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fisica e… Filling the terahertz gap Towards a new generation of sources, detectors and sensors in the “no man’s land” between photonics and electronics Gaetano Scamarcio1,2, Miriam Serena Vitiello3,4, Vincenzo Spagnolo1,2 1 Dipartimento Interateneo di Fisica, Università degli Studi di Bari “Aldo Moro” e Politecnico di Bari, Bari, Italy Consiglio Nazionale delle Ricerche, Istituto di Fotonica e Nanotecnologie, UOS Bari, Italy 3 Consiglio Nazionale delle Ricerche, Istituto di Nanoscienze and Laboratorio NEST, Pisa, Italy 4 Scuola Normale Superiore, Pisa, Italy 2 Recent breakthroughs in nanotechnologies are pushing up the efficiency of compact semiconductor laser sources and detectors of radiation in the terahertz (THz) range of the electromagnetic spectrum. At these frequencies, many molecules have their spectroscopic fingerprints due to rotational and vibrational transitions. Hence, long-dreamed killer applications of THz photonics such as sensing, imaging, diagnostics, coherent spectroscopy in strategic fields like medicine, microelectronics, petrochemical industry, forensic science, cultural heritage are coming true. 1 Introduction The terahertz region of the electromagnetic spectrum conventionally identifies radiation with frequencies in the range 100 GHz– 10 THz, or equivalently, wavelengths in the range 3 mm–30 µm, energies in the range 0.4–40 meV. From a fundamental point of view, both generation and detection of THz radiation in semiconductor heterostructures rely on the interaction with either bound or free electrons, in the realms of optics or electronics, respectively. The efficiencies of electronic devices based on the interaction with free electrons depends on the strength of oscillatory currents excited by the electric field of the optical waves, and dramatically drops beyond the microwaves region due to saturation of the electron velocity. In addition, the strength of photon absorption or emission associated with transitions between subbands created by quantum confinements in quantum well structures is governed by the strength of polarization currents, which drops with decreasing frequency in the infrared. Hence, due to inherent technical difficulties related with the lack of practical and efficient sources and detectors the THz electromagnetic range is still underdeveloped, a fact usually referred to as the existence of a so-called “THz gap”. On the other hand, while microwave technologies encroach the lower-frequency side, optical technologies have made tremendous advances in the last decade in the higher-frequency side. In this article, we will review recent milestones to fill the THz gap. We will present the state-of-the-art of quantum cascade lasers (QCLs), which are considered the most promising compact sources of THz wave [1]. The discovery that high-mobility graphene or low-dimensional semiconductor channels in field effect transistors (FETs) can act as plasma waves cavities to detect THz radiation [2] marks an outstanding progress in the field of long-wavelength photonics. The quest for smaller, faster and sensitive optoelectronics devices is a strong driving force behind developments in nanoscale science and technology. Remarkable progress has been recently achieved with the development of onedimensional [3] and two-dimensional [4] FETs in which high-mobility semiconductors can act as cavities for plasma waves to detect THz radiation via nonlinear rectification of the ac current induced by the oscillating radiation field. The recent innovation in the performance of photonic sources and detectors is now enabling THz technologies to be employed in an increasingly wide variety of applications, such as information and communications technologies, medical and biological sciences, global environmental monitoring, homeland vol31 / no3-4 / anno2015 > 43 fisica e… security, industrial quality and process controls. Most of the above applications exploits the THz spectroscopic features of materials. In fact, explosives, narcotics, and toxic gases have distinct spectral “fingerprints” and strong absorption bands associated with rotational and vibrational transitions across the THz range. Also, THz radiation is weakly absorbed by non-metallic and non-polarizing materials (e.g. textiles, paper, ceramics, plastics, undoped semiconductors), which makes these materials transparent while they are opaque at visible frequencies. Being non-ionizing, THz beams are safe for biological and medical imaging applications, unlike X-rays. All these properties makes THz technologies ideal for imaging and sensing applications. Among sensing techniques, quartz enhanced photoacoustic spectroscopy (QEPAS) based on QCLs has demonstrated its competitive advantages with respect to other spectroscopies [5]. Its recent first demonstration in the THz range will be reviewed [6]. 2 Quantum cascade lasers It is commonly accepted that the quantum cascade laser ([1] and refs. therein) is the most striking demonstration of bandgap engineering. In fact, in these devices all relevant physical parameters can be controlled by spatial confinement in semiconductor heterostructures thanks to the extreme precision achievable in the design via the quantum engineering of electronic wave functions on a nanometer scale and the state-of-the-art epitaxial material growth. QCLs are unipolar device that exploits inter-subband transitions between quantum confined electronic states of the conduction band. While the idea of using intersubband transitions for electromagnetic amplification was theoretically proposed long before their realization, it was the development of molecular beam epitaxy (MBE) or metalorganic vapor phase epitaxy (MOCVD) that provide unprecedented control of layer thickness down to a single atomic monolayer, necessary to put in practice the QCL concept. Indeed, a key feature of QCLs is the ability to create sharp discontinuities in the conduction band edges, thereby controlling the size quantization effects on charge carriers at nanometer scale, the related tuning of the electronic energy, the electron localization, the transport as well as the optical transition processes. The main feature of the QCL is the possibility to tune its emission frequency over a wide range by tailoring the thickness of quantum wells and barriers, while keeping the same semiconductor material system. This is markedly different with respect to conventional interband semiconductor lasers, in which the emission wavelength depends on the material bandgap and the gain is strongly temperature-dependent. Remarkably, the cascaded structure makes possible that each electron injected above threshold may generate a number of photons larger than one and even equal to the number of active region stages. For subband separations larger than longitudinal optical (LO) phonon energy ELO (32–36 meV in III-V semiconductors), Fig. 1 Best reported performance of quantum cascade lasers. Plot of the maximum reported operating temperatures as a function of the emission wavelength (or frequency, top axis). Reproduced with permission from ref [1], ©2015, OSA. 44 < il nuovo saggiatore g. scamarcio et al.: Filling the terahertz gap energy relaxation processes in QCLs are controlled by emission of LO phonons. The LO-mediated scattering is the dominant inter-subband scattering mechanism, with scattering times approximately 1 ps or less. For intersubband transitions in which the separation is less than ELO , emission of LO-phonons is energetically forbidden at low temperatures. Non-radiative relaxation is therefore dominated by the combination of electron-electron (e-e) scattering, electron-impurity scattering, and LO-phonon scattering of the high-energy tail of the subband electron distribution. However, even when devices are operated at low temperature, a non-equilibrium electron distribution may exist [7–11]. Intrasubband LO-phonon scattering is an important process in cooling the subband electron gas while the effect of intrasubband e-e scattering is to thermalize the electron distribution inside a particular subband. Although QCLs may be realized using almost any semiconductor material system, the best performance has been obtained by using either GaInAs/AlInAs grown on InP substrates in the mid-IR, or GaAs/AlGaAs grown on GaAs substrates in the THz. After two decades from their invention, QCLs operating in the mid-IR have reached impressive performance levels. Spectral coverage has been achieved at wavelengths in the range 3 μm–25 μm with the potential for large tunability [12]. In 2002, operation range of QCLs was extended to the THz region [13], although still at cryogenic temperatures. Figure 1 schematically shows the present state of available frequencies as a function of the temperature performances of QCLs. Room temperature, CW operation of mid-IR QCLs was achieved more than 10 years ago and the last decade has witnessed a dramatic improvement that has made it possible to reach emitted powers on the order of several watts. The wall-plug efficiency, i.e. the ratio between injected and emitted power, has reached values as high as 27% in pulsed mode and 21% in CW mode [14] with more than 5 W of CW output power at room temperature. These impressive performances are due to a combination of refined quantum engineering of the laser active region, together with a refined material growth, advanced processing solutions like buried heterostructures and epi-down mounting providing efficient management of dissipated heat. 3 Terahertz quantum cascade lasers Quantum cascade lasers offer a very promising route to cover the upper frequency range of the THz gap. At present, THz QCLs have been demonstrated to work in the frequency range 1.2–4.9 THz. Power levels of a few milliwatts are typical, and large area devices have recently demonstrated even watt-level output powers [15]. However, in spite of a decade of effort, THz QCLs are still limited to cryogenic operation. The highest reported temperatures in pulsed mode operations is 200 K [16], while in continuous-wave operation the upper bound is 129 K [17]. These performances have been achieved using the so-called resonant-phonon active region design, ([18] and refs. therein) schematically shown in fig. 2. Fig. 2 Terahertz quantum cascade laser. Scheme of the so-called resonant-phonon active region showing two modules of the calculated conduction band diagram (black line) obtained by the alternate growth of GaAs quantum wells and AlGaAs quantum barriers by molecular beam epitaxy. The colored lines show the square moduli of the wave functions obtained solving the Schrödinger equation in the envelope function approximation. The applied electric field correspond to the band alignment for lasing. Electrons are injected by tunneling into level 2. Laser action (wavy arrow) occurs between levels 2 and 1. Efficient depletion of level 1 is obtained by designing the energy difference between level 1 and the injector states (inj) equal to an LO-phonon energy (~ 36 meV). Reproduced with permission from ref. [18], ©2007, Nature Publishing group. vol31 / no3-4 / anno2015 > 45 fisica e… Looking for an explanation and possibly finding a correction for the poorer thermal performance of THz QCLs with respect to mid-IR ones, has been the subject of extensive investigation. On one side, THz QCLs are qualitatively different from the mid-IR ones for two reasons: the photon energy is smaller than the LO-phonon one; losses by free carrier scattering cannot be neglected, since they scale as λ2 and even lightly doped semiconductor layers can contribute significantly. This poses important constraints on the laser design. The calculations of active regions have to include the injection and depopulation of carriers with high precision into the closely spaced energy levels. In addition, the laser waveguides must be designed to minimize the overlap with doped regions. There is a general consensus on the major impediment to room temperature operation of THz QCLs. It is ascribed to the reduction of the optical gain at higher temperatures due to thermally activated energy relaxation paths allowing transitions between the upper and lower radiative states via optical-phonon emission [18, 19]. At low electronic temperatures this scattering path is suppressed, since ℏωTHz < ELO , (ELO = 36 meV in GaAs). However, at high temperatures electrons in the upper radiative subband gain sufficient in-plane kinetic energy to emit an LO-phonon and relax to the lower subband. This process, sketched in fig. 3, causes the upper-state lifetime to decrease exponentially with increasing electronic temperature. This in turn reduces the gain and eventually leads to laser shutoff. The most straightforward approach to improve the operating temperature is to explore novel active region designs within the GaAs/AlGaAs material system. Several designs have been demonstrated using an “indirect injection” scheme, where electrons are injected into the upper radiative state not by resonant tunneling, but by LO-phonon scattering from a higher-energy state [20]. This scheme, similar to the one successfully demonstrated also for mid-IR QCLs, [8, 21] adds flexibility to design and can in principle help to suppress sub-threshold parasitic leakage currents which plague THz QCLs. Good results have been obtained with this approach at lower frequencies and it seems likely that this scheme will be most useful for lasers emitting < 2.5 THz where parasitic currents are the most problematic. The lack of success in reaching room temperature operation using GaAs/AlGaAs-based QCLs is motivating the exploration of other III-V material systems. So far, none of these materials has matched the performance of GaAs/ AlGaAs designs. In principle, the use of III-nitride quantum wells is promising since GaN has an LO-phonon energy of 92 meV and hence thermally activated LO-phonon scattering should be dramatically suppressed at room temperature. Still, nitride quantum wells have proven to be very challenging to grow for vertical transport applications. Other more radical approaches are also actively explored. For example, implementing a cascade laser in quantum dots instead of quantum wells would allow to suppress non-radiative scattering by exploiting the so-called “phononbottleneck” mechanism to eliminate electronic states at the LO-phonon resonance energy, thereby suppressing both relaxation and dephasing scattering. So far, efforts to use selfassembled quantum dots embedded in quantum wells have not been successful at demonstrating lasing but has led to renewed interest in the “top-down” etching of semiconductor nanowires. Fig. 3 Scheme of the main process responsible for thermal quenching of lasing in THz QCLs. Calculated dispersion of the upper (2) and lower (1) laser subands. At high temperature, electrons in the subband 2 may acquire enough energy to scatter to subband 1 by non-radiative LO-phonon emission. 2 1 hot electron E ELO k 46 < il nuovo saggiatore g. scamarcio et al.: Filling the terahertz gap Because of these difficulties in achieving room temperature operation, there has been considerable effort to leverage the advances in high-power two-color mid-IR QC-lasers for intracavity THz difference frequency generation (DFG) at room-temperature. Indeed, the remarkable power performance of mid-IR QCLs has already allowed the development of room temperature, mid-IR–based THz sources relying on intra-cavity DFG [22, 23]. The highest reported output powers to date are 1.4 mW in pulsed mode and 3 μW in CW mode [23]. The challenges reside primarily in the conversion and overall wall-plug efficiency, which is ~10–5 or less, and the efficient extraction of the THz radiation, since free-carrier losses make the mid-IR laser cavity quite lossy for THz. Optical frequency combs [24] have revolutionized the fields of high-precision spectroscopy and metrology as certified by the attribution of the 2005 Nobel prize in physics to Ted Hänsch and John Hall. The extension of the comb concept to all the electromagnetic spectrum is currently involving a number of technologies, including QCLs. Mid-IR frequency combs have been already realized starting from pulsed sources by means of non-linear optics and frequency downconversion. In the last three years, a number of milestones have been achieved, starting with the first demonstration of comb operation of a broadband mid-IR QCL [25]. QCL comb operation has been recently extended also to the THz range, where the role of dispersion is much stronger than in mid-IR: the careful compensation of this effect has led to QCL combs at 3.5 THz [26], thereby opening the way to a very promising field of applications of high-resolution THz spectroscopy. 4 Terahertz detectors Conventional detectors of THz radiation are based on thermal sensing elements, that are typically very slow, e.g., Golay cells or pyroelectric elements that can be modulated up to 10–400 Hz. In addition, to keep low values of noise equivalent power they need deep cryogenic cooling at liquid He temperatures. Even those exploiting fast non-linear electronics (e.g., Schottky diodes) show a dramatic drop of their performance at frequencies larger than 1 THz. Recently, by exploiting the gate-modulation of the conductance channel induced by the incoming radiation, the possibility of using high-electron-mobility transistors, field effect transistor (FETs) and metal-oxide-semiconductor FETs as THz detectors has been demonstrated. Either single-pixel or even multi-pixel focal-plane array configurations have been realized in different material systems including Si, GaAs/ AlGaAs, InGaP/InGaAs/GaAs, and GaN/AlGaN. The recent progress in atomic-to-nanometer scale control of materials morphology, size, and composition allowed the fabrication of 2D-FETs based on graphene or 1D-FET based on semiconductor nanowires (NW), which are ideal building block for implementing detectors that could be well operated into the THz range also thanks to their typically ultra-small capacitance of the order of attofarad. A very promising scheme is based on the possibility to engineer the channel of a FET as a resonator for plasma waves, whose frequency can be properly tailored across the THz. In this way, gated regions hundreds-nm wide can support the propagation of collective density oscillations (plasma waves) at THz frequencies. Under these conditions, an ac-modulated THz wave funneled into the channel of a FET can give rise to a rectification effect and be detected as continuous (dc) source-drain voltage, which can be maximized by varying the gate bias VG . Rectification is due to the nonlinear response of the electron gas in the FET channel and is unrelated to extrinsic rectification mechanisms due to Schottky barriers at contacts or other circuital elements that respond in a non-ohmic manner. The induced plasma oscillations can propagate in the channel or be damped. In the first case, a strong resonant photo-response is achieved and channel materials having plasma damping rates lower than both the frequency of the incoming radiation and the inverse of the wave transit time in the channel are usually needed. This condition requires channel material mobility (μ) of at least several thousand cm2/Vs at frequencies > 1 THz. In the case of over-damping, the decay occurs on a distance shorter than the channel length. Eventually, a broadband THz detection is expected. In any case, in order to induce a non-zero drain-source voltage it is mandatory to have some asymmetry between source and drain. This is usually obtained using an asymmetric THz feeding configuration, which can be achieved either by using a special antenna or by an asymmetric design of S and D contacts or even engineering an anti-symmetric channel, as schematically shown in fig. 4. Using semiconductor NWs is a viable approach to develop THz plasma-wave detectors [27, 28]. This technology promises a rapid impact on spectroscopy and imaging applications. The versatility offered by the growth procedure and the possibility to lithographically “design” the required asymmetry, mandatory for the rectification of plasma waves in the transistor channel, make these devices highly appealing for the development of a compact multi-pixel detection system. As per the S-D asymmetry, it can be obtained by modifying the material morphology, composition, and doping or by properly shaping lowimpedance antennas. The latter system promises to show, even in a simple two-terminal configuration, high quantum efficiencies across the THz range. Furthermore, the inherent attofarad order capacitance of a NW and the related 10 THz cutoff frequency is ideal to push the devices at very high speeds (up to nanosecond scales). vol31 / no3-4 / anno2015 > 47 fisica e… 5 Terahertz optoacoustic trace gas sensors Recent breakthroughs in THz photonics and nanotechnology are enabling THz frequency research to be applied in a widespread range of sensing applications. Optical techniques are competitive with respect to conventional analytical ones and the THz region offers additional advantages in terms of selectivity, because gas molecules have clear absorption spectral “fingerprints”, arising from rotational quantum transitions. These spectra allow unambiguous, more efficient and accurate detection compared to the characteristic vibrational complex structures in the mid-IR. Consequently, the inherent high gas specificity together with the short response times (typically < 1 s) make THz optical sensors suitable for real-time in situ measurements. A number of THz optical systems based on quantum cascade lasers (QCLs) have been recently proposed for gas sensing or spectroscopy. In particular, for highresolution gas spectroscopy, they fully exploit the availability of distributed feedback or comb-assisted QCLs. Among trace-gas optical detection techniques, quartzenhanced photoacoustic spectroscopy (QEPAS) is very attractive since it is capable of record sensitivities using a compact and relatively low-cost acoustic detection module. Figure 5 schematically shows the principle of operation. In the photoacoustic effect, the absorption of light by molecules produces localized heating and increase of the gas pressure. Modulating the incident light intensity creates a pressure wave (sound) at the same frequency of the modulation. The resulting photoacoustic signal can be amplified by tuning the modulation frequency to one of the acoustic resonances of the system [29]. The key advantage of this technique is that no optical detector is required. In addition, QEPAS exploits the enhancement of acoustic signal by a sharply resonant high-quality factor quartz tuning fork (QTF) acting as piezoelectric acoustic transducer [5]. Efficient QEPAS sensors have been demonstrated for trace gas detection of several chemical species with a detection limit of a few parts per trillion in volume [30, 31]. The performance of different types of QEPAS sensors can be conveniently compared in terms of the normalized noise equivalent absorption (NNEA), taking into account the available optical laser power, the selected absorption line strength and the integration time. Significantly, the smallest NNEA values are expected in the THz region. In fact, THz absorption is dominated by rotational levels and their relaxation rates are up to three orders of magnitude faster than vibrational ones in the mid-IR. QEPAS signal strongly depends on the energy relaxation rates of the absorption gas species and the possibility to work with fast relaxing transition levels allows operating at low pressure, so taking advantage of the typically very high QTF Q-factors and enhancing the selectivity of the QEPAS sensor systems. In summary, QEPAS in the THz range promises unambiguous, more efficient and accurate detection compared to the mid-IR. Since the sensitivity of QEPAS technique is ultimately Fig. 4 (a) Image of InAs nanowires taken by a scanning electron microscope. The wire height is typically 1–2 µm; reproduced with permission from ref. [27], ©2012, ACS. (b) Schematics of a set-up to detect THz radiation emitted by a quantum cascade laser by a nanowire terahertz detector coupled with a patterned bow-tie antenna. (c) Antenna-coupled terahertz detector consisting in a log-periodic circular-toothed antenna patterned between the source and gate of a FET transistor. The drain is a metal line running to the bonding pad; reproduced with permission from ref. [3], ©2012, AIP Publishing LLC. 48 < il nuovo saggiatore g. scamarcio et al.: Filling the terahertz gap limited by the available optical power, QCLs represent the most promising THz light sources for QEPAS applications. Recently, by exploiting suitably designed custom QTFs and THz QCLs, we have reported the first demonstration of QEPAS sensor operating in the THz range [5, 32] using methanol as gas target and obtained a record NNEA levels as low as 2×10–10 cm–1 W/Hz1/2. In a subsequent report [33], we have clearly demonstrated the advantage of THz spectroscopy. We have selected hydrogen sulfide (H2S) which is a toxic gas present in oil, natural gas, volcanoes, hot springs, well water and food. H2S is also a threat to human health and exposure to high concentration levels will cause unconsciousness and even death. One of the most important potential applications of THz QEPAS sensors is H2S detection in natural gas. These types of measurements cannot be performed using near or mid-IR optical sensors, due to the ambiguity related to the dense absorption spectra of the buffer natural gas mixture (ethane, methane, propane and other hydrocarbons). The THz range offers a spectral region of simplified absorption features and hence leads to a potential solution for sensitive H2S detection in natural gas. The realized H2S THz QEPAS sensor has shown a value of the NNEA several times lower than the best result obtained for H2S in the mid and near-IR [33, 34]. The sensor minimum detection limit can be further improved by employing QCLs with higher emission power and with emission frequencies in resonance with stronger H2S absorption lines, potentially achieving a detectivity of a few tens of parts per billion. 6 Conclusion The terahertz range of the electromagnetic spectrum, where light-matter interaction phenomena based on charge transport and optical transitions merge, is offering exciting opportunities to explore a variety of physical effects as well as countless applications. The roadmap for the development of THz technologies include, among the others, applications in the fields of outdoor and indoor communications, security, drug detection, biometrics, food quality control, agriculture, medicine, semiconductors, air pollution. The realization of such applications demands high-power compact THz sources, more sensitive detectors, and more functional integrated THz systems. Recent breakthroughs in the fields of quantum cascade lasers, innovative nanowire-based FET detectors and optoacoustic spectroscopic systems are significantly contributing in filling the “THz gap” and bringing this so far underdeveloped region to a bright future of real life applications. Acknowledgments The described researches have been partially supported by the Italian Ministry for University and Research through the Italian national Projects Nos. PON02_00675, PON02_00576, PON 03 “SISTEMA”, and by the European Union through the MPNS COST Action “MP1204 TERA-MIR Radiation: Materials, Generation, Detection and Applications”. MSV acknowledges the program FIRB - Futuro in Ricerca 2010 RBFR10LULP “Fundamental research on Terahertz photonic devices”. Fig. 5 Pictorial view of terahertz quartz-enhanced photoacoustic sensor. 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His research interests include the development and applications of mid-infrared and terahertz quantum cascade lasers the study of optical, vibrational and transport properties of semiconductor structures at the nanoscale, the development of spectroscopic techniques for real-time monitoring of optoelectronic devices. He has been the recipient of an Award of the Italian Physical Society in 1989. 50 < il nuovo saggiatore [17] M. Wienold, B. Röben, L. Schrottke, R. Sharma, A. Tahraoui, K. Biermann and H. T. Grahn, Opt. Express, 22 (2014) 3334. [18] B.S. Williams, Nat. photon. 5 (2007) 17. [19] Y. Chassagneux, Q. J. Wang, S. P. Khanna, E. Strupiechonski, J.-R. Coudevylle, E. H. Linfield, A. G. Davies, F. Capasso, M. A. Belkin, and R. Colombelli, IEEE Trans. Terahertz Sci. Tech., 2 (2012) 83. [20] P. Patimisco, G. Scamarcio, M. V. Santacroce, V. Spagnolo, M. S. Vitiello, E. Dupont, S.R. Laframboise, S. Fathololoumi, G. S. Razavipour, Z. Wasilewski, Opti. Express, 21 (2013) 10172. [21] G. 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Spagnolo, P. Patimisco, S. Borri, G. Scamarcio, B.E. Bernacki and J. Kriesel, Opt. Lett., 37 (2012) 4461. [31] V. Spagnolo, P. Patimisco, S. Borri, G. Scamarcio, B.E. Bernacki and J. Kriesel, Appl. Phys. B, 112 (2013) 25. [32] P. Patimisco, S. Borri, A. Sampaolo, H.E. Beere, D.A. Ritchie, M.S. Vitiello, G. Scamarcio, and V. Spagnolo, Analyst, 139 (2014) 2079. [33] V. Spagnolo, P. Patimisco, R. Pennetta, A. Sampaolo, G. Scamarcio, M.S. Vitiello and F.K. Tittel, Opt. Express., 23 (2015) 7574. [34] M. Siciliani de Cumis, S. Viciani, S. Borri, P. Patimisco, A. Sampaolo, G. Scamarcio, P. De Natale, F. D’Amato and V. Spagnolo, Opt. Express, 22 (2014) 28222. Miriam Serena Vitiello Miriam Serena Vitiello received her PhD in physics from the University of Bari in 2006. Since 2010 she is a staff research scientist at the Nanoscience Institute of the National Research Council and Scuola Normale Superiore in Pisa. She published over 150 articles on high-impact international journals. Her research interest includes the development of THz QCLs, highfrequency nano-detectors, graphene and 2D materials photonics and optoelectronics, semiconductor nanowire quantum devices, THz optical waveguides, THz metrology and high-resolution spectroscopy. She has been the recipient of the Panizza Award of the Italian Physical Society in 2012. Vincenzo Spagnolo Vincenzo Spagnolo obtained his PhD in physics from the University of Bari in 1994. From 1997 to 1999, he worked as researcher of the National Institute of the Physics of Matter (INFM). Since 2004, he is assistant Professor of Physics at the Politecnico di Bari. His research activity is documented by more than 130 publications and 2 patents and include the following topics: quantum cascade lasers, spectroscopic techniques for real-time device monitoring, optoacoustic gas sensors.