Optical passive devices at THz frequencies

1. Graphene saturable absorbers

Saturable absorbers (SA) operating at terahertz (THz) frequencies can open new frontiers in the development of passively mode-locked THz micro-sources. We developed THz SAs by transfer coating and inkjet printing single and few-layer graphene films prepared by liquid phase exfoliation of graphite. Open-aperture z-scan measurements with a 3.5 THz quantum cascade laser show a transparency modulation ∼80%, almost one order of magnitude larger than that reported to date at THz frequencies.

(a) z-scan normalized transmittance of a water-based graphene saturable absorber probed with a 3.4 THz QCL (b) Transmission electron microscopy images of few-layer graphene flakes from water based inks.

1. V. Bianchi, T. Carey, L. Viti, L. Li, E.H. Linfield, A.G. Davies, A. Tredicucci, D. Yoon, P.G. Karagiannidis, L. Lombardi, F. Tomarchio, A.C. Ferrari, F. Torrisi & M.S. Vitiello, “Terahertz saturable absorbers from liquid phase exfoliation of graphite” Nat. Commun. 8, 15763 (2017).

2. Intersubband polariton based saturable absorbers

Intersubband (ISB) transitions in semiconductor multi-quantum well (MQW) structures are promising candidates for the development of saturable absorbers at terahertz (THz) frequencies. We developed polaritonic saturable absorbers at THz frequencies and exploit amplitude and phase-resolved two-dimensional (2D) THz spectroscopy on the sub-cycle time scale, to observe directly the saturation dynamics and coherent control of ISB transitions in a metal-insulator MQW structure.

(a) Schematic diagram of the THz saturable absorber structure (b) Experimental principle showing the two identical THz pulses with fields eA and eB and delayed by a time 𝜏, which prepare and interrogate the structure’s nonlinear response eAB.

1. J. Raab, C. Lange, J.L. Boland, I. Laepple, M. Furthmeier, E. Dardanis, N. Dessmann, L. Li, E.H. Linfield, G.A. Davies, M.S. Vitiello & R. Huber, “Ultrafast two-dimensional field spectroscopy of terahertz intersubband saturable absorbers”Optics Express 27, 2248 (2019).

3. THz waveguide adapters for efficient radiation out-coupling from double metal THz QCLs

We developed a novel approach to couple THz radiation from a double-metal QCL into an on-chip hollow rectangular waveguide feeding a triangular horn, with the specific aim of optimizing the optical beam divergence. The conceived novel extractor is composed of three parts: a series of slits patterned at the end of the laser top contact (slit coupler), a metallic waveguide section (feeder) assembled on top of the laser itself, and an adiabatic expansion of the feeder, forming a horn that radiates into an optical fiber or, alternatively, into free space.

(a) Device schematics: yellow areas indicate metallized surfaces, grey area correspond to GaAs. (b) Scanning electron microscope (SEM) image of the cleaved chip containing two waveguide couplers; (c) SEM image of the final assembly, taken from the cleaved facet side.

1. F. Castellano, H.L. Li, E.H. Linfield, A.G. Davies, H.E. Beere, D.A. Ritchie & M.S.Vitiello, “THz waveguide adapters for efficient radiation out-coupling from double metal THz QCLs” Optics Express, 23, 5190 (2015).