Extending the bandwidth of optical-tweezers interferometry

Abstract

High-resolution force and displacement measurements by laser interferometry, combined with optical tweezers in a light microscope, are frequently based on near-infrared lasers. With common silicon PN photodiodes the bandwidth of detection was found to be limited to about 5 kHz at 1064 nm laser wavelength. This is caused by the fact that silicon becomes increasingly transparent for wavelengths approaching the band gap energy, leading to the generation of charge carriers outside the depletion zone of the diode for wavelengths longer than about 850 nm. These charges have to diffuse before they can contribute to the photocurrent. In this technical note we demonstrate experimentally that the detection bandwidth can be extended to at least 100 kHz, either by using wavelengths below 850 nm, or by using different detectors at longer wavelengths: InGaAs PIN photodiodes or special-purpose fully depleted p-type silicon photodiodes. We measured the well-known power spectral density of the Brownian motion of micron-sized beads in optical tweezers and show that the optimized detectors do not cause attenuation within experimental noise. They are indeed linear enough to detect the weak inertial effects of the watery solvent on the power spectral density of the Brownian motion.

Single-molecule force spectroscopy: optical tweezers, magnetic tweezers and atomic force microscopy

“…Axial motion of the trapped particle also changes the interference pattern resulting in changes in the total intensity at the detector. Sub-nanometer spatial resolution and bandwidths in excess of 100 kHz have been achieved with this detection scheme 49,51 .…”

Calibrating bead displacements in optical tweezers using acousto-optic deflectors

“…15 We used this special purpose p-type, silicon QPD, operated at a reverse bias voltage of 100 V, to avoid suppression of high-frequency signals. 29 For fine control of the sample surface with respect to the trap, a piezo stage ͑P-517.3CL Physik Instrumente, FIG. 1.…”

Brownian Motion after Einstein: Some New Applications and New Experiments

“…So the differences in Fig. 1 between Einstein's simple theory and the hydrodynamically correct theory for Brownian motion in a fluid can be exposed experimentally [21,23,24]. The form of the thermal force in (12), on the other hand, remains a theoretical result.…”