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Wavelength beam-combining of four terahertz (THz) distributed-feedback quantum-cascade lasers (QCLs) is demonstrated using low-cost THz components that include a lens carved out of a plastic ball and a mechanically fabricated blazed grating. Single-lobed beams from predominantly single-mode QCLs radiating peak power in the range of$50-<\#comment/>170\mathrm{m}\mathrm{W}$are overlapped in the far field at frequencies ranging from$3.31-<\#comment/>3.54\mathrm{T}\mathrm{H}\mathrm{z}$. Collinear propagation with a maximum angular deviation of${0.3}^{\circ <\#comment/>}$is realized for the four beams. The total power efficiency for the focused and beam-combined radiation is as high as$25\mathrm{\%<\#comment/>}$. This result could pave the way for future commercialization of beam-combined monolithic THz QCL arrays for multi-spectral THz sensing and spectroscopy at standoff distances.

 

Plasmonic lasers suffer from low output power and divergent beams due to their subwavelength metallic cavities. We developed a phase-locking scheme for such lasers to significantly enhance their radiative efficiency and beam quality. An array of metallic microcavities is longitudinally coupled through traveling plasmon waves, which leads to radiation in a single spectral mode and a diffraction limited single-lobed beam in the surface normal direction. We implemented our scheme for terahertz plasmonic quantum-cascade lasers (QCLs) and measured peak output power in excess of$2\mathrm{W}$for a single-mode$3.3\mathrm{T}\mathrm{H}\mathrm{z}$QCL radiating in a narrow single-lobed beam, when operated at$58\mathrm{K}$in a compact Stirling cooler. We thereby demonstrated an order of magnitude increase in power and thirty-times higher average intensity for monolithic single-mode terahertz QCLs compared to prior work. The number of photons radiated from the cavity outnumber those absorbed within its claddings and semiconductor medium, which constitutes$><\#comment/>50\mathrm{\%<\#comment/>}$radiative efficiency and is significantly greater than that achieved for previous single-mode mid-infrared or terahertz QCLs.

 

To elucidate the secretary function of immune cells, we develop a nanoplasmonic circular interferometric biosensor based on intensity interrogation for label-free and dynamic sensing of molecular secretion. Exceptional sensitivity has been demonstrated through coupling free light and surface plasmon polariton (SPPs) waves, which generates a constructive and deconstructive interference pattern with high contrast and narrow linewidth when illuminated by white light. Alternatively, by adopting a narrow-band LED source and a CCD camera in this work, the transmission intensity of multiple sensing units is monitored simultaneously with a simple collinear optical setup. This intensity-modulated sensing platform yields a resolution of 4.1 × 10 −5 refractive index unit (RIU) with a high temporal resolution of 1 s and a miniaturized footprint as small as 9.8 × 9.8 μm 2 for a single sensing unit. By integrating the signals from multiple sensor units, the resolution of a 12 × 12 sensor array was found to reach 7.3 × 10 −6 RIU. We apply this sensor array to detect matrix metalloproteinase 9 (MMP-9) secretion from human monocytic cells, THP-1, at different time points after lipopolysaccharide (LPS) simulation and the results are in good agreement with enzyme-linked immunosorbent assay (ELISA) tests, but without the need for labeling. The spatial, temporal and mass resolutions of the sensor array are found to exceed other label-free technologies. These biomolecular arrays, incorporated in a microfluidic sensor platform, hold great potential for the study the dynamics and interplay of cell secretion signals and achieving a better understanding of single cell functions.