Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

New record achieved in terahertz pulse generation

13.02.2017

A group of scientists from TU Wien and ETH Zurich have succeeded in their attempts to generate ultrashort terahertz light pulses. With lengths of just a few picoseconds, these pulses are ideally suited to spectroscopic applications and enable extremely precise frequency measurements to be taken.

The unique properties of terahertz radiation mean it is of interest for a wide range of potential applications, including non-invasive medical imaging and the detection of hazardous substances. Terahertz waves can penetrate many materials that are opaque to visible light and, unlike X-radiation, do not pose a risk of damage to biological tissue.


Illustration of a broadband terahertz amplifier based on a quantum cascade laser.

TU Wien


Karl Unterrainer, Dominic Bachmann and Juraj Darmo at the laser lab of the Photonics Institute

TU Wien

In addition to this, many substances have a molecular fingerprint in the terahertz range, allowing them to be detected using spectroscopic methods. One efficient way of generating these terahertz waves is using quantum cascade lasers, which a working group led by Prof. Karl Unterrainer at the Photonics Institute at TU Wien has been researching and developing.

Quantum cascade lasers consist of a precisely defined sequence of several hundred semiconductor layers that measure just a few nanometres in thickness. This special construction means there is the freedom to select the exact energy state at which the electrons stay within the semiconductor structure. This allows the frequency of the laser light emitted to be adjusted to suit the application in question.

Creating a frequency comb with a broadband ‘laser sandwich’

With this special feature of being able to determine the laser wavelengths themselves, several quantum cascade structures with different emission frequencies can be stacked on top of one another, with the aim of generating broadband terahertz radiation. “Heterogeneous active zones of this kind are ideally suited for implementing broadband terahertz amplifiers and generating ultrashort terahertz pulses,” explains Dominic Bachmann from the Photonics Institute.

Plus, if the discrete laser lines are linked together to establish a fixed phase relationship between the laser modes, something known as a ‘frequency comb’ will be created. Frequency combs make it possible to take extremely precise measurements of the absolute frequency of the light being used, which is essential for a huge number of applications.

The discovery of the frequency comb more or less revolutionised optical metrology and was honoured with the Nobel Prize for Physics in 2005. Over the past four years, researchers have been working hard to generate a terahertz frequency comb using a quantum cascade laser as part of the EU project TERACOMB. Headed up by Dr Juraj Darmo from the Photonics Institute, the team of international research groups has succeeded in generating the first broadband terahertz frequency comb based on semiconductor technology.

Watching lasers at work

One method developed by the group led by Prof. Unterrainer makes it possible to analyse internal quantum cascade laser parameters during laser operation. This technique is based on time-resolved spectroscopy, with broadband terahertz pulses penetrating the sample to be measured. Based on femtosecond lasers, this technology can be used to collect the full information content relating to the time and frequency range with just one single measurement.

As a result, the scientists at the Photonics Institute have managed to quantify the optical gain coefficients as well as the optical dispersion in broadband terahertz quantum cascade lasers, improving their understanding of the complex dynamics at play. “These findings allow us to increase the laser bandwidth even further and to improve the efficiency of frequency combs,” explains Juraj Darmo.

Targeting losses

One unresolved issue with terahertz quantum cascade lasers had been the existence of laser lines with different propagation speeds. If there are laser modes with a higher lateral order, the intensity is distributed very unevenly between the laser lines, thereby reducing the usable bandwidth and preventing the generation of a frequency comb. In order to stop these modes from oscillating, the losses have to be increased to such an extent that they do not reach the laser threshold.

By adding a tailored lateral absorber to the edges of the laser resonator, the researchers managed to suppress the higher lateral modes entirely, without having any relevant impact on the fundamental modes. The result was an emission bandwidth covering a full octave, very even mode distribution in the middle at 700 GHz, and a frequency comb with a bandwidth of 440 GHz.

What's more, the lateral absorbers enable the generation of ultrashort terahertz pulses with pulse widths of less than 3 ps, which represents a new world record for terahertz pulses generated using a quantum cascade laser. “It was truly amazing to see how a relatively minor adjustment to the waveguide could bring about such a dramatic improvement,” explains Dominic Bachmann, who has just finished writing his dissertation on broadband quantum cascade lasers.

Original publications
Bachmann et al., “Short pulse generation and mode control of broadband terahertz quantum cascade lasers", Optica 3, 1087 (2016), DOI: 10.1364/OPTICA.3.001087.
Bachmann et al., "Dispersion in a broadband terahertz quantum cascade laser”, Appl. Phys. Lett. 109, 221107 (2016), DOI: 10.1063/1.4969065.

Links: http://thzlabs.tuwien.ac.at/
http://www.teracomb.org/

Picture download: https://www.tuwien.ac.at/dle/pr/aktuelles/downloads/2017/terahertzpuls

Further information:
Dr. Dominic Bachmann
Technische Universität Wien
Institut für Photonic
Gußhausstraße 27-29, 1040 Wien
T: +43-1-58801-38738
dominic.bachmann@tuwien.ac.at

Dr. Florian Aigner | Technische Universität Wien

More articles from Power and Electrical Engineering:

nachricht Researchers take next step toward fusion energy
16.11.2017 | Texas A&M University

nachricht Desert solar to fuel centuries of air travel
16.11.2017 | SolarPACES

All articles from Power and Electrical Engineering >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: A “cosmic snake” reveals the structure of remote galaxies

The formation of stars in distant galaxies is still largely unexplored. For the first time, astron-omers at the University of Geneva have now been able to closely observe a star system six billion light-years away. In doing so, they are confirming earlier simulations made by the University of Zurich. One special effect is made possible by the multiple reflections of images that run through the cosmos like a snake.

Today, astronomers have a pretty accurate idea of how stars were formed in the recent cosmic past. But do these laws also apply to older galaxies? For around a...

Im Focus: Visual intelligence is not the same as IQ

Just because someone is smart and well-motivated doesn't mean he or she can learn the visual skills needed to excel at tasks like matching fingerprints, interpreting medical X-rays, keeping track of aircraft on radar displays or forensic face matching.

That is the implication of a new study which shows for the first time that there is a broad range of differences in people's visual ability and that these...

Im Focus: Novel Nano-CT device creates high-resolution 3D-X-rays of tiny velvet worm legs

Computer Tomography (CT) is a standard procedure in hospitals, but so far, the technology has not been suitable for imaging extremely small objects. In PNAS, a team from the Technical University of Munich (TUM) describes a Nano-CT device that creates three-dimensional x-ray images at resolutions up to 100 nanometers. The first test application: Together with colleagues from the University of Kassel and Helmholtz-Zentrum Geesthacht the researchers analyzed the locomotory system of a velvet worm.

During a CT analysis, the object under investigation is x-rayed and a detector measures the respective amount of radiation absorbed from various angles....

Im Focus: Researchers Develop Data Bus for Quantum Computer

The quantum world is fragile; error correction codes are needed to protect the information stored in a quantum object from the deteriorating effects of noise. Quantum physicists in Innsbruck have developed a protocol to pass quantum information between differently encoded building blocks of a future quantum computer, such as processors and memories. Scientists may use this protocol in the future to build a data bus for quantum computers. The researchers have published their work in the journal Nature Communications.

Future quantum computers will be able to solve problems where conventional computers fail today. We are still far away from any large-scale implementation,...

Im Focus: Wrinkles give heat a jolt in pillared graphene

Rice University researchers test 3-D carbon nanostructures' thermal transport abilities

Pillared graphene would transfer heat better if the theoretical material had a few asymmetric junctions that caused wrinkles, according to Rice University...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Ecology Across Borders: International conference brings together 1,500 ecologists

15.11.2017 | Event News

Road into laboratory: Users discuss biaxial fatigue-testing for car and truck wheel

15.11.2017 | Event News

#Berlin5GWeek: The right network for Industry 4.0

30.10.2017 | Event News

 
Latest News

NASA detects solar flare pulses at Sun and Earth

17.11.2017 | Physics and Astronomy

NIST scientists discover how to switch liver cancer cell growth from 2-D to 3-D structures

17.11.2017 | Health and Medicine

The importance of biodiversity in forests could increase due to climate change

17.11.2017 | Studies and Analyses

VideoLinks
B2B-VideoLinks
More VideoLinks >>>