NANOPHOTONICS: Tiniest lasers aim for the optical-computing frontier

One of the major stumbling blocks for the oft-touted “optical computer” is simple: size. The continuing expansion in the fiber-optic telecommunications industry has made a niche for microscale lasers, and these advances have kept open the possibility of optical computing. Systems of lasers can in principle perform as digital switching networks, with potentially higher performance than their semiconductor counterparts-if only they could slim down. Optical fibers offer lots of bandwidth but are bulkier than simple wires. Optical sources have to compete with the semiconductor industry’s ever-shrinking transistor dimensions.

A promising new approach is at hand. Research by collaborators in the Netherlands and Korea has demonstrated the smallest electrically pumped lasers ever.1 The devices are metal-encapsulated nanocavities as small as 210 nm across that support low-power, high-Q lasing. The work is not just a single step down in size; the researchers claim that the process lends itself to resonators another order of magnitude smaller.

At the core of the devices is an epitaxially grown indium phosphide/indium gallium arsenide/indium phosphide pillar of about 300 nm height and varying diameter. It is surrounded by a 10 nm silicon nitride insulating layer fabricated by chemical-vapor deposition. The whole heterostructure is encapsulated in gold, forming a metallic cavity (see figure).


A nanolaser consists of an indium phosphide/indium gallium arsenide (InP/InGaAs) heterostructure embedded in a gold layer (top). A scanning-electron micrograph shows a nanolaser heterostructure before encapsulation in gold (bottom). (Courtesy of E. J. Geluk)
Click here to enlarge image

A small amount of the mode energy escapes through the bottom of the pillar, allowing the researchers to have a peek at what is happening inside the cavity (future applications may couple the energy out via metallic waveguides). They found that the heterostructure had well-confined modes resonant at a wavelength of 1408 nm and a linewidth of 0.3 nm. The cavity Q was just 48 at room temperature, and was dominated by optical losses in the gold-a result that has held back previous metallic-cavity nanolasers. However, that value increases at low temperature; the team measured Q values of 140 and 200 at temperatures of 77 and 10 K, respectively.

Possible use in plasmonics

But this story of the small gets better. “This is only the start of the miniaturization process,” says Martin Hill of the COBRA Research Institute (Eindhoven, The Netherlands) and lead author of the study. Finite-difference-time-domain simulations show that the devices can be made to cross an even more important size gap-that between photonics and plasmonics, the interaction of light with surface plasmons. The lure of this kind of subwavelength optical processing has formed speculation about the future of computing for years.

“Initially I thought of making the devices more as an interesting scientific exercise. However, in the beginning of 2007, I had the chance to sit back a bit and see how it fit into plasmonics,” Hill says. “The same structure and construction techniques can be used to make cavities that have a surface-plasmon-polariton gap mode.”

The key to that development is in varying the shape and dimensions of the pillar. “The plasmon-gap-mode waveguides provide the framework to do all sorts of complex things in VLSI (very-large-scale-integrated) photonic devices,” Hill says. “But the structure of the diode encapsulated in metal with lithographically defined cross section, and mode in the middle of the pillar defined by the wafer epitaxy, will form the basis for all this.” The researchers compared two devices with pillar diameters differing by 30 nm; their lasing wavelength was different by some 144 nm. By using a rectangular cross section, simulations predict that cavity size can be reduced to just a few tens of nanometers in height and in one lateral dimension.

The result is important because of the limitations imposed by optical losses in plasmonic systems. “In current subwavelength plasmonic systems, the signals are attenuated quickly, but with the active structures we show the loss could be overcome, allowing complex subwavelength optical systems to be made.” That means devices down to sizes of just tens of nanometers or less, with active, subwavelength waveguides that connect the tiny lasers.

Amnon Yariv of the California Institute of Technology (Pasadena, CA), is another past master of tiny indium-based lasers.2 He describes the recent research as a “hero experiment,” but warns that the metallic encapsulation leads to broad linewidths and reduced coherence. Hill’s researchers are convinced that their proof-of-principle demonstration is ripe for improvement, analogous to the situation for early bandgap-defect and microdisk lasers. The team is, for the moment, replacing the gold encapsulation layer with silver, an improvement that simulations predict will increase the room-temperature cavity Q to higher than that demonstrated at 77 K in the research.

D. Jason Palmer

REFERENCES

1. M.T. Hill et. al., Nature Photonics 1, 589 (October 2007).

2. J. Scheuer, et. al., Appl. Phys. Lett. 60, 289 (2005).

Most Popular Articles

50 YEARS OF GAS LASERS


Durable survivors evolve new forms

Webcasts

Laser Measurements Critical to Successful Additive Manufacturing Processes

Maximizing the stability of the variables going into any manufacturing process is what ensures ts consistency and high quality. Specifically, when a laser is...

Ray Optics Simulations with COMSOL Multiphysics

The Ray Optics Module can be used to simulate electromagnetic wave propagation when the wavelength is much smaller than the smallest geometric entity in the ...

Multichannel Spectroscopy: Technology and Applications

This webcast, sponsored by Hamamatsu, highlights some of the photonic technology used in spectroscopy, and the resulting applications.

Handheld Spectrometers

Spectroscopy is a powerful and versatile tool that traditionally often required a large and bulky instrument. The combination of compact optics and modern pa...
White Papers

All About Aspheric Lenses

The most notable benefit of aspheric lenses is their ability to correct for spherical aberration....

Wavelength stabilized multi-kW diode laser systems

Wavelength stabilization of high-power diode laser systems is an important means to increase the ...

Narrow-line fiber-coupled modules for DPAL pumping

A new series of fiber coupled diode laser modules optimized for DPAL pumping is presented, featur...
Technical Digests

ADHESIVES, SEALANTS, AND COATINGS: Solutions for optical technologies

A vast array of optical systems of various types and degrees of complexity require the use of adh...

WAVELENGTH-SWEPT LASERS: Dispersion-tuned fiber laser sweeps over a 140 nm range for OCT

By eliminating the use of mechanical tunable filters and instead tuning by intensity-modulation i...

Keeping pace with developments in photonic materials research

For demanding or custom spectroscopy solutions, care must be taken in selecting and integrating a...

HIGH-POWER FIBER LASERS: Working in the kilowatt regime

High-power materials-processing fiber lasers are available in an increasing variety of forms, as ...

Click here to have your products listed in the Laser Focus World Buyers Guide.

RELATED PRODUCTS

Social Activity
  •  
  •  
  •  
  •  
  •  
Copyright © 2007-2014. PennWell Corporation, Tulsa, OK. All Rights Reserved.PRIVACY POLICY | TERMS AND CONDITIONS