Enhanced Bandwidth in Radiation Sensors Operating at the Fundamental Temperature Fluctuation Noise Limit

Temperature-based radiation detectors are essential for long-wavelength detection, but they suffer from important bandwidth limitations. Their responsivity and, hence, their noise equivalent power (NEP) typically degrade at frequencies exceeding their characteristic thermal response time cutoff (τ_th), i.e., at ω > τ_th^-1. We show that this bandwidth limitation can be broken when a sensor operates at its fundamental temperature fluctuation noise limit. The key enabler of this demonstration is a nanomechanical sensor in which frequency stability is limited by fundamental temperature fluctuations over an unprecedentedly large bandwidth of 54 Hz. In this range, the sensor performance remains within a factor 3 of its peak detectivity (D_T^* = 7.4 × 10⁹ cm Hz^1/2 W^-1) even though the thermal cutoff frequency is 30 times lower (i.e., 1/2πτ_th = 1.8 Hz). We also derive expressions predicting this bandwidth enhancement for nanomechanical resonator-based sensors within a closed-loop frequency tracking scheme.