Unidirectional photonic wire laser

Photonic wire lasers are a new genre of lasers that have a transverse dimension much smaller than the wavelength. Unidirectional emission is highly desirable as most of the laser power will be in the desired direction. Owing to their small lateral dimension relative to the wavelength, however, the mode mostly propagates outside the solid core. Consequently, conventional approaches to attach a highly reflective element to the rear facet, whether a thin film or a distributed Bragg reflector, are not applicable. Herewe propose a simple and effective technique to achieve unidirectionality. Terahertz quantumcascade lasers with distributed feedback (DFB) were chosen as the platform of the photonic wire lasers. Unidirectionality is achieved with a power ratio of the forward/backward of about eight, and the power of the forward-emitting laser is increased by a factor of 1.8 compared with a reference bidirectional DFB laser. Furthermore, we achieved a wall plug power efficiency of ∼1%. Unidirectional emission from a ridge laser is highly desirable as such emission patterns will yield a nearly factor-of-two increase in the output power in the desired direction. In conventional Fabry– Perot laser devices, the unidirectional scheme can be easily implemented by the use of a high-reflectivity (HR) coating or distributed Bragg reflector attached to the rear facet. For a new genre of lasers termed photonic wire lasers1–3, which are of interests in applications such as ultrafast optical modulation4 and frequency tuning5–7, their cross-section is much smaller than the lasing wavelength8. As such, a large fraction of the lasing mode propagates outside the solid core. From the equivalence of displacement current and conduction current in the generation of electromagnetic waves, the electric field outside and along the solid core forms a distributed emitter. If the emission from this distributed emitter dominates that from the facet, then a HR coating of the rear facet will have a negligible effect on blocking the backward radiation, which results in a bidirectional emission pattern even with a HR coating. Here we demonstrate a novel scheme to achieve unidirectional emission from linear DFB lasers. By strategically placing monolithic reflectors relative to the DFB grating, we can enhance substantially the wave in the forward direction and suppress the wave in the backward direction. It is well known that the radiation pattern of a linear DFB laser is the product of the array factor (AF) and the element factor (EF). By nature, the periodicity of the DFB structure makes the AF symmetric in both forward and backward directions. Thus, the only way to achieve an asymmetric unidirectional emission is to design and implement a highly asymmetric EF. Terahertz quantum cascade lasers9 in metal–metal waveguides10 with a cross-section of about 10 μm, which is much smaller than the lasing wavelength of ∼80 μm, are chosen here as the photonic wire laser platform for their easier fabrication because of the long wavelengths. Furthermore, third-order DFB structures (instead of first-order and even-order DFBs, which are surface emitting) were chosen because of their tight (bidirectional) beam patterns11–13. In addition, although not essential for the demonstration of unidirectionality, here we also chose the structure of a third-order DFB coupled with an integrated microstrip antenna for a superior performance in the slope efficiency and a high wall-plug efficiency (WPE) of 0.57% in the single-mode operation14,15. A periodic antennacoupled third-order DFB structure, along with the computed symmetric bidirectional radiation pattern, is illustrated in Fig. 1. In a perfectly phase-matched third-order DFB structure, the periodicity is exactly λo/2, where λo is the free-space wavelength of the laser. The laser can be modelled as an array of evenly distributed radiators with a π-phase difference between adjacent radiators, which results in a tight bidirectional beam pattern13. The key conceptual development in this work is the deliberate design of an asymmetric element structure to ‘skew’ the bidirectional EF into a unidirectional one. The schematic and the computed overall emission beam patterns are shown in Fig. 2. In this scheme, we explicitly take advantage of a unique feature of photonic wire lasers, that a large portion of the mode propagates outside the solid core. As in the work in which the lasing frequency of a photonic wire laser can be tuned by accessing and manipulating the mode from outside the solid core6,7, here we place a reflector that can reflect a large fraction of the lasing mode propagating outside the solid core. Unidirectionality can be achieved by placing the reflector at an asymmetric point of the DFB grating. In this scheme, the EF is the field of a single aperture in the presence of all those passive reflectors. However, through simulations, we realized that the