Modulation of Second Harmonic Generation via Photocurrent Induced Heating

In addition to generating coherent light efficiently, modulating this light signal through the nonlinear process will be of practical interests and greatly extends its functionality for on-chip optical modulation. For this purpose, we fabricated a CdS NB device (Width: 5.6 m; Thickness: 0.35 m) with Ti/Au contacts (inset of fig. 6.9a) and studied SHG response to the photocurrent excited by an Argon laser.

Figure 6.9 Photocurrent modulated SHG output at the R-edge (L-R). A) SHG output when Ar+ laser was switched ON and OFF while the device was at 0 V. No change is seen in the SHG signal. Inset: Illustration of the device. B) SHG output as a function of photocurrent. Photocurrent in either direction leads to a reduction in SHG. C) SHG and current modulation by switching Ar+ laser ON/OFF with the bias voltage of 10 V. The SHG signal is modulated by the change in photocurrent. (d) Photoluminescence redshift in response with photocurrent, indicating heating taking place inside the NB. This can cause a change in the phase matching conditions in the NB and leading to a reduction in SHG.

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The Argon laser power was ~135 W and the wavelength ~458 nm. In this device, the photocurrent flows along the long-axis (or x-axis) and the SHG signal is waveguided along the short-axis (or z-axis). In order to reduce PL noise (500~520 nm) excited by the Argon laser, which may affect detected SHG signal, we studied SHG signal at 523 nm by tuning the fundamental wave at 1046 nm and shining it on the device far from the region where the Argon laser excites photocurrent and PL. As shown in fig. 6.9a, when the device is at 0 V bias (when there is no current), no SHG change is observed at the R-edge when the Argon laser is switched on and off. However, when the forward bias voltage is increased with the Argon laser on, the current through the device rises and the SHG output detected at the R-edge falls accordingly (fig. 6.9b). At the current of 3104 A/cm2, the SHG signal is changed by ~60% compared with that at 0 V. Similar behavior is also observed when the a reverse bias voltage is applied to the device.

The current present in the device consisted of both photocurrent (photo-excited carriers driven by the applied voltage) and free-current (free carriers driven by the applied voltage). To clearly evaluate the contribution of photocurrent, we fixed the bias voltage at 10V and monitored the time-varying SHG signal at the R-edge for several minutes within which we switched the Argon laser on and off several times. As shown in fig. 6.9c, when the Argon laser is switched off (time A to B), the current through the device is only free-current, ~ 0.87104 A/cm2, at the 10 V bias, and the SHG output is assumed to be at a highest level (initially 1 or 100%). When the Argon laser is turned on (B to C), the current increases up to ~2.6104 A/cm2 suddenly and the at the same time the SHG output falls down to ~ 50% its initial value. In this process, the efficient

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generation of photocurrent significantly changes the total current inside the device and subsequently results in the drop of the SHG output. From time C to D in figure 6.9c the current increases slightly as the Argon laser is kept on, while concurrently the SHG output goes down slightly. When the Argon laser was turned off from time D to E, the current dramatically decreases while SHG output jumps (~66%). Between time E and F photo-excited carriers generated by the Argon laser are gradually depleted through recombination, and the total current goes down slightly, which modulates the SHG output from ~66% (at E) to ~80% (at F). Similar SHG response is also observed in the subsequent cycles (from F to G) of tuning on and off the Argon laser. It takes several tens of seconds (from G to H) for the current and SHG output to resume their initial states (at A) after turning off the Argon laser. This slow response is likely due to the internal capacitance in the device. Therefore, we have demonstrated that the SHG output at the R- edge can be easily and quickly modulated by 60% via photocurrent.

In order to understand the underlying mechanism of modulating SHG, we measured PL spectrum at different currents. As shown in figure 6.9d, PL red-shifts and becomes weak with the increasing of the current. The red-shifted PL observed here is attributed to a current-induced thermal effect, that is, temperature will rise with increasing current21, then leading to the red-shifted PL. Therefore, the effective refractive index of the waveguide modes increases when current is applied18. Note that the material’s refractive index exhibits more dramatic changes versus temperature near exciton resonances rather than off-resonance20. The refractive index of the SHG signal (at 523 nm, near the exciton resonances) changes more than the index at the fundamental wave (at 1046 nm, off

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resonance) when temperature is elevated via photocurrent. As a consequence, the phase mismatch between the waveguide modes became worse. To confirm that heating is reducing the SHG output as opposed to photocuurent we directly heated the device from 300 K to 340 K (figure 6.10). We clearly observe that the output SHG signal at R goes down as the device is heating.

Note that the current has also been observed to change second-order nonlinear coefficients and modulate the SHG signal22. However, in that case, the current density in their device was much larger (~106 A/cm2) while current-induced SHG was only modulated by ~0.1% compared with the total SHG (or surface SHG). In contrast, the total current in our device is only ~104 A/cm2, which may not induce significant change of the second-order nonlinear coefficient ((2)). Therefore current modulated phase mismatch in the waveguide is a dominant factor to control SHG output at the R-edge rather than current induced change of (2).

Figure 6.10 SHG measured at the R edge as the temperature controlled by a heater is increased from 300 to 340 K. The SHG is observed to go down confirming our earlier hypothesis that the photocurrent induced heating is cause a change in SHG and not the current itself.

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