Because the people who make up the world of photonics are so skilled at creating new ideas and devices, as well as improving on existing photonics technologies, this year's Tech Review Top 20 list in truth should come with a supplementary list of 100 or so runners-up, maybe a few hundred honorable mentions, and numerous awards for just being unique. However, this editor must make his choices. So, take a look at the following 20 items and agree or disagree, but while you're at it, make sure to browse through the last 12 issues of Laser Focus World or look online and check back on your own favorites. (And drop us a line to give us your own opinions.)
A world in your glasses
Head-worn displays. With the high rate of "churn" in consumer communications and computing tech these days (anyone remember netbooks?), it is apt to begin with the intersection of photonics and the consumer world. Eyeglasslike head-worn displays, most famously pursued by Google in the form of the liquid-crystal-display-based Google Glass, are also being developed elsewhere, with the display technologies and optics taking other forms, such as organic light-emitting diode and liquid-crystal-on-silicon (see "Head-worn displays: Useful tool or niche novelty?" Laser Focus World, July 2013; http://bit.ly/HzQte5).
Understanding white-light LEDs. The standard white-light LED, which combines a gallium nitride (GaN)-based semiconductor emitter of blue light with a yellow-emitting phosphor, is already the most efficient source of white light for illumination that one can buy today. But there's one thing holding it back from becoming still more efficient, and that is the so-called "efficiency droop" that white LEDs suffer as the current is turned up. While the cause of this has been debated for years, researchers at University of California, Santa Barbara (UCSB) and the École Polytechnique (Palaiseau, France) have at last experimentally pinpointed the cause-perhaps leading the way to white LEDs with a luminous efficacy of up to 300 lm/W (see Fig. 1). (See "Cause of LED efficiency droop experimentally pinpointed at last," Laser Focus World online, April 2013; http://bit.ly/17bTXiE.)
The most powerful lasers and how to measure them. Moving to the industrial arena, lasers for materials processing are growing ever-more powerful—in fact, so powerful that their existence can precede the uses found for them, which of course drives research into how they can fully be exploited. A 100 kW fiber laser has been developed by IPG Photonics (Oxford, MA) and is now a commercial product; the first unit was shipped earlier this year to the NADEX Laser R&D Center (Nagoya City, Japan), where the laser's materials-processing capabilities will be explored. Especially for this laser, Ophir Photonics (North Logan, UT) has developed a laser power meter that can measure the full 100 kW, 1070 nm output (see "Ophir Photonics unveils 100 kW laser power meter, first unit is already in use," Laser Focus World, September 2013; http://bit.ly/1cSqubU).
Direct-diode systems. High-power direct-diode laser delivery, in which light from laser diodes is coupled into an optical fiber for up to kilowatt-level optical output, is very efficient due to the high (up to 70%) wall-plug efficiency of laser diodes. The biggest challenge is how to couple enough laser power into a small fiber to produce a bright-enough beam for practical use; a number of approaches are being tried (see "Making direct laser diodes shine more brightly," Laser Focus World, March 2013; http://bit.ly/I8wYJP).
VCSELs for spectroscopy. The use of vertical-cavity surface-emitting lasers (VCSELs) for tunable diode laser absorption spectroscopy (TDLAS or TDLS) has been accomplished by scientists at Physikalisch-Technische Bundesanstalt (PTB; Braunschweig, Germany), TU Darmstadt (Darmstadt, Germany), and the University of Duisburg-Essen (Duisburg, Germany) for measuring water vapor in internal-combustion engines, replacing distributed-feedback (DFB) lasers for the purpose. The VCSELs have a larger tuning range that does not diminish at high modulation rates, thus spanning the full absorption line (see "VCSELs benefit TDLAS combustion measurements," Laser Focus World, October 2013; http://bit.ly/1bbNK1N).
Laser-modeling software. How does one create a numerical physics-based model of a laser so that it can be characterized before building it (or while it is being experimentally optimized)? Synopsys (Ossining, NY) has developed software called Rsoft LaserMOD to model laser diodes and VCSELs using rate equations and measurement-driven models (see "Laser modeling software requires minimal parameter knowledge," Laser Focus World, September 2013; http://bit.ly/1elhAaE). And here's a bonus pick: Engineers at Simphotek (Newark, NJ), who created a laser- and amplifier-modeling suite that was featured in Laser Focus World's 2012 top-20 tech picks, have unveiled case studies that show some extremely complex laser and photonics-material physics being modeled quickly and straightforwardly (see "Rapid characterization and reference tools for active photonic materials," p. 24).
Big photonics shows results
Optics for extreme-ultraviolet lithography. One of the most massive photonics projects ever conceived, a decades-long effort that is still under development and yet is producing images at 12 nm resolution with a few nanometers of image distortion, is the one responsible for your lightweight laptop and your cool smartphone. Optical lithography is now reaching its limit with traditional light sources (excimer lasers) and is taking the next step: extreme-ultraviolet (EUV) lithography at a 13 nm wavelength. As always, the higher the numerical aperture (NA) of the optics, the higher the resolution (see Fig. 2). (See "High-NA EUV optics are on their way," Laser Focus World, July 2013; http://bit.ly/19DKAad.)
Thin-disk laser. The thin-disk laser, a laser configuration that can dump lots of heat from its gain material quickly and without beam distortions, has reached the 30 kW output level, exceeding the power level that the U.S. Department of Defense's Robust Electric Laser Initiative (RELI) considers to be a candidate for directed-energy weapons. This disk laser was developed by Boeing (Albuquerque, NM). (See "Thin-disk laser system from Boeing achieves directed-energy weapons status," Laser Focus World online, August 2013; http://bit.ly/HK0GnD.)
Higher laser-fusion output. Perhaps the biggest of them all is the National Ignition Facility (NIF; Livermore, CA), which strives to create not just breakeven laser fusion, but a way to safely test the U.S. nuclear weapons stockpile in the laboratory. This year, NIF generated a record yield of nearly 3 × 1015 fusion neutrons from a deuterium-tritium target, making progress toward a self-sustaining target burn—but not achieving ignition, as some in the popular press would have you believe (see "Good news from NIF," Laser Focus World online, September 2013; http://bit.ly/1bbPEzv, and "Progress at NIF, but no 'breakthrough," Laser Focus World online, October 2013; http://bit.ly/1fk2ikb).
In optics, invention leads the way
Monocentric lens. One of the most interesting cases in optical design is that of a monocentric lens, where all surfaces have the same center of curvature, removing any limit on field size (although the image "plane" is also a spherical section). A group at the University of California, San Diego has created a miniature monocentric that is used with a standard digital single-lens-reflex (DLSR) camera (see "Miniature monocentric camera records details of scene while maintaining extremely wide field of view," Laser Focus World online; September 2013; http://bit.ly/GzrsQ8).
OCT for optics manufacture. Optical coherence tomography (OCT), most well known for its use in creating detailed 3D images of biological tissue, has its uses elsewhere, too. Researchers at the University of Rochester (Rochester, NY) and elsewhere are using OCT to finely characterize polymer gradient-index lenses, helping them to refine the manufacturing process (see "OCT improves polymer gradient-index lens manufacture," Laser Focus World, June 2013; http://bit.ly/1bJhbbL).
Paper terahertz lens. Top-20 tech picks can be inspired by the fact that they are offbeat. Here's one that, while offbeat, is quite useful: the creation of terahertz focusing lenses from paper. Conceived at the Warsaw University of Technology (Warsaw, Poland) and the University of Savoie (Le Bourget du Lac, France), these Fresnel-zone-plate lenses were cut to a diameter of 100 mm and represent a quick and effective way to prototype a terahertz-optical system (see "Paper terahertz lens makes for quick prototyping," Laser Focus World, February 2013; http://bit.ly/1g37rh4).
Longitudinally oscillating light. Researchers at the Vienna University of Technology (TU Vienna) are, contrary to what most of us have been doing in photonics, making light oscillate in the longitudinal direction. They do this by trapping light within a bulge in optical fiber; the configuration has a practical aspect, as it allows strong coupling of light and matter via the light's evanescent waves at the surface of the fiber (see "Light captured in fiber oscillates in longitudinal direction," Laser Focus World online, May 2013; http://bit.ly/1baay3u).
Petabit-per-second fiber transmission. Who would have imagined that a single optical fiber could carry data at rates approaching a petabit per second? This feat was achieved earlier this year by a team led by Akihide Sano of NTT Corporation (Yokosuka, Atsugi, and Tsukuba, Japan); the fiber contains 12 cores in two rings in a technique called propagation-direction interleaving; the relative orientation of the rings minimizes intercore crosstalk. The researchers demonstrated a transmission rate of 409 Tbit/s in each direction, for a total of 818 Tbit/s (see "Multicore fiber transmits 818 Tbit/s over 450 km," Laser Focus World, August 2013; http://bit.ly/1bZ30AK).
Integrated photonics takes new forms
White light from bulk silicon. Bulk silicon is well known for its unsuitability as an emitter of light. However, a group at the University of Pennsylvania (Philadelphia, PA) has managed to get bulk silicon to produce broad-spectrum visible light; the trick is in the proper combination of nanowires and plasmonic nanocavities. The researchers are aiming next to electrically pump the configuration, which would make it useful in integrated photonics (see "Bulk silicon emits visible light for the first time," Laser Focus World, March 2013; http://bit.ly/187Kp2N).
Photonic integrated circuit for spectroscopy. A photonic integrated circuit (PIC) containing gallium antimonide (GaSb) lasers and photodetectors on a silicon-on-insulator (SOI) base has been developed as a stepping-stone toward a chip-based shortwave-infrared (SWIR) spectrometer. Developed at Ghent University (UGent; Ghent, Belgium) and Université Montpellier 2 (Um2; Montpellier, France), the PIC operates in the 2 μm wavelength region (the short-wavelength end of the molecular "fingerprint" region). Future devices could extend the range into the midwave IR (MWIR). (See "GaSb/SOI PICs target integrated SWIR spectroscopy," Laser Focus World, April 2013; http://bit.ly/1hNoWno.)
Phased optical array. Using standard CMOS fabrication techniques, researchers at the Massachusetts Institute of Technology (MIT; Cambridge, MA) have created a 64 × 64 phased array of optical nanoantennas on a millimeter-sized silicon chip; each nanoantenna is the optical version of the type of antenna that might be found in a phased radio array. Like a radio array, the optical phased array could potentially produce a beam of light that is electrically steerable at high speeds. A smaller prototype 8 × 8 array is already steering light at a 1.55 μm wavelength (see "Nanophotonic array is CMOS-compatible," Laser Focus World, March 2013; http://bit.ly/10shbHu).
Biophotonics and its benefits
Tunable VCSEL for OCT. Wavelength-tunable VCSELs that rely on microelectromechanical systems (MEMS) for tuning are extending the axial imaging reach of swept-source OCT from millimeters to tens of centimeters, allowing volumetric imaging of the entire eye, characterization of large manufactured parts, and Doppler OCT. Scan rates for the devices, which were developed by researchers at Thorlabs (Newton, NJ), Praevium Research (Santa Barbara, CA), and MIT, can top 1 MHz, while scan depths reach more than 15 cm (see "VCSELs accelerate new OCT applications," Laser Focus World, April 2013; http://bit.ly/1bacc58).
Zoom contact lens. A totally self-contained contact lens that allows the wearer to switch between normal and magnified vision was created this year by a group at the University of California, San Diego (UCSD; see Fig. 3). A prototype of the lens, which contains a liquid-crystal (LC) shutter and four coaxial aspheric reflectors, zooms to 2.8X and was tested on a mechanical eye. Along with its LC workings, the polymer lens includes diffractive chromatic aberration correction. The intent is to alleviate the vision problems related to age-related macular degeneration (AMD). (See "1-mm-thick telescopic contact lens magnifies by 2.8X," Laser Focus World online, July 2013; http://bit.ly/13j07FT).
Stopping epileptic seizures with light. The last pick in this year's list has all the signs of a lasting winner: technology drawn from a burgeoning field (optogenetics), the aim to halt the effects of a truly debilitating disease (epilepsy), striking early experimental results, and the opportunity to overcome many of the side effects of traditional techniques. In the experiment, an optical fiber was implanted in the brain of a mouse by an area that an electroencephalograph (EEG) had shown to be the locus of seizures. Application of amber light through the fiber terminated most seizures within a second. The researchers hope their work will lead to a better alternative to the conventional method of electrical stimulation (see "UC Irvine neuroscientists terminate epileptic seizures with implanted optical fibers," Laser Focus World online, January 2013; http://bit.ly/19DPBj1).
