Frequency Tuning

One of the important applications of second-order NLO materials is obtaining of tunable laser sources. Second harmonic generation or sum frequency generation systems lead to monochromatic sources. The optical parametric oscillators are based on the parametric generation of two waves with frequencies co (signal) and CO (idler). In noncentrosymmetric materials an incident photon with frequency co creates two photons satisfying the energy 

Tunable OPOs were already demonstrated with dilferent organic single crystals. Very recently Alshikh Khalil et al. [182] have shown the feasibility of an OPO in guided wave conhguration with poled polymer film. The pump was at 532 nm and the signal was at 853 nm. Internal optical gain of 1 dB was obtained after propagating on 5 mm with the pump power of 1.5 kW. 

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SFG [4.309, 4.310] uses visible and infrared lasers for generation of their sum frequency. Tuning the infrared laser in a certain spectral range enables monitoring of molecular vibrations of adsorbed molecules with surface selectivity. SFG includes the capabilities of SHG and can, in addition, be used to identify molecules and their structure on the surface by analyzing the vibration modes. It has been used to observe surfactants at liquid surfaces and interfaces and the ordering of interfacial... 

The results are shown in Figure 2. Spectra are plotted for various values of normalised input intensity yinc = n2 inc which can be considered as the strength of the nonlinearity. A positive nonlinearity, yjnc > 0, that shifts the spectra to the right was used. As expected the nonlinearity (spectral shift) is enhanced with increasing number of the periods. Moreover the structures with 2 and 3 periods exhibit bistable behavior for A, > Aq. In this case bistable switching is controlled by frequency tuning while input intensity is fixed. 

The frequency of solid-laser lines can often be tuned by temperature variationa) s for the ruby line where a shift of 20 cm has been obtained by varying the temperature between 77 °K and 300 °K. Semiconductor lasers can be frequency tuned by applying a variable mechanical pressure to the junction in the laser diode... 

Figure 7. (a) Three-level system Calculated ion dip and fluorescence dip spectra with pump laser frequency tuned to resonance with the 2 1 transition. Ionization is treated as... 

A mercury atom that was ionized by a weak electron beam was captured in a miniature Paul (radio frequency) trap that has internal dimensions of rQ s 466 pm and zQ s 330 pm. The rf trapping frequency was 21.07 MHz with a peak voltage amplitude of about 730 V. The ion was laser cooled by a few microwatts of cw laser radiation that was frequency tuned below the 6s Si -6p Pi electric dipole transition near 194 nm. When the Hg+ ion was cold and the 194 nm radiation had sufficient intensity to saturate the strongly allowed S-P transition, 2 x 10 photons/s were scattered. With our collection efficiency, this corresponded to an observed peak count rate of about 10 s-1 against a background of less than 50 s— -. 

The signal from the photomultiplier is passed to a phase-sensitive, and often frequency-tuned amplifier, which isolates the component attributable to light from the hollow cathode lamp from light emitted from the flame or stray daylight, as discussed in section 4. 

Fig. 2. Set-up of the ILP laser system. Intraeavity frequency-doubling is realized with a KTP crystal which, together with a Brewster plate, serves as a Lyot filter. This allows to frequency tune the laser by more than 500 GHz by changing the temperature of the KTP crystal. The 532 nm laser radiation, after passing an acousto-optical modulator (AOM), is directed into an external I2 fluorescence cell. A photomultiplier (PM) detects the fluorescence signal over a solid angle of almost 0.2 tt. The photodiode D is used to detect a fraction of the 532 nm laser beam to power stabilize the 532 nm light via the AOM...

Fuzessery ZM, HaU JC. 1996. Role of GABA in shaping frequency tuning and creating FM sweep selectivity in the inferior colliculus. J Neurophysiol 76 1059-1073. 

Suga N, Zhang YF, Yan J. 1997. Sharpening of frequency tuning by inhibition in the thalamic auditory nucleus of the moustached bat. J Neurophysiol 77 2098-2114. 

Fig. 10.16. Poincare surface of section of a strongly driven Morse oscillator at a laser field strength of 0.1 atomic units, with the frequency tuned to a two-photon resonance condition with excitation from the ground state to the fourth excited level. The plot indicates the presence of a resonance structure, away from which the motion lies on regular tori (after J.-P. Connerade et al. [584]).

Figure 9.20 The cis-bend and trans-bend normal mode frequencies tune into resonance at Nb 10. Similar to an inhomogeneous L-uncoupling perturbation, de-mixing after the level crossing cannot occur, and the qualitative character of the vibrational modes changes irreversibly from normal to local (from Jacobson and Field, 2000b).

Photoluminescence (PL) and lasing were excited by the radiation of a N2 laser (/ v = 3.68eV, /exc = 10 -10 W/cm, /= 1000 Hz, tp = 8ns), a HeCd laser (/zv = 3.81 eV) and by radiation of a dye laser with frequency tuning for direct excitation of the quantum wells. The electroluminescence was excited by voltage imposed to the ELT samples by stripe contacts. Reactive ion beam etching (RIBE, Ar+02) was used for the formation of the contacts on the n-type region of the device and for removing of a part of the cap layer. 

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