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1. Efficient and compact thermo-optic phase shifter in silicon-rich silicon nitride

   https://doi.org/10.1364/OL.431757  

   Nejadriahi, Hani ; Pappert, Steve ; Fainman, Yeshaiahu ; Yu, Paul ( September 2021 , Optics Letters)

   The design, fabrication, and characterization of low-loss ultra-compact bends in high-index ($n=3.1$at$\mathrm{\lambda <\#comment/>}=1550\mathrm{n}\mathrm{m}$) plasma-enhanced chemical vapor deposition silicon-rich silicon nitride (SRN) were demonstrated and utilized to realize efficient, small footprint thermo-optic phase shifter. Compact bends were structured into a folded waveguide geometry to form a rectangular spiral within an area of$65\times <\#comment/>65\text{µ<\#comment/>}{\mathrm{m}}^2$, having a total active waveguide length of 1.2 mm. The device featured a phase-shifting efficiency of$8\mathrm{m}\mathrm{W}/\mathrm{\pi <\#comment/>}$and a 3 dB switching bandwidth of 15 KHz. We propose SRN as a promising thermo-optic platform that combines the properties of silicon and stoichiometric silicon nitride.

    

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2. Mode multiplexer for guided optical and acoustic waves

   https://doi.org/10.1364/OL.404188  

   Dostart, Nathan ; Popović, Miloš A. ( October 2020 , Optics Letters)

   Integrated acousto-optic (AO) devices utilize the strong overlap of acoustic and optical fields in a waveguide to facilitate efficient photon–phonon (Brillouin) interactions. For example, acoustic waves offer a lossless modulation mechanism for light. “Brillouin active” photonic platforms are currently being developed that may see optical, acoustic, and AO waveguide circuits on the same chip, where guided light and sound come together in active interaction regions. A key missing component for such a platform is a device that can multiplex modes across these two physical domains. We propose and describe a new class of optical and acoustic components, the “acoustic–optical mode multiplexer” (AOMM), a device that takes respective optical and acoustic waveguides as input ports and couples their excited guided modes into a single, joint output waveguide. We show an example suspended silicon–silicon dioxide design that combines two optical modes and a spatially separate acoustic mode into a single, co-guided output port with low insertion loss down to 0.3 dB for both optical and acoustic modes, and reflection below$-<\#comment/>20\mathrm{d}\mathrm{B}$and$-<\#comment/>11\mathrm{d}\mathrm{B}$, respectively. The AOMM may enable new, efficient integrated AO devices, such as isolators and circulators, where the acoustic wave generation and opto-acoustic interaction are separated.

    

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3. Emission of R6G dye in Fabry–Perot cavities in weak and strong coupling regimes

   https://doi.org/10.1364/JOSAB.403612  

   Faruk, Md Omar ; Jerop, Nelly ; Noginov, Mikhail A. ( October 2020 , Journal of the Optical Society of America B)

   We have studied spectra and angular distribution of emission in Fabry–Perot cavities formed by two silver mirrors separated by a layer of poly (methyl methacrylate) polymer doped with rhodamine 6G (R6G) dye in low ($20\mathrm{g}/\mathrm{l}$) and high ($200\mathrm{g}/\mathrm{l}$) concentrations. The frequency of emission radiated to a cavity mode was larger at large outcoupling angles—the “rainbow” effect. At the same time, the angle of the strongest emission was also determined by the cavity size: the larger the cavity, the larger the angle. The angular distribution of emission is commonly dominated by two symmetrical lobes (located at the intersection of the three-dimensional emission cone with a horizontal plane) pointing to the left and to the right of the normal to the sample. Despite the strong Stokes shift in R6G dye, the branch of the cavity dispersion curve obtained in the emission experiment is positioned above the one obtained in the reflection (extinction) experiment. Some dye molecules are poorly coupled to cavity modes. Their emission has very broad angular distribution with the maximum at$\mathrm{\theta <\#comment/>}=0^{\circ <\#comment/>}$. The signatures of strong cavity–exciton coupling were observed at high dye concentration ($200\mathrm{g}/\mathrm{l}$) but not at low concentration ($20\mathrm{g}/\mathrm{l}$). The evidence of the effect of strong coupling on emission is exemplified by a strong difference in the angular distribution of emission in two almost identical cavities, one with and another without strong coupling. Most importantly, we have demonstrated the possibility to control the ground state concentration, the coupling strength, and the dye emission spectra with Q-switched laser pulses.

    

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4. Efficiency enhancement of ultrathin CIGS solar cells by optimal bandgap grading. Part II: finite-difference algorithm and double-layer antireflection coatings

   https://doi.org/10.1364/AO.474920  

   Ahmad, Faiz ; Civiletti, Benjamin J. ; Monk, Peter B. ; Lakhtakia, Akhlesh ( November 2022 , Applied Optics)

   In Part I [Appl. Opt.58,6067(2019)APOPAI003-693510.1364/AO.58.006067], we used a coupled optoelectronic model to optimize a thin-film${\mathrm{C}\mathrm{u}\mathrm{I}\mathrm{n}}_{1-<\#comment/>\mathrm{\xi <\#comment/>}}{\mathrm{G}\mathrm{a}}_{\mathrm{\xi <\#comment/>}}{\mathrm{S}\mathrm{e}}_2$(CIGS) solar cell with a graded-bandgap photon-absorbing layer and a periodically corrugated backreflector. The increase in efficiency due to the periodic corrugation was found to be tiny and that, too, only for very thin CIGS layers. Also, it was predicted that linear bandgap-grading enhances the efficiency of the CIGS solar cells. However, a significant improvement in solar cell efficiency was found using a nonlinearly (sinusoidally) graded-bandgap CIGS photon-absorbing layer. The optoelectronic model comprised two submodels: optical and electrical. The electrical submodel applied the hybridizable discontinuous Galerkin (HDG) scheme directly to equations for the drift and diffusion of charge carriers. As our HDG scheme sometimes fails due to negative carrier densities arising during the solution process, we devised a new, to the best of our knowledge, computational scheme using the finite-difference method, which also reduces the overall computational cost of optimization. An unfortunate normalization error in the electrical submodel in Part I came to light. This normalization error did not change the overall conclusions reported in Part I; however, some specifics did change. The new algorithm for the electrical submodel is reported here along with updated numerical results. We re-optimized the solar cells containing a CIGS photon-absorbing layer with either (i) a homogeneous bandgap, (ii) a linearly graded bandgap, or (iii) a nonlinearly graded bandgap. Considering the meager increase in efficiency with the periodic corrugation and additional complexity in the fabrication process, we opted for a flat backreflector. The new algorithm is significantly faster than the previous algorithm. Our new results confirm efficiency enhancement of 84% (resp. 63%) when the thickness of the CIGS layer is 600 nm (resp. 2200 nm), similarly to Part I. A hundredfold concentration of sunlight can increase the efficiency by an additional 27%. Finally, the currently used 110-nm-thick layer of${\mathrm{M}\mathrm{g}\mathrm{F}}_2$performs almost as well as optimal single- and double-layer antireflection coatings.

    

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5. Giant terahertz polarization rotation in ultrathin films of aligned carbon nanotubes

   https://doi.org/10.1364/OPTICA.422826  

   Baydin, Andrey ; Komatsu, Natsumi ; Tay, Fuyang ; Ghosh, Saunab ; Makihara, Takuma ; Noe, G. Timothy ; Kono, Junichiro ( May 2021 , Optica)

   For easy manipulation of polarization states of light for applications in communications, imaging, and information processing, an efficient mechanism is desired for rotating light polarization with a minimum interaction length. Here, we report giant polarization rotations for terahertz (THz) electromagnetic waves in ultrathin ($\sim <\#comment/>45\mathrm{n}\mathrm{m}$), high-density films of aligned carbon nanotubes. We observed polarization rotations of up to$\sim <\#comment/>{20}^{\circ <\#comment/>}$and$\sim <\#comment/>{110}^{\circ <\#comment/>}$for transmitted and reflected THz pulses, respectively. The amount of polarization rotation was a sensitive function of the angle between the incident THz polarization and the nanotube alignment direction, exhibiting a “magic” angle at which the total rotation through transmission and reflection becomes exactly 90°. Our model quantitatively explains these giant rotations as a result of extremely anisotropic optical constants, demonstrating that aligned carbon nanotubes promise ultrathin, broadband, and tunable THz polarization devices.

    

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