Characterizing TES Power Noise for Future Single Optical-Phonon and Infrared-Photon Detectors

In this letter, we present the performance of a 100 mu m x 400 mu m x 40 nm W Transition-Edge Sensor with a critical temperature of 40 mK. This device has a noise equivalent power of 1.5x10-18W/Hz, in a bandwidth of 2.6 kHz, indicating a resolution for Dirac delta energy depositions of 40 +/- 5 meV (rms). The performance demonstrated by this device is a critical step toward developing a O(100)meV threshold athermal phonon detector for low-mass dark matter searches.