Electro-optical modulator devices

When an electron is removed from a neutral oligomer, the distortion of molecular geometry (lattice relaxation) results in a raising of the former HOMO level (now denoted SOMO = singly occupied molecular orbital) and lowering of the LUMO level. The level below the former HOMO level (HOMO-1) forms the HOMO of the radical cation and dication. Two new optical transitions are allowed for the radical cation (polaron), from the new HOMO to SOMO and SOMO to LUMO. 

Upon further oxidation to form the dication, the unpaired electron is removed from the SOMO and the former SOMO and LUMO move further towards the centre of the gap as the geometry distortion becomes even stronger. Now, one new optical transition is expected for the dication, from the HOMO to new LUMO (former SOMO), at a higher energy than the lower energy transition of the radical cation. 

Probing changes in the optical absorption or reflection of an MIS device is very useful as a spectroscopic tool to determine the charged excitations in the solid state. The technique has already been used for investigation of conjugated polymers [221, 222] and demonstrated as the basis of an electro-optic modulator device [223]. 

Fignre 25. Schematic diagram of the experimental configuration for optical probing of field-induced charge within semi-transparent MIS diodes based on oligomers. 

By superimposing a small oscillatory (AC) voltage on the DC bias, it is possible to modulate the charge density within the device, either to probe the accumulation region, for negative DC bias or the edge of the depletion layer (i.e. the bulk of the oligomer layer) for positive DC bias. 

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Kajzar F, ChoUet PA (1997) Poled polymers and their application in second harmonic generation and electro-optic modulation devices. In Miyata S, Sasabe H (eds) Poledpol-ymers and their application to SHG and EO devices. Gordon 8c Breach, Amsterdam, p 1... 

In Fig. 5 a typical setup for resonant photoacoustic spectroscopy as used in the authors laboratory is shown. The radiation of a cw laser is intensity modulated by a mechanical chopper of high precision. An electro-optic modulation device may also be employed or the laser beam is modulated directly by modulation of its power supply. As already discussed, vibrational excitation with an IR laser, for example, causes a modulated pressure change in the resonator via fast vibrational relaxation. This acoustic signal is detected with a microphone, because these devices provide the highest sensitivity. Detectors employed in calorimetry to measure the heating of a mpte such as thermistors or thermophi are less sensitive and p( sess a slower rise... 

Electro-optical modulators are other examples whose efficiency is enhanced in the presence of ion-radicals. These devices are based on the sandwich-type electrode structures containing organic layers as the electron/hole-injecting layers at the interface between the electrode and the emitter layer. The presence of ion-radicals lowers the barrier height for the electron or hole injection. Anion-radicals (e.g., anion-radicals from 4,7-diphenyl-l,10-phenanthroline—Kido and Matsumoto 1998 from tetra (arylethynyl) cyclooctatetraenes—Lu et al. 2000 from bis (1-octylamino) perylene-3,4 9,10-bis (dicarboximide)s— Ahrens et al. 2006) or cation-radicals (e.g., cation-radicals from a-sexithienyl—Kurata et al. 1998 l,l-diphenyl-2-[phenyl-4-A/,A- /i(4 -methylphenyl)] ethylene— Umeda et al. 1990, 2000), all of them are electron or hole carriers. 

Crystalline lithium niobate was for many years the only material that could be used in devices, an example of its use being in electro-optic modulators for interfacing with fibre optic technologies. Unfortunately, this material is not very satisfactory because it is difficult to grow, and hence expensive, and only shows modest electrooptic properties. 

Figure 2 shows the tradeoffs between crystal and polymer organic NLO materials for device applications. Although cither type of materials could in principle be used for both applications, crystals are best for second harmonic generation, and poled polymers are best for electro-optic waveguide devices such as modulators and switches. 

Figure 2. Tradeoffs between polymer and crystal organic nonlinear optical materials. EO refers to applications for electro-optic waveguide devices such as modulators and switches. SHG refers to applications for frequency doubling of moderate and low power laser sources. A + indicates favored, - indicates disfavored, 0 indicates neither favored nor disfavored, and x indicates not relevant.

If electro-optic modulators and directional couplers are to be driven by the electrical outputs from very large-scale integration (VLSI) semiconductor chips, then the required drive or VK voltages of EO devices must be less than 6 V (for... 

Measurement of device bandwidths in the order of 100 GHz typically requires heterodyne detection and a stripline electrode configuration such as that illustrated in Fig. 31. The response of polymeric electro-optic modulators is typically flat to 100 GHz. Fall off above that frequency (Fig. 32) can be traced to resistive losses in millimeter wave transmission structures and in the metal electrodes. 

Propagation losses through active materials are a serious concern however, these typically contribute only a small fraction to the total insertion loss. The most serious problem relating to minimization of optical loss with use of electro-optic modulators is that of loss associated with mode mismatch between passive and active optical circuitry. When tapered transitions and other device structures discussed in this review are used to reduce optical loss associated with mode mismatch, total device insertion losses in the order of 4-6 dB are obtained. Without such adequate attention to coupling losses, insertion loss can be 10 dB or greater. 

Kikuchi and coworkers found the rapid electro-optical effect in polymer-stabilized blue phases and presented the major potential of such phases in application to display devices and optical modulation devices [45,46]. Tra-... 

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