Dec 13

Machining with Ultrafast Pulses

By JamesVanHowe CLEO: Applications No Comments »

(From Raydiance Inc)

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

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.

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)……..for the full original post click here.

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Apr 21

Postdeadline Papers show Emphasis in Broadband Light Generation, Biomedical Imaging, and Nanophotonics

By JamesVanHowe CLEO Conference, CLEO: Applications, CLEO: Expo No Comments »

Fig. 1. From P. Del'Haye, Nature, 450 1214, (2007). a) frequency comb spectrum, b) degenerate and non-degenerate four-wave-mixing among cavity modes, c) SEM image of torroidal microcavity

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

On Thursday May 5, from 8pm- 10pm, conference goers will be madly dashing from ballroom to ballroom to hear the latest breaking optics research- it’s like a geeky Black Friday for optical science. There are 36 talks in total, but because they are spread out among three sessions, you realistically can only hear 12. Trying to see more requires cat-like agility to maneuver around standing-room-only crowds. Good thing postdeadline abstracts were recently posted. Be sure to look through the agenda of sessions and plan your evening.

This year’s sessions include record breaking feats typical of CLEO postdealine papers: an ultralow 181 nA lasing threshold in a nanocavity laser (PDPA1), a whopping 4176% W-1cm-2 conversion efficiency for parametric fluorescence in a diode laser (PDPA3), a limit-pushing 1.5 mm imaging depth in a mouse brain cortex using a two-photon microscope (PDPB3), Mid-IR to keV X-ray supercontinuum generation (PDPC12), a noise figure less than 3 dB in a phase sensitive amplifier (PDPB10), and many others.

Though the sessions will host a wide variety of topics in fundamental and applied optics, some themes that emerge from this year’s postdeadline abstracts are papers that demonstrate broadband frequency generation, biomedical imaging (the postdeadline subcategory CLEO: Applications & Technology 1: Biomedical has the most papers of the three sessions), and nanoscale lasers and nano-photonic devices.

One of the papers on broadband frequency generation, PDPA4, “Mid-Infrared Frequency Combs Based on Microresonators,” by Wang et al. from a German, Swiss, and French collaboration (note one of et al’s s is Nobel Laureate Theodor Hansch), builds on previous work reported in a 2007 Nature paper to produce a monolithic comb generator in the Mid-IR. The reason for the microresonator is to get rid of the big Ti:Sapphire laser typically used to generate frequency combs in order to scale down cost, complexity and size of the comb generator. The high-Q microresonator, an example of which is shown in the Fig. 1, requires a simple CW pump. Besides being smaller, simpler, and potentially much cheaper, the microresonator has the advantage of producing comb spacings greater than 500 GHz (something unattainable by comb generators that use ultrafast pulsed seed sources like the Ti:Sapph).

Fig. 2. From Daylight Solutions, interesting molecules arranged by peak absorption wavelength

One compelling reason for building a comb generator in the Mid-IR is for ultrasensitive, broadband spectroscopy in an interesting spectral region for which there is a dearth of laser sources. Figure 2. from Daylight Solutions (CLEO booth 1526), a company that fabricates quantum cascade lasers between 3.0 and 20.0 microns, sorts molecules of interest by their peak spectral absorbance. These molecules are interesting for environmental monitoring (ozone, water, methane, carbon dioxide), threat and standoff detection (TNT, TATP, VX), and biomedical spectroscopy (glucose).

Fig. 3. From Kobat et al., Optics Exp., 17, 13354, (2009). a) Two-photon image of a mouse cortex with 775 nm excitation and 1280 nm excitation. b) Attenuation of fluorescence vs. depth for 775 nm and 1280 nm excitation.

 

 

 

 

 

 

 

 

New additions to this year’s postdeadline session are subcategories in CLEO: Applications and Technology, most notably CLEO: Applications & Technology 1: Biomedical. The four papers in this subcategory demonstrate pushing the limits on resolution, high-speed image acquisition, or penetration depth for different microscopic techniques. PDPB3, “In vivo two-photon imaging of cortical vasculature in mice to 1.5-mm depth with 1280 nm excitation,” by Kobat et al. shows record imaging depth in a mouse brain cortex using two-photon microscopy by cleverly using long-wavelength excitation. Typical two-photon microscopes use 800 nm, ultrafast pulses from a Ti:Sapphire laser to excite the tissue to be imaged. Photons may not make it to the depth of interest because of absorption or scattering. In brain tissue, scattering dominates over absorption between 350 nm -1300 nm. By using a longer excitation source, more photons can make it to the target allowing for deeper imaging…For the full original post, click here.

 

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Nov 24

Emerging THz technology could solve body-scanner controversy

By JamesVanHowe CLEO Conference, CLEO Tech Transfer, CLEO: Applications No Comments »

From TeraView press release Jan. 2010, THz Spectra of explosives (threats) and different clothing materials (non-threats).

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

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. What makes THz radiation useful for a body-scan is that it is readily transmitted through non-metallic and non-polar materials like clothing. 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 spectra, 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……for the full original post, click here.

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