Waveguide and light extraction

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.

Another approach is based on the design of distributed feedback waveguides which features a grating resonant with the third order Bragg condition [5]. 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 [6], 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 by pi with respect to the neighboring 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 the standard diffraction limit for the emission divergence in the lateral direction (Figure 3) [7].

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. This effect was used to tune the resonant frequency of the guided mode by means of nitrogen gas condensation for 20 GHz [8].

The approach with the patch array antenna (see Fig. 1), integrated on BCB polymer, has been optimized for surface emission of single mode lasers at specific wavelengths, interesting for astrophysical applications. For more details, visit the section QCLs for Astrophysical Applications.

Vivaldi antenna

A major drawback of double metal waveguide THz QCLs has been their poor far-field properties. We developed a broadband antipodal Vivaldi antenna [11], fabricated with a double-sided photolithography, metallization and lift-off process on a low-loss transparent polymer, as shown in Fig. 4.(b). The final antenna structure was aligned and positioned in front of a double metal waveguide THz QCL, as illustrated in Fig. 4.(a). Once the optical mode propagating inside the waveguide reaches the end facet, it remains confined between the two antenna flares. The antenna’s exponentially curved flare profile results in an adiabatic in-plane mode expansion, producing a narrow beam measured far-field pattern (23°x19°) with octave-spanning bandwidth, see Fig. 4.(c,d). The antenna also acts as a wave retarder, rotating the polarization from vertical towards horizontal polarization. The radiation mechanism is not based on a narrow resonance and is inherently broadband (designed for frequencies between 1.5-4.5 THz). Besides the significantly improved far-field of broadband devices, a major advantage of this approach is that the antenna does not affect the laser’s emission spectrum and current-voltage characteristics, as well as frequency comb operation, typically fragile to modifications of the waveguide structure.