Laser-direct-write techniques are one of the most versatile direct-write techniques, which uniquely enables the addition, removal and modification of a target materials without any physical contact. Additionally, these LDW techniques allow for the processing of complex materials with resolutions spanning more than three orders of magnitude, from millimetres to microns, and this makes these LDW process uniquely suited for fabrication of structures that are not possible via use of other techniques.
The LDW system normally consist of three key parts: a laser source, a beam delivery pathway, and substrate-material translation system. The heart of any LDW system is always the laser source. A wide range of lasers, from ultrafast pulsed lasers to continuous wave (c.w.) lasers, have been applied for LDW processing and the fundamental interaction of the laser with the target material is usually the main factor in the choice of the appropriate laser source and its corresponding parameters including the wavelength, pulse duration, divergence, beam quality etc., which directly determine the energy absorption efficiency and the subsequent material response.
To date, many kinds of LDW techniques have been used in science and engineering, and these techniques can be classified into three main categories: LDW subtractive procedures, where material is removed; LDW additive processes, where material is added; and finally, LDW modification techniques. The technique our group has developed and is described in detail below belongs to the last category, namely LDW modification wherein a laser is used to modify the target material or substrate to create user defined patterns within it. Our group has developed this technique with a focus on developing low-cost microfluidics-based sensors in paper that primarily have applications in point-of-care disease diagnostics.