## 1 Introduction  
Femtosecond pulsed laser deposition or femtosecond pulsed laser ablation (fs-PLA) is a versatile technique used in diverse applications. When performed in vacuum or gas environment, it is typically used for growing thin solid films or nanostructured materials like nanoparticles (NPs) of multiple elements, from polymers and biomaterials to oxides, carbides, nitrides, or metals [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11]. When applied in a liquid environment, fs-PLA allows the synthesis of colloidal solutions of NPs from the ablated material in a complex process that involves: (i) the absorption of the laser pulse by multiphoton absorption and direct photoionization; (ii) the ablation or fragmentation of the bulk material in the form of hot atoms, vapors, and liquid drops under non-equilibrium conditions; (iii) the expansion and quenching of the plasma plume; (iv) the expansion and collapse of the cavitation bubble; and (v) the growth and agglomeration of the NPs. A description of the full process can be found, for instance, in the paper by [PERSON] and [PERSON] [12].  
Diverse bulk materials have been applied to synthesize NPs by PLA in water or other liquid environments--either with femtosecond or nanosecond pulses--including metals [13; 14; 15; 16; 17; 18; 19; 20; 21; 22], metal alloys [23; 24; 25; 26], oxides [27; 28; 29], and other materials [30; 31; 32; 33]. Noble metal NPs like gold and silver have been extensively studied due to their optical properties. Particularly interesting is their localized surface plasmon resonance (LSPR); i.e., collective oscillations of the free electrons, excited by the absorption of photons in the visible spectrum. The LSPR depends on the plasmonic NP shape, size, size distribution, and concentration, as well as on the surrounding medium [34]. On the other hand, colloidal solutions containing Ag NPs are typically fabricated by chemical methods based on the reduction of silver salts [35; 36; 37]. Hence, the final product usually contains non-negligible amounts of reaction remnants, such as anions and reducing agents (mainly organic derivatives), which can interfere with subsequent stabilization and functionalization steps. PLA, on the other hand, is able to generate contaminant-free colloidal solutions of Ag NPs. Moreover, PLA also has the advantage of synthesis speed, because with this technique the particles can be obtained in a few minutes, compared to the several hours required by colloidal synthesis.  
The fabrication of Ag NPs in water by fs-PLA was first reported by [PERSON] et al. [16], and they studied the size distribution and compared their results to those previously obtained with nanosecond laser pulses. Unfortunately, those colloidal suspensions formed in pure water are highly unstable because the bare plasmonic NPs tend to aggregate, forming clusters that destabilize the solution--especially if high-concentration suspensions need to be prepared. This problem is also present for plasmonic NPs fabricated by colloidal synthesis [38; 39; 40], and is usually solved by using stabilizing molecules that inhibit the formation of aggregates. Stabilizer molecules such as sodium dodecyl sulphate [41; 42; 43], hexadecyltrimethylammonium bromide [43], cyclodext [44], various salts (e.g., KCl, NaCl, and NaOH) [20], or polyvinylpyrrolidone [17] have been successfully applied to perform PLA in liquids, and their effects in nanocrystal size and stability have been studied; however, the ablation conditions in these experiments usually lead to low-concentration solutions.  
In this work, we have fabricated Ag NPs by means of fs-PLA in purified water, obtaining highly-concentrated colloidal solutions. Afterwards, we studied the effect of two different stabilizers on the nanocrystal size distribution and concentration. The selected molecules were the ionic surfactant hexadecyltrimethylammonium bromide (CTAB) and the anionic polymer polyvinylpyrrolidone (PVP).