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 

Abstract-3D-Printed Phase Waveplates for THz Beam Shaping

Jan Gospodaric, Artem Kuzmenko, Anna Pimenov, Christian Huber, Dieter Suess, Stefan Rotter, Andrei Pimenov

https://128.84.21.199/abs/1803.07894v1

The advancement of 3D-printing opens up a new way of constructing affordable custom terahertz (THz) components due to suitable printing resolution and THz transparency of polymer materials. We present a way of calculating, designing and fabricating a THz waveplate that phase-modulates an incident THz beam ({\lambda}=2.14 mm) in order to create a predefined intensity profile of the optical wavefront on a distant image plane. Our calculations were performed for two distinct target intensities with the use of a modified Gerchberg-Saxton algorithm. The resulting phase-modulating profiles were used to model the polyactide elements, which were printed out with a commercially available 3D-printer. The results were tested in an THz experimental setup equipped with a scanning option and they showed good agreement with theoretical prediction

Optical computing with terahertz waves

Alexey Shuvaev, Andrei Pimenov, Florian Aigner, Georgy Astakhov, Mathias Mühlbauer, Christoph Brüne, Hartmut Buhmann and Laurens W. Molenkamp http://spie.org/x103717.xml?highlight=x2414&ArticleID=x103717   The polarization rotation of light passing through a topological insulator thin film can be tuned via gate voltage, opening the door for optical transistors in the terahertz range. 8 October 2013, SPIE Newsroom. DOI: 10.1117/2.1201309.005079 In today's computers, information is processed using electrical charges. Due to inherent physical limits, the shrinking of electron-based transistors to improve processing speeds is proving increasingly challenging. One approach to overcoming these limits is to replace electric currents with light. For this to work, we require tools that enable its effective manipulation. Light oscillates in many different directions perpendicular to the direction of propagation. Polarized light, however, oscillates in only one direction. In certain materials, an applied magnetic field can rotate the polarization direction of light passing through. This phenomenon, known as the Faraday effect, causes an angle of rotation that is usually minutely small. However, two years ago we observed a giant Faraday effect after passing terahertz (THz) light through thin mercury telluride films under an applied magnetic field: see Figure 1.1With a wavelength of the order of one millimeter, THz light resonates at a frequency equating to the clock speeds expected from computers two generations away. The benefits of this technology are not limited to computing: controlling beams of THz radiation is important in many applications, including airport security and biomedical sensing. However, changing the polarization direction of light without a large part of it being lost is difficult.   Figure 1. The oscillation direction of a polarized terahertz (THz) light wave is rotated as it passes through a thin film of mercury telluride (HgTe) under an external magnetic field. In our experiments, we exploit the strong interaction between THz radiation and a solid-state plasma. Because of this interaction, left- and right-handed circularly polarized THz waves have different refractive indices when a magnetic field is applied. This property was recently proposed as a tool by which it may be possible to realize broadband THz modulators based on electron-doped indium antimonide (InSb) crystals.2 There are, however, two problems in realizing such devices for practical application. First, the giant Faraday effect in InSb does not survive at room temperature due to strongly enhanced carrier scattering. Second, this approach requires fast modulation of a moderate applied magnetic field—several hundred milliTesla (mT)—which is technically challenging. In our experiments, we used thin films of mercury telluride (HgTe), a topological insulator. Topological insulators have attracted enormous interest over the past few years owing to the discovery of a number of nontrivial properties, including protected conductive states on the surface or edge of a sample.3 A number of unusual effects are theoretically predicted to occur during electrodynamic experiments on this material,4–7 including the universal Faraday effect and anomalous Kerr rotation (in which reflected radiation undergoes a large angle of polarization rotation). In addition, strain engineering of HgTe enables the suppression of effects from 3D carriers. When unstrained, HgTe is a zero gap semiconductor: there are always charge carriers present in the bulk material that disturb the observation of topological states. However, for strained thin films of HgTe, the energy gap becomes finite, which leads to a suppression of bulk carriers. Pure 2D electron behavior can therefore be expected from a strained sample, enabling the search for unusual electrodynamics, such as the giant Faraday effect.8 To achieve control over the Faraday effect in a constant magnetic field, preferably one that can be provided by a permanent magnet (<1T), we fabricated devices fitted with transparent gate electrodes: see Figure 2(a). In these devices, a thin film of HgTe is deposited on a cadmium telluride (CdTe) substrate. A semi-transparent gate made of ruthenium oxide (RuO2) is separated from the active layer of HgTe by a silicon nitride (Si3N4) insulator. This structure allows most of the light to pass through the sample while a gate voltage is applied. The electrodes enable us to change the carrier density (and thus the properties) of the electron plasma in the HgTe layer. By applying a moderate voltage of 9   Figure 2. Electric voltage control of THz radiation. (a) Schematic representation of experimental arrangement to control the Faraday rotation (θ) and Faraday ellipticity (η) by gate voltage. A thin film of HgTe is deposited on a cadmium telluride (CdTe) substrate. A semitransparent gate made of ruthenium oxide (RuO2) is separated from the active layer of HgTe by a silicon nitride (Si3N4) insulator. This material enables most of the light to pass through the sample while a gate voltage is applied. (b) Normalized detector signal as a function of gate voltage in a geometry with (i) parallel and (ii) crossed polarizers. B: Magnetic field. a.u.: Arbitrary units. When light is incident on a polarization filter, it passes through the filter or is blocked, depending on the polarization direction. In our device, the rotation of the beam of light passing through (and thus the electrical potential applied) determines whether or not a light signal is transmitted, thereby achieving the basic principle of a transistor. Figure 2(b)(i) shows experimentally observed changes in the component of the transmitted wave with the same polarization as the incident wave, while Figure 2(b)(ii) presents the variation in the component with the polarization orthogonal to the incident wave. Although it was not possible to completely block the radiation, a suppression of up to 30% was observed for the crossed signal. Our results were obtained at room temperature, suggesting the possibility of practical applications. These could include direct control of polarization state by gate voltage and/or magnetic field, and fast phase and amplitude modulation. We believe that our technology will enable a high modulation speed comparable to that currently obtained by high-electron-mobility transistors. In summary, we have been able to demonstrate that light can be tuned electrically. By passing THz radiation through a device based on a thin film of HgTe under a magnetic field, we achieved control of its Faraday rotation and ellipticity. This technology may find practical application in optical transistors of the future. We intend to further improve the modulation amplitude of our device (currently as large as several degrees per volt) by varying the barrier thickness and using a higher-quality HgTe layer. Alexey Shuvaev, Andrei Pimenov, Florian Aigner Vienna University of Technology Vienna, Austria Georgy Astakhov, Mathias Mühlbauer, Christoph Brüne, Hartmut Buhmann, Laurens W. Molenkamp University of Würzburg Würzburg, Germany References: 1. A. M. Shuvaev, G. V. Astakhov, A. Pimenov, C. Brüne, H. Buhmann, L. W. Molenkamp, Giant magneto-optical Faraday effect in HgTe thin films in the terahertz spectral range,Phys. Rev. Lett. 106, p. 107404, 2011. doi:10.1103/PhysRevLett.106.107404 2. T. Arikawa, X. Wang, A. A. Belyanin, J. Kono, Giant tunable Faraday effect in a semiconductor magneto-plasma for broadband terahertz polarization optics, Opt. Express20(17), p. 19484-19492, 2012. doi:10.1364/OE.20.019484 3. M. Z. Hasan, C. L. Kane, Colloquium: topological insulators, Rev. Mod. Phys. 82, p. 3045-3067, 2010. doi:10.1103/RevModPhys.82.3045 4. W.-K. Tse, A. H Macdonald, Giant magneto-optical Kerr effect and universal Faraday effect, Phys. Rev. Lett. 105(5), p. 057401, 2010. doi:10.1103/PhysRevLett.105.057401 5. W.-K. Tse, A. H. MacDonald, Magneto-optical Faraday and Kerr effects in topological insulator films and in other layered quantized Hall systems, Phys. Rev. B 84(20), p. 205327, 2011. doi:10.1103/PhysRevB.84.205327 6. J. Maciejko, X. L. Qi, H. D. Drew, S.-C. Zhang, Topological quantization in units of the fine structure constant, Phys. Rev. Lett. 105(16), p. 166803, 2010.doi:10.1103/PhysRevLett.105.166803 7. G. Tkachov, E. M. Hankiewicz, Anomalous galvanomagnetism, cyclotron resonance, and microwave, Phys. Rev. B 84(3), p. 035405, 2011. doi:10.1103/PhysRevB.84.035405 8. A. M. Shuvaev, G. V. Astakhov, C. Brüne, H. Buhmann, L. W. Molenkamp, A. Pimenov, Terahertz magneto-optical spectroscopy in HgTe thin films, Semicond. Sci. Tech. 27(12), p. 124004, 2012. doi:10.1088/0268-1242/27/12/124004 9. A. Shuvaev, A. Pimenov, G. V. Astakhov, M. Mühlbauer, C. Brüne, H. Buhmann, L. W. Molenkamp, Room temperature electrically tunable terahertz Faraday effect, Appl. Phys. Lett. 102(24), p. 241902, 2013.

Optical Transistors: The Next Step In Quantum Computing?

By Jim Pomager
http://www.photonicsonline.com/doc/optical-transistors-the-next-step-in-quantum-computing-0001

The world’s appetite for computing power is nearly insatiable, expecting microprocessors to handle ever-larger and more complex calculations quickly and efficiently. Until recently, the go-to method for achieving ongoing performance improvements was to continuously shrink the size of transistors, so you could keep cramming more of them onto each microprocessor chip. Generally speaking, the higher your transistor count, the greater the computing performance of your integrated circuit (IC).
However, we appear to be reaching the practical limits of Moore’s Law, which states that the number of transistors you can get onto an IC — largely due to reductions in transistor size — doubles approximately every two years. It’s going to be tough to make conventional electron-based transistors much smaller than they are now (though some are trying), and managing the corresponding heat generated by so many transistors packed into such a tight space is a major challenge.
An optical solution to the problem may be close(r) at hand, according to findings published by two different research groups in recent weeks. Teams at the Massachusetts Institute of Technology (MIT) Research Laboratory of Electronics and the Institute of Solid State Physics at the Vienna University of Technology (UT Vienna) are claiming novel breakthroughs in the development of optical transistors, which use light (photons) rather than electricity (electrons) to carry information. Theoretically, such optical transistors could improve heat/power management and processing speeds in conventional computers, and even facilitate the development of more effective quantum computers — the holy grail of computing.
Rotating Beams Of Light — More Efficiently
Back in 2011, a UT Vienna research team led by Professor Andrei Pimenov published results of an experiment demonstrating the Faraday effect — rotating the polarization plane of a beam of light — on a massive scale. The researchers effectively tuned a light beam’s polarization by applying a magnetic field to ultra-thin layers of mercury telluride as the beam (terahertz radiation, to be precise) passed through it. Used in conjunction with a polarization filter, which allows only light polarized at a particular angle to pass, this “light transistor” could thus control whether a beam would pass or be blocked, much like a traditional transistor would manage electron flow.
“This is the very principle of a transistor,” Pimenov stated. “The application of an external voltage determines whether current flows or not, and in our case, the voltage determines whether the light arrives or not.”

UT Vienna researchers applied an electromagnetic potential to a light beam traveling through special mercury telluride platelets, enabling them to rotate the beam’s polarization direction. [Credit: UT Vienna]

Unfortunately, this initial approach was less than efficient, requiring an external magnetic coil and very large electrical currents to generate the necessary magnetic field. In the results published earlier this month, however, Pimenov’s team showed it could overcome this problem by rotating the polarization using electrical potential, or the volume of electrons involved, rather than the strength of the magnetic field. As a result, the new optical transistor can be controlled using a permanent magnet and an electrical potential of less than one volt.
Creating A Single-Photon Optical Switch
The MIT group’s approach involved setting a pair of highly reflective mirrors a precise distance from one other, and filling the space in between them with supercooled cesium gas. In its “on” position, the optical switch allows light of a certain wavelength to pass through both the mirrors and the gas. But by throwing it into the “off” position — by firing a single “gate” photon into the gas, exciting one electron of one cesium atom into a higher energy state — only about 20% of the light is able to get through.
The MIT findings could help eventually address the transistor overheating issue in conventional microprocessors, at least in theory, because the single-photon approach is more energy efficient than the current electrical one. “One could imagine implementing a similar device in solid state — example, using impurity atoms inside an optical fiber or piece of a solid,” Vladan Vuletic, lead investigator and Lester Wolfe Professor of Physics at MIT. (The work was carried out in partnership with researchers from Harvard University and, of all places, UT Vienna.)
What really has technologists interested, though, is the potential application for the MIT optical transistor in quantum computing, particularly the issue of superposition. This concept ties my brain in a bit of a knot, but the gist is that where memory in classical computers is made up of bits, which must be in one of two states (for instance, 0 and 1) at any given time, in quantum computers it’s composed of a series of qbits, which can exist in both states simultaneously. Please don’t ask me to explain any further, but suffice it to say that photons are easier to keep in superposition than alternative particles being explored for qbits, like laser-trapped ions.
Coming Soon To A Computer Near You?
These two recent examples are far from the first demonstrated optical transistor concepts, and research into the technology is being conducted at McGill University, Purdue University, the Max Planck Institute, and elsewhere. Plus, both approaches are a long way from practical use — they are still comparatively large in scale and have rather particular operating requirements. (Sheets of mercury telluride and supercooled cesium gas aren’t exactly easy to come by.) So while these accomplishments may represent significant steps toward new computing techniques, don’t expect either to unseat the good old electrical transistor any time soon.
Main image credit: Christine Daniloff/MIT

TU Vienna develops light transistor

 

http://esciencenews.com/articles/2013/07/08/tu.vienna.develops.light.transistor

Light can oscillate in different directions, as we can see in the 3D cinema: Each lens of the glasses only allows light of a particular oscillation direction to pass through. However, changing the polarization direction of light without a large part of it being lost is difficult. The TU Vienna has now managed this feat, using a type of light -- terahertz radiation -- that is of particular technological importance. An electrical field applied to an ultra-thin layer of material can turn the polarisation of the beam as required. This produces an efficient transistor for light that can be miniaturised and used to build optical computers. Rotated light -- the Faraday effect

Certain materials can rotate the polarization direction of light if a magnetic field is applied to them. This is known as the Faraday effect. Normally, this effect is minutely small, however. Two years ago, Prof. Andrei Pimenov and his team at the Institute of Solid State Physics of TU Vienna, together with a research group from the University of Würzburg, managed to achieve a massive Faraday effect as they passed light through special mercury telluride platelets and applied a magnetic field.

At that time, the effect could only be controlled by an external magnetic coil, which has severe technological disadvantages. "If electro-magnets are used to control the effect, very large currents are required," explains Andrei Pimenov. Now, the turning of terahertz radiation simply by the application of an electrical potential of less than one volt has been achieved. This makes the system much simpler and faster.

It is still a magnetic field that is responsible for the fact that the polarisation is rotated, however, it is no longer the strength of the magnetic field that determines the strength of the effect, but the amount of electrons involved in the process, and this amount can be regulated simply by electrical potential. Hence only a permanent magnet and a voltage source suffice, which is technically comparatively easy to manage.

Terahertz radiation

The light used for the experiments is not visible: it is terahertz radiation with a wavelength of the order of one millimetre. "The frequency of this radiation equates to the clock frequency that the next but one generation of computers may perhaps achieve," explains Pimenov. "The components of today's computers, in which information is passed only in the form of electrical currents, cannot be fundamentally improved. To replace these currents with light would open up a range of new opportunities." It is not only in hypothetical new computers that it's important to be able to control beams of radiation precisely with the newly developed light turning mechanism: terahertz radiation is used today for many purposes, for example for imaging methods in airport security technology.

Optical transistors

If light is passed through a polarisation filter, dependent on the polarisation direction, it is either allowed to pass through or is blocked. The rotation of the beam of light (and thus the electrical potential applied) therefore determines whether a light signal is sent or blocked. "This is the very principle of a transistor," explains Pimenov: "The application of an external voltage determines whether current flows or not, and in our case, the voltage determines whether the light arrives or not." The new invention is therefore the optical equivalent of an electrical transistor.

Source: Vienna University of Technology, TU Vienna

Physicists Rotate Beams of Light in Terahertz domain

 

  

The magnetic field in the thin layer rotates the light waves. (Credit: Image courtesy of Vienna University of Technology)

http://www.sciencedaily.com/releases/2011/03/110330094149.htm
ScienceDaily (Mar. 30, 2011) — Controlling the rotation of light – this amazing feat was accomplished by means of a ultra thin semiconductor. This can be used to create a transistor that works with light instead of electrical current.

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Light waves can oscillate in different directions -- much like a string that can vibrate up and down or left and right -- depending on the direction in which it is picked. This is called the polarization of light. Physicists at the Vienna University of Technology have now, together with researchers at Würzburg University, developed a method to control and manipulate the polarization of light using ultra thin layers of semiconductor material.
For future research on light and its polarization this is an important step forward -- and this breakthrough could even open up possibilities for completely new computer technology. The experiment can be viewed as the optical version of an electronic transistor. The results of the experiment have now been published in the journal Physical Review Letters.
Controlling light with magnetic fields
The polarization of light can change, when it passes through a material in a strong magnetic field. This phenomenon is known as the "Faraday effect." "So far, however, this effect had only been observed in materials in which it was very weak," professor Andrei Pimenov explains. He carried out the experiments at the Institute for Solid State Physics of the TU Vienna, together with his assistant Alexey Shuvaev. Using light of the right wavelength and extremely clean semiconductors, scientists in Vienna and Würzburg could achieve a Faraday effect which is orders of magnitude stronger than ever measured before.
Now light waves can be rotated into arbitrary directions -- the direction of the polarization can be tuned with an external magnetic field. Surprisingly, an ultra-thin layer of less than a thousandth of a millimeter is enough to achieve this. "Such thin layers made of other materials could only change the direction of polarization by a fraction of one degree," says professor Pimenov. If the beam of light is then sent through a polarization filter, which only allows light of a particular direction of polarization to pass, the scientists can, rotating the direction appropriately, decide whether the beam should pass or not.
The key to this astonishing effect lies in the behavior of the electrons in the semiconductor. The beam of light oscillates the electrons, and the magnetic field deflects their vibrating motion. This complicated motion of the electrons in turn affects the beam of light and changes its direction of polarization.
An optical transistor
In the experiment, a layer of the semiconductor mercury telluride was irradiated with light in the infrared spectral range. "The light has a frequency in the terahertz domain -- those are the frequencies, future generations of computers may operate with," professor Pimenov believes. "For years, the clock rates of computers have not really increased, because a domain has been reached, in which material properties just don't play along anymore." A possible solution is to complement electronic circuits with optical elements. In a transistor, the basic element of electronics, an electric current is controlled by an external signal. In the experiment at TU Vienna, a beam of light is controlled by an external magnetic field. The two systems are very much alike. "We could call our system a light-transistor," Pimenov suggests.
Before optical circuits for computers can be considered, the newly discovered effect will prove useful as a tool for further research. In optics labs, it will play an important role in research on new materials and the physics of light.