  Laser direct writing is an attractive method for patterning 2D materials
without contamination. Literature shows that the femtosecond ablation threshold
of graphene across substrates varies by an order of magnitude. Some attribute
it to the thermal coupling to the substrates, but it remains by and large an
open question. For the first time the effect of substrates on femtosecond
ablation of 2D materials is studied using MoS$_{2}$ as an example. We show
unambiguously that femtosecond ablation of MoS$_{2}$ is an adiabatic process
with negligible heat transfer to the substrates. The observed threshold
variation is due to the etalon effect which was not identified before for the
laser ablation of 2D materials. Subsequently, an intrinsic ablation threshold
is proposed as a true threshold parameter for 2D materials. Additionally, we
demonstrate for the first time femtosecond laser patterning of monolayer
MoS$_{2}$ with sub-micron resolution and mm/s speed. Moreover, engineered
substrates are shown to enhance the ablation efficiency, enabling patterning
with low-power femtosecond oscillators. Finally, a zero-thickness approximation
is introduced to predict the field enhancement with simple analytical
expressions. Our work clarifies the role of substrates on ablation and firmly
establishes femtosecond laser ablation as a viable route to pattern 2D
materials.
