Optical trap, choosing the right condenser NA illuminate QPD

In summary, the conversation discusses illuminating a quadrant photo diode using a condenser/lens combination in an optical trap setup. The use of a 100x objective as a trap and a 10x objective as a condenser is mentioned. It is debated whether or not it is necessary to collect all the laser light scattered from a 1.25 NA objective, as a 0.25 NA condenser may not be able to do so. The idea that it is critical to image the whole laser onto the QPD is also questioned. The conversation ends with the suggestion to contact the speaker for further discussion.
  • #1
donroy81
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0
Hi, I am looking to illuminate a quadrant photo diode using a condenser/ lens combination in a optical trap setup. I see thorlabs has a kit that has a 100x objective as a trap and a 10x objective as a condenser. Does that mean that one doesn't need to collect all the laser light scattered from a 1.25 NA objective, because surely a 0.25 NA condenser cannot do that. I have always thought it is critical to image the whole laser onto the QPD but maybe it is not. Any inputs?
 
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  • #2
donroy81 said:
Hi, I am looking to illuminate a quadrant photo diode using a condenser/ lens combination in a optical trap setup. I see thorlabs has a kit that has a 100x objective as a trap and a 10x objective as a condenser. Does that mean that one doesn't need to collect all the laser light scattered from a 1.25 NA objective, because surely a 0.25 NA condenser cannot do that. I have always thought it is critical to image the whole laser onto the QPD but maybe it is not. Any inputs?

We are currently doing exactly the same thing (except my own scope instead of a kit).

The QPD does not need to collect the entire amount of light, only enough to detect the change in diffraction pattern as the trapped particle moves. We're still debugging the QPD data analysis software, but we seem to be able to generate useful datasets fairly easily.

My setup uses either a 63X NA 0.9 dipping or a 100X NA 1.47 immersion objective lens, and we defocus the condenser (NA 0.85 IIRC) slightly off Kohler conditions to (we hope) increase the sensitivity at the QPD.

PM me if you'd like to discuss more, let me know how it goes!
 

1. What is an optical trap?

An optical trap, also known as an optical tweezers, is a scientific instrument that uses a highly focused laser beam to trap and manipulate microscopic objects. This technique is based on the principle of light scattering and can be used to study the properties of individual cells, molecules, and nanoparticles.

2. How do I choose the right condenser NA for my optical trap?

The numerical aperture (NA) of a condenser lens is an important factor in determining the size and intensity of the laser beam used in an optical trap. A higher NA will result in a smaller and more intense beam, which can be useful for trapping smaller objects. However, a lower NA may be more suitable for larger objects or when working with delicate samples. Ultimately, the choice of condenser NA will depend on the specific requirements of your experiment.

3. What is the role of illuminate QPD in an optical trap?

Illuminate quadrant photodiode (QPD) is a type of position-sensitive detector that is commonly used in conjunction with optical traps. It is used to measure the position of the trapped object by detecting changes in the intensity of the laser beam as it is scattered off the object. This information is then used to adjust the position of the trap and maintain its stability.

4. How can I optimize the performance of my optical trap?

To optimize the performance of an optical trap, it is important to carefully choose the laser source, condenser lens, and position detection system. Additionally, regular calibration and maintenance of the instrument is necessary to ensure its accuracy and stability. It is also important to carefully consider the properties of the sample being studied and adjust the experimental conditions accordingly.

5. What are the limitations of using an optical trap?

While optical traps have revolutionized the study of microscopic objects, they do have certain limitations. These include the size and type of objects that can be trapped, as well as the potential for photodamage due to the intense laser beam. Additionally, the accuracy and stability of the trap may be affected by factors such as environmental vibrations or changes in temperature. Careful experimental design and optimization can help mitigate these limitations.

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