**1. Introduction**

A fluidic gallium-based liquid metal (LM) is an interesting material for flexible and stretchable electronics and has received much attention from researchers owing to its extraordinary electrical conductivity and outstanding mechanical properties [1–4]. It is known that various materials have been utilized for manufacturing flexible and stretchable electronics [5–7]. However, these materials are not flexible and stretchable in the bulk state and need to be treated further. Interestingly, LM has a fluidic nature at room temperature and thus has potential for various applications in stretchable electronics. With the rapid development of artificial and flexible applications and systems, such as flexible and wearable electronics [8,9], electronic skins [10–12], sensors [13,14], and energy harvesting and storage devices [15–17], LMs can be utilized for various applications in these fields. In particular, gallium-based LMs, such as eutectic gallium indium (EGaIn, Ga/In 85.8%/14.2%), have been intensively investigated in recent years because their toxicity is lower than that of mercury. For instance, gallium-based LMs can be used as high-elasticity droplets [18], self-powered liquid metal machines [12], conductive traces for circuit boards [19,20], soft electrodes for plasma [21], and reconfigurable antennas [22,23].

The patterning of LM film is another strategy for the fabrication of wearable, flexible, and stretchable devices. In contrast to other solid metals, manipulation of LM is difficult because of its high surface tension in the fluidic state and quick oxidation in air. To overcome this, various patterning methods for gallium-based LM have been developed. LM electrodes with patterned structures have been developed by many facile and cheap printing methods, including 3D printing [24], direct printing [25], inkjet printing [26], stencil printing [27], photolithography [28], masked deposition [29], microcontact

printing [30], laser patterning [31] and dielectrophoresis [32,33]. One of these methods, laser patterning, can be used with various materials and is a fast and simple method for fabricating devices [34,35]. Therefore, we tried to fabricate the desired LM patterns by the laser ablation method. It is expected that a thin LM film can be rapidly patterned by a fiber laser marker without fatal damage to the polydimethylsiloxane (PDMS) substrate because only metals can absorb energy at a wavelength of 1064 nm, while PDMS cannot.

In addition to the fabrication of conductive LM film, another main area of this work is the utilization of a newly formed metal oxide layer of LMs during the process. Most LMs based on gallium alloys are rapidly oxidized in contact with oxygen and form an ultrathin metal oxide layer by a self-limiting reaction [2,4,36]. It is known that a transparent Ga2O3 film is used as a photo-responsive film to measure low-density ultraviolet (254 nm and 365 nm) and visible light [37,38]. Furthermore, it has been reported that the oxidized layer could be exfoliated from the LMs with adhesive materials that are used as 2D materials for the semiconducting layer [39]. Thus, it is considered that the newly formed Ga2O3 film in this work can be separated with an adhesive material, and this layer would show photo-responsive performance.

In this work, we introduce a simple and reliable method to fabricate a flexible and transparent photodetector based on an LM. A large and thin conductive EGaIn film can be fabricated with compressed EGaIn microdroplets. The LM film is su fficiently conductive and can be rapidly patterned by laser ablation. In addition, a photo-responsive gallium oxide layer can also be separated with an adhesive PDMS substrate from a conductive LM film. A flexible and transparent photodetector was fabricated by combining the patterned LM electrode and the separated Ga2O3 film.
