THz Lasers and Technology
The operation of Quantum cascade lasers (QCLs) has been demonstrated over a broad frequency range of the Terahertz spectrum, from 5.3THz to 1.2THz [1,2,3]. Most potential applications of THz QCLs require a source that exhibits an excellent spatial and spectral control of the radiated emission. To date the best performance for THz QCLs, in terms of operating temperature and frequency coverage, have been obtained with the so called double metal waveguides (Schematic in Figure 1a) . These are very similar to microwave microstrip transmission lines: the active region is sandwiched between two metal layers providing a strong confinement of the optical mode in the gain medium. However, the subwavelength cross section of the waveguide, as well as the interferences from the front and back facets, cause highly non-directional far-field beam pattern and poor slope efficiency for these structures.
A strong interest is then dedicated to the design of suitable waveguide to improve the extraction efficiency and the laser beam quality, while keeping the advantage of the double metal waveguide. So far we have investigated two main approaches: an adjunction of a horn antenna at the facet of a double metal structure , or an integrated solution using a polymer basis .
The first is based on the fabrication of mode matching devices such as horn antennas. It has shown promising results (Figure 1) but it requires challenging technological efforts and in addition do not provide any spectral control of the laser emission.
The second approach is based on the design of distributed feedback waveguides which features a grating resonant with the third order Bragg condition . We show that an improvement of the extraction efficiency results in control of the laser emission wavelength and enhanced output power. Moreover, the grating can act as an array of phased linear sources , reshaping the typical wide and patterned far-field of double metal waveguides into a narrow beam of ~10° divergence in both directions, emitted along the direction of the device length (Figure 2). Moreover, thanks to the peculiar phase relation between the sources of this array, we demonstrate good beam quality even in the case of waveguides with sub-wavelength lateral dimensions (photonic-wire lasers). Each emitter is in fact shifted of pi respect to the neighbor one, unlike the case of surface emitting grating, and it is placed at a distance of lambda/2 from them. Thanks to this approach it is possible to design a waveguide that is not limited by standard diffraction limit for the emission divergence in the lateral direction (Figure 3) .
In a sub-wavelength device the mode is leaking out from the guiding cavity and is thus sensitive to the refractive index changes of the surrounding medium. Recently, this effect was used to tune the resonant frequency of the guided mode by means of nitrogen gas condensation for 20 GHz .
Contacts: Christopher Bonzon, Dana Turcinkova
1. Kohler, R., et al. Terahertz semiconductor-heterostructure laser. Nature 417, 156–159 (2002)
2. Williams, B.S. Terahertz quantum-cascade lasers. Nature Photonics 1, 517-525 (2007)
3. Scalari, G., et al. THz and sub-THz quantum cascade lasers . Laser and Photon. Rev. 3 , 45-66 (2009)
4. Belkin, M.A., et al. Terahertz quantum cascade lasers with copper metal-metal waveguides operating up to 178K. Opt. Express 16, 3242-3248 (2008).
5. M.I. Amanti, M. Fischer, G. Scalari, M. Beck and J. Faist, “Low-divergence single-mode terahertz quantum cascade laser,” Nature Photonics 3, 586-590 (2009)
6. C.A. Balanis, Antenna Theory (Wiley – Interscience, 2005)
7. M I. Amanti, G. Scalari, F. Castellano, M. Beck, and J. Faist, "Low divergence Terahertz photonic-wire laser," Opt. Express 18, 6390-6395 (2010)
8. D. Turcinkova, M.I. Amanti, F. Castellano, M. Beck and J. Faist, submitted
9. M.I. Amanti, M. Fischer, C. Walther, G. Scalari and J. Faist"Horn antennas for terahertz quantum cascade lasers." Elect. Lett. 43, 450 (2007)
10. C. Bonzon, I. C. Benea Chelmus, K. Ohtani, M. Geiser, M. Beck and J. Faist, "Integrated patch and slot array antenna for terahertz quantum cascade lasers at 4.7 THz." Appl. Phys. Lett. 104, 161102 (2014)
THz Cosmic Ray Detection
A heterodyne spectrometer is a powerful tool for astronomical observation and atmospheric sensing in the THz region. In particular, the frequency of 4.74 THz (≈63 μm) is of a great interest for astrophysics because it provides us information of the star formation process. THz QCL is a good candidate as a local oscillator of the heterodyne receiver at this frequency. Here we are developing single mode, continuous wave, high power 4.74 THz QCLs based on GaAs/AlGaAs. Bound-to-continuum four quantum wells architecture  is used for designing subbands structure in the active region (Fig.1). We have succeeded in lasing at this frequency range and typical threshold current density is 200 A/cm2 at 10 K in continuous wave mode (Fig.2). Recently we have improved the temperature performance and obtained the maximum operation temperature of 133 K, which is among the highest operating temperature in this spectral range (Fig.3). Toward single mode operation, we are currently developing first order distributed feedback (DFB) THz QCLs using the surface plasmon waveguide (Fig. 4) and third order DFB on the double metal waveguide. This is carried out within the Collaborative Research Council 956, funded by the Deutsche Forschungsgemeinschaft (DFG). 4.74 THz (63 μm) quantum cascade laser is a collaborative research project with Köln University supported by SFB956.
Contact: Keita Otani
 Maria I Amanti. Giacomo Scalari, Romain Terazzi, Milan Fischer, Matthias Beck, Jerome Faist, Alok Rudra, Pascal Gallo, and Eli Kapon, "Bound-to-continuum terahertz quantum cascade laser with a single quantum well phonon extraction/injection stage", New Journal of Physics 11, 125022 (2009).
Material Systems for THz QCLs
Despite the enormous progress in the development of the Terahertz quantum cascade lasers (THz QCL), there are still several issues that need to be solved before the THz QCL become mature technology for the possible industrial applications. One of them is the low operating temperature that is currently limited to cryogenic temperatures. Trying to improve the temperature performances of THz QCL by changing the base heterostructure material has lead to two opposite trends. On one hand, materials with a large optical phonon (LO) energy are of a great interest because they are expected to decrease the thermally activated LO phonon scattering rate which is believed to be a factor limiting operation temperature of present THz QCLs. A group-III nitride is a good candidate because of the large optical phonon energy ( ≈ 90 meV) compared to the one of GaAs ( ≈ 36 meV). On the other hand, materials with a small effective mass are attractive because they exhibit a large optical gain coefficient, as a small effective mass leads to a large oscillator strength and a small non-radiative scattering rate. InGaAs and InAs are suited materials for this purpose because of the smaller effective mass (0.043m0 for InGaAs and 0.023m0 for InAs) compared to the one of GaAs (0.067m0). Here we are studying on these two material systems: (1) InGaAs with ternary AlInAs and quaternary AlInGaAs barriers grown on InP substrate, and (2) InAs with quaternary AlGaAsSb barriers grown on InAs substrate.
We used four quantum wells bound-to-continuum structures in the active region, which makes it possible to inject electrons into the upper laser state with the higher selectivity. Our THz QCL based on InGaAs with ternary AlInAs barrier operated at 105 K in pulsed mode with 4 mW of optical power at helium temperatures . These lasers operated also in continuous wave mode. We have recently succeeded in lasing of THz QCLs based on InGaAs with quaternary AlInGaAs barriers . We are currently optimizing active layer structures and investigating carrier scattering processes in the active region which limits current laser performance. For InAs we established molecular beam epitaxy growth technique for strain compensation. The grown layer shows a high structural quality with the lattice constant matched to the substrate. Also thin AlSb/AlAs barrier on InAs was characterized by intersubband absorption measurement, which is important for band structure calculation in the active region . We have observed a very narrow intersubband THz electroluminescence (Linewidth: 0.6 meV) from InAs single quantum well, exhibiting a potential for THz QCL material .
Contact: Keita Otani
 M. Fischer, G. Scalari, K. Celebi, M. Amanti, Ch. Walther, M. Beck, and J. Faist, “Scattering processes in terahertz InGaAs/InAlAs quantum cascade lasers”, Applied Physics Letter 97, 221114 (2010).
 K. Ohtani, M. Beck, G. Scalari, and J. Faist, “InP Terahertz quantum cascade lasers based on quaternary AlInGaAs barriers”, submitted.
 F. Castellano, K. Ohtani, L. Nevou, and J. Faist, “Characterization of thin AlSb/AlAs barriers on InAs by mid-infrared intersubband absorption measurements”, Applied Physics Letter 102, 032103 (2013).
 K. Ohtani, M. Fischer, G. Scalari, M. Beck, and J. Faist, “Terahertz intersubband electroluminescence from InAs quantum cascade light emitting structures”, Applied Physics Letter 102, 141113 (2013).