Erratum

Greetings. If anyone is still paying attention, I wanted to clarify an error in post "Cutting-edge and Future Saturable Absorbers" originally posted on May 20. I was a little two-photon happy and incorrectly inserted a two-photon absorption process in the electron transition diagram. This has now been corrected. My apologies to author Amir Nevet for this mistake and many thanks to the author for clarifying the correct diagram of the second-order process. Best of luck to the authors developing this work. I know I am looking forward to the day when I can get my hands on a two-photon amplifier. If Nevet et al are taking pre-orders, I'd like mine fiber-coupled!

See you in Baltimore in 2011

I hope all conference attendees, presenters, and organizers have made it safely back home by now. Thanks to all of you for your hard work, great research, and participation in our field. Though the economy has not been on our side, one wouldn't know it from the innovations demonstrated in both the academic sphere and marketplace at this conference.

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.

Postdeadline Frenzy: Black is the new Black

Get ready for the post deadline frenzy beginning at 8:00 pm this evening. Make sure to stretch and warm-up properly as you race from room to room. Some areas that caught my interest when browsing through the abstracts were new spectroscopic techniques, novel sources for trace gas detection and molecular fingerprinting, and how to make something really, really, black.

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."

Cutting-edge and Future Saturable Absorbers

Khanh Kieu, from University of Arizona, began his talk, CTuII2,"Generation of sub-20fs pulses from an all-fiber carbon nanotube mode-locked laser system" emphasizing the importance of saturable absorbers (SA) in mode-locked lasers. The SA is the device that is responsible for locking the modes in a laser cavity, thereby allowing the creation of pulses. Without one, you'd just have a continuous wave laser. In general, a saturable absorber is any device that transmits higher intensities of light at expense of lower intensities. They can be active, passive, real, or artificial. In ultrafast fiber-lasers, the typical SA of choice is a semiconductor saturable absorber mirror, SESAM, (a real SA relying on material response) or nonlinear polarization evolution, NPE, (an artificial one making use of polarization tricks). Kieu's point is that it makes sense to spend time developing the component of the mode-locked laser that is responsible for mode-locking.

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

Expo, Idea Generation, and Multiphoton Microscopy

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 is a Mega-day at CLEO

All you have to do is flip to the schedule-at-a-glance in the conference program to see that today is a Mega-day or maybe I should say Tera-day at CLEO. Besides the technical program, the expo opens at 10:00 am, so does the history of the laser exhibit in the exhibition hall, the market focus presentations begin at 10:30 am, the welcome reception begins at 6:30 pm, and the day concludes with the Lasers Rock concert in the San Jose Civic auditorium (the Spanish mission-style building across from the convention center).

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?

Diagnosing Cancer with a Flashlight

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.

Jury Duty and SERS Spectroscopy

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).

Conference R&R: Exploring San Jose

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.