Fig. 1. Schematic setup of optical differential detection of nanometer-sized particles.

Fig. 2. Schematic of optical differential detection of nano-particles in a microfluidic channel using two tapered fibers for optical interference.

Fig. 3. (Color online) Differential waveforms of experimental signal, corresponding frames from a video recording with a test particle passing through the probe beam at four positions a–d.

Fig. 4. (Color online) In each chart a–d, the upper two curves refer to the direct measured PGC modulation waveforms, and the middle two curves refer to the normalized measured PGC modulation waveforms, while the lower green curve refers to the differential normalized waveform. Four graphs correspond to the four positions recorded by screen grabs a–d in Fig. 3, indicating change in waveforms as the test particle passing through the signal beam.

Fig. 5. (Color online) Measured phase shift as a function of the diameter of nano-particles (red, 900 nm polystyrene nanospheres; gray, 800 nm polystyrene nanospheres; green, 600 nm polystyrene nanospheres; blue, 500 nm polystyrene nanospheres; purple, 300 nm polystyrene nanospheres; yellow, 200 nm polystyrene nanospheres) as a function of the particles’ diameters with error bars of 14.5%, 14.8%, 19.5%, 20.3%, 22.0%, and 23.8%, respectively, while the dotted curve shows predicted values from the theoretical model^[25].