Laser chip mounted on a heat sink. The chip with several terahertz quantum-cascade lasers is soldered in the middle of a U-shaped contact pad with attached electrical leads. Courtesy of PDI
By: Forschungsverbund Berlin e.V. (FVB)
http://www.scientificcomputing.com/news/2014/04/new-record-quantum-cascade-laser-operation-temperature
For the observation of cold matter in the interstellar medium,
astronomers need instruments for the detection of terahertz radiation. Specific
high-resolution instruments are based on terahertz quantum-cascade lasers, but
operate only at cryogenic temperatures. Physicists have now developed a
terahertz quantum-cascade laser, which operates at significantly higher
temperatures than previously achieved. The new development allows for the use
of more compact cooling systems — also reducing the obstacles for many other
applications.
The wavelengths of terahertz radiation lie between the microwave
and infrared range. It penetrates many materials such as plastics and clothes.
At the same time, terahertz radiation is — due to its small energy —
non-ionizing and not dangerous for people. Applications of terahertz radiation
include non-destructive material testing and safety checks at airports.
For astronomers, terahertz radiation provides new insights in the
investigation of so-called cold matter. This kind of matter does not emit
visible light such as the stars, but electromagnetic radiation in the infrared
to microwave range. The German Aerospace Center (DLR) measures such emission
lines with high precision within the US-German SOFIA project. Due to the Doppler
shift of the detected frequencies, the researchers can determine the velocity
of the motion of cold matter through the galaxy. To reduce the absorption by
water in the earth atmosphere, the measurements are carried out from an
airplane. One key element of the detector system is a quantum-cascade laser
developed at the PDI.
In a joint project funded by the Investitionsbank Berlin,
researchers at the Paul Drude Institute (PDI) in Berlin Berlin ,
the Humboldt University
in Berlin , and the company Eagleyard Photonics
located also in Berlin
“One problem of the lasers are the low operating temperatures,
which are typically even below the temperature of liquid nitrogen of 77
Kelvin or -196 °C for continuous-wave operation”, explains Martin Wienold
from the PDI. “We achieved a new record: our lasers operate up to 129 Kelvin
(-144 °C) improving the previous record by more than 10 degrees.” This is
still rather cold, “but, in combination
with a significantly reduced power dissipation of the new lasers, it allows for
the use of much smaller mechanical coolers. Thereby, we will be able to reduce
the size of systems based on terahertz quantum-cascade lasers in the future —
an important point for flight missions such as SOFIA”, Wienold emphasizes.
The physicists at the PDI achieved the high operating temperatures
by developing a semiconductor heterostructure, which requires only a very low
driving power. The laser ridge is only about 10-15 microns high and 15 microns
wide, while the emission wavelength is about 100 microns. The active region is
confined by two metal layers, which are almost perfect mirrors in the terahertz
range. This combination results in very low power dissipation and operation at
low current densities and voltages.
“However, there has been an additional problem”, explains Martin
Wienold: “We achieved relatively high operating temperatures, but the strong
spatial confinement of the light in the laser resulted in an extremely
divergent beam profile”. The physicists solved the problem by applying a
concept from the early days of radio broadcasting. A grating on top of the
laser ridge — a so-called third-order grating — acts as a directive antenna,
which collimates the laser emission. “We are currently working on achieving
even higher operating temperatures”, says Wienold. “However, room temperature
operation will become difficult to achieve because of some physical limits.”
Quantum-cascade laser
Quantum-cascade lasers differ from common diode lasers by its
structure and the involved physical processes. Typical diode lasers emit light,
when electrons from the conduction band recombine with holes from the valence
band. Upon recombination, a photon is emitted with the energy of approximately
the semiconductors energy gap. Since the energy gap is determined by the used
semiconductor material, the wavelength of a diode laser is basically determined
by the material.
In a quantum-cascade laser, the electron remains in the conduction
band, and the laser transitions takes place between two confined subband states
within the conduction band. This performance is achieved by alternating
extremely thin semiconductor layers, resulting in so-called potential wells in
the conduction band. When an electric field is applied, the electrons move from
an energetically higher lying potential well to an energetically lower lying
potential well via the quantum mechanical tunneling effect. The electrons
tumble down from one potential well to the next potential well in such a way,
as falling down a staircase.
Citation: High-temperature,
continuous-wave operation of terahertz quantum-cascade lasers with metal-metal
waveguides and third-order distributed feedback. M. Wienold, B. Röben, L. Schrottke, R.
Sharma, A. Tahraoui, K. Biermann, and H. T. Grahn.Optics Express, Vol. 22,
Issue 3, pp. 3334-3348 (2014) http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-22-3-3334
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