From Joint IED Defeat Organization (JIEDDO) https://www.jieddo.dod.mil. Soldiers from the 713th Engineer Company, out of Valparaiso, Ind., conducted counter improvised explosive device training at Camp Atterbury Joint Maneuver Training Center Aug. 20. Photo by Staff Sgt. Matthew Scotten.
How can you accurately detect threats from chemical, biological, radiological, nuclear, or high-yield explosives (CBRNE) from a safe stand-off distance to protect or warn those in harms way?
Laser spectroscopy is the short answer, be it UV Raman, NIR Raman, Long Wave Absorption Spectroscopy, Laser-induced Breakdown Spectroscopy (LIBS), Photoacoustice Spectroscopy, Ultrafast Spectroscopy, just to name a few. However, what kind of spectroscopy you use to identify a threat is just the beginning to making a system that can function in rugged battlefield environments and accurately deliver the information you need in the time you need it.
Panelist Scott Robertson, Research Senior Manager at Lockheed Martin, posed just how difficult this can be with some specific targets of the type of systems needed in the field. One project whose objective was to analyze threats by the vapors and residues from vehicles needed a stand-off detection distance of 400 m, an entire scan, detect and process time of 1.0 second, with a false alarm rate of only 1 in one million, and packaged in a volume of 1 cubic meter. Another specification target was to be able to scan an area of 2,700 square meters per second while searching a road 100 m wide, while traveling 60 mph.
There are other constraints as well. Tom Stark (no relation to Tony from the Iron Man series), from Landmark Technologies Joint IED Defeat Organization, reminded the audience that 99.9% of the people in an area you want to scan are not the threat. You can't and don't want to blatently scan a crowd with a potentially dangerous high-power laser system. Another constraint therefore is laser safety, particularly eye safety. Add this to the checklist of specification targets and you start bumping up against fundamental limits for power needed to detect a spectroscopic signature of a threat, as well as selectivity and sensitivity for identification of molecules.
Augustus Fountain, Senior Research Scientist in Chemistry at Edgewood Chemical Biological Center, spoke to some of these issues. Fountain spoke about choosing the wavelength/spectroscopic for your method. In the UV you gain in sensitivity but loose in selectivity. The opposite is true as you move into the IR. Another problem to consider in system design is 1/r2 loss and atmospheric attenuation. What kind of time window do you have available for scanning? Is the analyte a mixture of compounds- harder to detect spectroscopically, or something simple? Scott Roberston echoed many of these remarks. Do you want to identify the threat or do you just want to know if it is going to kill you? The specific use and system dictate different constraints on what you design. Robertson also argued most users want the latter- "just give me a green or red indicator light," not a beautiful Raman spectrum that requires interpretation. More often you just want to know "threat or no threat" for fast decision making in an environment of potential threats.
Much of the panel discussion centered around the do's and don'ts of collaborating with companies for defense money and contracts or even directly submitting proposals to broad agency announcements from DoD. If you are a small business or researcher trying to connect with defense contractors or apply directly for money the advice was to follow the rules, connect with partners and collaborators early, ask lots of questions early, and once again follow the rules.
The panel did offer some specific areas where there is need for technology. Fountain spoke how the 785 nm laser has been inappropriately the workhorse for Raman. This wavelength region has many problems. He would like sources further into the IR or deeper into the UV, particularly solid state sources. Edwin Dottery, President of Alakai Defense Systems, pleaded for a UV laser source less than 250 nm. Specifically between 220-240 nm will be ideal for UV Raman.
The difficult obstacles to overcome for practical stand-off detection are worth the effort. The end-user is particularly important and worth the time, soldiers continually putting their lives in harms way as well as civilians who want to carry on a normal life and provide for their families without fear of attacks. Lasers just may make this possible.
Wavelength Modulation Spectrum using tunable 2.0 micron VCSEL; From JTh1L.6, A. Kahn et al,. "Open-Path Green House Gas Sensor for UAV Applications"
Today at CLEO I spent a large amount of time at the expo hunting down which companies were selling 2.0 micron wavelength products and why. In the technical program, there are a large number of contributed talks regarding 2.0 micron lasers, pulsed and continuous wave. On Monday I attended session CM1B, Ultrafast Mid-IR in which 5 out of 8 papers demonstrated ultrafast pulses about 2.0 microns. Today there was a session titled CTu2D,1.5 to 5 micron Lasers which also had 5 talks out of 8 showing laser systems operating near 2.0 microns. There have been and will be a handful of talks not pinned down to these topic categories as well:
-CM2B.2, "A Broadband 1850-nm 40-Gb/s Receiver Based on Four-Wave Mixing in Silicon Waveguides"
-CTu3M.7, "All-fiber 10-GHz Picosecond Pulse Generation at 1.9 microns without Mode-locking"
-JTh1L.6, "Open-Path Green House Gas Sensor for UAV Applications"
-CF1K.1, "Single-Frequency kHz-Linewidth 2-μm GaSb-Based Semiconductor Disk Lasers With Multiple-Watt Output Power"
-CF1N.4, "Double-wall carbon nanotube
Q-switched and Mode-locked Two-micron Fiber Lasers"
However, what we like to research and what we can actually bring to market are often two very different things. I am therefore excited that it is not just 2.0 micron papers that are cropping up at this years conference, but 2.0 micron products at the expo as well.
So why is anyone interested in light in the 2.0 micron region? My personal interest stems from a research talk I saw by analytical chemist, Mark Arnold, at University of Iowa. Arnold is trying to perform some hard analytical chemistry on 2.0 micron light shone through the skin on the back of one's hand. He hopes that by looking at the absorption spectra, he can measure blood glucose levels without having to draw blood. This noninvasive testing would be a boon to diabetics who are not thrilled about pricking their fingers regularly. Wavelengths that are helpful for pinning down glucose, but that are not absorbed as readily by tissue are 2.13 microns, 2.27 microns, and 2.33 microns.
In short, there are some interesting molecules around 2.0 microns on which to perform spectroscopy. For environmental sensing, there is 1877 nm, a well defined water absorption line, and 2004 nm, a good line for carbon dioxide detection, and many more.
Many of the companies I spoke with selling 2.0 micron components and sources confirmed such spectroscopic applications of their customers:
-Oz Optics now sells passive fiber components at 2.0 microns as well as DFB sources.
-Sacher Lasertchnik and Nanoplus make DFB lasers extending through the 2.0 micron region depending on your molecule of interest.
-Advalue Photonics makes thulium-based fiber laser systems and sells passive 2.0 micron products.
-New Focus will be developing tunable laser diodes about 2.0 microns in the next few months.
-Nufern and CorActive are selling Tm- and Ho-doped fiber for 2.0 micron amplification and for fiber sources.
-IPG sells a number of lasers from 2.0-2.8 microns based Cr:ZnSe as well as 2.0 micron fiber lasers using thulium doped fibers.
There are other advantages to 2.0 micron light as well. 2.2 microns is where the two-photon absorption coefficient in silicon drops to nothing. If you are interested in confining light to a silicon waveguide, doing so at 1550 nm could be the worst choice since it coincides with the peak two-photon absorption. However, above 2.2 microns allows higher throughput as well as access to other nonlinear effects like parametric amplification.
This is one of the pursuits of Thorlab startup, PicoLuz. Among other optical instrumentation, PicoLuz is developing 2.0 micron amplifiers which will eventually support its endeavors of a chip scale optical parametric amplification.
-Thorlab's Quantum electronics division is also selling a laser that is tunable about the gain bandwidth of thulium 1800-2000 nm, an FTIR spectrometer that goes out to 2500 nm, a handful of moderate speed long-wavelength detectors, passive fiber components for 2.0 microns, as well as pumps for Tm-amplifiers.
Another reason for generating 2.0 micron light is for opening up new spectral bandwidth for telecommunications or signal processing, whether on fiber or on silicon. To that end,
-Eospace is now offering 20 Gb/s speed LiNbO3 intensity and phase modulators at 2.0 microns.
Malcom Minty, a project manager from New Focus told me that New Focus was actively persuing thulium based laser systems in the 90's. The thought was that bandwidth would all be used up in the C- and L-band during the telecom boom, requiring expansion. Thulium has wide efficient gain and was a natural choice. Nufocus dumped the project as the the telecom bubble burst. Now they will be rejuvenating it, but more likely to sell to customers interested in spectrscopic pursuits.
Minty conjectured that 2.0 microns is becoming an interesting color to customers, vendors and researchers because it is in a region (or getting close to a region) of spectrscopic and biomedical interest. However, because it is close enough to the L-band, much telecom technology can still be used. It just squeaks by with some efficiency for detection on InGaAs-based detectors where as detection methods above 3.0 microns start getting tricky.
I think Minty likely has this right. If so I will be interested to see what new research we can carryout with 2.0 micron light while leveraging what we already know from fiber systems and telecom.
(One of seventeen youtube shorts from the program chairs highlighting hot topics for CLEO 2012)
For a few years now CLEO conference organizers have been posting youtube shorts highlighting contributed talks, symposia, research trends, and any new or unique directions for the upcoming conference. This year there are seventeen videos from the program chairs, all worth watching. However, for those who prefer text over A/V, I thought it might be helpful to highlight the highlights here.
Conference Program Stats
-The 2012 program has been selected from a record number of submissions.
-In just its second year, CLEO's new Technology and Applications Conference saw a 50 % increase in submissions.
-350 papers, 15 % of all submissions, live in the subcommittee sections "Nano-optics and Plasmonics" or "Micro- and Nano-Photonic Devices"
-Subcommittee section: "Fiber Amplifiers, Lasers and Devices" was the single committee that received the most submissions
In his youtube short, subcommittee Chair Ian Mckinnie of Lockheed Martin Coherent Technologies briefly discusses the two tracks of this subcommittee: 1) Ultrafast Laser Applications and 2) Instrumentation and Sensing.
Mckinnie talks about how the ultrafast program covers a broad range ultrafast laser applications spanning those performed at large facility-class systems to those on a bench top or operating table. These are exemplified by the tutorial talk, AW3J1, "Enabling Science at the Advanced Light Source X-ray Facility" that will be given by Roger Falcone of Lawrence Berkeley National Laboratory from 4:30-5:30 pm on May 9, and the invited talk AW3J4, "Applications of Ultrafast Lasers" by Mike Mielke of Raydiance Inc., also on May 9, but from 6:00-6:30 pm
The Advanced Light Source (ALS) is a large synchrotron source that produces laser light over an extremely broad spectrum including the hard-to-reach soft x-ray region. Falcone will be discussing the use of the coherent radiation at this user-facility for applications such as precise material processing and biomedical research.
On the other hand, Mielke will be discussing the use of compact fiber systems for micromachining and laser surgery. See blog post "Machining with Ultrafast Pulses" for some stunning videos and more information on these compact micromachining systems.
On the remote sensing side, Massayuki Fujita, from the Institute of for Laser Technology in Osaka, will be giving an invited talk on an application of remote sensing not typically found in the CLEO conference program- nondestructive inspection for heavy industrial processes. Fujita's talk, ATuG3 "Nondestructive Inspection for Heavy Construction" can be heard on Tuesday May 8, at 2:30 pm.
In his youtube short, subcommitee chair Yu Chen from University of Maryland mentions a number of specific talks you won't want to miss:
In the session "In vivo Imaging", there will be two talks on on image-guided spectroscopy. The first will be a tutorial talk by Brian Pogue of Dartmouth on integrating optical molecular spectroscopy techniques into standard medical imaging equipment, ATh4C1, "Image-Guided Spectroscopy of Cancer: Translating Optical Technology into Clinical Tools" on May 10, at 4:30 pm. The second will be an invited talk from Brian Benaron of Spectros Corporation, ATh4C4, "Molecular Spectroscopy and Imaging: A multibillion-dollar industry reshaping biotech and medicine" also on May 10, but 6:00 pm.
Chen also mentions the contributed talks from the In Vivo session by Saivash Yazdanfar of GE Global Research who will be speaking about fluorescence image-guided procedures in talk ATh4C2, "Fluorescence Image Guided Surgical Instruments and Contrast Agents for Intraoperative Visualization of Nerves" on May 10, 5:30 pm as well as contributed talk from Adam Straub of Cornell University who will be presenting work on increasing multi-photon image acquisition speed by a whopping two orders of magnitude, ATh4C3, "Multiphoton Multifoci Modulation Microscopy for High-Speed Fluorescence Lifetime Imaging"at 5:45 pm on May 10.
Chen goes on to highlight other talks in session "OCT and Microscopy" which will be held on Thursday May 10, from 2:00-4:00 pm, perhaps most notably talk JTh3J, "Recent advances in translating OCT into GI Endoscopy" by Brett Bouma of Massachusetts General Hospital, one of the pioneers of OCT. There will also be a host of cutting edge talks in session "Cellular Imaging and Therapy" 8:00-10:00 am on Thursday May 10. This session kicks off with an invited talk by Adam Wax from Duke University, ATh1M1, entitled "Coherence Imaging for Early Cancer Detection."
In his video short, subcommittee chair Eric Mottay of Amplitude Systemes discuses the two major trends of the Industrial Applications subcommittee: 1) micro- and nanofabrication techniques and 2) applications of graphene.
Talks in the latter category can be found in a joint session with CLEO: Science and Innovation subcommittee six in session "Graphene and Carbon Advanced Photonic Materials" which will be held form 11:00am-1:00 pm on May 8. This session will host talks presenting graphene-based devices such as detectors, modulators, and tunable resonators. Recall that Andre Geim and Konstantin Novoselov were awarded the 2010 Nobel Prize for showing the "exceptional" properties of graphene such as it being simultaneously the thinnest and strongest material, having better electrical conductivity than copper, better heat conduction than all other known materials, and having nearly 100 % transparency yet an extremely high density (so dense helium atoms cannot pass through). Be sure to see how this "magical" material is being translated into devices that may be on the market in the next three to five years.
On the other hand, the invited talks for this subcommitee all center around micro- and nano- fabrication processes. Arnold Gillner of the Fraunhofer Institute will discuss how ultrafast lasers can be used for surface processing at the micro- and nanoscale level for applications in light guiding, fabrication of low friction surfaces, or wear-resistant surfaces. His talk, ATu3L1, "Micromanufacturing and nano surface functionalisation with ultrashort pulsed lasers" is scheduled for May 8, at 4:30 pm. Additionally, Paul Webster from Queen's University will be discussing online monitoring during fabrication, particularly concerning the control of depth, in invited talk ATu3L5, "Inline Coherent Imaging: Measuring and Controlling Depth in Industrial Laser Processes," on May 8, at 5:45 pm and Rick Russo from Lawrence Berkeley National Laboratory will be speaking about real-time spectroscopy of a sample after it has been turned into a plasma through laser ablation in talk, AW1H3 "Laser Plasmas for Spectrochemistry" on May 9, at 11:00 am. CLEO Applications and Technology: Energy and Environment
In his video short, subcommittee chair Christian Wetzel from Rensselaer Polytechnich Institute discusses two trends in paper submissions 1) environmental sensing, particularly atmospheric sensing using quantum cascade lasers (QCL) and 2) Breakthroughs in LED lighting, for which many contributed papers address ways of overcoming "droop" (the reduction in efficiency by when driving with high current).
These two topics will be also be discussed indirectly and directly in the special symposium "50th Anniversary of the Semiconductor Laser" in which one of the pioneers of the QCL, Jerome Faist, from the Institute of Quantum Electronics in Zurich, will be giving an invited talk "Quantum Cascade Lasers: Coming of Age" as well in the plenary talk, "Development of nonpolar and semipolar InGaN/GaN light-emitting diodes (LEDs) and Laser Diodes" by solid-state lighting giant Steven Denbaars of University of California, Santa Barbara.
Wetzel mentions two must-see invited talks in his short. One is talk JTh4J1 "Hydrogen Generation using Nitride Photoelectrode" by Kazuhiro Ohkawa of Tokoyo University of Science on May 10, at 4:30 pm. Ohkawa will show results of solar powered water-splitting on a nitride-based electrode for which the incident photon-to-electron conversion efficiency (IPCE) is upwards of 70%. The other is JTh1L3, "III-Nitride Optochemical Nanosensors" in which Jörg Teubert from Justus-Liebig-University in Giessen will discuss a nitride-based nanosensor for spectroscopic measurement and ph detection.
In his youtube short, program co-chair René-Jean Essiambre of Bell Labs, Alcatel-Lucent discusses some of the trends of the various committees.
In subcommittee 11: Fiber Amplifiers, Lasers and Devices, Essiambre notes a trend in papers demonstrating lasers between 1.8-2.0 microns. This is a region where thulium and holmium give efficient and broad gain. Specifically, many submissions show increased wavelength tunability or higher-power operation. Since Essiambre mentions this track, I figured this would give me license to shamelessly promote my own contributed talk. I will be presenting a contributed paper in this category, CTu3M7, "All-fiber 10-GHz Picosecond-Pulse Generation at 1.9 μm without Mode-locking" which demonstrates an unconventional method for pulse generation in this spectral region.
What is so exciting about 2.0 micron light is that there are good gain media in this spectral region and it is just on the edge of the mid-IR for which spectral signatures for various interesting molecules have sharp unique absorption lines- the fingerprint region. Therefore, 2.0 micron sources may be good seed sources to frequency-shift to redder, more spectroscopically significant wavelengths. Two micron light also holds interest for silicon photonics since two-photon absorption, a hindrance for many processes involving tightly confined and/or pulsed light, drops off rapidly in silicon at 2.0 microns and beyond.
Essiambre also notes other trends in the various subcommittees in Science and Innovations. In subcommitee 12: Lightwave Communications and Optical Networks many submissions address new modulation formats and constellations, spatial multiplexing, and high spectral efficiency systems.
Subcommittee 13: Active Optical Sensing saw a focus on frequency combs, particularly making comb sources more accurate, with narrower line-widths, yet at the same time keeping them simple, portable, inexpensive and usable in harsh environments. This was also the trend for papers submitted to subcommittee 14: Optical Metrology. In addition, topics for this subcommittee address applications to astronomy, spectroscopy, and use for high precision standards, not to mention distribution of high-precision combs.
Check out the videos for more details and information. One of the marvelous things about CLEO is that it has so much breadth and hosts so many talks. However, this also makes it overwhelming and difficult to decide what to attend and to decipher new trends in research and applications. I recommend taking some time to hear what the chairs have to say so that they can make your work a little easier.
(Above: First room temp. CW semiconductor nanolaser with subwavelngth cavity presented at CLEO 2011. From K. Ding et al, CTuG2, CLEO 2011.)
The year 2012 marks the impressive 50th anniversary of the invention of the prolific and ubiquitous semiconductor laser. Almost every household in the industrialized world owns at least one, be it in a DVD player (maybe two if it is a Blue-ray), CD player, optical mouse or depend on them indirectly for long-distance phone service, digital cable, or internet access. Besides making telecommunications a practical possibility, semiconductor lasers have paved the way for the development of silicon photonics and will be pivotal in the future of optical information storage and processing. Despite their primary use in mass consumer markets for communications, information processing, mutimedia, and teasing cats (you can even get semiconductor laser pointers with phase masks and lens attachments that project images mice or fish on the floor for your feline to chase), many subfields have profited from the low-cost and small-footprint of these robust laser sources. Take for example the handful of semiconductor sources offered commercially by Thorlabs for optical coherence tomography, or the inexpensive semiconductor laser diode sources used by the Ozcan group for field-portable, ultra-low footprint, holographic microscopes.
There are too many other technologies and subfields to name that have profited as well. All you need to do is think of the numerous optics applications that live at telecom wavelengths near 1300 nm or 1550 nm or DVD player wavelengths, 405 nm and 635 nm. Such lasers offer unbelievable device characteristics at such a low price that researchers and venture capitalists often build their technologies to fit these wavelengths instead of the other way around.
Amnon Yariv and Pochi Yeh write in their 2007 edition of the book Photonics that,
"The semiconductor laser invented in 1961 is the first laser to make the transition from a research topic and specialized applications to the mass consumer market...It is by economic standards and the degree of its applications, the most important of all lasers."
To celebrate the most important laser of lasers, CLEO will be hosting a special symposium with talks from pioneers of semiconductor laser technology. The list of speakers and subjects has been well-crafted to paint not only a historical picture but to address current research and trends on this ever-evolving technology.
From a fundamentals perspective Russel Dupuis from Georgia Tech will be talking about device materials. Nobel Laureate Herbert Kroemer of University of California Santa Barbara will discuss the double heterostructure which is still the basic framework for almost all semiconductor light sources and solar cells and which without there would be no continuous wave (CW) lasing in semiconductor devices at room temperature. To this end, Morton Panish, formerly of Bell Laboratories, will describe the development of the first room temperature semiconductor laser.
(Above: Evolution of threshold current. From Nobel Laureate Z. Alferov, IEEE J. Sel. Top. Quant. Elec. 6, 832, 2000.)
Charles Henry, formerly of Bell Laboratories, will discuss the quantum well structure which was pivotal in reducing active layer thickness and therefore significantly reducing threshold current, see the figure above. Yasuhiko Arakawa from the University of Tokyo will discuss quantum dot lasers which reduced threshold densities even further and remains a developing area of semiconductor laser physics research.
On the more practical side, Jack Jewell, of Green VCSEL will discuss the vertical cavity surface emitting laser (VCSEL) which among other important device attributes may be the best laser for high-yield production. VCSELs are grown, processed, and tested in wafer-form allowing parallel fabrication and testing, minimizing labor and maximizing yield. They also take up less space on a wafer- about three times less than edge emitters of similar power and can be made in 2-D arrays. Jewell will likely discuss the benefits of lower power consumption of VCSELs for use in short-reach, high-speed networks. My understanding is that the "green" in "Green VCSEL" refers to environmental considerations not wavelength.
There will also be talks discussing the semiconductor laser's role in telecommunications, quantum cascade lasers, integrated and hybrid optical circuits, high-power devices, as well progress in nano laser structures with subwavelength volume (see the figure at the top).
Whether to learn the history, fundamental principles, pay homage to the pioneers, or to learn new trends, be sure to mark your calendar for the 50th Anniversary of the Semiconductor Laser symposium to celebrate "the most important of all lasers."
(Figure from R. Boyd and Z. Shi, Jan. 5, "News and Views" Nature, explaining temporal-cloaking)
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.
(From Raydiance Inc) As someone who has been trying to design novel ultrafast laser systems for the past eight years, my eyes were drawn to the title "Applications of Ultrafast Lasers" of Dr. Mike Mielke's talk from Raydiance, Inc. from the awesomely overwhelming list of invited speakers at CLEO 2012. Dr. Mielke's talk is one of a handful in CLEO's new Application and Technology conference which debuted last year in Baltimore in order to better bridge the gap between fundamental research and product commercialization.
To see what background information I could potentially find, I went to Raydiance's website to find a wealth of information on micromachining and a host of video shorts of ultrafast laser micromachining in action. They are so pleasing to watch, I couldn't help embedding many of them in this post.
Micromachinging with ultrafast lasers allows the removal of material without the introduction of heat (see the video above of laser micromachining on a match head without it igniting). Ultrafast lasers therefore give the advantages of laser machining- tailoring submicron features on the workpiece, without thermal collateral damage. For example, if you are going to have your dentist drill a tiny hole in one of your teeth (see the figure below) , you'd rather have her use the 350 fs laser shown in b) rather than 1.4 ns laser in a) in which the heat generated damages and fractures the tooth.
(Above: Drilling tooth enamel with a) 1.4 ns 30 J/cm2 laser pulses and b) with 350 fs 3 J/cm2 pulses. From B.C. Stuart et al LLNL.)
This is because drilling with the femtosecond pulses relies on an entirely different physical process for removal of material than nanosecond pulses. For long pulses (> 100 ps), photons are absorbed by the material and converted into heat. This eventually fractures, melts, or vaporizes material at (and nearby) the laser focus. On the other hand, if the pulse is fast enough (< 1 ps), the material is removed solely by photo-ionization. Rather than dumping energy into the material, electrons of target molecules are stripped off by the intense electric field of the pulse. No absorption takes place and therefore no heat is generated.
Because the mechanism for material removal using ultrafast pulses does not depend on the material properties as it does for thermal ablation, such as the melting point, conceivably any material can be machined using ultrafast pulses. This has allowed Raydiance to micromachine polymeric materials for manufacturing next-generation vascular stents and microfluidic devices (see the videos below).
(From Raydiance Inc)
Though micromachining using ultrafast lasers is not new, doing so in a robust workstation-platform is. Raydiance touts to have created the first "industrial grade" femtosecond laser platform. They have an impressive record and a current partnership with ROFIN GmbH for the development of industrial-grade femtosecond laser micromachining workstations. In the literature on their website they state, "A laser is not a solution. It might be the engine of a solution, however, 21st century manufacturing floors demand more: software integration, beam delivery, motion control, and visioning systems." As an "engine builder" myself it is helpful to know just what kind of engine is the most useful to workstation integration. Sometimes "engine builders" get caught up in making Formula One cars when what is most helpful is a reliable Hyundai sedan. Although not any pulse width, energy, and rep will do for athermal ablation, neither will a workstation without robust, continuous (thousands of hours 24/7), turn-key operation.
(From Raydiance Inc)
To that end, Raydiance's core platform, Smart Light, can simply be adapted (mainly turning down the power) for non-machining applications in defense and security such as remote sensing of hazardous chemicals and LADAR. Dr. Mielke's invited talk will likely emphasize Raydiance's pursuits in these areas since his talk is in the Government and Security subcategory. I will be interested to see what wavelength tuning options, wavelength conversion, or different center wavelengths Raydiance may be investigating for threat detection since Smart Light currently resides in the telecom C-band near 1550 nm and many absorption lines for molecules of interest live in the mid-IR. Until then, I hope you will enjoy, like me, these videos of lasers "vaporizing" material and leaving beautiful designs for very practical applications.
(Above: Setup of the metalens (soda cans) used to focus a sound wave to a size of 1/25 th of the wavelength of the waves used to generate the beam)
Professor Mathias Fink from ESPCI ParisTech and Institut Langevin doesn't fit the typical profile for a plenary speaker at an optics conference, which is precisely why why you won't want to miss his plenary talk at CLEO 2012 this May. Though acoustics is the consistent medium for his work, his research more broadly consists of understanding the nature of waves and how to get around the limits assumed by our conventional understanding, such as diffraction-limited focusing and imaging. Much of professor Fink's work since the late 1990's has been using time-reversal, the subject of his upcoming plenary talk, to achieve these ends.
For example, in the August 5, 2011 issue of Physical Review Letters, Fink and collaborators demonstrated that they could focus a sound wave to 1/25 th of the wavelength of the waves used to create the focused beam. Ironically, this novel feat was obtained using very conventional objects- soda cans and computer speakers.
The MacGyveresque experiment shown in the figure above uses a grid of soda cans, a group of subwavelength acoustic resonators, to act as a "metalens". When illuminated with a broadband field, this metalens allows subwavelength detail in the near-field to be encoded onto propagating waves. Essentially the metalens is a very good evanescent-to-propagating-wave converter, "unsticking" evanescent waves with subwavelength detail that are typically locked to the surface of the object (or source) of interest. This phenomenon is analogous to the generation of surface plasmons in near-field microscopy (see the August 16th post below). The propagating waves, now containing subwavelength information, can be detected in the far-field and time-reversed (essentially run backwards) in order to focus to subwavelength spots.
Time-reversal essentially amounts to phase-conjugation. However, unlike optical phase conjugation, time-reversal is broadband. Rather, time-reversal is phase conjugation for every frequency at once.
In order to experimentally employ time-reversal, one needs a time-reversal mirror (TRM). For an acoustic wave, a TRM is essentially an array of piezoelectric transducers spread over a surface through which the wave of interest propagates. Each transducer records the wave at its unique position and then is made to play back the time-reversed copy such that the each wave retraces its complex path back to the source. Professor Fink and collaborators first demonstrated the power of time-reversal in the mid 1990's when they focused sound to a much smaller spot size than allowed by the aperture of the transducer array producing it. They discovered that when the source was allowed to scatter many times off of a random array of steel rods, they could reverse the signal such that it came back to a smaller spot size than the original source. The long path lengths from multiple scattering effectively widened the focusing aperture. When they removed the steel rods, they could only focus to the predicted size limited by the aperture of the transducer array.
"If we want to reconstruct an exploded block from the various scattered pieces, a time-reversal mirror would be a device that precisely reverses the velocity of each debris particle as it crosses a closed surface surrounding the initial block. But before being sent back, each particle must be held for an appropriate delay time: To reconstitute the block, one has to send back first the slowest pieces, which had arrived last."
Time-reversing an exploding block is of course thermodynamically impossible, however, for waves which can be described completely by a limited amount of information, it is reality. The strangeness of the multiple scattering experiment performed by Fink et al in the 1990's, and current experiments, is that it is as if the exploding block is being time-reversed to be put back together into a block that is smaller than the original.
So what about time-reversal for optics? Subdiffraction focusing and imaging in the optical domain have already been shown using a variety of techniques without time-reversal (for example, see Frank Kuo's September 10th post). However, two recent articles by McCabe et al, and Vellekoop et al show the optical analog of Fink's 1990's work, in which a highly scattering medium combined with time-reversal (via spatial light modulators) can be used to enhance an optical focus. Another recent work by Xu et al from Washington University shows a technique called Time-Reversed Ultrasonically Encoded (TRUE) focusing in which only the encoded portion of light from a microscope focal volume is time-reversed back to the sample for clean focusing. In this case the time-reversal mirror consists a holographic technique using a photorefractive crystal to a phase-conjugate of the right bit of light back to the focus.
I'm not only looking forward to Fink's plenary talk to learn about other uses of time-reversal in optics, but to generate ideas of what other wave phenomena may be borrowed from fields like acoustics, microwave communication, and quantum mechanics and visa-versa. After all, it's just the same wave equation.
