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Optical Nanosensors
Published in Vinod Kumar Khanna, Nanosensors, 2021
The main physical phenomena exploited for optical chemical sensing are absorption, fluorescence, chemical luminescence, Raman scattering, and plasmon resonance. Optical or light absorption refers to the process in which energy of light radiation is transferred to a medium. An absorption spectrum is the absorption of light as a function of wavelength or frequency. Fluorescence is the emission of electromagnetic radiation, especially of visible light, stimulated in a substance by the absorption of incident radiation and persisting only as long as the stimulating radiation is continued. Chemical luminescence is the emission of light by a substance, caused by chemical means. Raman scattering is the inelastic scattering of a photon, which creates or annihilates an optical phonon. Plasmon resonance is concerned with the excitation of surface plasmons by light.
Basic Principles of Fluorescence
Published in Guy Cox, Fundamentals of Fluorescence Imaging, 2019
An absorption spectrum is obtained by measuring the transmission of light as a function of wavelength. The combination of the resonance condition and the absorption strength gives rise to an absorption spectrum with a well-characterized intensity for every different wavelength. Figure 1.7 shows the absorption spectrum of a fluorescence reference standard, Rhodamine 6G (also called Rhodamine B, or just R6G), dissolved in ethanol. The spectrum shows a very strong absorption between 490 and 550 nm, with a maximum near 530 nm, where the molar extinction coefficient is 116,000 M−1 cm−1. This transition corresponds to the R6G molecule (shown as an inset) being excited from its ground electronic state, S0, to its first excited electronic state, S1. The width of the transition is a reflection of the range of vibrational energy that can accompany the electronic excitation. The Jablonski diagram in Fig. 1.6 is a good representation of this transition in R6G. The spectrum also shows a number of other absorption features at shorter wavelength. These correspond to R6G being excited into higher and higher electronic states, but these transitions are not shown in Fig. 1.6.
Nature of Light
Published in George K. Knopf, Kenji Uchino, Light Driven Micromachines, 2018
When looking at a light source for activating a photo-responsive material or optically driving a microscale machine, there are two prominent spectra to consider: the emission spectra and the absorption spectra. An emission spectrum is emitted by a light source whereas the absorption spectrum arises from light striking a surface or passing through an absorbing medium. The light emission from a source can be spontaneous (e.g., commercial neon signs, fluorescent materials, gas discharge, or flash lamps) or stimulated (e.g., laser, microwave maser). Figure 3.2a shows the emission spectra of several common light sources including daylight, incandescent, fluorescent, and LED. Note that natural daylight has a very broad spectrum, whereas artificial light generated by the LED is narrow. The absorption spectrum of a typical artificial light detector, the photodiode, is illustrated in Figure 3.2b.
Ultrafast third-order nonlinear optical properties of self-assembled poly[[4,4-bis(4-sulphobutyl)-4H-cyclopenta[2,1-b:3,4-b’]dithiophene-2,6-diyl]-1,4-phenylene sodium sa multilayer films
Published in Journal of Modern Optics, 2021
Jie Zong, Xingzhi Wu, Wenfa Zhou, Yinglin Song
From Figure 2, it is found that there is a characteristic absorption peak, located in the visible region, the height of the peak increases with the bilayer number, which indicates the progress of self-assembly. It is clear that the light is not scattered, which shows that the thin films are optically flat and homogenous [14]. Absorption spectra provide information about the electronic transitions occurring in the molecules. PCPDTPhSO3Na/PDDA film showed a clear polaron absorption. Absorption spectra exhibit the maximum absorption appears at 470 nm, corresponding to π–π* transitions in the polymer main chain from the highest occupied molecular orbital to the lowest unoccupied molecular orbital.
Composite of discotic liquid crystal and gold nanoparticles: A comperative band gap study
Published in Phase Transitions, 2018
The band gap can be estimated by the Tauc relation. It is a convenient way of studying the optical absorption spectrum of a material. According to the Tauc relation, the absorption coefficient for the material is given by Tauc et al. [17,18]where is incident photon energy, α is the absorption coefficient, (, where d and A are the width of the cell and absorbance, respectively) and is the band gap related to the particular transition in the material. The exponents depends upon the nature of transition and it may have values 1/2, 2, 3/2 and 3 corresponding to the allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions, respectively. Band gaps were then calculated from the Tauc plots by fitting a line through the linear portion of the band edge region. A sample having a direct band gap shows a linear dependence where the Kubelka–Munk function first shows distinct increase if n = 2, while one having an indirect gap will shows a linear dependence when n = 1/2. Here the best fitting is characterized by the value of chi-square (χ2) and correlation coefficient (R2). For finest fitted curve the value of χ2 must tend to 0 and the value of R2 should tend to 1. By the method of fitting, the best-fit values of different parameters of Equation (1) are obtained. The Tauc plot is plotted with hν along the X-axis and (αhν)2 along the Y-axis [19]. The band gap at room temperature is found by extrapolating the X-axis. The Tauc plot of a sample defines the optical band gap as represented as the region A in Figure 2.