Predictable surface emission patterns in terahertz photonic-crystal quantum cascade lasers

Terahertz photonic integrated circuit for frequency tuning and power modulation

The quantum cascade laser is a powerful solid-state source of terahertz-frequency radiation. However, integrating multiple photonic functions into a monolithic platform in this frequency range is non-trivial due to the scaling of photonic structures for the long terahertz wavelengths and the low frequency tuning coefficients of the quantum cascade lasers. Here, we have designed a simple terahertz-frequency photonic integrated circuit by coupling a racetrack resonator with a ridge laser in the longitudinal direction to design a notch filter. The transmission properties of this filter structure are dependent on the phase matching and losses in the coupled racetrack and results in a comb of stopband frequencies. We have optimized the comb separation by carefully selecting the cavity dimensions of the racetrack resonator to suppress longitudinal modes in the ridge laser enabling single-mode emission. The emission frequencies and output power from laser are controlled through appropriate control of drive currents to the ridge and the racetrack resonator. The emission frequency is electrically tuned over ∼81 GHz exploiting Stark shift of the gain as a function of drive current at the ridge laser, coinciding with an output power variation of ∼27% of the peak power (at a heat sink temperature of 50 K). The output power from the ridge also varied by ∼30% and the frequency was tuned by a further 10 GHz when the driving conditions at the ridge laser are invariant and the current at the racetrack resonator was varied. To our best knowledge, this is the first report of a frequency engineering, tuning and power modulation of terahertz-frequency quantum cascade lasers using a photonic integrated circuit.

Two-mode surface plasmon lasing in hexagonal arrays

We demonstrate surface-plasmon lasing in hexagonal metal hole arrays with a semiconductor gain medium. The device can be tuned between two laser modes, with distinct wavelengths, spatial distributions, and polarization patterns, by changing the size of the optically pumped area. One of the modes exhibits a six-fold polarization pattern, while the mode observed for larger pump spots has a rotationally symmetric polarization pattern. We explain the mode tuning by the differences of in-plane and radiative out-of-plane losses of the modes. The spatial and polarization properties of the modes are conveniently described by a sum of vectorial orbital angular momentum beams with orbital, spin, and total angular momentum j=ℓ+s.

Low divergence single-mode surface-emitting concentric-circular-grating terahertz quantum cascade lasers

We report the design, fabrication and experimental characterization of surface-emitting terahertz (THz) frequency quantum cascade lasers (QCLs) with distributed feedback concentric-circular-gratings. Single-mode operation is achieved at 3.73 THz with a side-mode suppression ratio as high as ~30 dB. The device emits ~5 times the power of a ridge laser of similar dimensions, with little degradation in the maximum operation temperature. Two lobes are observed in the far-field emission pattern, each of which has a divergence angle as narrow as ~13.5° × 7°. We demonstrate that deformation of the device boundary, caused by anisotropic wet chemical etching is the cause of this double-lobed profile, rather than the expected ring-shaped pattern.

Optical properties of metal-dielectric-metal microcavities in the THz frequency range

We present an experimental and theoretical study of the optical properties of metal-dielectric-metal structures with patterned top metallic surfaces, in the THz frequency range. When the thickness of the dielectric slab is very small with respect to the wavelength, these structures are able to support strongly localized electromagnetic modes, concentrated in the subwavelength metal-metal regions. We provide a detailed analysis of the physical mechanisms which give rise to these photonic modes. Furthermore, our model quantitatively predicts the resonance positions and their coupling to free space photons. We demonstrate that these structures provide an efficient and controllable way to convert the energy of far field propagating waves into near field energy.

Surface-emitting mid-infrared quantum cascade lasers with high-contrast photonic crystal resonators

We have developed surface-emitting single-mode quantum cascade lasers which employ high-contrast photonic-crystal resonators. The devices operate on band-edge states of the photonic band-structure. The mode profile and polarization characteristics of the band-edge modes are calculated by three-dimensional finite-difference time-domain simulation. Experimentally, the spectral properties, the far-field patterns, and the polarization characteristics of the lasers are determined and compared with simulations. The good agreement between the simulations and the experiments confirms that the hexapolar mode at the Gamma-point band-edge gives rise to lasing. By using a novel and advanced fabrication method, deep and vertical PhC holes are fabricated with no metal redeposition on the sidewalls, which improves the laser performance with respect to the current status. The angular of the output beam is approximately 15 masculine, and the side mode suppression ratio of the single mode emission is about 25 dB. The threshold current density at 78 K and the maximum operation temperature are 7.6 kA/cm2 and 220 K, respectively. The performance is mainly limited by the loss induced by surface plasmon waveguide, which can be overcome by using an optimized dielectric waveguide structure.

Electrically switching transverse modes in high power THz quantum cascade lasers

The design and fabrication of a high power THz quantum cascade laser (QCL), with electrically controllable transverse mode is presented. The switching of the beam pattern results in dynamic beam switching using a symmetric side current injection scheme. The angular-resolved L-I curves measurements, near-field and far-field patterns and angular-resolved lasing spectra are presented. The measurement results confirm that the quasi-TM(01) transverse mode lases first and dominates the lasing operation at lower current injection, while the quasi-TM(00) mode lases at a higher threshold current density and becomes dominant at high current injection. The near-field and far-field measurements confirm that the lasing THz beam is maneuvered by 25 degrees in emission angle, when the current density changes from 1.9 kA/cm(2) to 2.3 kA/cm(2). A two-dimension (2D) current and mode calculation provides a simple model to explain the behavior of each mode under different bias conditions.

Polarized single-lobed surface emission in mid-infrared, photonic-crystal, quantum-cascade lasers

We report single-mode, surface-emitting, mid-IR, photonic-crystal (PhC), quantum-cascade lasers with linearly polarized and highly directional single-lobed emission. A metallic square-lattice photonic crystal with elliptical air holes and pi phase shift was used as the resonator. The 2D feedback coupling--necessary for the operation of the photonic-crystal resonator--is induced by the mismatch between the modes supported by metalized and nonmetalized regions and yields single-mode output with a side-mode suppression ratio >30 dB. The elliptical air holes modify the relative intensities of the TM field components (H(x) and H(y)) in the PhC plane, thus yielding linearly polarized emission. The pi phase shift allows the system to produce a single-lobed pattern in the far field with a narrow divergence angle (2.4 degrees x 1.8 degrees). The emission is perfectly orthogonal to the device surface, and the maximum operating temperature--still limited by the metallic ohmic losses--is 240 K.

Distributed feedback ring resonators for vertically emitting terahertz quantum cascade lasers

We present distributed-feedback Terahertz quantum cascade lasers operating in a double-metal ring waveguide. High power collimated emission in a single spectral mode is observed in the vertical direction. A double-slit configuration is employed to achieve both good electrical contacts and efficient power out-coupling. The optical properties of the devices are interpreted with the aid of finite element simulations.