Scanning force microscope using an optical trap

An optical force microscope, comprising:

• a fluid chamber adjacent a sample to be sensed;
• a sensing probe located in said chamber and supported by said fluid;
• a laser light source producing a light beam having an axis of propagation;
• means focusing said laser light beam at a focal region in said fluid to produce an optical trap which confines said probe coaxially with said light beam and positions the probe in close proximity to said sample;
• and
• detector means positioned to receive laser light scattered from said probe for determining displacement of said probe.

An optical scanning force microscope sensitive to small forces, comprising:

• a housing having walls composed of an optically transparent material;
• a fluid medium within said housing;
• a laser light source generating a light beam at a predetermined wavelength;
• means for focusing said light beam with sufficient convergence to generate at least one gradient force optical trap at a predetermined focal region within said fluid medium;
• a probe particle immersed in said fluid medium and having a generally non-spherical, prolate shape carried by said optical trap at a position of stable local equilibrium near the focal region generated by said laser light source, said probe having dimensions comparable to the size of said focal region and having a sharp tip with an apex dimension substantially less than the wavelength of said laser light source, said optical trap providing three dimensional confinement and accurate positioning of the probe;
• a light detector disposed for receiving laser light directly related to the displacement of the sensing probe relative to the focal region said laser light source and outputting an electrical signal directly related to the power of said received laser light, the detector including a lens collecting the beam emitted by said laser light source and producing a conjugate focus at a predetermined position, the detector further including a photodiode having a limiting aperture truncating the light beam collected by said light focusing lens;
• a scan stage means for creating transverse scan movement supporting said housing closely adjacent to said means focusing said light beam;
• and
• vertical position adjustment means for adjusting the position of said probe tip.

In a method for sensing displacement of a sensing probe in a scanning force microscope, comprising the steps of:

• dispersing multiple sensing probes in a fluid medium contained within a housing;
• generating a laser light beam at a predetermined wavelength:
• focusing said light beam with sufficient convergence to generate at least one gradient force optical trap in a predetermined region within said fluid medium to produce stable, three dimensional confinement and accurate positioning of one of said sensing probes near a sample surface to be sensed;
• disposing a light detector to receive laser light from said laser source scattered by said sensing probe in said optical trap;
• producing an electrical signal from said detector directly related to the power of said received laser light, said electrical signal being directly related to the displacement of the sensing probe relative to the optical trap;
• scanning the sensing probe over the sample surface while operating the laser light source;
• and
• determining from said electrical signal physical properties of the sample.