We design and fabricate large-area (1.1 mm × 1.1 mm) photonic crystal quantum cascade lasers, enabling single-mode (wavelength ∼ 8.5 µm) surface emission at room temperature, with a maximum peak power up to 176 mW. The beam divergence is < 1 • and without side-lobes. Moreover, by introducing asymmetry into the photonic crystal pillar shape, a single-lobed far-field is realized. The photonic band structure is measured with high spectral (0.72 cm −1 ) and angular (0.1 • ) resolution by using the photonic crystal quantum cascade laser itself as a detector.Quantum cascade lasers (QCLs) are semiconductor laser sources that operate in the both mid-infrared [1] and terahertz region [2]. Nowadays, QCLs are the sources of choice for many laser-based applications, e.g., trace gas spectroscopy [3], process control [4], and biological sensing [5]. Surface emitting lasers are advantageous for their beam shape and the ease of two-dimensional (2D) integration [6]. Based on intersubband transitions, the selection rule of QCL determines the light to be transverse magnetic (TM) polarized.Therefore, conventional vertical cavity surface emission design is not suitable for QCLs.In order to realize a surface-emitting QCL, novel coupling structures have been implemented, for example, photonic crystal (PhC) [7-10], second-order distributed feedback (DFB) grating [11,12], microdisk and ring resonators [13][14][15]. Compared to these other 1 arXiv:1811.08786v1 [physics.optics]

Room temperature surface emission is realized on a large area (1.5 mm × 1.5 mm) photonic crystal quantum cascade laser (PhC-QCL) driven under pulsed mode, at the wavelength around 8.75 µm. By introducing in-plane asymmetry to the pillar shape and optimizing the current injection with a grid-like window contact, the maximum peak power of the PhC-QCL is up to 5 W. The surface emitting beam has a crossing shape with 10 • divergence.

Topological charges are the winding numbers of polarization vectors around the vortex centers of far-field radiation. In this work, the topological charge of photonic crystal modes is theoretically analyzed using an envelope function approach. A group of modes is discovered with unique polarization properties dictated by their non-trivial envelope functions. Experimentally, lasing operation on such mode is demonstrated in an electrically pumped mid-infrared photonic crystal surface-emitting laser with high slope efficiency. The topological charge is directly observed from the polarization properties of single-mode laser emission.

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