Photonic engineering of terahertz quantum cascade lasers (QCLs)

1. Continuous-wave highly-efficient low-divergence THz wire lasers

Quantum cascade lasers (QCLs) operating at THz frequencies have undergone rapid development since their first demonstration. Typically, continuous-wave (CW) operation is required to target application needs, combined with a low divergent spatial profile in the far-field, and a fine spectral control of the emitted radiation. This, however, is very difficult to achieve in practice both when single-mode emission and multimode emission are required. We recently demonstrated a novel distributed feedback wire THz QCL, in which feedback is provided by a lateral sinusoidal corrugation of the cavity, defining the emission frequency, combined with an array of holes in the top waveguide metallization, used for light extraction (a,b). This new architecture overcomes all the present technological limits in optimizing the performance of THz QCLs, and has led to the achievement of low-divergent beams (10˚) (c), single-mode emission, very high slope-efficiencies (250 mW/A), and stable CW operation.

a) Schematic diagrams of the laterally corrugated wire laser with a periodic sinusoidal corrugation. (b) Scanning electron microscope (SEM) image of a fabricated device, having average width of 40 µm. (c) Far-field emission patterns measured at 20 K whilst driving the devices in continuous wave.

1. S. Biasco, K. Garrasi, F. Castellano, L. Li, H. E Beere, D. A Ritchie, E.H Linfield, A G. Davies & M.S Vitiello, “Continuous-wave highly-efficient low-divergence terahertz wire lasers” Nat. Commun. 9, 1122 (2018).

2. Frequency tunable, continuous-wave random lasers at THz frequencies

Random lasing has long been extensively studied theoretically and experimentally reported in a number of different systems, such as optically pumped suspended microparticles in laser dye and fine powders. Quantum cascade lasers (QCLs) represent a promising platform for the integration of aperiodic photonic patterns with the aim of controlling the intra-cavity propagation of light and its extraction into the free space. We have conceived and devised random THz QCLs, exploiting a broadband active material and a double-metal waveguide (a-c), operating for the first time in continuous wave (CW) with remarkably high optical powers (b), a very collimated far field profile (c) and a rich sequence of optical modes distributed over a 500 GHz bandwidth.

(a) SEM image of a fabricated random laser. (b) Far-field emission patterns measured at 20 K whilst driving the devices in continuous wave; (c) CW Voltage-current density (V-J) and power-current density (L-J) characteristics of the random THz laser.

1. S. Biasco, H.E Beere, D.A. Ritchie, L. Li, E. H Linfield, A.G. Davies & M.S. Vitiello, Nature Light Science & Applications; in press (2019).

3. Broadband heterogeneous THz frequency QCL

We demonstrate a broadband, heterogeneous terahertz frequency quantum cascade laser by exploiting an active region design based on longitudinal optical (LO)-phonon-assisted interminiband transitions. We obtain continuous wave laser emission with a threshold current density of ~120 A/cm2, a dynamic range of ~3.1, and an emission spectrum spanning from 2.4 to 3.4 THz at 15 K.

Typical CW LIV characteristics at 15 K of a 60 mm x 2.9 mm DM waveguide heterogeneous THz QCL with 2 nm absorbers on both sides of the device ridge. Inset: emission spectrum of the device at 2.1 x Jth.

1. L.H. Li, K. Garrasi, I. Kundu, Y.J. Han, M. Salih, M.S. Vitiello, A.G. Davies & E.G. Linfield,“Broadband heterogeneous terahertz frequency quantum cascade laser”Electronics Letters 54, 1229 (2018).

4. Broadly tunable THz QCLs

Tunable oscillators are a key component of almost all electronic and photonic systems. We developed a novel approach to achieve reliable, repeatable and broad tuneability in THz quantum cascade laser emitters, by exploiting the strong coupling between two different cavity mode concepts: a distributed feedback one-dimensional photonic resonator (providing gain), and a mechanically actuated wavelength-size microcavity (providing tuning). The result is a continuously tunable, single-mode emitter covering a 162 GHz spectral range, centered on 3.2 THz with a few tens of MHz resolution and an unprecedented compact and simple architecture.

(a) Scanning electron microscope image of a dual-slit DFB QCL. The absorbing boundary is visible around the grating. (b) Schematic diagram of the external cavity arrangement. The movable mirror was milled from an aluminum block and was then laid on top of the piezoelectric actuator.

1. F. Castellano, V. Bianchi, L.H. Li, J.X. Zhu, A. Tredicucci, E.H. Linfield, A.G. Davies & M.S. Vitiello, “Tuning a microcavity-coupled terahertz laser“, Appl. Phys. Lett. 107, 261108 (2015).

5. Photonic engineering of 1D and 2D QCL resonators

We optimized the extraction efficiently and collimation of the output radiation of THz QCLs by developing single mode and multimode quasi-crystalline resonators, in which the distinction between symmetric (vertically radiative, but low quality factor, Q) and antisymmetric (non-radiative, high Q) modes is fully overcome, reaching 70 mW peak output powers with characteristic surface-emitting conical beam profiles, result of the rich quasi-crystal Fourier spectrum.

Photonic engineering solutions to control the emission frequency and the output beam direction of DFB THz QCLs independently have been also developed. Single-mode THz emission at angles tuned by design between 0° and 50° was realized, leading to an original phase-matching approach, lithographically independent, for highly collimated THz QCLs.

(a) SEM image of a prototype quasi-crystal resonator device. The lattice spatial length scale a have been lithographically designed at each vertex of a Penrose pattern and imprinted into the top Cr/Au metallization of the THz QCL (see inset). (b) Far-field measured from the device with a=21 μm; (c) Spatial dependence of the effective refractive index of a bi-periodic DFB THz QCL (d) Spatial spectra of the grating of panel (c). Two light cones (dashed lines) are drawn, corresponding to the two bandgap modes of a first order grating with spatial periodicity of 712 cm-1. The two modes are predicted to be at 3.1 THz (red, inner circle) and 3.35 THz (green, outer circle), and to radiate at different angles, as indicated by the arrows.

1. M.S. Vitiello, M. Nobile, A. Ronzani, A. Tredicucci, F. Castellano, A. Talora, L.H. Li, E.H. Linfield & A.G. Davies, “Photonic quasi-crystal terahertz lasers” Nat. Commun. 5, 5884 (2014)2. S. Biasco, L. Li, E. H. Linfield, A.G. Davies & M. S. Vitiello, “Multimode, Aperiodic Terahertz Surface-Emitting Laser Resonators” Photonics 3, 32 (2016)3. F. Castellano, S. Zanotto, L. H. Li, A. Pitanti, A. Tredicucci, E.H. Linfield, A.G. Davie & M.S. Vitiello, “Distributed feedback terahertz frequency quantum cascade lasers with dual periodicity gratings”Appl. Phys. Lett. 106, 011103 (2015).

6. Frequency and amplitude modulation of ultra-compact THz QCLs

We developed an unprecedented compact architecture to induce both frequency and amplitude self-modulation in a THz QCL. By engineering a microwave avalanche oscillator into the laser cavity, which provides a 10 GHz self-modulation of the bias current and output power, we demonstrate multimode laser emission centered around 3 THz, with distinct multiple sidebands. The resulting microwave amplitude and frequency self-modulation of THz QCLs opens up the, so far unexplored, potential for engineering integrated self-mode-locked THz lasers, with impact in fields such as nano- and ultrafast photonics and optical metrology.

1. F. Castellano, L.H. Li, E.H. Linfield, A.G. Davies & M.S. Vitiello, “Frequency and amplitude modulation of ultra-compact terahertz quantum cascade lasers using an integrated avalanche diode oscillator” Sci. Rep. 6, 23053 (2016).