We report on the realization of a monolithically integrated master-oscillator power-amplifier architecture in a terahertz quantum cascade laser (THz-QCL) with a metal-metal waveguide. The master-oscillator section is a first-order distributed feedback (DFB) laser. Instead of using a thick anti-reflection coating, we exploit a diffraction grating together with an absorbing boundary in the power-amplifier section to efficiently extract the laser radiation and suppress the self-lasing in it. The devices demonstrate a stable generation and power amplification of single-mode emission. The amplification factor is about 5, and the output power is approximately twice that of the standard second-order DFB lasers fabricated from the same material. Emission beam pattern with a divergence angle of $18 Â 40 is achieved. Our work provides an avenue for the realization of singlemode THz-QCLs with high output power and good beam quality. Published by AIP Publishing.[http://dx.doi.org/10.1063/1.4969067] Terahertz quantum cascade lasers (THz-QCLs) with single-mode emission are highly desired for applications such as spectroscopy, trace gas detection, and free space communication.1-3 Intensive studies have been devoted to develop unique photonic coupling structures in order to approach a high output power and a directional beam pattern for singlemode THz-QCLs. [4][5][6][7][8][9][10][11][12][13][14][15][16][17] Related results include third-order distributed feedback (DFB) lasers, 5,6 second-order DFB THzQCLs with dual slits, 7 THz-QCLs with a graded photonic heterostructure resonator, 9-11 phase-locked arrays of secondorder DFB lasers, 12-14 etc. However, to ensure that lasing occurs on transverse and longitudinal single-mode, the dimensions of the photonic coupling structure must be limited, thus restricting the available output power and increasing the divergence of the emitted beam.Monolithically integrated master-oscillator power-amplifier (MOPA) is a promising architecture to realize single-mode emission with a high output power and a near diffractionlimited beam quality. It has been implemented in near-infrared diode lasers and also in mid-infrared quantum cascade lasers. [18][19][20][21][22] In order to suppress the self-lasing in the poweramplifier (PA) section and to fully exploit the material gain, the front facet of the PA section needs to be coated by antireflection (AR) layers. However, extending the MOPA architecture to the THz frequency range is a challenge because of the lack of suitable AR coatings.Only a few researches related to MOPA devices based on THz quantum cascade structures have been reported. 23,24 In these devices-which are based on a single plasmon waveguide-the master oscillator (MO) section is a Fabry-P erot laser and is separated from the PA section by a deep air gap. The PA section is another Fabry-P erot cavity with one facet angled or coated by a thick parylene layer. The Fabry-P erot MO section intrinsically results in multimode emission, while the deep air gap degrades the coupling efficiency o...

A terahertz master-oscillation power-amplifier quantum cascade laser (THz-MOPA-QCL) is demonstrated where a grating coupler is employed to efficiently extract the THz radiation. By maximizing the group velocity and eliminating the scattering of THz wave in the grating coupler, the residue reflectivity is reduced down to the order of 10. A buried DFB grating and a tapered preamplifier are proposed to improve the seed power and to reduce the gain saturation, respectively. The THz-MOPA-QCL exhibits single-mode emission, a single-lobed beam with a narrow divergence angle of 18° × 16°, and a pulsed output power of 136 mW at 20 K, which is 36 times that of a second-order DFB laser from the same material.