Quantum dots self-tune their color for ultra-efficient nano lasers

Apr 14, 2006 feature
Quantum dots self-tune their color for ultra-efficient nano lasers
Fig. A. In this design of the ultra-efficient photonic crystal nano laser, a top-view electron microscope image shows the drilled hole pattern in a GaAs membrane. The three missing holes in a row at the center define the nano cavity, which confines the light. To finely tune the electromagnetic field, a waveguide (w) has been inserted and neighboring holes have been resized and shifted. Photo Credit: Stefan Strauf.

The laser, one of the most valuable scientific instruments, is getting smaller and more efficient. Scientists have designed a miniature laser, whose nanoscale dimensions and low optical losses will be instrumental in future developments of integrated photonic circuits.

Compared with a regular flashlight, a laser emits a very intense and strongly collimated beam of monochromatic light. These properties arise from an interaction between the gain medium and an optical cavity. In order for a laser to generate its tiny colored spot, the active gain medium, which amplifies the beam, must provide photons that are all emitted at the same wavelength in a process called “stimulated emission.”

While generating sufficient stimulated emission usually requires a great deal of gain material, scientists have discovered a method to create a nano laser with only a few tiny objects called “quantum dots.” Physicists from UC Santa Barbara and the University of Pavia in Italy have designed a new nano-device that works with only two to four quantum dots. The design, which provides ultra-efficient lasing performance, is published in a recent Physical Review Letters.

“While conventional quantum dot lasers require many layers with thousands of dots, this new design takes advantage of a quantum dot property that effectively self-tunes the dots’ photon emission wavelength into resonance with the cavity,” explained co-author Stefan Strauf to PhysOrg.com. When the gain medium resonates with the cavity, a laser can be created.”

Individual quantum dots have very sharp transition energy, and a large number of dots will display a broad emission bandwidth due to their natural size variations. While this broad emission makes them an ideal gain material for large volume lasers, device miniaturization presents a challenge since the sharp transitions of a few quantum dots no longer resonate with the laser cavity.

The team of scientists found a way out of this dilemma by embedding the dots in a photonic crystal nano cavity, which enables light to be confined in a very small volume. A photonic crystal can be made by drilling many holes within a thin membrane of a semiconductor material such as GaAs (see figure). This particular design provides a finely tuned distribution of the electromagnetic field inside the nanocavity, which optimizes the overlap of the embedded quantum dots with the field profile and increases the quality of the cavity.

“This optimization process is a lot like the skillful fine-tuning of a violin producing resonance tones, but for light instead of sound,” said Strauf.

Quantum dots self-tune their color for ultra-efficient nano lasers
Fig. B and C. (B) This image shows a close-up of the resulting field profile around the tuned nano cavity region, where the yellow-white color is the highest field strength. (C) This atomic-force microscope image shows a typical layer of the low-density quantum dot gain material that is embedded underneath the surface of the membrane. The bright spots are individual InAs quantum dots, about 20 nm in size. The dashed blue circles indicate that there are on average only a few dots spatially positioned within the field profile of the nano cavity. Photo Credit: Stefan Strauf.

This design drastically inhibits the emission of the dots at their natural sharp energies and forces them to interact with electronic carriers in their immediate surroundings. Because this interaction provides additional energy, the dots can self-tune their emission color into resonance with the cavity. Since this is a very pronounced interaction, the nano lasers display a hundredfold improvement of their lasing threshold values compared with any other semiconductor laser.

In addition to the record low lasing threshold produced with a couple of quantum dots, the scientists found a much higher optical efficiency than exhibited in high density quantum dot devices. Measuring the optical efficiency with the spontaneous emission coupling factor (which has a theoretical limit of 1.0), the team found that the new method exhibited a value of 0.85, while multi-layer quantum dot lasers have a factor ranging from 0.1-0.2.

“The record low lasing thresholds combined with the high efficiencies make these nano lasers useful for future applications in integrated photonic circuits on a chip and for bio sensing of individual molecules,” said Strauf. “Such nano lasers may now be formed with only a few or even a single quantum dot.”

Citation: Strauf, S. et al. Self-Tuned Quantum Dot Gain in Photonic Crystal Lasers. Physical Review Letters 96, 127404 (2006).

By Lisa Zyga, Copyright 2006 PhysOrg.com

Explore further: Greenland darkening to continue, predicts CCNY expert Marco Tedesco

Related Stories

Scientists succeed in linking two different quantum systems

Mar 30, 2015

Physicists at the Universities of Bonn and Cambridge have succeeded in linking two completely different quantum systems to one another. In doing so, they have taken an important step forward on the way to a quantum computer. ...

Nanovectors combine cancer imaging and therapy

Feb 09, 2015

Researchers at Imperial College London and the Laboratoire de chimie de la matière condensée de Paris (CNRS/Collège de France/UPMC) have designed and developed hybrid gold-silica nanoparticles, which are ...

Rediscovering spontaneous light emission

Feb 03, 2015

Berkeley Lab researchers have developed a nano-sized optical antenna that can greatly enhance the spontaneous emission of light from atoms, molecules and semiconductor quantum dots. This advance opens the ...

Recommended for you

United States, China team explore energy harvesting

23 hours ago

Six authors have described their work in harvesting energy in a paper titled "Ultrathin, Rollable, Paper-Based Triboelectric Nanogenerator for Acoustic Energy Harvesting and Self-Powered Sound Recording." ...

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.