now so that you can arm yourself with those good questions to advance and broaden your research. Some brief searching on the CLEO website last night already got me thinking about some new directions for MID-IR work as well as resurrecting some old ideas I had about novel pulsed sources using optical-phase locking.
To start your planning, I suggest going to the Hot Topics categories midway down on the main conference page. Typically I am cynical about what seem like cute little multimedia ploys to spice-up dry, technical subject-matter like the You Tube shorts on this page. However, these are well delivered and helpful. They are worth a listen. I found myself playing them all even though many were out of my sub-field. The list of tutorials and invited talks beside each short was particularly useful.
In one of the shorts (above), Peter Smowton, the Semiconductors Subcommittee Chair, from Cardiff University teased my interest in prompting what he thinks could be a "controversial" talk, CTuKK1, "Direct Observation of Two-Photon Gain in Semiconductors." This is the first observation of two-photon gain in a solid. Be sure to show up at 4:45 pm on Tuesday, May 18, in room A6 to watch the drama unfold.
In another short, Konstantin Vodopyanof, the General Chair from Stanford and Brian Applegate, the Biophotonics Subcommittee Chair, from Texas A&M University prompted me to schedule William Moerner's tutorial on supper-resolution into my itinerary, 4:45 on Thursday, May 20, in room A4. What is super you may ask? Less than 100 nm. Impressive.
After browsing Hot Topics, you may try what I did and build up a conference itinerary.
If you are going to CLEO you're a nerd already, embrace it and do some nerdy planning too.
Tuesday, April 20, 2010
Photo from Marco Fulle, National Geographic Daily News
In the lead-up to CLEO I have been trying to do some work in the MID-IR wavelength range. I have been waiting for some CaF2 lenses that were due to arrive a few days ago only to find that they were held up by the ash plume from Eyjafjallajokull's recent eruptions. It turns out that the distribution center from where my lenses were to be shipped is in Germany.
Searching the news for more information, I learned about the woes of poor stranded European travelers (rock stars, film-makers and pro-wrestlers included; it seems ash plumes don't discriminate),how to pronounce the name of the Icelandic volcano that caused all of this trouble (by the way it is EY-ya-fyat-lah-YOH-kuht), debate among officials about the safety of flying through ash plumes, but most interesting to the scientist in me, volcanic lightning.
I stumbled across these stunning pictures on the NY Times blog Lens from Icelandic photographer Ragnar Th. Sigurdsson. These photos that show the explosions, ash, and volcanic lightning from Eyjafjallajokull seem to be more like something from a George Lucas film (like that fiery lava planet in Star Wars: Episode III) than reality.
I then did some browsing on volcanic lightning to find some more photos and some explanation . The same ingredients in the eruption are found in thunderstorms: water droplets, ice, and particles all interacting. The ash plume provides lots of surface area for charging. I am reminded of stories I've read about the dust bowl in the plains of the U.S. during the 1930s when people refused to shake hands for fear of large static shock and that they also dragged chains from the tailgates of their cars to ground charge build-up. I guess it should be a no-brainer that lots of moving dust = lots of static build up.
I wish those still delayed in Western Europe safe and uninterrupted travel. May your delays be short. Maybe my lenses will arrive soon too...