Operating Characteristics of Surface-Emitting Quantum Cascade Lasers Using Photonic Crystals with Low-Symmetric Pillars

Quantum cascade lasers (QCLs) are semiconductor emitters that cover the mid-infrared (MIR) wavelength range ^[1] . Surface-emitting semiconductor lasers can be easily arrayed, and their use in sensing applications has increased dramatically in recent years. In addition, compared to the edge-emitting type, there is no cleavage process that lowers the yield, and the inspection can be performed on the wafer, surface-emitting lasers are excellent in productivity ^{[2], [3]} . QCLs emit TM-polarized light from the principle of light emission, surface-emitting lasers can be realized by using photonic crystals (PCs). Surface emission of QCL was realized with PC ^[4]–[6] . We fabricated quantum cascade lasers with PC of low-symmetric pillars as unit cell, and evaluated and analysed the oscillation characteristics. An active layer composed of InGaAs / InAlAs strained quantum wells designed for the oscillation wavelength of 4 μm was grown by molecular beam epitaxial growth, and a PC composed of InGaAs / InP was formed by buried regrowth (Fig. 1(a)). The PC is a low-symmetric pentagonal prism shape. Low-symmetry allows lower oscillation threshold for modes that allow radiation in the direction normal to the surface. Fig. 1(b) shows the characteristic of current-light output power of the surface-emitting QCL driven by a pulse current at 77K. Fig. 1(c) shows a far-field image. The light intensity is unimodal, and it can be seen that only the mode that permits radiation in the plane direction oscillates. A very narrow emission angle of less than 1°, which is characteristic of surface-emitting lasers with a large emission area, has been observed.