Hello Dear Researchers, I am confused about the Difference between Second-harmonic generation and Two-photon absorption? Can anyone describe the two phenomena briefly in the physical picture? What is the difference in measuring them?? Thanks a lot.
#Photonics #Optics #Physics #Nonlinear Optics
Milan's answer formally covers the difference in perturbative orders between the two effects. However, it is worth noting that 2PA leaves the material in an excited state from which it often relaxes internally before decaying radiatively, meaning it can deposit some energy in the absorbers. Also, the emission resulting from 2PA is incoherent and is emitted sometime after the absorption.
SHG, on the other hand, is a virtual process (classically it can be envisioned as coming from the coherent excitation the material's polarization oscillating at twice the frequency of the incident fundamental wave) and does not deposit energy into the medium. Because it is a coherent process, phenomena like phase matching matter and it only occurs during the time the incident wave is present in the medium.
Hello Kimi Chen. Second-harmonic generation (SHG) and two-photon absorption (2PA) are nonlinear optical processes. SHG is the second-order nonlinear process, while 2PA is the third-order nonlinear process.
The second-order nonlinear processes occur in the non-centrosymmetric (crystal) nonlinear optical materials. In centrosymmetric media, such as silicon, the lowest order nonlinearity is the third order. You can find more about this in the book Robert Boyd, Nonlinear Optics.
In SHG, two photons at the same frequency are combined in the 2nd order nonlinear material to create a new photon at the second harmonic (double frequency). Analogous of the SHG in the third-order nonlinear processes would be the third-harmonic generation (THG). In 2PA, two photons at the same frequency are simultaneously absorbed.
Since the 2PA absorption is the third-order nonlinear process, the absorption depends on the intensity. You can estimate 2PA by measuring transmission while changing the incident intensity. In SHG you will measure the output signal at the double frequency.
Milan's answer formally covers the difference in perturbative orders between the two effects. However, it is worth noting that 2PA leaves the material in an excited state from which it often relaxes internally before decaying radiatively, meaning it can deposit some energy in the absorbers. Also, the emission resulting from 2PA is incoherent and is emitted sometime after the absorption.
SHG, on the other hand, is a virtual process (classically it can be envisioned as coming from the coherent excitation the material's polarization oscillating at twice the frequency of the incident fundamental wave) and does not deposit energy into the medium. Because it is a coherent process, phenomena like phase matching matter and it only occurs during the time the incident wave is present in the medium.
To, Kimi chen
a. Corresponding to second order non linear susceptibility, the quadratic term of polarisation corresponding to two laser input with , frequencies w and p , for example on a surface contains DC field , SFG, SHG and DFG. For example, take P(2)= (constant terms)*(E_1Coswt + E_2Cospt)^2 and calculate the terms to find SHG .
b. ''In the process of two-photon absorption an atom makes a transition from its ground state to an excited state by the simultaneous absorption of two laser photons''- From the book of Robert Boyd, Nonlinear optics.
Also I believe Milan and Michael has already answered your question nicely.
Hello,
SHG, like TPA, also depends on two photons interacting simultaneously with a target. Unlike TPA, however, in which two photons are absorbed by the target to produce a single photon of fluorescent emission with some energy loss, with SHG there is no absorption of photons by the target.
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