One of the important nowadays challenges of solid state physics and technology is related to so called "Terahertz gap". In terms of the frequency of electromagnetic radiation this gap extends from 0.3 THz to 10 THz. This frequency region still lacks of convenient compact solid state devices that could emit or detect THz radiation in a selective and/or tunable way. Such sources are necessary for emerging applications such as THz medical imaging, chemical and biological sensing, wide-band telecommunications systems, etc.
The "Terahertz gap" can also be seen as a frontier between optics and electronics. The cut-off frequency of nanometer transistors was recently pushed up to a few hundreds of GHz and the THz limit becomes realistic for electronic devices. From the high frequency (optics) side the semiconductor lasers (quantum cascade lasers) show possibility of the efficient operation not only in traditional 30 THz range but also in the frequency down to 2-4 THz range.
The proposed project is conceived with the purpose to merge the competences of different scientific European groups, to exchange ideas and experimental means between adjacent sub domains of THz research. The project focuses on stimulating information exchange and collaborations between researches from fundamental solid state physics and researches from the complementary domains like physics of nanoscale/high frequency devices or biophysics, together with representatives of the industry.
RESEARCH PROJECT DESCRIPTION
This project proposes the continuation of the Terahertz Science and Technology related GDR-E and GDR actions 2005-2009. Even if the important progress has been achieved in the last few years – main objectives and motivations remain valid.
Progress in the modern semiconductor physics and technology was obtained mainly thanks to better understanding and controlling of the electronic properties of low-dimensional systems. Intense investigations of confined electron systems performed during the last two decades have lead to spectacular new applications and devices like single-electron transistors or Quantum Cascade Lasers.
One of the important nowadays challenges of solid state physics and technology is related to so called "Terahertz gap". In terms of the frequency of electromagnetic radiation this gap extends from 0.3 THz to 10 THz. This frequency region still lacks of convenient compact solid state devices that could emit or detect THz radiation in a selective and/or tunable way. Such sources are necessary for emerging applications such as THz medical imaging, chemical and biological sensing, wide-band tele-communications systems, etc.
The "Terahertz gap" can also be seen as a frontier between optics and electronics. The cut off frequency of nanometer transistors was recently pushed up to a few hundreds of GHz and the THz limit becomes realistic for electronic devices. From the high frequency side (optics) the semiconductor lasers (quantum cascade lasers) shows possibility of the efficient operation not only in traditional 30THz range but also in the frequency down to 2-4 THz range.
Lack of convenient THz sources and detectors lead also to the "Terahertz gap" in knowledge of basic properties of many solid state and biological species/medias.
During the last years, many research centers in France and all over the world, have devoted important part of their activity to solve problems related to THz generation and detection in semiconductor systems.
Let us consider a few examples for the sake of the demonstration:
A promising concept of THz electronics utilizing plasma waves in a gated 2D electron gas (2DEG) has been proposed in the early 90-ties. It is based on the fact that the resonant plasma modes oscillating at THz frequencies can be generated by the current flowing in submicron 2D systems. In the gated 2D structures the carrier density can be changed and therefore the frequency of THz oscillations can be tuned.
Recent experimental observations and theoretical studies (see, e.g., M.S. Shur and J. Q. Lu, IEEE Trans. Microwave Theory and Techniques, 2000, 48, p.75; X. G. Peralta, S. J. Allen, M.C. Wanker, N. E. Harff, J. A. Simmons, M. P. Lilly, J. L. Reno, P. J. Burke, J. P. Eisenstein, Appl. Phys. Lett. 81, 1627 (2002); W.Knap, Y.Deng, S.Rumyantsev, M. S. Shur, Appl. Phys. Lett. 81, 4637 (2002).) have revealed that resonant detection and emission of terahertz radiation can be effectively induced by excitation of plasma oscillations in the electron channel of nanometer size field-effect transistors (FET).
Also the first broadband room temperature operating focal plane arrays have been realized in Silicon Nanotransistor MOSFET Technology.
The results of those studies raised many theoretical, technological and experimental problems which have to be resolved in order to understand deeper the underlying fundamental physical mechanism of these phenomena and optimize the material and device design for realizing plasma-wave sensitive detectors and tunable sources in terahertz frequency range.
Another promising THz source is the quantum cascade laser. Although in the mid-infrared region 5 < lambda< 10 μm (60 > f > 30 THz) these devices have been in development for more than ten years, it is only recently that the first THz-laser has been reported at 67 μm (4.4 THz) (R. Kohler et al., Nature, 417, 156 (2002)). Since then, several technologically important milestones have been achieved: continuous wave (cw) operation up to 100 K has been demonstrated (S. Kumar et al., Appl. Phys. Lett. 84, 2494, (2004)) and emission wavelength extended to more than 250μm. Optical power of a few tens of mW is routinely observed at 10K and only a few mW at liquid nitrogen temperature. The fundamental wavelength limits are not clear today but it is foreseeable to extend them down into the frequency region around 1THz. Lower frequencies are probably difficult to achieve. However, an extra in-plane confinement, imposed by a perpendicular magnetic field which can greatly enhance properties of the lasers, has been investigated. Under this condition, THz quantum cascade lasers have been demonstrated at very long wavelengths (700 GHz). [A. Wade, et al. Nature Photon. 3, 41-45 (2009).
When the magnetic field is greater than 6 T, the ratio of the magnetic confinement to the photon energy becomes large. At temperatures below 20 K, the devices are then characterized by a very low threshold current density, with values as low as Jth = 1 A/cm2, and an increase of gain by an order of magnitude. It is assumed that the device operates in the quantum Hall regime, where the carriers are localized on edge states originating by a combination of magnetic confinement and disorder.
We would like also to bring the attention to the possible THz technology developmentsThe researchers from Japan have developed recently a few systems for THz based security application. As an example: the instrument to check mail through imaging the content of the envelopes has been successfully integrated in the Japan Custom Institutions. This instrument can be improved using new sources and detectors existing in European laboratories (e.g., quantum cascade lasers combined with plasma wave detectors). Another example of possible technological implementation concerns the Canadian partner who has developed high responsivity Quantum Well Photodetectors and demonstrated the system of THz communication based on these detectors and Quantum cascade laser sources.
Existing research efforts in the THz domain are currently the following (non-exhaustive list):
• Hot electron lasers in bulk materials - nitrides
• Mixing and multiplication in non-linear elements
• Quantum wells based Intersubband THz detection
• Two wavelength lasers for THz differential generation
• Quantum wells Intersubband emitters fountain lasers
• THz limits for silicon nanotransistors
• Control of spontaneous emission dynamics in terahertz sources
• High mobility 2DEG and nanofabrication for terahertz devices
• Electronic transport mechanisms in terahertz nanostructures
• THz QC lasers
• Interaction of THz radiation with biological systems and tissues
• THz metamaterials
The list of examples is surely not complete but it shows important diversity. It seems important in the present stage to propose a continuation of the GDR project in order to enhance the exchange of information and creation of common projects and collaborations between groups and laboratories of different countries.
The proposed project is conceived for the purpose to join competences of different scientific European scientific groups, to exchange ideas and experimental means between adjacent sub domains of THz field.
The development of THz activity is in great part driven by possible biological/medical applications. The energy of the THz photon corresponds to the energies of bound inside heavy molecules, such as biomolecules. The research in THz range can bring interesting new information in molecular biology, as for example the conformation studies of proteins or the hydration phenomena. For the use in the cellular biology the design, fabrication and tests of new BioMEMS dedicated to the THz electromagnetic spectroscopy on living cells was performed. The measurements were done with different type of cells for establishing the first dielectric spectroscopy database in this field. Moreover, the dynamical measurements of the internalization of informative molecules inside the cell membrane could bring complementary knowledge in cellular physiology.
To intensify the exchange of information small 2-3 groups seminars and exchange of researchers between laboratories will be organized. During these seminars, meetings and exchanges of researchers common experiments will be discussed, planned and their results will be interpreted.
The school "Semiconductor sources and detectors of THz radiation" will be organized to give the basic courses understood able also on the PhD student level.
The project is directed by researchers coming from fundamental physics and by people of the complementary domains of devices and processing at nanoscales, together with representatives of the industry. The creation of joint "start up" of industrial enterprise is planned by the French and Russian sides. Further industrial contacts and collaborations for exploitable technological development are expected.