symmetric and antisymmetric band-edge modes exist in distributed feedback surface-emitting semiconductor lasers, with the dominant difference being the radiation loss. Devices generally operate on the low-loss antisymmetric modes, although the power extraction efficiency is low. Here we develop graded photonic heterostructures, which localize the symmetric mode in the device centre and confine the antisymmetric modes close to the laser facet. This modal spatial separation is combined with absorbing boundaries to increase the antisymmetric mode loss, and force device operation on the symmetric mode, with elevated radiation efficiency. Application of this concept to terahertz quantum cascade lasers leads to record-high peakpower surface emission ( > 100 mW) and differential efficiencies (230 mW A − 1 ), together with low-divergence, single-lobed emission patterns, and is also applicable to continuous-wave operation. such flexible tuning of the radiation loss using graded photonic heterostructures, with only a minimal influence on threshold current, is highly desirable for optimizing secondorder distributed feedback lasers.

We demonstrate semiconductor terahertz (THz) resonators with sub-wavelength dimensions in all three dimensions of space. The maximum confinement is obtained for resonators with a diameter of 13 lm, which operate at a wavelength of %272 lm. This corresponds to a k eff /6 confinement, where k eff is the wavelength inside the material (or k/20, if the free space wavelength is considered). These highly sub-wavelength devices operate on the fundamental magnetic resonance, which corresponds to the fundamental oscillation mode of split-ring resonators and is usually inactive in purely optical resonators. In this respect, these resonators are another step towards the hybridization of optics and electronics at THz frequencies. As a proof of principle for cavity quantum electrodynamics experiments, we apply these resonators to THz intersubband polaritons.

We demonstrate efficient surface-emitting terahertz frequency quantum cascade lasers with continuous wave output powers of 20-25 mW at 15 K and maximum operating temperatures of 80-85 K. The devices employ a resonant-phonon depopulation active region design with injector, and surface emission is realized using resonators based on graded photonic heterostructures (GPHs). GPHs can be regarded as energy wells for photons and have recently been implemented through grading the period of the photonic structure. In this paper, we show that it is possible to keep the period constant and grade instead the lateral metal coverage across the GPH. This strategy ensures spectrally singlemode operation across the whole laser dynamic range and represents an additional degree of freedom in the design of confining potentials for photons. V C 2014 AIP Publishing LLC.

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Surface emitting photonic-crystal quantum cascade lasers operating at Ϸ 7.3 m are demonstrated. The photonic crystal resonator is written solely on the top metallization layer. The mismatch between the modes supported by metallized and nonmetallized regions yields enough optical feedback to achieve laser action. The devices exhibit single-mode emission with a side mode suppression ratio of Ϸ20 dB, the wavelength is lithographically tunable across a range of almost 70 cm −1 , and the radiation is emitted from the surface. The maximum operating temperature is 220 K. The divergence of the output beam, which is doughnut-shaped, is approximately 9°.

We demonstrate the generation of high order terahertz (THz) frequency sidebands (up to 3rd order) on a near infrared (NIR) optical carrier within a THz quantum cascade laser (QCL). The NIR carrier is resonant with the interband transition of the quantum wells composing the QCL, allowing the nonlinearity to be enhanced and leading to frequency mixing. A phonon depopulation based QCL with a double metal cavity was used to enhance the intracavity power density and to demonstrate the higher order sidebands. The 1st order sideband intensity shows a linear dependence with THz power corresponding to a single THz photon, while the second order sideband has a quadratic dependence implying a two THz photon interaction and hence a third order susceptibility. These measurements are compared to the photoluminescence and the QCL bandstructure to identify the states involved, with the lowest conduction band states contributing the most to the sideband intensity. We also show that the interaction for the second order sideband corresponds to an enhanced direct third order susceptibility v (3) of $7 Â 10 À16 (m/V) 2 , two orders of magnitude greater than the bulk value. V C 2013 AIP Publishing LLC. [http://dx

Photonic-crystal lasers operating on Γ-point band-edge states of a photonic structure naturally exploit the so-called "nonradiative" modes. As the surface output coupling efficiency of these modes is low, they have relatively high Q factors, which favor lasing. We propose a new 2D photonic-crystal design that is capable of reversing this mode competition and achieving lasing on the radiative modes instead. Previously, this has only been shown in 1D structures, where the central idea is to introduce anisotropy into the system, both at unit-cell and resonator scales. By applying this concept to 2D photonic-crystal patterned terahertz frequency quantum cascade lasers, surfaceemitting devices with diffraction-limited beams are demonstrated, with 17 mW peak output power. In the terahertz (THz) frequency range of the electromagnetic spectrum, several PhC design strategies have been applied to quantum cascade laser (QCL) technologies in recent years [5,6]. The goal has been to improve the shape and quality of the strongly divergent emission [5,6] and the power efficiency of the devices by using the confinement and/or the dispersion properties of periodically patterned (in 1D or 2D) metal-semiconductor-metal structures. To date, a coherent single-lobed emission with reduced divergence has been obtained in edgeemitting devices, using third-order distributed feedback (DFB) gratings [7,8], and in surface-emitting devices, using 2D PhCs [9,10] and second-order DFBs [11,12]. However, efficient power extraction and, hence, wallplug efficiency (WPE) from DFB-or PhC-patterned QCLs has been elusive. WPEs for these devices fall well below the current state-of-the-art for THz QCLs exploiting unpatterned single-plasmon waveguides, where values in the 0.5%-1% range are currently achieved at 10 K [5].The problem in achieving high WPE has a fundamental origin. 1D and 2D PhCs support two classes of modes at the high symmetry points in the band structure where PhC lasers usually operate. These modes are associated with either symmetric or antisymmetric electric field profiles on the scale of the unit cell. When located above the light line, both modes will give rise to emission but will result in constructive or destructive interference. THz frequency QCLs (and indeed all QCLs) are TM polarized, i.e., the electric field is aligned parallel to the growth (z) axis. Since surface emission originates from the in-plane components of the field in the holes/slits of 2D/1D PhCs, it is only the magnetic field component that sources the radiation [13]. As a general rule, antisymmetric modes with respect to E z are symmetric with respect to the in-plane H field in the slits/holes of the DFB/PhC and are, hence, labeled "radiative." Conversely, symmetric E z modes are antisymmetric with respect to the in-plane H field and are, hence, termed "nonradiative." In fact, radiative modes at the Γ point (k in-plane 0) naturally exhibit an emission maximum in the direction normal to the device surface. As a consequence, the following relation is always true:...

Terahertz (THz) and sub-THz frequency emitter and detector technologies are receiving increasing attention, underpinned by emerging applications in ultra-fast THz physics, frequency-combs technology and pulsed laser development in this relatively unexplored region of the electromagnetic spectrum. In particular, semiconductor-based ultrafast THz receivers are required for compact, ultrafast spectroscopy and communication systems, and to date, quantum well infrared photodetectors (QWIPs) have proved to be an excellent technology to address this given their intrinsic ps-range response However, with research focused on diffraction-limited QWIP structures (lambda/2), RC constants cannot be reduced indefinitely, and detection speeds are bound to eventually meet un upper limit. The key to an ultra-fast response with no intrinsic upper limit even at tens of GHz is an aggressive reduction in device size, below the diffraction limit. Here we demonstrate sub-wavelength (lambda/10) THz QWIP detectors based on a 3D split-ring geometry, yielding ultra-fast operation at a wavelength of around 100 {\mu}m. Each sensing meta-atom pixel features a suspended loop antenna that feeds THz radiation in the ~20 m3 active volume. Arrays of detectors as well as single-pixel detectors have been implemented with this new architecture, with the latter exhibiting ultra-low dark currents below the nA level. This extremely small resonator architecture leads to measured optical response speeds - on arrays of 300 devices - of up to ~3 GHz and an expected device operation of up to tens of GHz, based on the measured S-parameters on single devices and arrays