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Related Concept Videos

Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)01:20

Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)

Two NMR-active nuclei bonded to a central atom can be involved in geminal or two-bond coupling. Geminal coupling is commonly seen between diastereotopic protons in chiral molecules and unsymmetrical alkenes, among others.
The central atom need not be NMR-active because its electrons are affected by the electron polarization of the spin-active atoms. However, spin information is transmitted less effectively than in one-bond coupling, and 2J values are usually weaker than 1J values. The energy of...
Double Resonance Techniques: Overview01:12

Double Resonance Techniques: Overview

Double resonance techniques in Nuclear Magnetic Resonance (NMR) spectroscopy involve the simultaneous application of two different frequencies or radiofrequency pulses to manipulate and observe two distinct nuclear spins. One important application of double resonance is spin decoupling, which selectively suppresses coupling with one type of nucleus while observing the NMR signal from another nucleus, simplifying the spectrum and enhancing resolution.
Spin decoupling is usually achieved by...
Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)01:22

Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)

Vicinal or three-bond coupling is commonly observed between protons attached to adjacent carbons. Here, nuclear spin information is primarily transferred via electron spin interactions between adjacent C‑H bond orbitals. This generally favors the antiparallel arrangement of spins, so 3J values are usually positive.
The extent of coupling depends on the C‑C bond length, the two H‑C‑C angles, any electron-withdrawing substituents, and the dihedral angle between the involved orbitals. The...
¹H NMR: Long-Range Coupling01:27

¹H NMR: Long-Range Coupling

The coupling interactions of nuclei across four or more bonds are usually weak, with J values less than 1 Hz. While these are usually not observed in spectra, the presence of multiple bonds along the coupling pathway can result in observable long-range coupling.
In alkenes, spin information is communicated via σ–π overlap, as seen in allylic (four-bond) and homoallylic (five-bond) couplings. These coupling interactions are stronger when the σ bond is parallel to the alkene π orbitals.
Interference and Diffraction02:18

Interference and Diffraction

Interference is a characteristic phenomenon exhibited by waves. When two electromagnetic waves interact with their peaks and troughs coinciding, a resulting wave with enhanced amplitude is produced. This is known as constructive interference. In this case, the two waves interacting are in phase with each other.
Phase Contrast and Differential Interference Contrast Microscopy01:26

Phase Contrast and Differential Interference Contrast Microscopy

Phase-Contrast Microscopes
In-phase-contrast microscopes, interference between light directly passing through a cell and light refracted by cellular components is used to create high-contrast, high-resolution images without staining. It is the oldest and simplest type of microscope that creates an image by altering the wavelengths of light rays passing through the specimen. Altered wavelength paths are created using an annular stop in the condenser. The annular stop produces a hollow cone of...

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Writing Bragg Gratings in Multicore Fibers
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Unidirectional complex grating assisted couplers.

Maxim Greenberg, Meir Orenstein

    Optics Express
    |June 2, 2009
    PubMed
    Summary
    This summary is machine-generated.

    We developed a new method for unidirectional grating assisted couplers using gain-loss modulation. This eliminates signal reversibility, enabling one-way light transmission between waveguides.

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    Area of Science:

    • Photonics and Waveguide Optics
    • Nonlinear Optics
    • Materials Science

    Background:

    • Grating assisted couplers are essential for integrated optics.
    • Reversibility in couplers limits their application in optical signal processing.
    • Modulating material properties offers a path to control light propagation.

    Purpose of the Study:

    • To introduce a novel concept for unidirectional and irreversible grating assisted couplers.
    • To eliminate signal reversibility in waveguide couplers.
    • To enable directional light transmission in photonic devices.

    Main Methods:

    • Utilizing a gain-loss modulated medium for periodic perturbations.
    • Applying matched modulation of both refractive index (real part) and loss/gain (imaginary part).
    • Analyzing self-coupling coefficients and proposing a feasible implementation.

    Main Results:

    • Achieved unidirectional energy transfer between coupler modes.
    • Demonstrated one-way light transmission from one waveguide to another.
    • Successfully disabled inverse light transmission, confirming irreversibility.

    Conclusions:

    • The proposed gain-loss modulation technique effectively creates unidirectional grating assisted couplers.
    • This method provides a robust solution for eliminating signal reversibility in waveguide devices.
    • The concept is implementable, offering new possibilities for optical signal routing and control.