parallel fed and serial fed patch antenna arrays on a benzocyclobutene (BCB) polymer layer are integrated with a 70 μm wide, dry etched, double metal waveguide quantum cascade laser, operating at about 1.9 THz. The BCB surrounds the quantum cascade laser ridge and is planarized to fit precisely its height. The patch antenna arrays emit a linearly polarized, highly symmetric beam perpendicular to the antenna plane. The beams have a FWHM angle of 49° () and 35° (). Both measurements and simulations indicate coupling factors to a Gaussian beam of over 90%. The antenna design is strongly governed by the high thickness () and the low dielectric constant () of the BCB substrate. Because the patch array has a very low input reflectivity of to over the 1.7–2.1 THz operation band, the laser needs a partially transmitting reflector to maintain the -factor of the active medium resonator to assure lasing in the antennas operation band. By changing the dimensions of the reflector, the facet transparency can be designed in a wide range from fully transmissive to highly reflective.
© 2016 Optical Society of America
Metal–metal waveguide quantum cascade lasers (QCLs) are attractive for applications because of their low-power dissipation and good c.w. operation properties, especially at lower operation frequencies. Although these lasers typically emit high output powers up to the milliwatt range [1–3], the application of these lasers is limited because of the poor beam quality . The subwavelength size of the laser facet is responsible for the poor far field, as well as for its high reflectivity (). A lens combined with an anti-reflection coating , active leaky-wave antennas , an on-chip horn antenna [7,8], or integration to a rectangular waveguide [9,10] was used to improve the far-field and/or the power extraction. Unfortunately, those solutions often imply complex manufacturing and integration work. Narrow third-order distributed feedback lasers  and antenna-coupled photonic wire lasers  show a narrow far field and have an intrinsically single-mode emission, but the beam may not be entirely linearly polarized, and the lasers may be prone to frequency detuning due to residual gas deposition .
To overcome the high facet reflectivity, a metal–metal QCL with a linear patch antenna array was developed  based on a BCB polymer layer planarization process described in . This approach demonstrates improved power extraction by an increased threshold current, slope efficiency, and emitted power. In our new approach, we change to a vertically emitting, two-dimensional patch antenna array to improve the far field. Furthermore, we add a partially transmitting reflector to assure lasing in the antenna’s operation band. The patch antenna array is fabricated by structuring a gold layer on top of the BCB substrate which is planarized to the height of the laser ridge with . The high thickness of the BCB substrate, together with the low dielectric constant , entirely governs the design of the power distribution network because under these conditions, () the field is only loosely confined to the microstrip line. All microstrip bends, microstrip T-junctions, and other discontinuities will radiate power into the substrate and into free space. This power is lost for the actual antenna beam. Thus, it is imperative to minimize microstrip bending angles. Therefore, it is impossible to use complex power distribution and phase balance microstrip networks to feed every patch antenna with the correct power fraction and phase, as is usually done in patch array designs for instance in radar applications. On the other hand, the substrate properties enable a wide patch antenna bandwidth of 15%–20% [17,18].
Two power distribution networks which minimize radiative losses are proposed. The first is a parallel fed patch array, where all patches are directly connected to the distribution network [Figs. 1(a) and 1(b)]. The second is a serial fed patch array, where the patches are arranged in three branches [Figs. 1(c) and 1(d)], similar to the serial fed patch array structure in . In both structures, all patches are fed in phase by designing the electrical path differences between the patches to be multiples of at the design center frequency of 1.9 THz. We use elliptic instead of rectangular patches because simulations reveal a 1%–3% increase of antenna efficiency over the antenna bandwidth from 1.7 to 2.1 THz. Furthermore, elliptic patches allow smaller bending angles in the central feed of the parallel feeding network.
Simulations with CST Microwave Studio  allow us to estimate the total efficiency of the antenna to for the patch array. The input reflection of the bare antenna structure is about to over the 1.7–2.1 THz bandwidth [compare Fig. 2(d)], accounting for loss. Losses due to surface wave generation in the substrate are comparatively low () due to the low dielectric constant and due to the in-phase (even mode) excitation of the patches . The high conductivity of the gold film at cryogenic temperatures contributes only a small fraction of , whereas the lossy BCB substrate ()  dissipates of the available power. The antenna efficiency of the array is higher (), mainly due to the shorter path lengths in the structure.
Figures 2(a) and 2(b) show the far-field properties of both antenna structures. As expected, the array has a broader beam lobe. For the array, the E-field distribution has been extracted from the simulation, as shown in Fig. 2(c). The main lobe shows polarization along the x-axis. The far-field simulation yields E-field vectors in spherical coordinates with full phase information. This vector field is fitted to a Gaussian field distribution. For the array, the FWHM angle of the Gaussian beam is 35°, corresponding to a beam waist radius of . is in the order of the overall antenna dimensions [compare Fig. 1(d); blue, dashed circle].
The maximum power coupling to a Gaussian beam (Gaussicity) is calculated by evaluating the normalized overlap integral of the simulated E-field data with the fitted Gaussian field distribution following . For the array, the coupling yields 87%–93% in the 1.7–2.1 THz range. For the array, the coupling is slightly higher, , due to the smaller side lobes [compare Figs. 2(a) and 2(b)].
Because the electrical lengths of the feeding network change with frequency, the beam steers with toward the QCL ridge [compare Fig. 2(b)].
Since the patch array itself has a very low input reflection of [Fig. 2(d)], we introduce a reflector between the antenna and the QCL ridge consisting of a short BCB section with length L1, followed by a section of active medium with length L2. The dimensions L1 and L2 are chosen to center the maximum reflection in the operation band of the antenna. In addition, the amount of power transmitted to the antenna can be chosen with L1 and L2 in a wide range. If very low reflection coefficients are desired, a quarter wave transformer or a taper section as published in  can be used. To equalize the reflected power over the band, it is important to choose length L3 accordingly.
We fabricated QCLs with both antenna structures and partial reflectors with transmission. In Figs. 3(a) and 3(b) the beam shape of the antenna structures is presented. A solid circle marks the FWHM of a Gaussian beam shape, fitted to the actual power measurements, while a dashed circle marks the FWHM of the calculated E-field distribution of the corresponding CST model.
To evaluate the Gaussicity for the measured beam shapes, we calculate the magnitude of the E-field by taking the square root of the measured power. The measurements with a wire grid polarizer show that the main lobe is entirely linearly polarized. Only the phase of the wavefront cannot be accessed. Since the power was measured in the far-field distance , where is the confocal distance, the assumption of a spherical phase front is justified. Thus, we can again calculate the power coupling of the main beam by evaluating the overlap integral  with the fitted Gaussian beams which yields 98% for the array and 96% for the array. The actual power coupling to a Gaussian beam is less because the beam scan does not cover the full side lobe pattern, and the power detector does not see the actual polarization and phase of the side lobes.
In conclusion, both patch array variants enable metal–metal QCLs to emit linearly polarized beams with high efficiency and over 90% coupling to a Gaussian beam. The integrated reflector allows us to optimize the output reflectivity according to the given application.
Deutsche Forschungsgemeinschaft (DFG) (CRC956).
1. R. Koehler, A. Tredicucci, F. Beltram, H. E. Beere, E. H. Linfield, A. G. Davies, D. A. Ritchie, R. C. Iotti, and F. Rossi, Nature 417, 156 (2002).
2. G. Scalari, C. Walther, M. Fischer, R. Terazzi, H. Beere, D. Ritchie, and J. Faist, Laser Photon. Rev. 3, 45 (2009). [CrossRef]
3. C. Walther, G. Scalari, M. I. Amanti, M. Beck, and J. Faist, Science 327, 1495 (2010). [CrossRef]
4. A. J. L. Adam, I. Kašalynas, J. N. Hovenier, T. O. Klaassen, and J. R. Gao, Appl. Phys. Lett. 88, 151105 (2006). [CrossRef]
5. A. Lee, Q. Qin, S. Kumar, B. Williams, Q. Hu, and J. Reno, Opt. Lett. 32, 2840 (2007). [CrossRef]
6. A. Tavallaee, B. Williams, P. Hon, T. Itoh, and Q. Chen, Appl. Phys. Lett. 99, 141115 (2011). [CrossRef]
7. M. Amanti, M. Fischer, C. Walther, G. Scalari, and J. Faist, Electron. Lett. 43, 573 (2007). [CrossRef]
8. W. Maineult, P. Gellie, A. Andronico, P. Filloux, G. Leo, C. Sirtori, S. Barbieri, E. Peytavit, T. Akalin, J. Lampin, H. Beere, and D. Ritchie, Appl. Phys. Lett. 93, 183508 (2008). [CrossRef]
9. A. Valavanis, Y. Han, N. Brewster, P. Dean, R. Dong, L. Bushnell, M. Oldfield, J. Zhu, L. Li, A. Davies, B. Ellison, and E. Linfield, Electron. Lett. 51, 919 (2015). [CrossRef]
10. M. Justen, K. Ohtani, D. Turcinkova, F. Castellano, M. Beck, U. Graf, D. Büchel, M. Schultz, and J. Faist, “Mechanically robust waveguide-integration and beam shaping of terahertz quantum cascade lasers,” IEEE Trans. Terahertz Sci. Technol. (submitted for publication).
11. M. Amanti, M. Fischer, G. Scalari, M. Beck, and J. Faist, Nat. Photonics 3, 586 (2009). [CrossRef]
12. T. Kao, X. Cai, A. Lee, J. Reno, and Q. Hu, Opt. Express 23, 17091 (2015). [CrossRef]
13. K. Ohtani, D. Turčinková, C. Bonzon, I. Benea-Chelmus, M. Beck, and J. Faist, ““High performance 4.7 THz GaAs quantum cascade lasers based on four quantum wells,” New J. Phys. (submitted for publication).
14. C. Bonzon, I. C. B. Chelmus, K. Ohtani, M. Geiser, M. Beck, and J. Faist, Appl. Phys. Lett. 104, 161102 (2014). [CrossRef]
15. H. Zhang, G. Scalari, J. Faist, L. Dunbar, and R. Houdré, Appl. Phys. Lett. 108, 093104 (2010).
16. E. Perret, N. Zerounian, S. David, and F. Aniel, Microelectron. Eng. 85, 2276 (2008). [CrossRef]
17. D. Pozar, IEEE Trans. Antennas Propag. 31, 740 (1983). [CrossRef]
18. G. Kumar and K. Ray, Broadband Microstrip Antennas (Artech House, 2003).
19. D. Palaferri, Y. Todorov, Y. Chen, J. Madeo, A. Vasanelli, L. Li, A. Davies, E. Linfield, and C. Sirtori, Appl. Phys. Lett. 106, 161102 (2015). [CrossRef]
20. CST AG, CST Microwave Studio, 2014, www.cst.com.
21. P. Goldsmith, Quasioptical Systems (IEEE, 1998).