OTHER SECOND-ORDER NLO APPLICATIONS

Among the requirements for E/0 devices based on 2nd-order NLO effects is the absence of symmetry, frequently achieved by aligning the dipole moments of chromophores (which are responsible for the absorptions creating the nonlinearities), a process known as poling . A common method of poling is to include a 2nd-order-NLO-active CP in a polymer matrix, heat the latter to above its glass transition temperature, apply a strong electric field, and then rapidly cool the matrix to lock in the asymmetry. Such methods with CPs have met with limited success The 

A novel type of device configuration first apparently proposed by Inganas and Lundstrom [865], and developed to a considerable degree of refinement by Chandrasekhar et al. [866], is one which is based on interfacing a CP to an inorganic semiconductor (SC). This utilizes the well known electrochromic properties of the CP. The SC may be a material such as CdS or Ai-Si. In this context, it should be noted that the CP, in its doped form, is also a SC, but its semiconducting properties are not the primary focus of interest in such SC/CP interfaces- rather, its electro-chromic properties are. 

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. 

In later work, Chandrasekhar et al. [866] constructed devices having the interface shown in Fig. 18-4. The sequential steps involved in electrochromic switching of the CP are indicated in the figure legend. 

The /2-type semiconductor bands are bent upward at the polymer-SC interface, as shown. 0/R is the electronic level corresponding to the 0/R redox potential rest state (a) laser excitation (b) charge transport in polymer (i.e., polymer switching) (c). The processes occurring in chronological order are laser excitation of the SC (sub-nanosecond) (b) charge transfer to polymer (subnanosecond) (b) oxidation-reduction of initial polymer layers (subnanosecond risetime falltime from ca. 10 ns to microseconds) (c) charge transport in bulk polymer (slow process) (c). 

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