China
Africa
Explore chapters and articles related to this topic
Optically Induced Nonlinear Waveguides
Published in María L. Calvo, Vasudevan Lakshminarayanan, Optical Waveguides, 2018
Photorefractive materials6 offer an intensity-dependent saturable nonlinearity that has extensively been used for generating spatial solitons.24 The photorefractive effect describes the nonlinear generation of a refractive index change depending on incident light. In its most common definition, it encompasses a local photoexcitation of charge carriers, several charge transport mechanisms giving rise to local static fields and finally a change of the refractive index via the linear electrooptic effect as a result of the generated fields. Photorefractivity has been known for more than three decades but was initially only considered a hindrance to the observation of nonlinear optical effects and hence called optical damage.25
P
Published in Philip A. Laplante, Comprehensive Dictionary of Electrical Engineering, 2018
photon noise fundamental noise due to the quantum nature of light; a statistical variation in optical intensity due to measurements of discrete number of photons. photopic formally, a description of luminances under which human cone cells are active. Informally, describing daylight luminances. photorefractive beam fanning a photorefractive phenomenon in which a beam of coherent light is scattered into a fanned pattern by a photorefractive crystal (e.g., barium titanate, lithium niobate). When a laser beam passes through a photorefractive crystal, significant scattering often occurs. The scattered light appears to be asymmetrical with respect to the beam except for propagation along the c-axis. For laser beams of moderate power, the scattered light appears to develop slowly in time and eventually reaches a steady-state scattering pattern. This is known as photorefractive beam fanning. The beam fanning originates from an initial scattering due to crystal imperfections. The initially scattered light is amplified due to the physical overlap and the energy coupling between the incident beam and the scattered beam. Beam fanning often occurs in highly efficient photorefractive media, even if the material is near-perfect. Photorefractive beam fanning plays an important role in the initiation of many phase conjugators and resonators, even through the fanning itself can be a source of noise in many experimental measurements. photorefractive crystal crystalline solids that exhibit photorefractive effect. The photorefractive effect is observed in many electro-optic crystals, including BaTiO3 , KNbO3 , LiNbO3 , Sr1-x Bax xNb2 O6 (SBN), Ba2-x Srx K1-y Na y
Adaptive Optics and Phase Conjugate Reflectors
Published in Chunlei Guo, Subhash Chandra Singh, Handbook of Laser Technology and Applications, 2021
Michael J. Damzen, Carl Paterson
The photorefractive effect is a change in the refractive index due to the optical transfer of charge in the medium leading to change in refractive index. The effect was first observed in lithium niobate (LiNbO3) and was a detrimental effect due to its change in the propagation properties of the light. The basic mechanism is due to the photoexcitation of electrons (or holes) from donor (or acceptor) sites to the conduction (or valence) band, where they are free to migrate by drift or diffusion before recombining into an empty trap site. The dominant rate of charge photoexcitation is in the bright intensity regions with preferential trapping in dark regions. Hence, with an intensity interference pattern, the charge is redistributed to dark fringes. The imbalance of charge leads to a space-charge electric field ESC that modulates the refractive index ∆n through the electro-optic (Pockels) effect ∆n = 1/2n3reffESC, where n is the refractive index and reff is the effective electro-optic coefficient. Hence, good choices of photorefractive media exhibit high photoconductivity and have high electro-optic coefficient; examples include barium titanate (BaTiO3), bismuth silicon oxide (BSO) and lithium niobate (LiNbO3). Later, interest has been centred on photorefractive polymer materials where the composite polymer material can be engineered to have particular donor and trapping species, but they tend to require high externally applied electric fields and have high absorption.
A Graphene based bimetallic plasmonic waveguide to increase photorefractive effect
Published in Waves in Random and Complex Media, 2021
The photorefractive effect is a nonlinear optical phenomenon in which the refraction index of a medium is changed by the illumination of a beam of light with a spatial intensity variation. This effect was first discovered in 1966 during the study of the transmission of laser beams through some crystals, such as LiNbO3 and BaTiO3. It was discovered that the presence of laser beams inside these crystals causes index inhomogeneity. This index inhomogeneity, in turn, causes distortions in the wavefront of the transmitted laser beam. So, this effect was first referred to as an optical damage [23] and is now known as the photorefractive effect. This effect gives rise to many interesting effects and applications, such as phase conjugation [24,25], light-induced waveguiding [26], beam amplification [27], and two-wave mixing [28]. The photorefractive effect can be described by the following four processes: Generation of charge carriers through photo-excitation by an inhomogeneous illumination. Charge carriers can either be electrons excited from trap levels, induced by donor atoms, in the material energy bandgap into the conduction band, or holes excited into the valence band (from acceptor atoms). So, photorefractive material is usually doped by both donor and acceptor impurities.Transport of the excited charge carriers from the bright regions into dark regions due to diffusion or drift, induced by an external or the internal space-charge field. In some materials photo-galvanic effect, the appearance of an electric current in a homogeneous crystal under illumination, is also considered.In the dark zones, the excited charge carriers recombine into trapped states such as impurities. This process continues until the diffusion current is exactly balanced by the drift current.A space charge electric field, ESC, is generated due to this charge redistribution. This electric field, in turn, causes a change in the refractive index.