Casos de Uso del Sistema y Diagramas de Secuencia

Figure5.4shows results obtained in the measurement of THz output power trends from a 50 µm-gap coplanar stripline (CPS) LT-GaAs PCA pumped by the two distributed feedback (DFB) LD system with average wavelengths around 848 nm.

The observed THz output power trend is quadratic as we would expect. In this measurement, the frequency difference between the two optical signals was tuned

Figure 5.4: Bolometer-measured THz output power trend from an 50 µm-gap CPS LT-GaAs PCA pumped by the dual-DFB laser setup at three different

optical powers with increasing applied voltage to the PCA.

to 1 THz. The polarisation of the combined beam was closely monitored using a linear polariser at the fibre output and the input polarisations of the beams were equalised by using a half-wave plate at the fibre input from one of the LDs. The absolute polarisation of the combined dual-mode beam is not a critical factor here because the PC gap was much larger than the pump wavelength [9], but they must both match to ensure electric dipole modulation in the same direction for photomixing. The output power is relatively low here due to the very large PC gap and subsequently low gain – 50 µm is well above the typically-used CW PCA gap width. However, this setup was used in initial testing because the commercially-available pre-mounted LT-GaAs THz PCA provided by TeraVil, Lithuania [10] was a suitable reference sample. Moreover, such results could be used to confirm the successful alignment of the pump system and allow the straightforward replacement of PCA devices for further tests in the same setup.

Figure 5.5 shows results obtained in the measurement of the THz output sig-nal power from the same 50 µm-gap CPS LT-GaAs PCA pumped using the Ti:Sapphire laser. Again we observed quadratically increasing THz power trends as the Ti:Sapphire pump power or PCA bias was increased. This was tested up to a PCA bias of 60 V and average pump power of 50 mW, at which point a THz output power just below 1.3 µW was observed. This is a typical perfor-mance level from such a device, and corresponds to an optical-to-THz conversion

Figure 5.5: Bolometer-measured THz output power trend from a 50 µm-gap CPS LT-GaAs PCA pumped by the Ti:Sapphire laser with increasing applied

voltage to the antenna.

efficiency of 2.6 × 10⁻⁵.

5.1.2 QD-Based Antenna Tests with Bolometer Detection

Figure5.6 shows results obtained in the measurement of THz output power from a 5 µm-gap CPS QD (Structure 1) PCA pumped using the two-DFB LD system.

These particular measurements were taken over a relatively modest PCA bias and pump power range. As such, the measurement range does not indicate a proper quadratic output trend but the results presented in this plot were repeatable and averaged over multiple measurements to verify the accuracy of the plot. The output power in these measurements is relatively low, but again this is due to the highly unoptimised nature of the antenna contacts and the low expected conversion efficiency of CW regime operation (see Section1.2.4.2).

Figure5.7 shows results obtained in the measurement of THz output power from a broadband 5 µm-gap log-periodic QD (Structure 2) PCA pumped using a tun-able QD LD in the double-Littrow configuration, and the corresponding optical spectrum which was used for this test. Again, the use of a 5 µm-gap PCA of any geometry is not optimal but the THz output power trend was repeatable and quadratically increased with increasing PCA bias. Interestingly, it was found that

Figure 5.6: Bolometer-measured THz output power trend from a 5 µm-gap CPS PCA over QD Structure 1 pumped by the dual-DFB laser setup with (a) increasing applied voltage to the PCA and (b) increasing optical pump power.

the highest conversion efficiency from this device was obtained while pumping the structure at wavelengths between the peak QD ES PL energy and the pho-tomixer cavity resonance peak at ∼ 1220 nm. This is perhaps further indication that the higher degeneracy and gain saturation of the ES is better suited to sup-porting carrier generation, relaxation and recombination mechanisms. Indeed, it has been previously demonstrated that the fast component of carrier recovery in such InAs:GaAs QD heterostructures may be further reduced by excitation of the dots near the ES PL peak(s) [11]. This observation is in agreement with the concepts of carrier dynamics in bandgap-engineered QD heterostructures that were discussed in Section 2.2.5. These discussions indicated the likeliness of ac-celerated ultrafast carrier dynamics when optically exciting carriers to excited

QD states and applying a lateral E-field across the active volume. Such effects are investigated further in Section5.3. No observable THz EM signals were gen-erated when the PCA was pumped around the QD GS energy. It is possible that optical-to-THz signal conversion processes were initiated in this regime, but carrier escape and recombination will likely have occurred over longer timescales and the subsequent output signal power in this case may have simply been too low.

Additionally, it is worth noting that throughout the PC heterostructure QD lay-ers are not simply ‘bunched’ at the pump wave nodes (as might be the case for QD-SESAMs) but are distributed periodically throughout the entire standing wave of the internal PCA ‘cavity’ (see Figure 4.2). Therefore, some layers are subject to higher optical excitation than others. If we consider that this effect is crucial in the carrier dynamics for effective saturation control of ultrafast QD-SESAMs as discussed in Section2.2.3, we begin to see the possibility of QD layers being saturated at different rates throughout the volume. Therefore, it is pos-sible that some layers may be more susceptible to pump absorption and carrier generation and others more susceptible to carrier capture and relaxation. The variation in the layer-to-layer behaviour in this case would depend on the pump wavelength (and corresponding standing wave pattern), the respective availabil-ity of energy states in each layer, and would be most noticeable at the QD ES energies due to their higher degeneracy and saturation fluence.