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1. Quasi-monolithic heterodyne laser interferometer for inertial sensing

   https://doi.org/10.1364/OL.473476  

   Zhang, Yanqi ; Guzman, Felipe ( September 2022 , Optics Letters)

   We present a compact heterodyne laser interferometer developed for high-sensitivity displacement sensing applications. This interferometer consists of customized prisms and wave plates assembled as a quasi-monolithic unit to realize a miniaturized system. The interferometer design adopts a common-mode rejection scheme to provide a high rejection ratio to common environmental noise. Experimental tests in vacuum show a displacement sensitivity level of$11\mathrm{p}\mathrm{m}/\sqrt{\mathrm{H}\mathrm{z}}$at$100\mathrm{m}\mathrm{H}\mathrm{z}$and as low as$0.6\mathrm{p}\mathrm{m}/\sqrt{\mathrm{H}\mathrm{z}}$above$1\mathrm{p}\mathrm{m}$. The prototype unit is$20\mathrm{m}\mathrm{m}\times <\#comment/>20\mathrm{m}\mathrm{m}\times <\#comment/>10\mathrm{m}\mathrm{m}$in size and weighs$4.5\mathrm{g}$, allowing subsequent integration in compact systems.

    

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2. Phase-locked terahertz plasmonic laser array with 2  W output power in a single spectral mode

   https://doi.org/10.1364/OPTICA.390852  

   Jin, Yuan ; Reno, John_L ; Kumar, Sushil ( June 2020 , Optica)

   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.

    

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3. Electro-optic interface for ultrasensitive intracavity electric field measurements at microwave and terahertz frequencies

   https://doi.org/10.1364/OPTICA.384160  

   Benea-Chelmus, Ileana-Cristina ; Salamin, Yannick ; Settembrini, Francesca_Fabiana ; Fedoryshyn, Yuriy ; Heni, Wolfgang ; Elder, Delwin_L ; Dalton, Larry_R ; Leuthold, Juerg ; Faist, Jérôme ( May 2020 , Optica)

   Electro-optic quantum coherent interfaces map the amplitude and phase of a quantum signal directly to the phase or intensity of a probe beam. At terahertz frequencies, a fundamental challenge is not only to sense such weak signals (due to a weak coupling with a probe in the near-infrared) but also to resolve them in the time domain. Cavity confinement of both light fields can increase the interaction and achieve strong coupling. Using this approach, current realizations are limited to low microwave frequencies. Alternatively, in bulk crystals, electro-optic sampling was shown to reach quantum-level sensitivity of terahertz waves. Yet, the coupling strength was extremely weak. Here, we propose an on-chip architecture that concomitantly provides subcycle temporal resolution and an extreme sensitivity to sense terahertz intracavity fields below 20 V/m. We use guided femtosecond pulses in the near-infrared and a confinement of the terahertz wave to a volume of$V_{\mathrm{T}\mathrm{H}\mathrm{z}}\sim <\#comment/>{10}^{-<\#comment/>9}({\mathrm{\lambda <\#comment/>}}_{\mathrm{T}\mathrm{H}\mathrm{z}}/2{)}^3$in combination with ultraperformant organic molecules ($r_{33}=170\mathrm{p}\mathrm{m}/\mathrm{V}$) and accomplish a record-high single-photon electro-optic coupling rate of$g_{\mathrm{e}\mathrm{o}}=2\mathrm{\pi <\#comment/>}\times <\#comment/>0.043\mathrm{G}\mathrm{H}\mathrm{z}$, 10,000 times higher than in recent reports of sensing vacuum field fluctuations in bulk media. Via homodyne detection implemented directly on chip, the interaction results into an intensity modulation of the femtosecond pulses. The single-photon cooperativity is$C_0=1.6\times <\#comment/>{10}^{-<\#comment/>8}$, and the multiphoton cooperativity is$C=0.002$at room temperature. We show$><\#comment/>70\mathrm{d}\mathrm{B}$dynamic range in intensity at 500 ms integration under irradiation with a weak coherent terahertz field. Similar devices could be employed in future measurements of quantum states in the terahertz at the standard quantum limit, or for entanglement of subsystems on subcycle temporal scales, such as terahertz and near-infrared quantum bits.

    

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4. Efficient and highly tunable second-harmonic generation in Z-cut periodically poled lithium niobate nanowaveguides

   https://doi.org/10.1364/OL.393445  

   Chen, Jia-Yang ; Tang, Chao ; Ma, Zhao-Hui ; Li, Zhan ; Meng_Sua, Yong ; Huang, Yu-Ping ( July 2020 , Optics Letters)

   Thin-film lithium-niobate-on-insulator (LNOI) has emerged as a superior integrated-photonics platform for linear, nonlinear, and electro-optics. Here we combine quasi-phase-matching, dispersion engineering, and tight mode confinement to realize nonlinear parametric processes with both high efficiency and wide wavelength tunability. On a millimeter-long, Z-cut LNOI waveguide, we demonstrate efficient ($1900\pm <\#comment/>500\mathrm{\%<\#comment/>}{\mathrm{W}}^{-<\#comment/>1}{\mathrm{c}\mathrm{m}}^{-<\#comment/>2}$) and highly tunable ($-<\#comment/>1.71\mathrm{n}\mathrm{m}/\mathrm{K}$) second-harmonic generation from 1530 to 1583 nm by type-0 quasi-phase-matching. Our technique is applicable to optical harmonic generation, quantum light sources, frequency conversion, and many other photonic information processes across visible to mid-IR spectral bands.

    

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5. Improving carrier transport in AlGaN deep-ultraviolet light-emitting diodes using a strip-in-a-barrier structure

   https://doi.org/10.1364/AO.394149  

   Velpula, Ravi_Teja ; Jain, Barsha ; Bui, Ha_Quoc_Thang ; Shakiba, Fatemeh_Mohammadi ; Jude, Jeffrey ; Tumuna, Moses ; Nguyen, Hoang-Duy ; Lenka, Trupti_Ranjan ; Nguyen, Hieu_Pham_Trung ( June 2020 , Applied Optics)

   This paper reports the illustration of electron blocking layer (EBL)-free AlGaN light-emitting diodes (LEDs) operating in the deep-ultraviolet (DUV) wavelength at$\sim <\#comment/>270\mathrm{n}\mathrm{m}$. In this work, we demonstrated that the integration of an optimized thin undoped AlGaN strip layer in the middle of the last quantum barrier (LQB) could generate enough conduction band barrier height for the effectively reduced electron overflow into the$p\text{-}\mathrm{G}\mathrm{a}\mathrm{N}$region. Moreover, the hole injection into the multi-quantum-well active region is significantly increased due to a large hole accumulation at the interface of the AlGaN strip and the LQB. As a result, the internal quantum efficiency and output power of the proposed LED structure has been enhanced tremendously compared to that of the conventional$p\text{-}\mathrm{t}\mathrm{y}\mathrm{p}\mathrm{e}$EBL-based LED structure.

    

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