Resonance second harmonic generation

Figure Bl.22.7. Left resonant second-harmonic generation (SHG) spectrum from rhodamine 6G. The inset displays the resonant electronic transition induced by the two-photon absorption process at a wavelength of approximately 350 nm. Right spatially resolved image of a laser-ablated hole in a rhodamine 6G dye monolayer on fused quartz, mapped by recording the SHG signal as a function of position in the film [55] SHG can be used not only for the characterization of electronic transitions within a given substance, but also as a microscopy tool.

Heinz, T, Chen, C., Ricard, D., and Shen, Y. (1982). Spectroscopy of molecular monolayers by resonant second-harmonic generation. Phys. Rev. Lett., 48 478 - 481. 

Liipke, G., Bottomley D., and van Driel, H. (1994). Resonant second-harmonic generation on Cu(lll) by a surface-state to image-potential-state transition. Phys. Rev. 8,49 17303-17306. 

With commercially available YDFL as pumps, powers > 40 W at 1178 nm are feasible. This sets an upper limit to the conversion efficiency needed in the subsequent second harmonic generation. Numerical simulations for the amplifier and resonator Raman laser configuration indicate feasibility of the system with sufficient SBS suppression. ESO has assembled the amplifier configuration, and has demonstrated up to 4 W CW at 1178 nm. ESO s goal is to have compact and turnkey commercial fiber lasers for LGS/AO within 3 years. 

In LB films not only the interaction of chromophores but also their orientation can be controlled at the molecular level. Molecular orientation of chromophores has been determined by several methods including polarized UV/vis or IR absorption, second harmonic generation (SHG), Electron Spin Resonance (ESR), or resonance Raman scattering. We have measured the incident angle and polarization angle dependencies of polarized UV/vis absorption to study the molecular orientation of alloxazine, porphyrin, and carbazolyl chromophores, or 4,4 -bipyridinium radical cations in LB films[3-12]. Usually in-plane components of transition dipoles of chromophores are... 

One of the most important theoretical contributions of the 1970s was the work of Rudnick and Stern [26] which considered the microscopic sources of second harmonic production at metal surfaces and predicted sensitivity to surface effects. This work was a significant departure from previous theories which only considered quadrupole-type contributions from the rapid variation of the normal component of the electric field at the surface. Rudnick and Stern found that currents produced from the breaking of the inversion symmetry at the cubic metal surface were of equal magnitude and must be considered. Using a free electron model, they calculated the surface and bulk currents for second harmonic generation and introduced two phenomenological parameters, a and b , to describe the effects of the surface details on the perpendicular and parallel surface nonlinear currents. In related theoretical work, Bower [27] extended the early quantum mechanical calculation of Jha [23] to include interband transitions near their resonances as well as the effects of surface states. 

By simultaneously injecting both beams into a DR1 PMMA film, within hours a self-written QPM nonlinearity of 90 pm V1 was permanently inscribed, allowing further use of the film as a second harmonic generator. The limitation is that the harmonic is generated near its absorption peak and hence only very thin films with a small net conversion efficiency can be used [116]. Detuned from resonance, the nonlinearity is naturally significantly decreased. Nonetheless this method remains potentially very attractive. 

SERBS Surface-enhanced resonance Raman scattering SHG Second harmonic generation... 

Here Xo (2w), x (wiiw2) and Xo (O) are the non-resonant values of the hyperpolarisabilities. Thus second harmonic generation is resonantly enhanced at both the fundamental and the harmonic of the optical transition, sum and difference frequency generation at the fundamentals and the sum and difference frequencies, and the rarely observed optical rectification only at the fundamental frequency. The term 3 in the expansion gives rise to effects such as third harmonic generation, x(3) -3oj oj, oj,u>), electric field induced second harmonic generation, x(3) (- 2w 0,w, oj), the optical Kerr effect, x(3) (-oj oj, oj, -cj), etc. that will display resonances at oj, 2oj and 3u>. 

The use of optical methods which probe interface electronic and vibrational resonances offers significant advantages over conventional surface spectroscopic methods in which, e.g. beams of charged particles are used as a probe, or charged particles emitted from the surface/interface after photon absorption are detected. Recently, three-wave mixing techniques such as second-harmonic generation (SHG) have become important tools to study reaction processes at interfaces. SHG is potentially surface-sensitive at nondestructive power densities, and its application is not restricted to ultrahigh vacuum (UHV) conditions.However, SHG suffers from a serious drawback, namely from its lack of molecular selectivity. As a consequence, SHG cannot be used for the identification of unknown surface-species. 

Jordan P et al (2005) Surface-enhanced resonance Raman scattering in optical tweezers using co-axial second harmonic generation. Opt Express 13(11) 4148-4153... 

Because of the wide analytical range already accessible with second harmonic generation, many elements routinely determined by conventional AAS in analytical flames or furnaces can also be determined by AAS with diode lasers. The availa-blility of laser diodes with lower wavelengths will only make the approach cheaper, as then second harmonic generation will become superfluous. The elements now accessible with X > 630 nm with resonance lines are already manifold Li, Na, Al, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Ga, Rb, Sr, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Cr, Ba, La, Hf, Ta, W, Re, Ir, Pt, Tl, Pb, Nd, Sm, Eu, Gd, Ho, Tm, Yb and Lu. Also U and some of the actinides can be determined. Important elements such as Be, Mg, As and Hg with diodes emitting in the blue region will eventually become accessible. 

Puccetti, G. Electric field induced second harmonic generation/third harmonic generation measurements on molecules with extended charge transfer Absorption domain and strong resonance effects, J. Chem. Phys. 102, 6463-6475 (1995)... 

---