Difference-frequency generation

Rayleigh scattering, ordinary refraction Sum-frequency generation Difference-frequency generation Four-wave mixing... 

Figure Bl.22.8. Sum-frequency generation (SFG) spectra in the C N stretching region from the air/aqueous acetonitrile interfaces of two solutions with different concentrations. The solid curve is the IR transmission spectrum of neat bulk CH CN, provided here for reference. The polar acetonitrile molecules adopt a specific orientation in the air/water interface with a tilt angle that changes with changing concentration, from 40° from the surface nonnal in dilute solutions (molar fractions less than 0.07) to 70° at higher concentrations. This change is manifested here by the shift in the C N stretching frequency seen by SFG [ ]. SFG is one of the very few teclnhques capable of probing liquid/gas, liquid/liquid, and even liquid/solid interfaces.

Materials are also classified according to a particular phenomenon being considered. AppHcations exploiting off-resonance optical nonlinearities include electrooptic modulation, frequency generation, optical parametric oscillation, and optical self-focusing. AppHcations exploiting resonant optical nonlinearities include sensor protection and optical limiting, optical memory appHcations, etc. Because different appHcations have different transparency requirements, distinction between resonant and off-resonance phenomena are thus appHcation specific and somewhat arbitrary. 

This shows that there are now oscillating dipoles at frequencies (coi + CO2) and (cOi — CO2), which give rise to SFG and difference frequency generation (DFG), respectively. When C0i = C02, Eq. (5.7) becomes ... 

In addition, it should be noted that the experimental setup is different between the 20 kHz and the high frequency systems. Therefore, the type of the cavitation would be different between them. It has been reported that the 20 kHz system consisting of a hom type sonicator generates predominantly transient cavitation whereas the systems used for higher frequencies generate stable cavitation [34]. 

From the form of the polarization it is clear that in order to observe any nonlinear optical effect, the input beams must not be copropagating. Furthermore, nonlinear optical effects through the tensor y eee requires two different input frequencies (otherwise, the tensor components would vanish because of permutation symmetry in the last two indices, i.e., ytfl eee = Xijy ) For example, sum-frequency generation in isotropic solutions of chiral molecules through the tensor y1 1 1 has been experimentally observed, and the technique has been proposed as a new tool to study chiral molecules in solution.59,61 From an NLO applications point of view, however, this effect is probably not very useful because recent results suggest that the response is actually very low.62... 

Third-order susceptibilities of the PAV cast films were evaluated with the third-harmonic generation (THG) measurement [31,32]. The THG measurement was carried out at fundamental wavelength of 1064 nm and between 1500 nm and 2100 nm using difference-frequency generation combined with a Q-switched Nd YAG laser and a tunable dye laser. From the ratio of third-harmonic intensities I3m from the PAV films and a fused quartz plate ( 1 thick) as a standard, the value of x(3) was estimated according to the following equation derived by Kajzar et al. [33] ... 

Two rf fields411. The restriction E, = (Em + Ek)/2 for the intermediate energy level E may be dropped if photons of two different frequencies generate a DQT. The corresponding transition probability is then given by... 

F ure 2.21 Different types of three-wave interaction in a nonlinear media (at the photon level) (a) sum-frequency generation and (b) the optical parametric generation. 

A. Tadjeddine, A. Le Rille, Sum and difference frequency generation at electrode surfaces in Interfacial Electrochemistry. Theory, Experiment and Applications (Ed. A. Wieckowski), Marcel Dekker, New York-Basel, 1999, p. 317. 

Another relatively new technique is the infrared visible sum frequency generation (SFG) spectroscopy and the difference frequency generation (DFG) spectroscopy [13], both of which represent vibrational spectroscopic techniques sensitive only to the material located at interfaces. 

Initial laser pulse generation is achieved with the use of a twin-tube excimer laser in which one channel is a XeCl laser oscillator dehvering 15-ns, 308-nm, 80-mJ pulses for the driving of two dye lasers needed for difference frequency generation. The second channel is used for amphfication of subpicosecond 308-nm pulses that become pump pulses. 

The general condition of second-order NLO effects involves the interaction of two distinct waves of frequencies coi and C02 in an NLO material. In this case, polarization occurs at sum (coi + C02) and difference (coi — C02) frequencies. This electronic polarization will therefore reemit radiation at these frequencies, with contributions that depend on x , which is itself frequency-dependent. The combination of frequencies is called sum (or difference) frequency generation (SFG). SHG is a special case of SFG, in which the two frequencies are equal. 

Emission spectroscopy utilizes the characteristic line emission from atoms as their electrons drop from the excited to the ground state. The earliest version of emission spectroscopy as applied to chemistry was the flame test, where samples of elements placed in a Bunsen burner will change the flame to different colors (sodium turns the flame yellow calcium turns it red, copper turns it green). The modem version of emission spectroscopy for the chemistry laboratory is ICP-AES. In this technique rocks are dissolved in acid or vaporized with a laser, and the sample liquid or gas is mixed with argon gas and turned into a plasma (ionized gas) by a radio frequency generator. The excited atoms in the plasma emit characteristic energies that are measured either sequentially with a monochromator and photomultiplier tube, or simultaneously with a polychrometer. The technique can analyze 60 elements in minutes. 

This was achieved in quartz 9 ) and gallium phosphide 98). Two beams of different frequency generated through stimulated Raman scattering in appropriate liquids were directed to the crystal in directions calculated from the polariton... 

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