Optical modulation processes in thin films based on thermal effects of surface plasmons
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Experimental schemes for SPAC (top left) and SP-current coupling (top right). The Kretschmann configuration is utilized to excite SP using a 29.5 nm gold film, optimized for the wavelength of the pump laser beam . In top left a linearly polarized HeCd line is used as a probe beam. The absorption curve for the IR is displayed in bottom left. At the resonance angle , the absorption is almost total for polarization and is below 3% for the polarization. In the case of the HeCd beam incident at (bottom right), 46 and 8% absorptions, for and polarization, respectively, are observed.
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Reflected beams profiles showing the mutual effect of SPAC (a)–(d) and the effect of the probe beam polarization state (e)–(h). (a) and (b) display the horizontal and vertical HeCd reflected beam profiles, respectively, without (solid curves) and with (dotted curves) IR. (c) and (d) represent the IR reflected beam profiles without (solid curves) and with (dotted curves) HeCd influence. In this case, the used incident power levels are 20 mW for the HeCd and 165 mW for the IR, and the two beams are polarized. (e) and (f) show the HeCd reflected beam profiles for (solid curves) and (dashed curves) polarization with no IR, while (g) and (h) display the corresponding curves with IR (with an output power equals to 1.4 W). The abscissas ( and ) give the beam dimensions, while the ordinate represent the variations of the profiles intensity in the two directions.
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SP excitation in thin gold films in the presence of resistive heating with the aim of providing further modulation processes related to SPAC. (a) shows the variations of the resistance of the foil (nominal ) as a function of time, while imposing a constant 5 V potential across it. After a fast increase of the resistance due to the applied potential, the resistance reaches a stationary value ( for this particular potential). At 850 s, the IR laser is turned on at full power (1.5 W) for one minute, and then, for two minutes at 950 and 1250 s. The thermal contribution, induced by the laser, shows a new increase in the resistance, which reaches a new limit value of , and returns to its original value as the laser is turned off. In (b), we monitor the resistance of the same foil at three different potentials, while the IR laser power is increased in regular steps. For each step, the resistance increases to reach a stable value. In (c), the signal for a 3 V potential is plotted as a function of the IR beam modulation frequency at different power levels.
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