Progress in broadband QCL active regions and external cavity tuning Yuan Ren and Harvey Beere Cavendish Laboratory, University of Cambridge JJ Thomson Avenue, Cambridge, CB3 0HE

External Cavity Tuning 5.5x10 10 cm _ jpg Schematic figure from S. Kumar

External Cavity Tuning 5.5x10 10 cm -2 Operating dynamic range varies dramatically with doping Suggests doping too low in previous structure growths Operating dynamic range varies dramatically with doping Suggests doping too low in previous structure growths

External Cavity Tuning 5.5x10 10 cm -2 Operating dynamic range varies dramatically with doping Suggests doping too low in previous structure growths Operating dynamic range varies dramatically with doping Suggests doping too low in previous structure growths

External Cavity Tuning 5.5x10 10 cm -2 Operating dynamic range varies dramatically with doping Suggests doping too low in previous structure growths Operating dynamic range varies dramatically with doping Suggests doping too low in previous structure growths Pisa THz1.9 %SP laser +SiO2+ mirror MIT THz3.8 %SP laser + Si lens + LDPE + grating Cambridge THz5.0 %SP laser + Si lens + parylene C + grating

Broadband Anti Reflection Coating 5.5x10 10 cm -2 Operating dynamic range varies dramatically with doping Suggests doping too low in previous structure growths Operating dynamic range varies dramatically with doping Suggests doping too low in previous structure growths Three-dimensional photonic grating structure

Broadband Anti Reflection Coating 5.5x10 10 cm -2 Operating dynamic range varies dramatically with doping Suggests doping too low in previous structure growths Operating dynamic range varies dramatically with doping Suggests doping too low in previous structure growths Transparent indium tin oxide (ITO) thin film

Broadband THz QCLs Design 5.5x10 10 cm -2 Operating dynamic range varies dramatically with doping Suggests doping too low in previous structure growths Operating dynamic range varies dramatically with doping Suggests doping too low in previous structure growths Three Hybrid AR stacks, 2.1THz, 2.6THz and 3.0THz in Metal-Metal Waveguide Turčinková et al. Appl. Phys. Lett. 99, (2011) Heterogeneous Principle

Broadband THz QCLs Design 5.5x10 10 cm -2 Operating dynamic range varies dramatically with doping Suggests doping too low in previous structure growths Operating dynamic range varies dramatically with doping Suggests doping too low in previous structure growths 1THz broad emission from single device achieved: 2.2THz to 3.2THz (30dB range) Sequential ‘turning on’ of individual gain designs Heterogeneous Hybrid Short Bound to Continuum: EV1xxx ETH Zurich 2.08mm x 160  m ridge

Broadband THz QCLs Design 5.5x10 10 cm -2 G G Heterogeneous Hybrid Short Bound to Continuum: V771 UCAM (x30; x30; x60; x30; x30) 1.4mm x 140  m ridge 600GHz broad emission (10dB range): 2.45THz to 3.05THz 850GHz broad emission (20dB range): 2.3THz to 3.15THz 115K maximum operating temperature consistent with ETH Zurich wafer

Broadband THz QCLs Design 5.5x10 10 cm -2 V771 UCAM UCAM V771 ETH Zurich EV1xxx Same sequential ‘turning on’ of individual gain designs Less obvious spectral hole around 2.8THz (linear scale)

Broadband THz QCLs Design 5.5x10 10 cm -2 Operating dynamic range varies dramatically with doping Suggests doping too low in previous structure growths Operating dynamic range varies dramatically with doping Suggests doping too low in previous structure growths Heterogeneous Hybrid Short Bound to Continuum: EV1913 ETH Zurich – optimised doping Three Hybrid AR stacks (2.1THz, 2.6THz and 3.0THz) – modified gain profile Bandwidth extending over 1.6THz: 1.64THz to 3.3THz (40dB range) – 950GHz (20dB range) Rösch et al. Nature Photonics 9, 42 (2015)

Summary External Cavity – 150 GHz tuning range achieved – Result limited by gain bandwidth of QCL – First batch of matched field designs grown and characterised Heterogeneous Active Regions – Tri-stack and Bi-Stack Hybrid 4QW with optimised doping tested – Tri-stack with optimised gain profile grown (to be tested) Broadband Anti-Reflection Coating – Different approaches are under development

Broadband QCLs: Hybrid 4QW 5.5x10 10 cm -2 Operating dynamic range varies dramatically with doping Suggests doping too low in previous structure growths Operating dynamic range varies dramatically with doping Suggests doping too low in previous structure growths Higher threshold currents but also higher operating current density range Maximum temperature of operation similar (130K compared to 150K) Initial optimal doping 3.0x10 16 cm -3

Broadband QCLs: Hybrid 5.5x10 10 cm -2 Operating dynamic range varies dramatically with doping Suggests doping too low in previous structure growths Operating dynamic range varies dramatically with doping Suggests doping too low in previous structure growths