Ultrafast photonics

T. Hasan, F. Torrisi, Z. Sun, D. Popa, V. Nicolosi, G. Privitera, et ah, Solution-phase exfoliation of graphite for ultrafast photonics, Physica Status Solidi (B), 247 (2010) 2953-2957. 

Charge transfer NLO polymers offer the promise of truly unique properties properties that mimic the performance of photorefractive materials, but on ultrafast (picosecond) time scales. Such materials would enable ultrafast photonic applications (e.g. ultrafast optical switching and ultrafast image processing) that are impossible today with any known class of materials. 

T. Nagamura, H. Sakaguchi, S. Muta, and T. Ito, Ultrafast photon-mode recording by novel photochromic polymer via photoinduced electron transfer, Appl. Phys. Lett. 63, 2762-2764 (1993). 

In the Ingan and Lundstrdm work, an AZ-Si/poly(N-Me-pyrrole) SC/CP interface was used, with application as a fast optical memory envisioned. In the write step of this memory, the CP was oxidized by illuminating the SC. In the erase step, a negative (cathodic or reducing) potential was applied to the CP, returning it to its original, de-doped state. This device however required the use of liquid electrolyte, and switching times were of the order of ms. It thus did not utilize the solid state and ultrafast photonic capabilities of such an interface. 

The dynamics of fast processes such as electron and energy transfers and vibrational and electronic deexcitations can be probed by using short-pulsed lasers. The experimental developments that have made possible the direct probing of molecular dissociation steps and other ultrafast processes in real time (in the femtosecond time range) have, in a few cases, been extended to the study of surface phenomena. For instance, two-photon photoemission has been used to study the dynamics of electrons at interfaces [ ]. Vibrational relaxation times have also been measured for a number of modes such as the 0-Fl stretching m silica and the C-0 stretching in carbon monoxide adsorbed on transition metals [ ]. Pump-probe laser experiments such as these are difficult, but the field is still in its infancy, and much is expected in this direction m the near fiitiire. 

Passino S A, Nagasawa Y, Joo T and Fleming G R 1996 Photon echo measurements in liquids using pulses longer than the electronic dephasing time Ultrafast Phenomena X ed P Barbara, W Knox, WZinth and J Fujimoto (Berlin Springer) pp 199-200... 

Several laser systems have been used in our time-resolved PM measurements. For the ultrafast measurements, a colliding pulse mode-locked (CPM) dye laser was employed [11]. Its characteristic pulsewidth is about 70 fs, however, its wavelength is fixed at 625 nin (or 2.0 cV). For ps measurements at various wavelengths two synchronously pumped dye lasers were used (12], Although their time resolution was not belter than 5 ps, they allowed us to probe in the probe photon energy range from 1.25 cV to 2.2 cV. In addition, a color center laser... 

Figure 1.3. Real-time femtosecond spectroscopy of molecules can be described in terms of optical transitions excited by ultrafast laser pulses between potential energy curves which indicate how different energy states of a molecule vary with interatomic distances. The example shown here is for the dissociation of iodine bromide (IBr). An initial pump laser excites a vertical transition from the potential curve of the lowest (ground) electronic state Vg to an excited state Vj. The fragmentation of IBr to form I + Br is described by quantum theory in terms of a wavepacket which either oscillates between the extremes of or crosses over onto the steeply repulsive potential V[ leading to dissociation, as indicated by the two arrows. These motions are monitored in the time domain by simultaneous absorption of two probe-pulse photons which, in this case, ionise the dissociating molecule.

If the pulse from laser 1 is ultrafast, the bond-energizing step occurs in a veiy short time, about 10 fs (lfs = 10 S). As the bond stretches through the specific length at which it absorbs photons from laser 2, the molecule can absorb a photon from the second laser beam. This absorption causes the transmitted intensity of laser 2 to fall rapidly as the bond stretches. When the bond breaks, photons from laser 2 are no longer absorbed and the transmitted intensity returns to its original value. By measuring the time it takes for this to occur, chemists have determined that it takes about 200 fs for a chemical bond to break. 

Recently, Zewail and co-workers have combined the approaches of photodetachment and ultrafast spectroscopy to investigate the reaction dynamics of planar COT.iii They used a femtosecond photon pulse to carry out ionization of the COT ring-inversion transition state, generated by photodetachment as shown in Figure 5.4. From the photoionization efficiency, they were able to investigate the time-resolved dynamics of the transition state reaction, and observe the ring-inversion reaction of the planar COT to the tub-like D2d geometry on the femtosecond time scale. Thus, with the advent of new mass spectrometric techniques, it is now possible to examine detailed reaction dynamics in addition to traditional state properties." ... 

All the nucleic acid bases absorb UV radiation, as seen in Tables 11-1, 11-2, 11-3, 11-4, and 11-5, making them vulnerable to the UV radiation of sunlight, since the energy of the photons absorbed could lead to photochemical reactions. As already mentioned above, the excited state lifetimes of the natural nucleobases, and their nucleotides, and nucleosides are very short, indicating that ultrafast radiationless decay to the ground state takes place [6], The mechanism for nonradiative decay in all the nucleobases has been investigated with quantum mechanical methods. Below we summarize these studies for each base and make an effort to find common mechanisms if they exist. 

The discussion in this chapter is limited to cyanine-like NIR conjugated molecules, and further, is limited to discussing their two-photon absorption spectra with little emphasis on their excited state absorption properties. In principle, if the quantum mechanical states are known, the ultrafast nonlinear refraction may also be determined, but that is outside the scope of this chapter. The extent to which the results discussed here can be transferred to describe the nonlinear optical properties of other classes of molecules is debatable, but there are certain results that are clear. Designing molecules with large transition dipole moments that take advantage of intermediate state resonance and double resonance enhancements are definitely important approaches to obtain large two-photon absorption cross sections. 

Wiesner, B. (2005). Coumarinylmethyl esters for ultrafast release of high concentrations of cyclic nucleotides upon one- and two-photon photolysis. Angew. Chem. Int. Ed. 44, 7887-7891. 

Femtosecond solvation dynamics experiments in water [147] clearly hint at the existence of a bimodal response of the solvent to a change in solute charge density that is produced by photon absorption for instance. Water appears to show an ultrafast component in the fl/ kT timescale and a slow component due to diffusive motions whose timescale would be in the 1/y range. 

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