# A Wideband Termination Based on Laser-Scribed Lossy Microstrip Line Structures
## 4 Results and Discussions  
The laser-scripted open-end termination is demonstrated in Figure 6a. As investigated in the previous studies [13], laser irradiation of graphene oxide film turns its color from brown to metallic grey, which implies the formation of rGO. The insets demonstrate the scanning electron microscope images of the GO and laser-scribed rGO film. During the continuous wave laser irradiation, the single-photon absorption produces a photothermal effect with a strong heat accumulation, leading to an increase in temperature at the vicinity of the focal point. This enables photothermal reduction, where the oxygen-containing groups mounted on the GO sheets are removed via thermal decomposition [22], as the FTIR spectra presents in Figure 6b. The GO spectrum shows several characteristic peaks which are attributed to the epoxide, hydroxyl, and carboxylic groups [23]. The peak at 1619 cm\({}^{-1}\) corresponds to the vibration of aromatic C=C bonds. The broadband around 3380 cm\({}^{-1}\) is due to the stretching vibration of the O-H. The peak at 1724 cm\({}^{-1}\) is assigned to the C=O stretching vibration, and the peaks at 1060 cm\({}^{-1}\) and 1220 cm\({}^{-1}\) are attributed to the C-O stretch vibrations. Obviously, the peak strength of the oxygen-containing groups is weakened for the laser-scribed rGO. As the result, the C/O ratio is gradually increased from 2.09 to 4.06 with increased laser power, as we previously reported [13], whereas the oxygen concentration decreases. By tuning the laser reduction degree, the oxygen content could be tuned, thus the conductivity of the rGO film could also be changed within a certain range. The sheet resistance was obtained from the formula \(R_{\mathrm{s}}=Rw/l\), where the resistance \(R\) was measured using a Keithley 2400 source meter, and \(w\) and \(l\) are the width and length of the electrode, respectively. The measured sheet resistances are plotted in Figure 7, illustrating a tunable feature from around 1.2 k\(\Omega/\square\) to 175 \(\Omega/\square\) with increasing laser power. The corresponding conductivity ranges from 75.4 S/m to 517 S/m, which is obviously lower than that of silver-based conductors (>10\({}^{3}\) S/m) [24]. Note that the laser scripted rGO film provides a well-matched sheet-resistance range to the device design (20 \(\Omega/\square\)-1.2 k\(\Omega/\square\)) for the goal of high absorption and low reflection. Therefore, a wide laser power range from 25 mW to 150 mW can be chosen for the LDW. The thin GO film without laser irradiation illustrates a very low conductivity, which is similar to a dielectric. The influences of the remaining GO on the S-parameters can be ignored. Figure 6: (a) Laser-scripted open-end termination. The insets are the scanning electron microscope (SEM) images of the GO and laser-scribed rGO film with a laser power of 40 mW. The scale bar indicates 20 \(\upmu\)m. (b) the Fourier transform infrared spectroscopy (FTIR) spectra of the GO and laser-scribed rGO film. The fabricated terminations were examined by a vector network analyzer (ZVA67, Rohde & Schwarz) from 0.2 GHz to 30 GHz. A variation of the laser power from 40 mW to 50 mW was demonstrated by three samples with corresponding sheet resistances of 410 \(\Omega/\square\), 383 \(\Omega/\square\), and 365 \(\Omega/\square\), respectively, simulating the fabrication-error-caused variations of the sheet resistance. The measured reflections ( \(\mathrm{[S_{11}]}\)) in Figure 8 show high return losses. All samples provide a termination level of over \(-10\) dB at frequencies above 4.0 GHz, and a terminal level of over \(-15\) dB at frequencies above 6.0 GHz. This confirms the efficacy of the design and the fabrication process, and the functionality of the resulting terminations. Additionally, the S-parameters of the laser-scribed termination at two-port configurations is measured, as plotted in Figure 9. The terminations can provide over a \(-10\) dB attenuation level ( \(\mathrm{[S_{21}]}\)) at frequencies above 8.5 GHz.