  Nanomechanical photothermal sensing has significantly advanced
single-molecule/particle microscopy and spectroscopy, and infrared detection
through the use of nanomechanical resonators that detect shifts in resonant
frequency due to photothermal heating. However, the relationship between
resonator design, photothermal sensitivity, and response time remains unclear.
This paper compares three resonator types - strings, drumheads, and trampolines
- to explore this relationship. Through theoretical modeling, experimental
validation, and finite element method simulations, we find that strings offer
the highest sensitivity (with a noise equivalent power of 280 fW/Hz$^{1/2}$ for
strings made of silicon nitride), while drumheads exhibit the fastest thermal
response. The study reveals that photothermal sensitivity correlates with the
average temperature rise and not the peak temperature. Finally, the impact of
photothermal back-action is discussed, which can be a major source of frequency
instability. This work clarifies the performance differences and limits among
resonator designs and guides the development of advanced nanomechanical
photothermal sensors, benefiting a wide range of applications.
