Apparatus and method for sizing nanoparticles based on optical forces and interferometric field detection

Abstract

Light from a laser source is split into a reference arm and a detection arm. The light in the detection arm is focused into a channel containing particles to be detected and is backscattered by the particles. The light in the reference arm is attenuated. The attenuated and backscattered light are caused to interfere and detected by a split detector so that the effects of background light can be subtracted out, while the backscattered light is detected to detect the particles.

1 . A method for detecting a particle in a location, the method comprising: 
 (a) emitting a beam of electromagnetic radiation;    (b) splitting the beam of electromagnetic radiation into a first component and a second component;    (c) directing the first component into a reference arm;    (d) directing the second component into the location;    (e) receiving light backscattered from the location;    (f) causing the backscattered light to interfere with the first component from the reference arm to produce an interference intensity distribution;    (g) detecting the interference intensity distribution with a detector comprising a plurality of components; and    (h) detecting the particle in accordance with a difference among detection signals form the plurality of components.    
   
   
       2 . The method of  claim 1 , wherein step (h) comprises deriving a particle detection signal from a difference between the detection signals from two of said components.  
   
   
       3 . The method of  claim 1 , wherein the reference arm comprises an attenuator, and wherein step (c) comprises attenuating the first component with the attenuator.  
   
   
       4 . The method of  claim 3 , wherein the attenuator is adjustable, and wherein step (c) comprises adjustably attenuating the first component with the adjustable attenuator.  
   
   
       5 . The method of  claim 4 , wherein the adjustable attenuator is adjusted to maximize a signal-to-noise ratio of the difference among the detection signals as a function of a degree of attenuation of the first component.  
   
   
       6 . A system for detecting a particle in a location, the system comprising: 
 a source of a beam of electromagnetic radiation;    a beam splitter for splitting the beam of electromagnetic radiation into a first component and a second component;    a reference arm receiving the first component from the beam splitter;    focusing optics, receiving the second component from the beam splitter, for directing the second component into the location and for receiving light backscattered from the location, thereby causing the backscattered light to interfere with the first component from the reference arm to produce an interference intensity distribution;    a detector comprising a plurality of components for detecting the interference intensity distribution; and    a data acquisition system for detecting the particle in accordance with a difference among detection signals form the plurality of components.    
   
   
       7 . The system of  claim 6 , wherein the data acquisition system derives a particle detection signal from a difference between the detection signals from two of said components.  
   
   
       8 . The system of  claim 6 , wherein the reference arm comprises an attenuator.  
   
   
       9 . The system of  claim 8 , wherein the attenuator is adjustable.  
   
   
       10 . The system of  claim 9 , wherein the adjustable attenuator comprises a half-wavelength plate.  
   
   
       11 . The system of  claim 6 , wherein the focusing optics comprise an objective lens.