The fast modulation of lasers is a fundamental requirement for applications in optical communications, high-resolution spectroscopy and metrology. In the terahertz-frequency range, the quantum-cascade laser (QCL) is a high-power source with the potential for high-frequency modulation. However, conventional electronic modulation is limited fundamentally by parasitic device impedance, and so alternative physical processes must be exploited to modulate the QCL gain on ultrafast timescales. Here, we demonstrate an alternative mechanism to modulate the emission from a QCL device, whereby optically-generated acoustic phonon pulses are used to perturb the QCL bandstructure, enabling fast amplitude modulation that can be controlled using the QCL drive current or strain pulse amplitude, to a maximum modulation depth of 6% in our experiment. We show that this modulation can be explained using perturbation theory analysis. While the modulation rise-time was limited to ~800 ps by our measurement system, theoretical considerations suggest considerably faster modulation could be possible.
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Showing posts with label John Cunningham. Show all posts
Showing posts with label John Cunningham. Show all posts
Saturday, February 15, 2020
Abstract-High-speed modulation of a terahertz quantum cascade laser by coherent acoustic phonon pulses
Friday, February 14, 2020
Using sound and light to generate ultra-fast data transfer
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The terahertz quantum cascade laser on its mounting. A pair of tweezers shows how small the device is. Credit:
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Researchers have made a breakthrough in the control of terahertz quantum cascade lasers, which could lead to the transmission of data at the rate of 100 gigabits per second—around one thousand times quicker than a fast Ethernet operating at 100 megabits a second.
What distinguishes terahertz quantum cascade lasers from other lasers is the fact that they emit light in the terahertz range of the electromagnetic spectrum. They have applications in the field of spectroscopy where they are used in chemical analysis.
The lasers could also eventually provide ultra-fast, short-hop wireless links where large datasets have to be transferred across hospital campuses or between research facilities on universities—or in satellite communications.
To be able to send data at these increased speeds, the lasers need to be modulated very rapidly: switching on and off or pulsing around 100 billion times every second.
Engineers and scientists have so far failed to develop a way of achieving this.
A research team from the University of Leeds and University of Nottingham believe they have found a way of delivering ultra- fast modulation, by combining the power of acoustic and light waves. They have published their findings today in Nature Communications.
John Cunningham, Professor of Nanoelectronics at Leeds, said: "This is exciting research. At the moment, the system for modulating a quantum cascade laser is electrically driven—but that system has limitations.

"Ironically, the same electronics that delivers the modulation usually puts a brake on the speed of the modulation. The mechanism we are developing relies instead on acoustic waves."
A quantum cascade laser is very efficient. As an electron passes through the optical component of the laser, it goes through a series of 'quantum wells' where the energy level of the electron drops and a photon or pulse of light energy is emitted.
One electron is capable of emitting multiple photons. It is this process that is controlled during the modulation.
Instead of using external electronics, the teams of researchers at Leeds and Nottingham Universities used acoustic waves to vibrate the quantum wells inside the quantum cascade laser.
The acoustic waves were generated by the impact of a pulse from another laser onto an aluminium film. This caused the film to expand and contract, sending a mechanical wave through the quantum cascade laser.
Tony Kent, Professor of Physics at Nottingham said "Essentially, what we did was use the acoustic wave to shake the intricate electronic states inside the quantum cascade laser. We could then see that its terahertz light output was being altered by the acoustic wave."
Professor Cunningham added: "We did not reach a situation where we could stop and start the flow completely, but we were able to control the light output by a few percent, which is a great start.
"We believe that with further refinement, we will be able to develop a new mechanism for complete control of the photon emissions from the laser, and perhaps even integrate structures generating sound with the terahertz laser, so that no external sound source is needed."
Tuesday, May 28, 2019
Abstract-Substrate Integrated Bragg Waveguide: a New Transmission-Line Medium for Millimeter-Wave and Terahertz Systems Integration
Binbin Hong, Naixing Feng, Guo Ping Wang, Viktor Doychinov, Roland Clarke, Nutapong Somjit, John Cunningham, Ian Robertson
We demonstrate an air-core single-mode hollow waveguide that uses a Bragg reflector structures in place of the vertical metal walls of the standard rectangular waveguide or via holes of the so-called substrate integrated waveguide. The high-order modes in the waveguide are substantially suppressed by a modal-filtering effect, making the waveguide operate in the fundamental mode over more than one octave. Numerical simulations show that the propagation loss of the proposed waveguide can be lower than that of classic hollow metallic rectangular waveguides at terahertz frequencies, benefiting from a significant reduction in Ohmic loss. To facilitate fabrication and characterization, a proof-of-concept 20 to 45 GHz waveguide is demonstrated, which verifies the properties and advantages of the proposed waveguide. A zero group-velocity dispersion point is observed at near the middle of the operating band. This work offers a step towards a new transmission-line medium that can be integrated within a substrate, either in integrated circuit or multi-chip module technology, for broadband millimeter-wave and terahertz applications.
Sunday, November 13, 2016
Abstract-Free-space terahertz radiation from a LT-GaAs-on-quartz large-area photoconductive emitter
Free-space terahertz radiation from a LT-GaAs-on-quartz large-area photoconductive emitter
David R. Bacon, Andrew D. Burnett, Matthew Swithenbank, Christopher Russell, Lianhe Li, Christopher D. Wood, John Cunningham, Edmund H. Linfield, A. Giles Davies, Paul Dean, and Joshua R. Freeman
We report on large-area photoconductive terahertz (THz) emitters with a low-temperature-grown GaAs (LT-GaAs) active layer fabricated on quartz substrates using a lift-off transfer process. These devices are compared to the same LT-GaAs emitters when fabricated on the growth substrate. We find that the transferred devices show higher optical-to-THz conversion efficiencies and significantly larger breakdown fields, which we attribute to reduced parasitic current in the substrate. Through these improvements, we demonstrate a factor of ~8 increase in emitted THz field strength at the maximum operating voltage. In addition we find improved performance when these devices are used for photoconductive detection, which we explain through a combination of reduced parasitic substrate currents and reduced space-charge build-up in the device.
Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
Full Article | PDF ArticleMonday, October 26, 2015
Probing the properties of individual nanoscale objects
By Helen Knight
http://www.theengineer.co.uk/more-sectors/electronics/news/probing-the-properties-of-individual-nanoscale-objects/1021281.article
The high frequency electronic properties of single nanoscale objects can now be measured, thanks to a technique designed to manipulate terahertz radiation.
Carbon nanotubes, quantum dots and other nanoscale objects are so small that it has previously been impossible to study them individually with terahertz radiation.
Instead, researchers have had to study the nanoscale objects in bulk, according to Prof John Cunningham of Leeds University, who led the research.
But if we are to continue to produce smaller and smaller electronic systems, we will need to understand exactly how they work at the nanoscale, where devices can exhibit different properties from largescale devices.
Now the team, with funding from the EPSRC and Leeds University, have developed a technique in which a nanostructure is used to filter the terahertz waves. By passing the terahertz radiation through a tiny region of semiconductor, with gates on its surface, the researchers are able to control the spectrum of the radiation passing through it.
The device consists of a nanostructure embedded within a microscopic waveguide, where it is split into three cavities.
A voltage is applied to the device, which controls how electrons inside the cavities oscillate. This in turn determines the frequency of electromagnetic radiation that each cavity can transmit.
“We can control the properties of the terahertz radiation moving through the object using the gate,” said Cunningham.
Tuning the radiation in this way allows the waves to be shaped or modified, meaning information can be encoded in the signal, he said. “When the terahertz radiation is passed down the waveguide, it interacts with the nanoscale object.”
This allows the researchers to study even single nanoscale objects. “The technique in principle allows you to measure almost any nanoscale electronic object,” said Cunningham.
It could be used to measure the properties of graphene, for example, or ultrafast transistors built from nanostructures, and experiments on both of these objects are already underway.
The research has been published in the journal Scientific Reports.
Thursday, October 22, 2015
New technique to manipulate terahertz waves
http://www.engineering.leeds.ac.uk/faculty/news/2015/new-technique-to-manipulate-terahertz-waves.shtml
Research funded by the EPSRC and the University of Leeds has developed a new technique to manipulate terahertz waves, the part of the electromagnetic spectrum between infrared and microwaves.
While previously it was possible for terahertz light to be manipulated by arrays of nanostructures, the researchers have now found a way to use a single nanostructure as a filter for terahertz waves, whose properties depend on how much voltage is applied.
The researchers found that, by passing terahertz radiation through a tiny region of semiconductor, with gates on its surface, they could control the spectrum of the radiation passing through it.
The new technique embeds the nanostructure in a microscopic waveguide, in which it is split into three sections or “cavities”.
A voltage they apply controls how electrons in these cavities oscillate, and therefore the frequency of electromagnetic radiation that each cavity can transmit.
The new technique has exciting research applications, as it can be applied to the study of almost any nanoscale electronic component.
It could be used to measure the properties of ultrafast transistors built from nanostructures, for example, or of graphene (a single atomic layer of carbon). Experiments on both are now underway.
Professor John Cunningham, from the School of Electronic and Electrical Engineering, who led the research, said: “This has really exciting potential, because terahertz waves have a wide range of possible uses, but until now researchers have found it hard to make a compact single component which can control the terahertz waves which pass through it.
“Such tuning is a prerequisite for many applications, since it allows the waves to be shaped or modified, allowing information to be encoded in the signal.”
Dr Chris Wood, a University of Leeds Research Fellow, added: “We are delighted with these results. Our work in the area is at the forefront of on-chip terahertz science and technology worldwide, and I very am grateful for the long-term support offered by the University which has allowed us to bring this complex 5-year project to completion.”
The research is published today in Scientific Reports.
Further information
Contact University of Leeds press office on pressoffice@leeds.ac.uk or call 0113 343 4031
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