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Concept: Nonlinear optics


Second Harmonic Generation (SHG) microscopy recently appeared as an efficient optical imaging technique to probe unstained collagen-rich tissues like cornea. Moreover, corneal remodeling occurs in many diseases and precise characterization requires overcoming the limitations of conventional techniques. In this work, we focus on diabetes, which affects hundreds of million people worldwide and most often leads to diabetic retinopathy, with no early diagnostic tool. This study then aims to establish the potential of SHG microscopy for in situ detection and characterization of hyperglycemia-induced abnormalities in the Descemet’s membrane, in the posterior cornea.

Concepts: Optics, Diabetes mellitus, Laser, Diabetes, Cornea, Diabetic retinopathy, Nonlinear optics, Descemet's membrane


The coupling between DNA molecules and quantum dots can result in impressive nonlinear optical properties. In this paper, we theoretically demonstrate the significant enhancement of Kerr coefficient of signal light using optical pump-probe technique when the pump-exciton detuning is zero, and the probe-exciton detuning is adjusted properly to the frequency of DNA vibration mode. The magnitude of optical Kerr coefficient can be tuned by modifying the intensity of the pump beam. It is shown clearly that this phenomenon cannot occur without the DNA-quantum dot coupling. The present research will lead us to know more about the anomalous nonlinear optical behaviors in the hybrid DNA-quantum dot systems, which may have potential applications in the fields such as DNA detection.

Concepts: Optics, Light, Refractive index, Polarization, Nonlinear optics, Kerr effect, Kerr-lens modelocking, Filament propagation


We investigate a hybrid electro-optomechanical system that allows us to realize controllable strong Kerr nonlinearities even in the weak-coupling regime. We show that when the controllable electromechanical subsystem is close to its quantum critical point, strong photon-photon interactions can be generated by adjusting the intensity (or frequency) of the microwave driving field. Nonlinear optical phenomena, such as the appearance of the photon blockade and the generation of nonclassical states (e.g., Schrödinger cat states), are demonstrated in the weak-coupling regime, making the observation of strong Kerr nonlinearities feasible with currently available optomechanical technology.

Concepts: Electron, Electromagnetism, Quantum mechanics, Optics, Light, Electromagnetic radiation, Condensed matter physics, Nonlinear optics


Time-reversal symmetry is important to optics. Optical processes can run in a forward or backward direction through time when such symmetry is preserved. In linear optics, a time-reversed process of laser emission can enable total absorption of coherent light fields inside an optical cavity of loss by time-reversing the original gain medium. Nonlinearity, however, can often destroy such symmetry in nonlinear optics, making it difficult to study time-reversal symmetry with nonlinear optical wave mixings. Here we demonstrate time-reversed wave mixings for optical second harmonic generation (SHG) and optical parametric amplification (OPA) by exploring this well-known but underappreciated symmetry in nonlinear optics. This allows us to observe the annihilation of coherent beams. Our study offers new avenues for flexible control in nonlinear optics and has potential applications in efficient wavelength conversion, all-optical computing.

Concepts: Photon, Optics, Light, Refractive index, Electromagnetic radiation, Laser, Nonlinear optics, Superposition principle


Strain engineering is widely used in material science to tune the (opto-)electronic properties of materials and enhance the performance of devices. Two-dimensional atomic crystals are a versatile playground to study the influence of strain, as they can sustain very large deformations without breaking. Various optical techniques have been employed to probe strain in two-dimensional materials, including micro-Raman and photoluminescence spectroscopy. Here we demonstrate that optical second harmonic generation constitutes an even more powerful technique, as it allows extraction of the full strain tensor with a spatial resolution below the optical diffraction limit. Our method is based on the strain-induced modification of the nonlinear susceptibility tensor due to a photoelastic effect. Using a two-point bending technique, we determine the photoelastic tensor elements of molybdenum disulfide. Once identified, these parameters allow us to spatially image the two-dimensional strain field in an inhomogeneously strained sample.

Concepts: Diffraction, Optics, Fundamental physics concepts, Laser, Materials science, Nonlinear optics, Deformation, Angular resolution


The sensitive response of the nematic graphene oxide (GO) phase to external stimuli makes this phase attractive for extending the applicability of GO and reduced GO to solution processes and electro-optic devices. However, contrary to expectations, the alignment of nematic GO has been difficult to control through the application of electric fields or surface treatments. Here, we show that when interflake interactions are sufficiently weak, both the degree of microscopic ordering and the direction of macroscopic alignment of GO liquid crystals (LCs) can be readily controlled by applying low electric fields. We also show that the large polarizability anisotropy of GO and Onsager excluded-volume effect cooperatively give rise to Kerr coefficients that are about three orders of magnitude larger than the maximum value obtained so far in molecular LCs. The extremely large Kerr coefficient allowed us to fabricate electro-optic devices with macroscopic electrodes, as well as well-aligned, defect-free GO over wide areas.

Concepts: Electric charge, Crystal, Fundamental physics concepts, Vector space, Phase transition, Liquid crystal, Nonlinear optics, Lars Onsager


Molecular dynamics of formamide solutions of alkali metal halide salts were investigated using the time-resolved ultrafast optical Kerr effect (OKE) to observe the effects of ion solvation on the dynamics of a nonaqueous high-permittivity H-bonding solvent. The picosecond orientational and ultrafast intermolecular dynamics of liquid formamide as a function of concentration of NaI and KI are compared with the temperature effect on the pure solvent. The effect of a range of other salts at fixed concentration is also recorded. Transient OKE and corresponding low-frequency (THz) Raman spectra of the solutions revealed differences in the solvent dynamics caused by ion solvation. Increasing concentrations of NaI and KI have the effect of slowing down the diffusive reorientation and reducing the librational frequencies of formamide, with cation-related effects observed on the THz Raman spectrum. These effects are discussed in terms of an ion perturbation of the H-bonding structure in the solution. This approach provides a valuable means of investigating the dynamics, structure, and interactions in complex, interacting systems.

Concepts: Refractive index, Concentration, Chemistry, Solution, Nonlinear optics, Kerr effect, Halide, Magneto-optic Kerr effect


Optomechanical phenomena in photonic devices provide a new means of light-light interaction mediated by optical force actuated mechanical motion. In cavity optomechanics, this interaction can be enhanced significantly to achieve strong interaction between optical signals in chip-scale systems, enabling all-optical signal processing without resorting to electro-optical conversion or nonlinear materials. However, current implementation of cavity optomechanics achieves both excitation and detection only in a narrow band at the cavity resonance. This bandwidth limitation would hinder the prospect of integrating cavity optomechanical devices in broadband photonic systems. Here we demonstrate a new configuration of cavity optomechanics that includes two separate optical channels and allows broadband readout of optomechanical effects. The optomechanical interaction achieved in this device can induce strong but controllable nonlinear effects, which can completely dominate the device’s intrinsic mechanical properties. Utilizing the device’s strong optomechanical interaction and its multichannel configuration, we further demonstrate all-optical, wavelength-multiplexed amplification of radio-frequency signals.

Concepts: Optics, Fundamental physics concepts, Particle physics, Signal processing, Nonlinear optics, Photonics, Radio frequency, Narrowband


In 1950, a quarter of a century after his first-ever nonlinear optical experiment when intensity-dependent absorption was observed in uranium-doped glass, Sergey Vavilov predicted that birefringence, dichroism and polarization rotatory power should be dependent on light intensity. It required the invention of the laser to observe the barely detectable effect of light intensity on the polarization rotatory power of the optically active lithium iodate crystal, the phenomenon now known as the nonlinear optical activity, a high-intensity counterpart of the fundamental optical effect of polarization rotation in chiral media. Here we report that a plasmonic metamaterial exhibits nonlinear optical activity 30 million times stronger than lithium iodate crystals, thus transforming this fundamental phenomenon of polarization nonlinear optics from an esoteric phenomenon into a major effect of nonlinear plasmonics with potential for practical applications.

Concepts: Optics, Light, Refractive index, Polarization, Birefringence, Nonlinear optics, Optical rotation


A heterodyne grating-based interferometer for three-degree-of-freedom (3-DOF) displacement measurement is proposed. This technique has the merits of both heterodyne interferometry and grating interferometry. A heterodyne light beam is obtained using an electro-optic modulating technique for amplitude modulation. While the heterodyne light beam is normally incident into a transmission-type 2D grating, two detection parts for in-plane and out-of-plane displacement measurements will be obtained. The heterodyne light beam is utilized to carry the optical phase variation that results from grating displacement in three directions. The experimental results demonstrate that the proposed interferometer is capable of sensing the displacement of a motion stage in 3-DOF. The resolution and range of the measurement can achieve up to nanometric and millimetric levels.

Concepts: Light, Phase, Modulation, Nonlinear optics, Radio, Astronomical interferometer, Aperture synthesis, Optical heterodyne detection