High power surface emitting terahertz laser with hybrid second- and fourth-order Bragg gratings

Abstract

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.

Photonic Engineering Technology for the Development of Terahertz Quantum Cascade Lasers

“…Nevertheless, most of these designs output a comparable power level to their DM ridge equivalents. [135] Copyright 2018, The Authors, published by Springer Nature. As shown in Figure 9a, the magnetic field of these band-edge modes is antisymmetric around the emission aperture, and thus destructive interference around the emission apertures suppresses the radiative efficiency, leading to a higher resonator Q factor to lase but low output power.…”

“…By gradually reducing the grating period from the center toward each end of the laser ridge, the photonic band diagram is similar to the band structure of a type-II semiconductor quantum well with a potential well for photon confinement in the center. [135] The additional fourthorder gratings were introduced to tailor the radiative efficiency of antisymmetric and symmetric band-edge modes at will as shown in Figure 9h and both of the modes become radiative in this sense. Consequently, the symmetric mode will be promoted to lase due to lower total loss and narrow emission lobe can be obtained because of its in-phase outcoupling nature.…”

Highly efficient surface-emitting semiconductor lasers exploiting quasi-crystalline distributed feedback photonic patterns

“…The possibility of easily switching from singlefrequency-mode to multimode emission, while maintaining high output powers and a large slope efficiency, clearly unveils that our photonic quasi-crystal provides a robust performance enhancement for both regimes, very differently from all previously reported architectures that instead operate either with a single laser mode or a broad bandwidth [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39]43 .…”

“…This problem can be addressed, however, by devising architectures in which the resonator periodicity is deliberately perturbed by including localised defects 13 or by implementing hybrid DFB patterns, exploiting a combination of second-and fourth-order gratings that are chosen to maximise the surface-emitted power (up to 170 mW) and slope efficiency (up to 993 mW/A) for single-spatial-mode lasers 32 . The first approach has been demonstrated in dual periodicity gratings 18 , laterally corrugated wire lasers with a surface extraction grating 28 , graded photonic heterostructures (peak powers of 100 mW and slope efficiencies 230 mW/A) 33,34 , and nondeterministic disordered structures such as random THz lasers with intrinsically multimode emission both in 1D 35 and 2D configurations [36][37][38][39] (see the performances in Table 1) and a unique low spatial coherence of the emitted optical modes.…”

Double-metal waveguide terahertz difference-frequency generation quantum cascade lasers with surface grating outcouplers

“…This type of grating outcoupler can deliver a diffractionlimited single-lobe beam pattern without additional means to control the optical field structure in the laser cavity which is used in the DFB grating outcouplers, such as p phase-shift in the center of the second-order Bragg grating, 18 third-order Bragg grating, 19 dual-slit DFB grating, 22 graded photonic heterostructures, 23 plasmon-enhanced absorption of antisymmetric modes, 24 and hybrid second-and fourth-order Bragg grating. 25 However, unlike devices with DFB grating outcouplers, our devices need to have a separate structure to select the mid-IR pump frequencies. This is done by external diffraction gratings in the present work, but frequencyselection may also be achieved by implementing mid-IR DFB gratings in the device structure in addition to the THz outcoupling grating.…”