Abstract
We report the development of on-chip optical components designed to improve the out-coupling of double-metal terahertz (THz) frequency quantum cascade lasers (QCLs). A visible reshaping of the optical beam is achieved, independent of the precise waveguide configuration, by direct incorporation of cyclic-olefin copolymer (COC) dielectric optical fibers onto the QCL facet. A major improvement is further achieved by incorporating a micromachined feed-horn waveguide, assembled around the THz QCL and integrated with a slit-coupler. In its first implementation, we obtain a ± 20° beam divergence, offering the potential for high-efficiency radiation coupling from a metal-metal waveguide into optical fibers.
Highlights
Whilst the visible, infrared and microwave ranges of the electromagnetic spectrum benefit from established signal routing and processing components, the terahertz (THz) frequency range still requires a suitable framework of equivalent components
Low divergence double-metal devices have been realized over the last few years, principally by exploiting lithographic approaches based on engineering third-order distributed feedback gratings [8], plasmonic collimators [9], circular resonator approaches for vertical emission [10], photonic crystals [11] and quasi-crystals [12]
It is worth mentioning that the assembly of the waveguide coupler on the quantum cascade lasers (QCLs) is achieved by gluing it with photoresist, meaning that it is a reversible process which allows one to disassemble the coupler without damaging the QCL ridge by removing the photoresist with acetone
Summary
Whilst the visible, infrared and microwave ranges of the electromagnetic spectrum benefit from established signal routing and processing components, the terahertz (THz) frequency range still requires a suitable framework of equivalent components. One of the key issues in THz QCL technology is, the divergence of the outcoming beams: in a standard edge emitting configuration the radiation is emitted in a beam with approximately 30° divergence for single-plasmon waveguides [6], whereas an almost isotropic emission, rich in fringes, is seen for the case of double-metal [7] waveguides. This is principally due to diffraction at the subwavelength-sized laser facet, and interference between emissions from the two cleaved facets. Low divergence double-metal devices have been realized over the last few years, principally by exploiting lithographic approaches based on engineering third-order distributed feedback gratings [8], plasmonic collimators [9], circular resonator approaches for vertical emission [10], photonic crystals [11] and quasi-crystals [12]
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