On-the-Fly Calibrated Measure and Remote Control of Temperature and Viscosity at Nanoscale

Abstract

A novel on-the-fly calibration method of optical tweezers is presented, which enables in situ control and measure of absolute temperature and viscosity at nanoscale dimensions. Such noncontact measurement and control at the nanoscale are challenging as the present techniques only provide off-line measurements that do not provide absolute values. Additionally, some of the present methods have a low spatial resolution. We simultaneously apply the high temporal sensitivity of position autocorrelation and equipartition theorem to precisely measure and control in situ temperature and the corresponding microrheological property around the focal volume of the trap at high spatial resolution. The femtosecond optical tweezers (FOTs) use a single-beam high repetition rate laser for optical trapping to result in finer temperature gradients in comparison to the continuous-wave laser tweezers. Such finer temperature gradients are due to the additional nonlinear optical (NLO) phenomena occurring only at the nanoscale focal plane of the FOTs. Because NLO processes are laser peak power-dependent, they promote an effective study of physical properties occurring only at the focal plane. Using FOTs at optically benign near-infrared wavelengths, we demonstrate microrheological control and measurement in water by adding a highly absorbing yet low fluorescent dye (IR780).

Figure 1

FOT setup. WP: half-wave plate; PBS: polarizing beam splitter; L1: concave lens (f: 10 cm); L2: collimating convex lens (f: 20 cm); DM(1,2): dichroic mirror; O: objective lens; PZS: piezoelectric sample stage; PZC: piezoelectric controller; DAQ: data acquisition card; C: condenser lens; GF: green filter; L3: focusing lens (f: 5 cm); QPD: quadrant photodiode; SM: silver mirror; RF: red filter; CCD: camera (charge-coupled device); PC: personal computer.

Figure 2

(a) Measured differential light scattering through our buffered fluorophore-coated polystyrene bead sample solution showing the average particle distribution size to be 550 nm. (b) Absorption spectrum of the IR780 dyes.

Figure 3

NPAF of the 550 nm radius trapped particle in linear-log plot with a delay time (τ) (a) at different trapping laser powers within (a) 1.25 × 10^–5 (M) concentration of the IR780 dye in water and (b) 3.33 × 10^–5 (M) concentration of the IR780 dye in water.

Figure 4

NPAF of the 550 nm radius trapped particle in linear-log plot with delay time (τ) at different trapping wavelengths and within (a) 1.25 × 10^–5 (M) concentration of the IR780 dye in water and (b) 3.33 × 10^–5 (M) concentration of the IR780 dye in water.

Figure 5

(a) Temperature around trapped bead vs trapping laser power (at 780 nm) experimental data (red circle) and its linear fit (blue line) at an IR780 dye concentration of 3.33 × 10^–5 (M) and experimental data (green circle) and its linear fit (orange line) at an IR780 dye concentration of 1.25 × 10^–5 (M). (b) Temperature around trapped bead vs trapping wavelength (at 25 mW power) experimental data (red circle) and corresponding Lorentzian fit (red line) at an IR780 dye concentration of 3.33 × 10^–5 (M) and experimental data (green circle) and its Lorentzian fit (green line) at an IR780 dye concentration of 1.25 × 10^–5 (M).