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1. Lasing with multimode guided waves with exceptional degeneracy

   https://doi.org/10.1117/12.2634283  

   Mealy, Tarek; Herrero Parareda, Albert; Furman, Nathaniel; Nikzamir, Alireza; Capolino, Filippo (October 2022, Proceedings Volume PC12196, Active Photonic Platforms 2022; PC121961M (2022))

   Subramania, Ganapathi S.; Foteinopoulou, Stavroula (Ed.)

   We will discuss two kinds of exceptional points of degeneracy in waveguides and their respective application in lasers. Such exceptional points occur in waveguides with balanced loss and gain (e.g., PT symmetry), and in waveguides without loss and gain (e.g., periodic Si waveguides). Waveguides with such exceptional points have a strong degeneracy of their wavenumbers and polarization states that enables specific wave physics, only found in these degenerate systems. We will discuss advantages and disadvantages of both concepts to conceive laser regimes, related to high power, high spectral purity, high efficiency, etc, and show some realistic designs involving Si ridge waveguides. 

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2. A multi-layer platform for low-loss nonlinear silicon photonics

   https://doi.org/10.1063/1.5115234  

   MacFarlane, Neil; Kossey, Michael_R; Stroud, Jasper_R; Foster, Mark_A; Foster, Amy_C (November 2019, APL Photonics)

   We demonstrate four-wave mixing (FWM) interactions in a-Si:H waveguides in a multilayer integrated silicon photonic chip. The a-Si:H waveguides are accessed through interlayer couplers from waveguides composed of SiNx. The interlayer couplers achieve a coupling of 0.51 dB loss per transition at the target wavelength of 1550 nm. We observe greater idler power extraction and conversion efficiency from the FWM interaction in the interlayer-coupled multilayer waveguides than in single-material waveguides. 

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3. Optimized virtual optical waveguides enhance light throughput in scattering media

   https://doi.org/10.1038/s41467-023-40864-z  

   Pediredla, Adithya; Scopelliti, Matteo Giuseppe; Narasimhan, Srinivasa; Chamanzar, Maysamreza; Gkioulekas, Ioannis (December 2023, Nature Communications)

   Abstract Ultrasonically-sculpted gradient-index optical waveguides enable non-invasive light confinement inside scattering media. The confinement level strongly depends on ultrasound parameters (e.g., amplitude, frequency), and medium optical properties (e.g., extinction coefficient). We develop a physically-accurate simulator, and use it to quantify these dependencies for a radially-symmetric virtual optical waveguide. Our analysis provides insights for optimizing virtual optical waveguides for given applications. We leverage these insights to configure virtual optical waveguides that improve light confinement fourfold compared to previous configurations at five mean free paths. We show that virtual optical waveguides enhance light throughput by 50% compared to an ideal external lens, in a medium with bladder-like optical properties at one transport mean free path. We corroborate these simulation findings with real experiments: we demonstrate, for the first time, that virtual optical waveguides recycle scattered light, and enhance light throughput by 15% compared to an external lens at five transport mean free paths. 

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4. Fabrication of waveguides in flexible glass via femtosecond laser micromachining and visualization of ultrafast dynamics of the laser-glass interaction

   Agnes, J; Nagar, G; Dempsey, D; Batista, N; Sutherland, J; Shim, B (November 2021, 63rd Annual Meeting of the APS Division of Plasma Physics)

   We fabricate waveguides in Corning® flexible glass using Femtosecond Laser Micromachining (FLM) and visualize the ultrafast plasma dynamics which lead to waveguide formation via time-resolved interferometry. Due to minimal thermal effects and highly-nonlinear optical processes [1], FLM is an ideal tool to fabricate waveguides in glass with high precision and without post processing. We optimize laser fabrication of waveguides by varying scanning speed and pulse energy and, in particular, achieve waveguides with circular cross-sections using slit beam shaping [2]. Further optimization requires investigation of the underlying dynamics of how structural changes in glass are made during and after laser-glass interactions. Thus, we visualize the creation and recombination of plasma in glass which leads to the formation of waveguides using time- resolved interferometry [3]. [1] Rafael R. Gattass and Eric Mazur, Nature Photonics 2, 219–225 (2008)); [2] M. Ams et al. Opt. Express 13, 5676-5681 (2005); [3] G. C. Nagar, D. Dempsey, and B. Shim, Communications Physics 4, 96 (2021). 

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5. Overcoming the tradeoff between confinement and focal distance using virtual ultrasonic optical waveguides

   https://doi.org/10.1364/OE.404986  

   Scopelliti, Matteo_Giuseppe; Huang, Hengji; Pediredla, Adithya; Narasimhan, Srinivasa_G; Gkioulekas, Ioannis; Chamanzar, Maysamreza (November 2020, Optics Express)

   A conventional optical lens can be used to focus light into the target medium from outside, without disturbing the medium. The focused spot size is proportional to the focal distance in a conventional lens, resulting in a tradeoff between penetration depth in the target medium and spatial resolution. We have shown that virtual ultrasonically sculpted gradient-index (GRIN) optical waveguides can be formed in the target medium to steer light without disturbing the medium. Here, we demonstrate that such virtual waveguides can relay an externally focused Gaussian beam of light through the medium beyond the focal distance of a single external physical lens, to extend the penetration depth without compromising the spot size. Moreover, the spot size can be tuned by reconfiguring the virtual waveguide. We show that these virtual GRIN waveguides can be formed in transparent and turbid media, to enhance the confinement and contrast ratio of the focused beam of light at the target location. This method can be extended to realize complex optical systems of external physical lenses and in situ virtual waveguides, to extend the reach and flexibility of optical methods. 

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