Optical attenuation signatures of Bacillus subtillis in the THz region

This letter reports the use of a photomixing sweep oscillator to characterize samples of B. subtillis spores at room temperature between 200 and 1200 GHz. Dilute (10% by mass) and concentrated samples were tested, the former using fine-grained polyethylene powder as a low-loss dielectric matrix. The dilute sample displays many weak transmission undulations, but the three centered at 418, 1037 and 253 GHz were strong enough to remain after normalization. The concentrated sample displays a much stronger broadband attenuation, precluding accurate measurements above ∼500 GHz. Two broad signatures remain between 250 and 290 GHz, and between 410 and 440 GHz, respectively. The results suggest that single-bioparticle electromagnetic resonance, perhaps from surface phonons, occurs in the dilute sample but is smeared in the concentrated sample.