A surface-emitting distributed feedback (DFB) laser with second-order gratings typically excites an antisymmetric mode that has low radiative efficiency and a double-lobed far-field beam. The radiative efficiency could be increased by using curved and chirped gratings for infrared diode lasers, plasmon-assisted mode selection for mid-infrared quantum cascade lasers (QCLs), and graded photonic structures for terahertz QCLs. Here, we demonstrate a new hybrid grating scheme that uses a superposition of second and fourth-order Bragg gratings that excite a symmetric mode with much greater radiative efficiency. The scheme is implemented for terahertz QCLs with metallic waveguides. Peak power output of 170 mW with a slope-efficiency of 993 mW A−1 is detected with robust single-mode single-lobed emission for a 3.4 THz QCL operating at 62 K. The hybrid grating scheme is arguably simpler to implement than aforementioned DFB schemes and could be used to increase power output for surface-emitting DFB lasers at any wavelength.

Plasmonic lasers suffer from low output power and divergent beams due to their subwavelength metallic cavities. We developed a phase-locking scheme for such lasers to significantly enhance their radiative efficiency and beam quality. An array of metallic microcavities is longitudinally coupled through traveling plasmon waves, which leads to radiation in a single spectral mode and a diffraction limited single-lobed beam in the surface normal direction. We implemented our scheme for terahertz plasmonic quantum-cascade lasers (QCLs) and measured peak output power in excess of 2 W for a single-mode 3.3 T H z QCL radiating in a narrow single-lobed beam, when operated at 58 K in a compact Stirling cooler. We thereby demonstrated an order of magnitude increase in power and thirty-times higher average intensity for monolithic single-mode terahertz QCLs compared to prior work. The number of photons radiated from the cavity outnumber those absorbed within its claddings and semiconductor medium, which constitutes > 50 % radiative efficiency and is significantly greater than that achieved for previous single-mode mid-infrared or terahertz QCLs.

A new tuning mechanism is demonstrated for single-mode metal-clad plasmonic lasers, in which refractive-index of the laser's surrounding medium affects the resonant-cavity mode in the same vein as refractive-index of gain medium inside the cavity. Reversible, continuous, and mode-hop-free tuning of ∼ 57 GHz is realized for single-mode narrow-beam terahertz plasmonic quantum-cascade lasers (QCLs), which is demonstrated at much more practical temperature of 78 K. The tuning is based on post-process deposition/etching of a dielectric (Silicon-dioxide) on a QCL chip that has already been soldered and wire-bonded onto a copper mount. This is a considerably larger tuning range compared to previously reported results for terahertz QCLs with directional far-field radiation patterns. The key enabling mechanism for tuning is a recently developed antenna-feedback scheme for plasmonic lasers, which leads to generation of hybrid surface-plasmon-polaritons propagating outside the cavity of the laser with a large spatial extent. The effect of dielectric deposition on QCL's characteristics is investigated in detail including that on maximum operating temperature, peak output power and far-field radiation patterns. Single-lobed beam with low divergence (< 7 • ) is maintained through the tuning range. The antenna-feedback scheme is ideally suited for modulation of plasmonic lasers and their sensing applications due to the sensitive dependence of spectral and radiative properties of the laser on its surrounding medium.

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Bonnet polishing has been widely used in structured surface generation, form correction and surface finishing. The inflated flexible tool is able to match the local surface to realize surface finishing. However, after surface finishing, the polishing induced form change is still not clear, especially for complex freeform surfaces with millimeter-level spatial periods. This paper presents a simulation and experimental study on form-preserving capability of bonnet polishing. The concept of form-preserving capability is discussed firstly. Subsequently, the relative surface speed and contact condition on ripple structures are calculated and analyzed. The results demonstrate that the contact condition of bonnet tool on structured surfaces has the main influence on form-preserving capability. Then, the influence of various factors on contact pressure difference is investigated. Compared with experimental results, the linear correlation between form change and the contact pressure difference is obtained, indicating that formpreserving capability can be strengthened by reducing contact pressure difference. Under optimal conditions, the form change can be controlled less than 20 nm, which proves the superior form-preserving capability of bonnet polishing.