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Optics (physics.optics)

Tue, 16 May 2023

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1.Plasmonic photoconductive terahertz focal-plane array with pixel super-resolution

Authors:Xurong Li, Deniz Mengu, Aydogan Ozcan, Mona Jarrahi

Abstract: Imaging systems operating in the terahertz part of the electromagnetic spectrum are in great demand because of the distinct characteristics of terahertz waves in penetrating many optically-opaque materials and providing unique spectral signatures of various chemicals. However, the use of terahertz imagers in real-world applications has been limited by the slow speed, large size, high cost, and complexity of the existing imaging systems. These limitations are mainly imposed due to the lack of terahertz focal-plane arrays (THz-FPAs) that can directly provide the frequency-resolved and/or time-resolved spatial information of the imaged objects. Here, we report the first THz-FPA that can directly provide the spatial amplitude and phase distributions, along with the ultrafast temporal and spectral information of an imaged object. It consists of a two-dimensional array of ~0.3 million plasmonic photoconductive nanoantennas optimized to rapidly detect broadband terahertz radiation with a high signal-to-noise ratio. As the first proof-of-concept, we utilized the multispectral nature of the amplitude and phase data captured by these plasmonic nanoantennas to realize pixel super-resolution imaging of objects. We successfully imaged and super-resolved etched patterns in a silicon substrate and reconstructed both the shape and depth of these structures with an effective number of pixels that exceeds 1-kilo pixels. By eliminating the need for raster scanning and spatial terahertz modulation, our THz-FPA offers more than a 1000-fold increase in the imaging speed compared to the state-of-the-art. Beyond this proof-of-concept super-resolution demonstration, the unique capabilities enabled by our plasmonic photoconductive THz-FPA offer transformative advances in a broad range of applications that use hyperspectral and three-dimensional terahertz images of objects for a wide range of applications.

2.Dirac points, new photonic band gaps and effect of magnetically induced transparency in dichroic cholesteric liquid crystals with wavelength dependent magnetooptical activity parameter

Authors:A. H. Gevorgyan

Abstract: We investigated the properties of dichroic cholesteric liquid crystals (CLCs) being in external static magnetic field directed along helix axis. We have shown that in the case of the wavelength dependence of magneto-optic activity parameter, new features appear in the optics of dichroic CLCs. We have shown that in this case new Dirac points appear, moreover, at some Dirac points photonic band gaps (PBGs) appear, at others, lines of magnetically induced transparency (MIT). In this case a polarization-sensitive transmission band appears too. At certain values of the helix pitch of the CLC and of the magnitude of the external magnetic field three PBGs of different nature appear, a transmittance band, two narrow lines of MIT and one broadband MIT. This system is non-reciprocal and the nonreciprocity changes over a wide range, it is observed both for reflection and transmittance and for absorption. The soft matter nature of CLCs and their response to external influences lead to easily tunable multifunctional devices that can find a variety of applications. They can apply as tunable narrow-band or broad-band filters and mirrors, a highly tunable broad/narrow-band coherent perfect absorber, transmitter, ideal optical diode, and other devices.

3.Tunable all-optical logic gates based on nonreciprocal topologically protected edge modes

Authors:Jie Xu, Panpan He, Delong Feng, Yamei Luo, Siqiang Fan, Kangle Yong, Kosmas L. Tsakmakidis

Abstract: All-optical logic gates have been studied intensively for their potential to enable broadband, low-loss, and high-speed communication. However, poor tunability has remained a key challenge in this field. In this paper, we propose a Y-shaped structure composed of Yttrium Iron Garnet (YIG) layers that can serve as tunable all-optical logic gates, including, but not limited to, OR, AND, and NOT gates, by applying external magnetic fields to magnetize the YIG layers. Our findings demonstrate that these logic gates are based on topologically protected one-way edge modes, ensuring exceptional robustness against imperfections and nonlocal effects while maintaining extremely high precision. Furthermore, the operating band of the logic gates is shown to be tunable. In addition, we introduce a straightforward and practical method for controlling and switching the logic gates between "work", "skip", and "stop" modes. These findings have important implications for the design of high-performance and precise all-optical integrated circuits.

4.Plasmonic sensing using Babinet's principle

Authors:Joseph Arnold Riley, Michal Horák, Vlastimil Křápek, Noel Healy, Victor Pacheco-Peña

Abstract: Developing methods to sense local variations in nearby materials, such as their refractive index and thickness, is important in different fields including chemistry and biomedical applications, among others. Localized surface plasmons (LSPs) excited in plasmonic nanostructures have demonstrated to be useful in this context due to the spectral location of their associated resonances being sensitive to changes near the plasmonic structures. In this manuscript, Babinet's principle is explored by exploiting LSP resonances excited in complementary metal-dielectric cylindrical plasmonic structures (plasmonic particle-dimers and aperture-dimers in our case). Both plasmonic structures are evaluated numerically and experimentally using Electron Energy Loss Spectroscopy (EELS), providing a full physical understanding of the complementary nature of the excited LSP resonances. The studied plasmonic structures are then exploited for dielectric sensing under two configurations: when a thin dielectric film is positioned atop the plasmonic structures and when the analyte surrounds/fills the plasmonic particles/apertures. The complementary sensing performance of both proposed structures is also evaluated, showing the approximate validity of the Babinet principle with sensitivities values of up to 700 nm/RIU for thin dielectric sensing.

5.Time derivatives via interconnected waveguides

Authors:Ross Glyn MacDonald, Alex Yakovlev, Victor Pacheco-Peña

Abstract: Electromagnetic wave-based analogue computing has become an interesting computing paradigm demonstrating the potential for high-throughput, low power, and parallel operations. In this work, we propose a technique for the calculation of derivatives of temporal signals by exploiting transmission line techniques. We consider multiple interconnected waveguides (with some of them being closed-ended stubs) forming junctions. The transmission coefficient of the proposed structure is then tailored by controlling the length and number of stubs at the junction, such that the differentiation operation is applied directly onto the envelope of an incident signal sinusoidally modulated in the time domain. The physics behind the proposed structure is explained in detail and a full theoretical description of this operation is presented, demonstrating how this technique can be used to calculate higher order or even fractional temporal derivatives. We envision that these results may enable the development of further time domain wave-based analogue processors by exploiting waveguide junctions, opening new opportunities for wave-based single operators and systems.

6.Generation of an ultrahigh-repetition-rate optical half-cycle pulse train in the nested quantum wells

Authors:Mikhail Arkhipov, Anton Pakhomov, Rostislav Arkhipov, Nikolay Rosanov

Abstract: We propose a simple quantum system, namely, a nested quantum-well structure, which is able to generate a train of half-cycle pulses of a few-fs duration, when driven by a static electric field. We theoretically investigate the emission of such a structure and its dependence on the parameters of the quantum wells. It is shown that the production of a regular output pulse train with tunable properties and the pulse repetition frequencies of tens of THz is possible in certain parameter ranges. We expect the suggested structure can be used as an ultra-compact source of subcycle pulses in the optical range.

7.Experimental parameters' Uncertainty limits for z-scan and f-scan techniques

Authors:Esteban Marulanda, Edgar Rueda

Abstract: In this paper, we present an analytical study of the relationship between the statistical distribution of a physical parameter and the uncertainties in the physical quantities used to determine it through indirect measurement. We investigate two possible methods for determining the physical quantity: linear regression and inversion of the equation in the parameter. Our analysis focuses on finding the limits of "small" uncertainties to guarantee a Gaussian distribution to the indirect physical quantity. Also, we introduce the "reliability cone" concept to describe the dependence of errors on the physical parameters uncertainties. We propose a new probability distribution for significant uncertainties and define the first three moments. We apply these methods to the z-scan and f-scan techniques, presenting the most sensitive parameters for the nonlinear two-photon absorption coefficient measurement. Finally, we implement our findings on experimental data of the two-photon absorption coefficient in CdSe.

8.Single-Input Polarization-Sensitive Optical Coherence Tomography Through a Catheter

Authors:Georgia L. Jones, Qiaozhou Xiong, Xinyu Liu, Brett E. Bouma, Martin Villiger

Abstract: Intravascular polarimetry with catheter-based polarization-sensitive optical coherence tomography (PS-OCT) complements the high-resolution structural tomograms of OCT with morphological contrast available through polarimetry. Its clinical translation has been complicated by the need for modification of conventional OCT hardware to enable polarimetric measurements. Here, we present a signal processing method to reconstruct polarization properties of tissue from measurements with a single input polarization state, bypassing the need for modulation or multiplexing of input states. Our method relies on a polarization symmetry intrinsic to round-trip measurements and uses the residual spectral variation of the polarization states incident on the tissue to avoid measurement ambiguities. We demonstrate depth-resolved birefringence and optic axis orientation maps reconstructed from in-vivo data of human coronary arteries. We validate our method through comparison with conventional dual-input state measurements and find a mean cumulative retardance error of 13.2deg without observable bias. The 95% limit of agreement between depth-resolved birefringence is 2.80 x 10^(-4), which is less than the agreement between two repeat pullbacks of conventional PS-OCT (3.14 x 10^(-4)), indicating that the two methods can be used interchangeably. The hardware simplification arising from using a single input state may be decisive in realizing the potential of polarimetric measurements for assessing coronary atherosclerosis in clinical practice.

9.Controlling light propagation in multimode fibers for imaging, spectroscopy and beyond

Authors:Hui Cao, Tomáš Čižmár, Sergey Turtaev, Tomáš Tyc, Stefan Rotter

Abstract: Light transport in a highly multimode fiber exhibits complex behavior in space, time, frequency and polarization, especially in the presence of mode coupling. The newly developed techniques of spatial wavefront shaping turn out to be highly suitable to harness such enormous complexity: a spatial light modulator enables precise characterization of field propagation through a multimode fiber, and by adjusting the incident wavefront it can accurately tailor the transmitted spatial pattern, temporal profile and polarization state. This unprecedented control leads to multimode fiber applications in imaging, endoscopy, optical trapping and microfabrication. Furthermore, the output speckle pattern from a multimode fiber encodes spatial, temporal, spectral and polarization properties of the input light, allowing such information to be retrieved from spatial measurements only. This article provides an overview of recent advances and breakthroughs in controlling light propagation in multimode fibers, and discusses newly emerging applications.