**2. Direct laser writing**

The DLW method mainly uses an ultrafast laser (femtosecond, fs or picosecond, ps lasers) [7] but recently continuous wave (cw) lasers are also used for specific materials [8]. The laser

**Figure 1.** Schematic of direct laser writing setup.

source is tightly focused to a diffraction limited focal spot, using an objective made of different numerical apertures (NAs) forming a high intensity of laser beam. The transparency at the wavelength and spot size uniformity of the used laser beam as well as the sample uniformity is highly necessary to obtain good quality structures. However, due to the high-intensity generation from the tight focusing conditions, the non-linear process is triggered at the focal spot, which can lead to the photo-polymerization [9], microexplosion [10], photoreduction [11] and micro-machining process [12] in the material leaving remaining area unmodified. The sample moves with the help of three-dimensional (3D) translation stage according to the preprogrammed pattern design. The schematic of the DLW setup is given in **Figure 1**.
