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Perfect absorbers with extremely high absorption are of crucial importance for many applications such as harvesting solar energy and eliminating stray light. A key point for their applications is ultralow photon escaping rate (PER) with easy fabrication process, which enables to maximize energy utilization or minimize the influence of stray light. An ultrathin perfect absorber (total thickness of 226 nm) is reported in this work with dielectric plasmonic nanocomposite (titanium nitride (TiN) embedded in aluminum nitride (AlN)) and antireflective coating (ARC). It can be fabricated by low‐cost one‐step sputtering approach in a very large area (200 × 200 mm2 for demonstration) on almost any substrates, including flexible polyethylene‐terephthalate. The average PER is only 0.4% in the whole visible range of 400–750 nm. As an example, the absorber is used in a laser scanning confocal microscope to eliminate the strong laser stray light and enhance the signal‐to‐noise ratio, which improves the clarity and contrast of imaging remarkably. It can not only match the requirement of harvesting solar energy, but also is suitable for stray light elimination of weak signal detection instruments, etc. [ABSTRACT FROM AUTHOR]