## Materials and Methods
### Methods  
Plate preparation:  
High purity copper was used for the study in the form of 3 mm thick disks of 60 mm diameter both as standalone material or in combination with gold. High purity gold was used in magnetron sputtering equipment (Cresington 108) as target for the bimetallic study. In this way, a thin film of gold was deposited on the polished copper surface. The highly fluorescent dye R6G was used as pure powder for the basic Raman spectroscopy. An aqueous solution of R6G with 10\({}^{-4}\) M molar concentration was prepared using distilled water at room temperature for the SERS amplification study. The Cu plates were optically polished and then washed with ethanol after a 10 min sonication to remove impurities from the surface. After this, one plate was coated with a thin film of 200 nm thickness of Au with the DC magnetron sputtering system. Further mentions of this resulting product of bulk Cu coated with 200 nm of Au are called simply Cu-Au. Thin film thickness measurements were taken during the film deposition using a thickness measurement system equipped with a quartz crystal, as provided by the deposition system. Both coated and uncoated plates were then laser irradiated. A Lumera Hyperrapid 50 (diode pumped Nd:YAG laser), with 1064 nm wavelength, was used for the substrate fabrication. The laser system delivered pulses of 8 ps duration, fixed 500 kHz pulse repetition rate and adjustable average power up to a maximum of 50 W, corresponding to 100 \(\upmu\)J average pulse energy, with a Gaussian (TEM00) energy distribution and beam quality factor (M\({}^{2}\)) higher than 1.4, according to the datasheet. The laser beam was directed using a galvanometric scanner and focused on a circular spot of \(72.5\pm 3.5\)\(\upmu\)m fixed diameter (value at 1/e\({}^{2}\) of the maximum intensity of the Gaussian profile of the laser beam [20]) by an f-theta lens where f = 160 mm for all experiments. Laser ablation experiments:  
Laser markings on the plate were created by ablating tightly packed parallel lines, which formed an ablated square of 3 by 3 mm. A total of 49 such squares were laser ablated onto the surface of each plate, arranged into a 7 by 7 matrix. Each row of squares was ablated using a different laser fluence and each column, with a different laser marking speed. This way, each plate was ablated with 49 squares formed with different combinations of laser fluence and laser writing speeds. The laser fluences range from (\(0.3\pm 0.02\)) J/cm\({}^{2}\) up to (\(2.42\pm 0.22\)) J/cm\({}^{2}\). Variation of the laser fluence was achieved by using the power setting provided by the control system, which varied the pulse energy in steps of 0.3 J/cm\({}^{2}\). The value of 1.8 J/cm\({}^{2}\) was skipped. The laser marking speeds ranged from 100 mm/s to 1000 mm/s. Considering that the pulse repetition rate is a constant 500 kHz, the laser spot diameter is fixed approximately at 72.5 \(\upmu\)m and the trajectory is always linear, the laser writing speed parameter is described in its equivalent average number of overlapping pulses. It expresses the average number of pulses of the calculated spot diameter which would overlap on any given point within the linear trajectory of the laser markings. A higher laser writing speed will yield a lower number of overlapping pulses and vice versa. The lowest laser writing speed, 100 mm/s, is equivalent to approximately 360 overlapping pulses, while the highest writing speed, 1000 mm/s, is equivalent to approximately 36 overlapping pulses. Intermediary laser writing speeds were in increments of 150 mm/s, producing a total of 7 different laser writing speeds. Consequently, the calculated values for the number of overlapping pulses were approximated as follows: 36; 42; 51; 65; 90; 144 and 360.