A hybrid plasmonic waveguide terahertz quantum cascade laser

Degl'Innocenti, R. and Shah, Y. D. and Wallis, R. and Klimont, A. and Ren, Y. and Jessop, D. S. and Beere, H. E. and Ritchie, D. A. (2015) A hybrid plasmonic waveguide terahertz quantum cascade laser. In: Proceedings 2015 European Conference on Lasers and Electro-Optics - European Quantum Electronics Conference, CLEO/Europe-EQEC 2015. Optical Society of America (OSA), DEU. ISBN 9781467374750

Abstract

We present a terahertz quantum cascade laser emitting around 2.9 THz, based on a new hybrid plasmonic waveguide design. The resultant optical mode provides a performance commensurate with a metal-metal (M-M) waveguide, while improving the far-field pattern. The poor quality of the beam emission out of a M-M QCL, is a direct consequence of the high subwavelength modal confinement creating a large mismatch with the optical mode propagating in free space. The ultimate aim is to achieve the beam pattern as that obtained from a SP waveguide with the temperature performance of a M-M waveguide. Our approach consists in the realization of a hybrid plasmonic-dielectric waveguide based on an alternative wafer bonding technique utilizing a low loss polymer as a bonding/waveguide layer. The process is compatible with arbitrary metal thicknesses between cladding and laser active region, thus allowing a controlled mode leakage into the cladding/bonding dielectric layer. The improvement of the beam pattern emission is attributed to reduction in the subwavelength confinement. The material which at the same time provides the wafer bonding and the cladding layer, is the resin Cyclotene (BCB) 4026-46 from Dow company and it was chosen because of its mechanical strength and thermal stability and its low losses in the THz. The BCB bonding process was done on a bound to continuum emitting at 2.9 THz. First, an arbitrary thick metalllic layer (Ti/Au) is thermally evaporated on top of the active region wafer. The wafer bonding process, described in details elsewhere [1], consists of several steps of material spinning and thermal curing on top of the active region wafer and on the host substrate one. In the final step the two wafers are placed in contact by flipping one wafer on top of the other at the BCB layer interface, and then thermally curing. The final sample is thermally and mechanically stable, and can be process by using selective chemical etching processes normally used in the fabrication of M-M THz QCLs. A 1.31 mm long, 120 μm wide ridge QCL was fabricated by standard photolithography followed by chemical wet etching. The laser was then In-mounted on a copper block, wire bonded and tested on a cold-finger continuous flow Helium cryostat, while the power measurements have been carried out with a Golay cell and lock-in amplifier.