Ying-Hui Liu

Coupled ridge waveguide distributed feedback quantum cascade laser arrays

A coupled ridge waveguide quantum cascade laser (QCL) array consisting of fifteen elements with parallel integration was presented. In-phase fundamental mode operation in each element is secured by both the index-guided nature of the ridge and delicate loss management by properly designed geometries of the ridges and interspaces. Single-lobe lateral far-field with a nearly diffraction limited beam pattern was obtained. By incorporating a one-dimensional buried distributed feedback grating, the in-phase-operating coupled ridge waveguide QCL design provides an efficient solution to obtaining high output power and stable single longitudinal mode emission. The simplicity of this structure and fabrication process makes this approach attractive to many practical applications.

Phase-locked array of quantum cascade lasers with an integrated Talbot cavity

We show a phase-locked array of three quantum cascade lasers with an integrated Talbot cavity at one side of the laser array. The coupling scheme is called diffraction coupling. By controlling the length of Talbot to be a quarter of Talbot distance (Zt/4), in-phase mode operation can be selected. The in-phase operation shows great modal stability under different injection currents, from the threshold current to the full power current. The far-field radiation pattern of the in-phase operation contains three lobes, one central maximum lobe and two side lobes. The interval between adjacent lobes is about 10.5°. The output power is about 1.5 times that of a single-ridge laser. Further studies should be taken to achieve better beam performance and reduce optical losses brought by the integrated Talbot cavity.

High efficiency, single-lobe surface-emitting DFB/DBR quantum cascade lasers.

We demonstrate a surface-emitting quantum cascade laser (QCL) based on second-order buried distributed feedback/distributed Bragg reflector (DFB/DBR) gratings for feedback and outcoupling. The grating fabricated beneath the waveguide was found to fundamentally favor lasing in symmetric mode either through analysis or experiment. Single-lobe far-field radiation pattern with full width at half maximum (FWHM) of 0.18° was obtained along the cavity-length direction. Besides, the buried DFB/DBR grating structure successfully provided an efficient vertical outcoupling mechanism with low optical losses, which manages to achieve a high surface outcouping efficiency of 46% in continuous-wave (CW) operation and 60% in pulsed operation at room temperature. Single-mode emission with a side-mode suppression ratio (SMSR) about 25 dB was continuously tunable by heat sink temperature or injection current. Our work contributes to the realization of high efficiency surface-emitting devices with high far-field beam quality that are significantly needed in many application fields.

Development of Low Power Consumption DFB Quantum Cascade Lasers

We report on the development of λ ~ 4.97 μm continuous-wave operation of substrate emitting distributed feedback quantum cascade lasers. A low threshold power consumption of 1.27 W is obtained at 20 °C, along with an ultralow threshold current density of 0.81 kA/cm² by depositing the high reflection coatings on both facets. Single-lobed far-field radiation pattern with a low divergence angle of about 0.24° × 20.34° is obtained. Reliable single-mode emission with a side-mode suppression ratio about 25 dB is observed among the working temperature range of 10 °C-30 °C.

Top Grating, Surface-Emitting DFB Quantum Cascade Lasers in Continuous-Wave Operation

We demonstrate room temperature, continuous-wave operation of top grating based surface-emitting distributed feedback quantum cascade lasers at λ ~ 4.8 μm. The second-order metal/semiconductor grating structure allows efficient surface emission for the transverse-magnetic-polarized light. The CW output power from the substrate facet reached 94 mW at 25 °C, with a low threshold current density of 1.21 kA/cm2. Single-mode emission with a side-mode suppression ratio about 30 dB is continuously tunable by the heat sink temperature or injection current. A far-field radiation pattern with a small divergence angle of 0.58° × 16.2° is obtained. Our work may contribute to the fabrication approach for substrate-emitting distributed feedback lasers based on a very simple processing.

Small divergence substrate emitting quantum cascade laser by subwavelength metallic grating.

Metallic periodic structure in subwavelength scale offers an exciting way to couple light into surface plasmons (SPs), thus manipulating the properties of near-field optics. We show that subwavelength metallic grating (SMG) defined on the substrate side of substrate emitting quantum cascade lasers enables far-field improvement in mid-infrared spectrum. The SMG is designed to tailor the interaction of SPs with single mode transverse magnetic light. The experiment results are in good agreement with the simulated model. A far-field full width at half maximum (FWHM) divergence angle of 3.9 ° in the direction perpendicular to the laser waveguide layers is obtained, improved by a factor of 8.5 compared with traditional surface emitting device.

Room temperature continuous wave quantum dot cascade laser emitting at 7.2 μm

We demonstrate a quantum cascade laser with active regions consisting of InAs quantum dots deposited on GaAs buffer layers that are embedded in InGaAs wells confined by InAlAs barriers. Continuous wave room temperature lasing at the wavelength of 7.2 μm has been demonstrated with the threshold current density as low as 1.89 kA/cm2, while in pulsed operational mode lasing at temperatures as high as 110 °C had been observed. A phenomenological theory explaining the improved performance due to weak localization of states had been formulated.

Low Power Consumption Distributed-Feedback Quantum Cascade Lasers Operating in Continuous-Wave Mode above 90°C at λ ∼ 7.2 μm*

We report on the design and fabrication of λ ~ 7.2 μm distributed feedback quantum cascade lasers for very high temperature cw operation and low electrical power consumption. The cw operation is reported above 90°C. For a 2-mm-long and 10-μm-wide laser coated with high-reflectivity on the rear facet, more than 170 mW of output power is obtained at 20°C with a threshold power consumption of 2.4 W, corresponding to 30mW with a threshold power consumption of 3.9 W at 90°C. Robust single-mode emission with a side-mode suppression ratio above 25 dB is continuously tunable by the heat sink temperature or injection current.