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.
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.
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.