## 2 DBQW RTD Photo-Detectors
### DC Photo-Detection Characteristics  
Photo-detection characterization of RTD-PDs consisted in coupling light into the waveguide cleave ends (endfire coupling) using a lensed single mode optical fiber (spot diameter of \(5\pm 0.5\)\(\upmu\)m and coated on the tip). The lensed fibers and the RTD-PD devices were mounted on microblock 3-axis translation stages, in an endfire arrangement where both input optical fibers and samples could be translated and rotated independently. The light source employed for optical detection characterization was a Phonetics Tunics laser diode tunable in the wavelength region 1,457-1,599 nm, with up to 10 mW optical output power, that could be used for high frequency modulation with a bandwidth up to 1 GHz. RTD-PD devices mounted on the brass fixture were characterized directly on-chip by probing the RTD CPW contacts using a ground-signal-ground CPW probe with a pitch size of 125 \(\upmu\)m.  
Due to the polarization dependence of the RTD waveguide detectors, laser light was coupled to the waveguide using a polarization controller and the position of the input fiber was optimized to obtain the highest photocurrent from the device (and hence a high response to light modulated signals), ensuring a maximized overlap between the optical mode from the input fiber and the DBQW region. In Figure 3a is shown the I-V characteristics for both dark and illuminated conditions showing the photocurrent gradually turned on as the bias was increased to the valley voltage. Consequently, the NDR of the RTD-PD shifts to lower voltages, although the effect of the current increase shown in Figure 3a is not very pronounced because of the low optical power received by the RTD due to the small coupling efficiency as detailed next in this sub-section. Also shown is the external responsivity (the photocurrent generated divided by the optical power in fiber) as a function of the bias voltage. Note that a bias voltage of only 0.5 V was required to efficiently collect the photogenerated carriers. The results show clearly a higher responsivity close in or in the valley region which confirms the detection mechanism analyzed previously. Figure 3b presents the photogenerated valley current plotted  
Figure 3: **(a)** Dark RTD-PD I-V characteristic and I-V curve with illumination at 1,550 nm for input optical power in fiber around 5 mW. The estimated responsivity as a function of the bias voltage is also plotted. **(b)** Photogenerated valley current as a function of illumination intensity and the linear regression. _versus_ the optical power in fiber. The straight line in the figure is the linear regression obtained from the data. The photogenerated valley current increases approximately linearly with the illumination power level (_dI_/_dP_\({}_{i}\)) with a slope of 0.29 mA/mW, and up to 10 mW in-fiber optical power level. No saturation of the photogenerated valley current was observed, indicating that photoexcited RTD waveguide detectors can operate at moderate incident power light conditions. The quantum efficiency in photo-detectors is defined as the fraction of photons creating electron-hole pairs:  
\[\eta_{ph}=\frac{I_{ph}}{q\phi} \tag{1}\]  
where \(I_{ph}\) is the photocurrent, and \(\phi=\lambda P(\lambda)\)/\(hc\) is the photon flux, with \(\lambda\) the operation wavelength, and \(h\) and \(c\) being the Planck constant and the speed of light in the vacuum, respectively.