Semiconductor Laser's Golden Anniversary

(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 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 "the most important of all lasers."

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

(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.

Machining with Ultrafast Pulses

(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.

Using Soda Cans to Beat the Diffraction Limit

(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.

In a 1997 physics today article, Fink explains time-reversal using an analogy of an exploding block:

"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.

Call for Papers

October 1, marked the official call for papers for CLEO 2012 in San Jose, CA. I've decided to include a recurring gimmick in my past blog submissions for CLEO- a countdown clock. Mark your calendars for December 5, for submitting contributed work- there is still a good 63 days left to collect good data, put finishing touches on new instruments, or simulate new phenomena.

The official CLEO website has already posted plenary speakers. You can visit Expocad which will take you through the expo map, giving you booth information as you hover your mouse over different areas of the map. Stay tuned to this blog and CLEO's other various social media in the lower right-hand corner of the main page for the latest information for authors, attendees, speakers, students, and exhibitors.

Photonics for Global Health

(Left: Reflection images of a histopathology slide corresponding to skin tissue using a low-cost, portable, lens-free off-axis holographic microscope. Right: Conventional reflection-mode microscope image of the same specimen using a 4X objective lens (NA: 0.1). Image from Biomedical Optics Express)

Research performed in the Ozcan group at UCLA holds a unique place in the field of optics and photonics. Besides the typical pursuit of advancing optical technology, another major initiative of this photonics group is solving problems of global world health, particularly in resource-poor countries.

Early September marked a milestone for the UCLA group as they published work on a compact, low-cost (~$100 USD of parts), dual-mode microscope with 2 micron resolution in Biomedical Optics Express (also written up in a recent OSA press release). The key to making such a low-footprint, low-cost, lab-grade device is using holographic microscopy. The image information stored in a hologram (the interference of the reflected or transmitted light from the specimen with a reference beam) requires no lenses, drastically reducing the weight, size, and overall expense of the device. A computer reconstructs the wavefront reflecting from (or transmitting through) the sample instead of a lens (see fig below). The impact to world health will be increased blood-diagnostics, water quality tests, tissue screening and analysis, and other imaging diagnostics in areas where microscopes currently are not available due to cost and/or remoteness of location. Getting more microscopes into the hands of health workers may have large impacts for heading off disease outbreaks as well as treatments for individuals.
















(Schematic of the 200 gram microscope developed by the Ozcan group in reflection mode. LD: laser diode, PH: pin hole, BC: Beamsplitting Cube. Note the two AA batteries as the power source as well as for scale. Image from M. Lee, O. Yaglidere, and A. Ozcan, Biomedical Optics Express, 2, 2721 (2011). )

The idea of using holograms in microscopy is not new. In fact it was the quest for higher resolution in electron microscopy which prompted Dennis Gabor to devise wavefront reconstruction by holography in 1948. Gabor coined the word "hologram" which translates "whole message" to emphasize the amount of information that is stored in this very special interference pattern. For a brief history of holography from its roots in microscopy, its development through radar, and its boom in mainstream art and media in the 60's and 70's , see Jeff Hecht's 2010 OPN article.

What makes the Ozcan group's work so special is not the use of a fundamentally new technique, but clever and impressive engineering. This holographic microscope is small, inexpensive, and can work in both transmission and reflection mode. The transmission mode of the current device is similar to an earlier work by the Ozcan group- a cell-phone microscope. In the summer of 2010, the UCLA group published work in Lab on a Chip demonstrating a clever attachment to an ordinary cell-phone which could convert it into lab-grade microscope (see the youtube short below). By employing digital holographic microscopy, the group was able to produce a 38 gram attachment without any lenses, lasers, or bulky optics, which when incorporated with the cell phone camera, produced hologram on the cell phone detector array. The idea is that the hologram data would be sent over the same cell phone to the closest hospital/analysis station, a computer would process the hologram to extract the image information, and then the image would be sent back to the same phone, all within seconds of placing the sample to be analyzed into the device.



Though the current device cannot be so easily integrated onto a phone, the additional benefit of reflection-mode operation makes up for its "bulkiness." By operating in reflection-mode, the new microscope is additionally suited for imaging optically dense media like tissue, something not possible using in-line transmission holography due to spatial distortions in the reference wave. The developers decided to keep a transmission-mode an option, however, since it produces a larger field-of-view then its reflection counter-part and is easier to align and operate.

Once again a computer is needed to reconstruct the image from the hologram. However, hologram data could be sent to the nearest processing center if the field-worker is not carrying a laptop already. My thoughts immediately lead to the computer produced by Quanta through the One Laptop per Child initiative (OLPC). The XO laptop costs approximately $200 and can run on power sources such as solar, human power, generators, wind or water power. Though the aim of OLPC is to close the digital gap for children of resource-poor nations, I wonder if an XO equivalent could be developed to bridge the gap in digital medicine, not just on a records basis, but for data acquisition and processing for field-portable medical instruments like the microscope produced by the Ozcan group. I can imagine this $100 microscope interfaced with a $200 laptop.

What is exciting about this work is that it underscores the beauty and power of cross-discipline connections. Though lensless holographic microscopy is not new, using it as the foundation for a low-cost, field-portable devices is. To learn about more innovations like these, be sure to visit sessions this spring at CLEO Applications and Technology: Biomedical (just in its second year) in San Jose.

Year of the Plasmon

(Left: August Cover of Nature Materials showing Liu et al s work on gas sensing using plasmonic response from a triangular nanoantenna. The work in the Nature article was expanded from that presented in CLEO 2011, postdeadline session)

This year may not be a flush for the market but it is looking good for plasmonics. Expansion of the the work shown in CLEO 2011, Postdeadline paper "Nanoantenna-enhanced gas sensing in a single tailored nanofocus," from Na Liu et al. just took the August cover of Nature Materials (see the figure above). Additionally, plasmonics has had a solid recent run of the main-stream physics circuit after the publication of two Physics Today articles earlier this year in February and July.

The July issue of Physics Today features an article by Lukas Novotny from University of Rochester in which he reviews near-field optics, the broader category where plasmonics resides. Earlier in the year, Mark Stockman of Georgia State University wrote a very accessible and informative article on nanoplasmonics that took the cover of the February issue of Physics Today. The cover shows a 13th century stained glass window of Sainte Chappelle in Paris whose yellow and red brilliance are assumed to come from nanoplasmonic resonances of silver and gold nanoparticles in the glass. The optical effect of how the red changes over the length of the window is said to have purposely been designed to mimic the flowing blood of Christ.





(Above: Sketch of Edward Synge's proposed near-field microscope. The red dot denotes the gold nanoparticle. Picture from L. Novotny, Phys. Today, 64, 47 (2011))

Novotny's July article also offers a romantic insight into the history of near-field optics and plasmonics. Novotny, recounts how in 1928, Edward Synge wrote a "prophetic letter" to Einstein proposing a near-field microscope (see Figure above) to optically image a biological sample below the diffraction limit. Synge's proposed microscope, which could not be realized until 1982 (by Dieter Phol's group at IBM of Switzerland), looks eerily familiar to current techniques used for the development of plasmonic devices and sensing- the use of metallic nanoparticles to generate surface plasmons in order to enhance a probing optical field. The two Physics Today articles are must-reads for those who need a crash-course on plasmonics.

A plasmon is created when the electrons on a metal surface are periodically displaced with respect to the lattice ions by an external, driving, optical field, creating an "electron oscillator." The frequency of the surface plasmon depends not on the driving field, but instead upon the restoring force and effective mass of the electrons in the metal. Changing the size and geometry of the metal structure will alter the restoring force and thereby the plasmon frequency. Using metallic nanostructures of the right size (smaller than the skin depth of the metal but bigger than the distance an electron moves during on optical cycle, 2-20 nm) the electric field due to the plasmon becomes highly localized in the immediate vicinity of the outer surface of the nanostructure (see the Figure below). By coupling the surface plasmon to propagating optical radiation, nanoscale information from the plasmon can be encoded micron-sized optical waves as it is in near-field microscopy. The highly localized field can also be used for a number of sensing techniques like SERS by which the interaction of a probe beam with a molecule is significantly enhanced due to the presence of nearby nanostructures. The cover article from Nature Materials uses a standard plasmonics approach by using redshifted plasmon response itself from a gold triangle structure for ultra-sensitive detection of hydrogen.





(Above: Diagram of plasmon dynamics on a 10 nm silver nanosphere. Eo represents the external light field, the black arrows represent the electric field from displaced electrons, the plasmon field, and the red arrows show the field inside the sphere. Picture from M. Stockman, Phys. Today, 64, 39 (2011) )

The exciting field of plasmonics has applications and positive repercussions in other fields as well. Tumor cells have been found to readily take up nanoparticles. By illuminating tissues with non-lethal IR light, the heat generated from enhanced local-fields of the high-Q nanostructures selectively kills cancer cells. Plasmon-enhanced solar energy conversion entails using metallic structures to better localize light for solar concentration. The opening tutorial, "Solar Energy Applications of Plasmonics," by Professor Harry Atwater of Caltech in CLEO:QELS session "Frontier Applications of Plasmoincs" during CLEO 2011, addressed this burgeoning new field.

There is no doubt that CLEO 2012 will host a number of technical and invited talks, both fundamental and applied, on the subject of plasmonics. After reading the Physics Today articles, I think I will have to add a lecture or two on plasmonics in my junior-level E&M class this fall. I will definitely have to attend some plasmonic talks next CLEO to learn more about this extremely interesting work that saddles fundamental physics and cutting-edge applications.