Integration of electronics and photonics for future applications requires an efficient conversion of electrical to optical signals. The excellent electronic and photonic properties of graphene make ...it a suitable material for integrated systems with extremely wide operational bandwidth. In this paper, we analyze the novel geometry of modulator based on the rib photonic waveguide configuration with a double-layer graphene placed between a slab and ridge. The theoretical analysis of graphene-based electro-absorption modulator was performed showing that a 3 dB modulation with ~ 600 nm-long waveguide is possible resulting in energy per bit below 1 fJ/bit. The optical bandwidth of such modulators exceeds 12 THz with an operation speed ranging from 160 GHz to 850 GHz and limited only by graphene resistance. The performances of modulators were evaluated based on the figure of merit defined as the ratio between extinction ratio and insertion losses where it was found to exceed 220.
Resonant Akhmediev breathers Chowdury, Amdad; Tan, Dawn T H
Scientific reports,
05/2024, Letnik:
14, Številka:
1
Journal Article
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Modulation instability is a phenomenon in which a minor disturbance within a carrier wave gradually amplifies over time, leading to the formation of a series of compressed waves with higher ...amplitudes. In terms of frequency analysis, this process results in the generation of new frequencies on both sides of the original carrier wave frequency. We study the impact of fourth-order dispersion on this modulation instability in the context of nonlinear optics that lead to the formation of a series of pulses in the form of Akhmediev breather. The Akhmediev breather, a solution to the nonlinear Schrödinger equation, precisely elucidates how modulation instability produces a sequence of periodic pulses. We observe that when weak fourth-order dispersion is present, significant resonant radiation occurs, characterized by two modulation frequencies originating from different spectral bands. As an Akhmediev breather evolves, these modulation frequencies interact, resulting in a resonant amplification of spectral sidebands on either side of the breather. When fourth-order dispersion is of intermediate strength, the spectral bandwidth of the Akhmediev breather diminishes due to less pronounced resonant interactions, while stronger dispersion causes the merging of the two modulation frequency bands into a single band. Throughout these interactions, we witness a complex energy exchange process among the phase-matched frequency components. Moreover, we provide a precise explanation for the disappearance of the Akhmediev breather under weak fourth-order dispersion and its resurgence with stronger values. Our study demonstrates that Akhmediev breathers, under the influence of fourth-order dispersion, possess the capability to generate infinitely many intricate yet coherent patterns in the temporal domain.
Projected demands in information bandwidth have resulted in a paradigm shift from electrical to optical interconnects. Switches, modulators and wavelength converters have all been demonstrated on ...complementary metal-oxide semiconductor compatible platforms, and are important for all optical signal and information processing. Similarly, pulse compression is crucial for creating short pulses necessary for key applications in high-capacity communications, imaging and spectroscopy. In this study, we report the first demonstration of a chip-scale, nanophotonic pulse compressor on silicon, operating by nonlinear spectral broadening from self-phase modulation in a nanowire waveguide, followed by temporal compression with an integrated dispersive element. Using a low input peak power of 10 W, we achieve compression factors as high as 7 for 7 ps pulses. This compact and efficient device will enable ultrashort pulse sources to be integrated with systems level photonic circuits necessary for future optoelectronic networks.
Nonlinear graphene plasmonics Ooi, Kelvin J. A.; Tan, Dawn T. H.
Proceedings of the Royal Society. A, Mathematical, physical, and engineering sciences,
10/2017, Letnik:
473, Številka:
2206
Journal Article
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The rapid development of graphene has opened up exciting new fields in graphene plasmonics and nonlinear optics. Graphene's unique two-dimensional band structure provides extraordinary linear and ...nonlinear optical properties, which have led to extreme optical confinement in graphene plasmonics and ultrahigh nonlinear optical coefficients, respectively. The synergy between graphene's linear and nonlinear optical properties gave rise to nonlinear graphene plasmonics, which greatly augments graphene-based nonlinear device performance beyond a billion-fold. This nascent field of research will eventually find far-reaching revolutionary technological applications that require device miniaturization, low power consumption and a broad range of operating wavelengths approaching the far-infrared, such as optical computing, medical instrumentation and security applications.
Theoretical investigations of graphene-based electro-optic plasmonic modulators with a dielectric ridge were analyzed and the results were presented. The effects of different ridge materials and ...different spacer dielectric functions were analyzed, showing that a 3 dB modulation with a 65 nm-long waveguide is possible with dielectric-loaded surface plasmon polariton waveguides (DLSPPWs), resulting in an energy per bit only 0.08 fJ bit. The figure of merit, defined as the ratio between the extinction ratio and the insertion loss, was found to be about 5.2 with a low refractive index ridge and increased to over 17.3 for a high refractive index Si ridge, compared to 3.5 calculated and measured with photonic graphene-based waveguides. Additionally, it was shown that further improvement in terms of the figure of merit is possible with long-range dielectric-loaded surface plasmon polariton waveguides (LR-DLSPPWs), where it was calculated to exceed 72. For such waveguides, a 3 dB modulation was achieved with 10 μm-long waveguides with an energy per bit of 15.8 fJ bit. The wavelength dependence of the graphene conductivity and, consequently, the attenuation of the waveguide were analyzed for different chemical potentials. It was shown that gate voltage applied across the graphene layers shifts the attenuation curve to shorter wavelengths, with the 3 dB modulation bandwidth exceeding 15 THz for a 12 μm-long DLSPP waveguide.
CMOS‐compatible nonlinear optics platforms with negligible nonlinear losses and high nonlinearity are of great merit. Silicon, silicon nitride and Hydex glass have made significant headway in ...nonlinear optical signal processing, though none of these platforms possesses the highly sought after combination of high nonlinearity and negligible nonlinear losses. In this manuscript, we present a nonlinear optics platform based on silicon‐rich nitride, deposited at a low temperature of 250°C compatible with back‐end CMOS processing. The silicon‐rich nitride is designed and engineered in composition to have a bandgap of 2.05 eV, such that the two‐photon absorption edge is well below 1.55 μm. The designed and developed waveguides have a nonlinear parameter of 550 W−1/m, 500 times larger than that in silicon nitride waveguides, while at the same time not possessing two‐photon and free‐carrier losses. Using 500‐fs pulses, we generate supercontinuum exceeding 0.6 of an octave.
We present a nonlinear optics platform based on silicon rich nitride, deposited at 250°C and compatible with back‐end CMOS processing. The silicon rich nitride is engineered to have a band gap of 2.05 eV, such that the two photon absorption edge is well below 1.55 μm. Developed waveguides have a nonlinear parameter of 550W−1/m and negligible nonlinear losses. Using 500 fs pulses, we generate supercontinuum exceeding 0.6 of an octave.
Mono-layer graphene integrated with optical waveguides is studied for the purpose of maximizing E-field interaction with the graphene layer, for the generation of ultra-large nonlinear parameters. It ...is shown that the common approach used to minimize the waveguide effective modal area does not accurately predict the configuration with the maximum nonlinear parameter. Both photonic and plasmonic waveguide configurations and graphene integration techniques realizable with today's fabrication tools are studied. Importantly, nonlinear parameters exceeding 10(4) W(-1)/m, two orders of magnitude larger than that in silicon on insulator waveguides without graphene, are obtained for the quasi-TE mode in silicon waveguides incorporating mono-layer graphene in the evanescent part of the optical field. Dielectric loaded surface plasmon polariton waveguides incorporating mono-layer graphene are observed to generate nonlinear parameters as large as 10(5) W(-1)/m, three orders of magnitude larger than that in silicon on insulator waveguides without graphene. The ultra-large nonlinear parameters make such waveguides promising platforms for nonlinear integrated optics at ultra-low powers, and for previously unobserved nonlinear optical effects to be studied in a waveguide platform.
A well-established connection exists between increased gait variability and greater fall likelihood in Parkinson's disease (PD); however, a portable, validated means of quantifying gait variability ...(and testing the efficacy of any intervention) remains lacking. Furthermore, although rhythmic auditory cueing continues to receive attention as a promising gait therapy for PD, its widespread delivery remains bottlenecked. The present paper describes a smartphone-based mobile application ("SmartMOVE") to address both needs.
The accuracy of smartphone-based gait analysis (utilizing the smartphone's built-in tri-axial accelerometer and gyroscope to calculate successive step times and step lengths) was validated against two heel contact-based measurement devices: heel-mounted footswitch sensors (to capture step times) and an instrumented pressure sensor mat (to capture step lengths). 12 PD patients and 12 age-matched healthy controls walked along a 26-m path during self-paced and metronome-cued conditions, with all three devices recording simultaneously.
Four outcome measures of gait and gait variability were calculated. Mixed-factorial analysis of variance revealed several instances in which between-group differences (e.g., increased gait variability in PD patients relative to healthy controls) yielded medium-to-large effect sizes (eta-squared values), and cueing-mediated changes (e.g., decreased gait variability when PD patients walked with auditory cues) yielded small-to-medium effect sizes-while at the same time, device-related measurement error yielded small-to-negligible effect sizes.
These findings highlight specific opportunities for smartphone-based gait analysis to serve as an alternative to conventional gait analysis methods (e.g., footswitch systems or sensor-embedded walkways), particularly when those methods are cost-prohibitive, cumbersome, or inconvenient.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Silicon based nonlinear photonics has been extensively researched at telecom wavelengths in recent years. However, studies of Kerr nonlinearity in silicon at mid-infrared wavelengths still remain ...limited. Here, we report the wavelength dependency of third-order nonlinearity in the spectral range from 1.6 μm to 6 μm, as well as multi-photon absorption coefficients in the same range. The third-order nonlinear coefficient n2 was measured with a peak value of 1.65 × 10(−13) cm2/W at a wavelength of 2.1 μm followed by the decay of nonlinear refractive index n2 up to 2.6 μm. Our latest measurements extend the wavelength towards 6 μm, which show a sharp decrement of n2 beyond 2.1 μm and steadily retains above 3 μm. In addition, the analysis of three-photon absorption and four-photon absorption processes are simultaneously performed over the wavelength range from 2.3 μm to 4.4 μm. Furthermore, the effect of multi-photon absorption on nonlinear figure of merit in silicon is discussed in detail.
Silicon-rich nitride films are developed and explored using an inductively coupled plasma chemical vapor deposition system at low temperature of 250 °C with an ammonia-free gas chemistry. The ...refractive index of the developed silicon-rich nitride films can increase from 2.2 to 3.08 at 1550 nm wavelength while retaining a near-zero extinction coefficient when the amount of silane increases. Energy dispersive spectrum analysis gives the silicon to nitrogen ratio in the films. Atomic force microscopy shows a very smooth surface, with a surface roughness root-mean-square of 0.27 nm over a 3 μm × 3 μm area of the 300 nm thick film with a refractive index of 3.08. As an application example, the 300 nm thick silicon-rich nitride film is then patterned by electron beam lithography and etched using inductively coupled plasma system to form thin-film micro/nano waveguides, and the waveguide loss is characterized.
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