IR Laser Mixing, Optical “Magic” Yields THz Waves with Relative Efficiency

By mixing and manipulating an infrared laser, a team led by researchers at TU Wein (Vienna University of Technology) has generated a wide band of terahertz waves with relatively high efficiency.

Bill Schweber

https://www.electronicdesign.com/technologies/analog/article/21131170/ir-laser-mixing-optical-magic-yields-thz-waves-with-relative-efficiency

The terahertz segment of the electromagnetic spectrum exists in a sort of “twilight zone” between about 300 GHz (0.3 THz) to 3 THz (note that the upper boundary value is somewhat arbitrary, and some consider it to be 30 THz). It’s bounded below by millimeter-wave RF (30 to 300 GHz) and above by optical wavelengths. Electromagnetic radiation below the terahertz band can be created by many sources and emitted by antennas, while the optical radiation above the terahertz band can be developed by solid-state lasers.

However, the major part of the terahertz span is largely a void. Although it’s governed by Maxwell’s equations just as the rest of the spectrum, it’s relatively difficult to generate these waves as they’re too high in frequency for even most leading-edge electronic components and too low for optical sourcing. Due to this and other problems, the terahertz region is far less explored and exploited than the RF or optical areas (see “References” at end of article).

Why even investigate the terahertz part of the spectrum? Among the many reasons are because it’s there, of course; because it potentially has useful and fascinating characteristics; and it may be needed for 6G, 7G, or 8G wireless links. After all, 20 years ago, if you had told someone that there would be mass-market consumer products operating at ~10 GHz by the first part of the 21st century, you would have likely been called a wild-eyed dreamer.

Power Problems

However, the problem remains in terms of efficiently generating useful levels of power in the terahertz band. There are tabletop sources based on either optical rectification in electro-optic crystals or two-color filamentation in gases and liquids. With optical rectification, terahertz pulses with energy up to 0.9 mJ and THz conversion efficiency (ratio of generated THz energy to the input laser pulse energy) up to 3.7% have been generated, but the pulses are long (several picoseconds) with narrow spectra.

Unfortunately, the optical-damage threshold of electro-optic crystals prevents a significant increase in power. Furthermore, while there are no issues with optical damage when using the two-color filamentation approach, it generates only less-energetic near-infrared (NIR) laser pulses, and with a very low conversion efficiency of ~0.01%.

Efficient Source Breakthrough

Now, a research group based at TU Wein (Vienna University of Technology), in close cooperation with a team from the Institute of Electronic Structure and Laser (IESL) Foundation for Research and Technology-Hellas (FORTH) in Heraklion, Greece, and some help from Texas A&M University at Qatar, has developed a source that’s both fairly efficient and can generate waves across the entire terahertz slice of spectrum. The experiment, with results they claim has broken previous records in these areas, was inspired by a theory developed in Texas A&M University, which predicted that long-wavelength laser pulses could be used to achieve extremely efficient terahertz generation in air plasma.

In the setup schematic (Fig. 1), the generation of the terahertz waves begins by sending infrared laser light through a nonlinear medium, where part of the infrared radiation is transformed into optical radiation at twice the initial frequency. The two radiation waves are then superimposed, creating an electromagnetic wave with an electric field having a very specific asymmetric shape.

The setup for THz generation by two-color mid-infrared filaments (a). After the quarter wave plate (QWP), the 3.9-μm laser pulse passes through the gallium-selenide (GaSe) crystal and generates the second harmonic pulse. The two-color laser pulse is focused by an off-axis parabolic mirror (OPM1) in ambient air and forms a filament where the THz radiation is generated. A parabolic mirror (OPM2) gathers the emitted THz pulse and guides it into one of the detection setups. The metallic disk (MD) blocks the on-axis mid-infrared radiation and generated supercontinuum, while the conically emitted THz radiation passes around it. A set of lowpass filters filter out the remained unwanted radiation and prevent the saturation of the pyroelectric detector (PED) by the intense THz pulses. THz detection setups (b-d): The parabolic mirror (OPM3) focuses the THz pulse on the PED to measure its energy (b). The wire grid polarizer (GP), placed before the OPM3, makes it possible to characterize the THz polarization. For the electro-optic measurements, the parabolic mirror (OPM4) focuses the THz pulse into a 55-μm thick gallium phosphide (GaP) crystal (c). A pair of GPs reduces the THz field strength to ensure a linear response. The 680-nm synchronized probe pulse focuses into the GaP crystal through a hole in OPM4 and then is guided to the balanced detection setup. For the cross-phase modulation experiment, the GaP crystal is replaced by the 1-mm-thick zinc-telluride (ZnTe) crystal and the 761-nm probe pulse is guided to the spectrometer. The Michelson interferometer measures the THz field autocorrelation and consists of a pellicle beam splitter (PBS) and two flat mirrors: fixed (M1) and movable (M2) (d). At the exit, a parabolic mirror (OPM5) focuses the radiation on the PED. (Source: TU Wein)

But that’s only the start of this process. The intense wave “rips” electrons out of the molecules in the air, turning the air into a glowing plasma. The special shape of the wave's electric field accelerates the electrons in such a way that they produce the desired terahertz radiation.

As noted by team member Claudia Gollner (Fig. 2), “Our method is extremely efficient with 2.3% of the supplied energy converted into terahertz radiation. That is orders of magnitude more than can be achieved with other methods. This results in exceptionally high terahertz energies of almost 200 microjoules and terahertz-field amplitudes exceeding 100 MV per cm.”

Researcher Claudia Gollner adjusts the optoelectronic setup in the lab at TU Wein’s photonics institute. (Source: TU Wein)

She also added that their approach develops many wavelengths throughout the terahertz range to be emitted simultaneously, and the larger spectrum of different terahertz wavelengths yields the shorter and more intense pulses.

The details, including a review of existing methods of terahertz generation and their characteristics, are in their paper “Observation of extremely efficient terahertz generation from mid-infrared two-color laser filaments” published in Nature Communications, along with even-more detailed, equation-laced Supplementary Information.

References

IEEE Spectrum, “Wireless Industry’s Newest Gambit: Terahertz Communication Bands”

IEEE Spectrum, “Terahertz Waves Could Push 5G to 6G”

IEEE Spectrum, “The Truth About Terahertz”

Terahertz science discloses the ultrafast photocarrier dynamics in carbon nanotubes

Comparison of computed peak values of THz emission and photocurrent with experimental data.CREDIT @2020 American Chemical Society

https://www.eurekalert.org/pub_releases/2020-04/ou-tsd042020.php

OSAKA UNIVERSITY
A team of researchers from Osaka University, TU Wien, Nanyang Technological University, Rice University, University of Alberta and Southern Illinois University-Carbondale comes closer to unraveling the physics of quasiparticles in carbon nanotubes.

Carbon nanotubes (CNTs), a model one-dimensional (1D) material made up entirely of carbon atoms, have attracted considerable attention ever since their discovery because of the unique properties arising from quantum confinement effects. CNTs have been labeled as one of the materials for next-generation optoelectronic devices. Critical towards this advancement is understanding how quasiparticles - theoretical particles used to describe observable phenomena in solids - behave and interact with each other in a 1D system. This requires a fundamentally different model compared to a conventional 3D material like silicon as a consequence of the reduced dimensionality in CNTs.

"It was difficult to develop a terahertz radiation device with an external high electric field in a specific direction to CNT," says corresponding author Masayoshi Tonouchi.

By combining different experimental techniques, the team was able to directly probe the creation of free charge carriers in CNTs at different time scales after photoexcitation. Very complex interactions that involve different quasiparticles occur after the initial photoexcitation. These processes change over time, and being able to probe one of the quasiparticles makes it easier to understand the whole process.

Together with state-of-the-art simulations, the team was able to identify two key mechanisms that explain their data and helped them develop a detailed microscopic model describing quasiparticle interactions in a strong electric field in CNTs.

"We proposed a model in which electron-hole bound quasiparticles excited in the high energy E22 exciton band diverge to the low energy band and play a role in ultrafast electrical conduction. This model successfully explained the experimental facts and led to the clarification of the physical properties of CNTs."

Their results shed light on a number of long-standing issues in CNT ultrafast dynamics, moving us closer towards the realization of advanced optoelectronics based on CNTs and other low-dimensional materials.

###

The article, "Terahertz Excitonics in Carbon Nanotubes: Exciton Autoionization and Multiplication," was published in ACS Nano Letters at DOI: https://doi.org/10.1021/acs.nanolett.9b05082.

The perfect terahertz beam - thanks to the 3D printer

Jan Gospodaric and Andrei Pimenov 

https://www.myscience.org/news/2018/the_perfect_terahertz_beam_thanks_to_the_3d_printer-2018-tuwien

TU Wien has succeeded in shaping terahertz beams with extremely high precision. All that is needed for this is a simple plastic screen from a 3D printer.

Terahertz radiation can be used for a wide variety of applications and is used today for airport security checks just as much as it is for material analysis in the lab. The wavelength of this radiation is in the millimetre range, meaning that it is significantly larger than the wavelength of visible light. It also requires specialised techniques to manipulate the beams and get them into the right shape. At TU Wien, shaping terahertz beams is now something of a resounding success: with the help of a precisely calculated plastic screen produced on the 3D printer, terahertz beams can be shaped as desired.

Like lenses - only better
"Normal plastic is transparent for terahertz beams, in a similar way as glass is for visible light," explains Prof. Andrei Pimenov from the Institute of Solid State Physics at TU Wien. "However, terahertz waves slow down a little when they pass through plastic. This means that the crests and troughs of the beam become a little displaced - we call that phase shifting."

This phase shifting can be used to shape a beam. Exactly the same thing happens - in a much simpler form - with an optical lens made of glass: when the lens is thicker in the middle than on the edge, a light beam in the middle spends more time in the glass than another beam that simultaneously hits the edge of the lens. The light beams in the middle are therefore more phase delayed than the light beams on the edge. This is exactly what causes the shape of the beam to change; a wider beam of light can be focussed on a single point.

And yet the possibilities are still far from being exhausted. "We didn’t just want to map a wide beam to a point. Our goal was to be able to bring any beam into any shape," says Jan Gosporadic, a PhD student in Andrei Pimenov’s team.

The screen from the 3D printer
This is achieved by inserting a precisely adapted plastic screen into the beam. The screen has a diameter of just a few centimetres, its thickness varying from 0 to 4 mm. The thickness of the screen must be adjusted step by step so that the different areas of the beam are deflected in a controlled way, resulting in the desired image at the end. A special calculation method has been developed in order to obtain the desired screen design. From this we can then produce the matching screen from an ordinary 3D printer.

"The process is amazingly simple," says Andrei Pimenov. "You don’t even need a 3D printer with an especially high resolution. If the precision of the structure is significantly better than the wavelength of the radiation used, then it’s enough - this is no problem for terahertz radiation with a 2mm wavelength." 

In order to highlight the possibilities offered by the technique, the team have produced different screens, including one which brings a wide beam into the shape of the TU Wien logo. "This shows that there are hardly any geometric limits to the technology," says Andrei Pimenov. "Our method is relatively easy to apply, which leads us to believe that the technology will be rapidly introduced for use in many areas and that the terahertz technology that is currently emerging will make it a bit more precise and versatile." 

Original publication:3D-printed phase waveplates for THz beam shaping, J. Gospodaric, A. Kuzmenko, Anna Pimenov, C. Huber, D. Suess, S. Rotter, and A. Pimenov; Appl. Phys. Lett. 112, 221104 (2018); doi: 10.1063/1.5027179 

QCL-based THz sources generate 'record' ultrashort pulses

TUW and ETHZ researchers add lateral absorber to laser resonator, creating emission bandwidth across a full octave.

http://optics.org/news/8/2/16

Broadband terahertz amplifier based on a quantum cascade laser.

Researchers at TU Wien (Vienna, Austria) and ETH Zurich (Switzerland) have succeeded in generating ultrashort terahertz waves. With lengths of just a few picoseconds, these pulses are suited to spectroscopic applications and can enable extremely precise frequency measurements to be taken, says the team.

A working group led by Prof. Karl Unterrainer at the Photonics Institute at TUW has been developing quantum cascade lasers (QCLs) as an efficient means of generating terahertz waves. QCLs consist of a precisely defined sequence of several hundred semiconductor layers a few nanometers thick.

This construction means there is the possibility for the operator to select the exact energy state at which the electrons remain within the semiconductor structure. This in turn allows the frequency of the laser light emitted to be tuned to suit various inspection and spectroscopic applications.

‘Laser sandwich’

With the capability of being able to determine the laser wavelengths themselves, several quantum cascade structures with different output 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,” said Dominic Bachmann from the Photonics Institute.

Furthermore, if the discrete laser lines are linked together to establish a fixed phase relationship between the laser modes, a “frequency comb” is formed. 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 revolutionized optical metrology and was honoured with the Nobel Prize for Physics in 2005. Over the past four years, researchers worldwide have been working to generate a terahertz frequency comb using a quantum cascade laser as part of the EU project TERACOMB, completed in 2015. Led by Dr Juraj Darmo from the Vienna Photonics Institute, the team of international research groups has succeeded in generating the first broadband terahertz frequency comb based on semiconductor technology.

Karl Unterrainer, Dominic Bachmann and Juraj Darmo at the Vienna Photonics Institute.

Vienna breakthough

A method developed by the group led by Prof. Unterrainer makes it possible to analyze 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.

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.

To prevent these modes from oscillating, the losses must 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 TUW/ETHZ researchers suppressed the higher lateral modes entirely, without having any relevant impact on the fundamental modes. The result was an emission bandwidth covering a full octave, even mode distribution in the middle at 700 GHz, and a frequency comb with a bandwidth of 440 GHz.

Furthermore, the lateral absorbers enable the generation of ultrashort terahertz pulses with pulse widths of less than 3 ps, which represents what they say is 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,” said Dominic Bachmann, who has just finished writing his dissertation on broadband quantum cascade lasers.

New record achieved in terahertz pulse generation

http://www.alphagalileo.org/ViewItem.aspx?ItemId=172465&CultureCode=en

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. 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.