THORLABS EDU-OT2 Bedienungsanleitung

EDU-OT2

EDU-OT2/M

Portable Optical Tweezers

User Guide

Portable Optical Tweezers Kit

Table of Contents

Chapter 1 Warning Symbol Definitions.......................................................1

Chapter 2 Safety............................................................................................2

Chapter 3 Product Description ....................................................................3

Chapter 4 Principles of Optical Tweezers...................................................5

4.1. Dipole Approach in the Rayleigh Scattering Regime R <<



....... 5

4.2. Geometrical Optics Approach in the Mie Regime R >>



........... 7

Chapter 5 Kit Components.........................................................................17

5.1. Trapping Laser Source.............................................................17

5.2. Beam Expander.......................................................................18

5.3. Right-Angle Mirrors ................................................................19

5.4. Sample Positioning System .....................................................20

5.5. Microscope .............................................................................21

5.6. Additional Components...........................................................23

5.7. Screws Included ......................................................................24

Chapter 6 Setup and Adjustment...............................................................25

6.1. Assembly ................................................................................25

6.1.1. Trapping Laser Source............................................................................26

6.1.2. Beam Expander ......................................................................................28

6.1.3. Microscope..............................................................................................33

6.1.4. Sample Positioning System ....................................................................40

6.2. Controller and Software Setup ................................................44

6.2.1. LED Controller.........................................................................................44

6.2.2. Laser Controller.......................................................................................44

6.2.3. Camera ...................................................................................................47

6.2.4. Sample Positioning Stages .....................................................................50

6.2.5. Kinesis Settings.......................................................................................53

6.3. Camera Adjustment ................................................................54

6.4. Beam Adjustment ...................................................................55

6.5. Trapping, Microscope Focus, and Final Alignment...................59

Portable Optical Tweezers Kit

Chapter 7 Experiments...............................................................................63

7.1. Creating a Sample.................................................................. 63

7.2. Setting the Correct Focus Level............................................... 64

7.3. Arranging the Silica Beads ..................................................... 65

7.4. Manipulating a Dairy Cream Particle in a Cream/Water

Emulsion ................................................................................ 66

7.5. The Holding Force of the Optical Trap .................................... 69

7.5.1. Brownian Motion......................................................................................69

7.5.2. The Maximum Holding Force ..................................................................71

Chapter 8 Teaching Tips ............................................................................73

Chapter 9 Control via Game Controller ....................................................81

Chapter 10 Troubleshooting........................................................................84

Chapter 11 Regulatory..................................................................................86

Chapter 12 Thorlabs Worldwide Contacts..................................................87

Portable Optical Tweezers Kit Chapter 1: Warning Symbol Definitions

Rev B, July 8, 2019 Page 1

Chapter 1 Warning Symbol Definitions

Below is a list of warning symbols you may encounter in this manual or on your device.

Symbol

Description

Direct Current

Alternating Current

Both Direct and Alternating Current

Earth Ground Terminal

Protective Conductor Terminal

Frame or Chassis Terminal

Equipotentiality

On (Supply)

Off (Supply)

In Position of a Bi-Stable Push Control

Out Position of a Bi-Stable Push Control

Caution: Risk of Electric Shock

Caution: Hot Surface

Caution: Risk of Danger

Warning: Laser Radiation

Portable Optical Tweezers Kit Chapter 2: Safety

Page 2 MTN012639-D02

Chapter 2 Safety

Warning

The class 3B laser diode used in this kit can emit more than 50 mW of optical power,

which can cause damage to the eyes if viewed directly. The laser driver is equipped with

a key switch and safety interlock, which should be used appropriately to avoid injury.

Additionally, we recommend wearing appropriate laser safety glasses

when using this kit.

Portable Optical Tweezers Kit Chapter 3: Product Description

Rev B, July 8, 2019 Page 3

Chapter 3 Product Description

For many people, moving and controlling objects with a beam of light sounds more like the

“tractor beams” of science fiction than reality. However, optical tweezers are devices that

allow precisely that kind of manipulation. Many areas of research use them to measure

small forces on the order of piconewtons

. More exotic applications include the control of

tiny microgears

. Biologists use optical tweezers to manipulate different types of molecules

and cells

. In-vitro fertilization of ova is a typical application example –sperm can be

inserted into ova without mechanical contact, thus maintaining a sterile environment.

In a lab course, various demonstrations and experiments can be performed with an optical

tweezers setup. This kit can be used to carry out basic experiments such as moving small

spheres or cells through a solution. The kit can also be used for more advanced

experiments such as investigating the Brownian motion of objects, and measuring the

optical forces of the tweezers.

The working principle can be explained using concepts usually known to undergraduate

students, such as geometric optics, basic theory of Brownian motion, and Stokes’ friction.

It is an intriguing experience to be able to control objects with a laser beam –and not only

for students!

This Optical Tweezers Kit can be assembled into a complete and fully operating

experimental setup with which particles onthe orderof micronscan betrapped andmoved.

The beam path is schematically depicted in Figure 1. It is possible to perform a variety of

experiments using a number of different particles such as polystyrene beads, glass beads,

or starch grains from ordinary corn flour. A special feature of this setup is that it is portable.

It can be moved from room to room without needing disassembly or major readjustment,

making it ideally suited to demonstrate the principle of optical tweezers to students in

seminars or lecture halls.

We recommend using the OTKBTK sample kit with the setup. The performance was

optimized for the sample slides and the cover glasses provided with the OTKBTK. For

simplicity, we designed the tweezers system to work without immersion oil.

K. SVOBODA, S.M. BLOCK: Optical trapping of metallic Rayleigh particles, Optics Letters 19

(1994) 13, 930-932

S.L. NEALE, M.P. MACDONALD, K. DHOLAKIA, T.F. KRAUSS: All-optical control of microfluidic

components using form birefringence, Nature materials 4 (2005), 530-533

J.E. MOLLOY, M.J. PADGETT: Lights, action: optical tweezers, Cont. Phys. 43 (2002) 43, 241-

258

Portable Optical Tweezers Kit Chapter 3: Product Description

Page 4 MTN012639-D02

The light path through the optical tweezers experiment.

Sample

Position

LED

Laser

Camera

Microscope Beam Path

Laser Beam Path

Portable Optical Tweezers Kit Chapter 4: Principles of Optical Tweezers

Rev B, July 8, 2019 Page 5

Chapter 4 Principles of Optical Tweezers

To describe the function of optical tweezers, we will examine the force that a focused laser

beam with a Gaussian intensity profile (the TEM00 mode) exerts on an object, which is near

or in the focus. Usually one also assumes that the object is a bead, which consists of a

dielectric, linear, isotropic, and spatially and chronologically non-dispersive material. In the

experiments described below, micron-sized beads made of polystyrene are primarily used.

It is customary to describe the force of the laser on the object by separating it into two

components. One component, the scattering force, acts along the direction of beam

propagation. The second component acts along the intensity gradient and is therefore

called the gradient force. The gradient force can act in different directions with respect to

the beam. As the laser has a Gaussian intensity profile, the gradient force can act

orthogonally to the beam, but it can also act parallel to the beam, as the laser is focused

and therefore also has an intensity gradient along the beam axis. These two components

and their relationship to one another are the defining factors for whether or not a particle

can be trapped by the optical trap. Stable optical tweezers are only obtained if the gradient

force, which pulls the object in the direction of the focus, is greater than the scattering

force, which pushes the particle in the direction of the beam away from the focus.

The various theoretical approaches to describe optical trapping can roughly be divided

according to the areas in which they are valid. The relationship of the radius (or diameter

) of the bead to the wavelength of the incident laser beam is the dividing factor. The

case  is theoretically very complex and shall therefore not be dealt with here. The

two extreme cases for very large and very small particles are summarized below:

4.1. Dipole Approach in the Rayleigh Scattering Regime R << 

The first case we will consider is when the radius of the bead is significantly smaller than

the wavelength of the incident laser beam.Then, the electrical field 

󰇍



󰇛󰇜is approximately

spatially constant with respect to the particle and the situation can be portrayed as follows:

As the bead is assumed to be dielectric, one can imagine it as a collection of point

dipoles. Due to their polarizability, a dipole moment is induced in each of the point

dipoles by the incident laser beam. Due to the linearity of the material, the following applies:

 

󰇍



󰇛󰇜

(1)

Here, is the location of the i-th point dipoles and 

󰇍



󰇛󰇜is the electrical field strength at

this location. In addition, the electrical field of the laser appears to be approximately

spatially constant for the bead due to the condition  , meaning that at a certain point

in time the strength of the electrical field is equally great for all point dipoles of the bead.

As a result, the induced dipole moment is equally great for all point dipoles. The

polarization 

resulting from the induced dipole moments is then

Portable Optical Tweezers Kit Chapter 4: Principles of Optical Tweezers

Page 6 MTN012639-D02



 







󰇍



 

󰇍



(2)

where is the electrical susceptibility, is the electrical constant, and is the volume of

the bead. The potential energy of one of the point dipoles with dipole moment in the

electrical field 

󰇍



is 

󰇍



. Because there are point dipoles in a bead with the volume

, the energy density in the bead is defined by:











󰇍





󰇍





󰇍





󰇍



(3)

The occurrence of the gradient force, which is a force component that is directed in the

direction of the intensity gradient of the incident electrical field, can be explained when one

observes this potential energy of the bead in the electrical field. Equation (2) states that



is proportional to 

󰇍



. Therefore, according to equation (3), is proportional to 

󰇍



and

thus to intensity 

󰇍



of the incident field. The force exerted on the particle by the

incident field is proportional to the gradient of the potential energy  and therefore

proportional to the intensity gradient . The following equations describe the gradient

force:

  





(4)



󰇧

󰇨

(5)

 



(6)

Here, is the polarizability of the dipoles and is the relationship of the refraction index

of the particles, (polystyrene in our case) to the refraction index of the surrounding

medium, (water in our case).

The destabilizing scattering force component is explained by the scattering of the incident

light at the particle. The force action is created by the absorption and isotropic re-emission

of the light by the bead. As  , the conditions are fulfilled for Rayleigh scattering. The

resulting force can be stated as follows:

 



(7)



󰇧

󰇨

(8)

Portable Optical Tweezers Kit Chapter 4: Principles of Optical Tweezers

Rev B, July 8, 2019 Page 7

Here, is the speed of light in vacuum and is the incident intensity and is the scattering

cross-section of the incident light. It is important to note that the scattering force is

proportional to the intensity and points in the beam direction.

4.2. Geometrical Optics Approach in the Mie Regime R >> 

This ray optical approach deals with the second possible extreme case. Here, we will

assume that the radius of the bead is much larger than the wavelength of the incident

laser. In this range, the conditions of geometric (ray) optics are fulfilled and one can think

of the laser beam as a bundle of rays. Typically, this assumption is valid for beads with a

radius that is greater than . The basics of the theoretical derivation of the gradient

and scattering force in accordance with this model can be found in a work by Ashkin

The particle properties of light must now be taken into account, namely that light can

transfer momentum to an object in the form of photons. The force action of a beam on a

particle can be explained using Newton’s second law: the force on a particle is exactly

equal to the change in the momentum of the particle over time:

 



(9)

The following equation describes the change in momentum 

 of a beam over time in a

medium with a refractive index :



 

 

󰇛󰇜

(10)

Here, 󰇛󰇜is the intensity distribution in the beam cross-section. Often, a Gaussian profile

is used, in which the intensity decreases in a Gaussian distribution from the center of the

beam outward. This is also the case in our setup.

If a beam with the power  hits a sphere at an angle of , part of the beam will be

reflected and part of it will reach the interior of the sphere through transmission (see Figure

2). For the power of these two partial beams, the following is in effect:

   

(11)

   

(12)

Here, is the reflectivity and is the transmissivity. The transmitted beam transports

momentum into the sphere in accordance with the equation (10).

Ashkin A., Forces of a single-beam gradient laser trap on a dielectric sphere in the ray

optics regime. In: Biophys. J. 61 (1992) 2, 569-582

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