First Images in Liquid Environment

By using NCLR cantilever, we obtained about 0.20V amplitude in liquid and we are able to scan biological samples from now on!

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In Fukuma’s article [1] we see that when we coated the back of the cantilever with carbon material, it is possible to get higher efficiency in conversation of light to heat. We tried to use NCHR (346 kHz) carbon coated cantilever. We see that it is possible to get improvement. However, Fukuma’s article suggests coating only the back of the cantilever. Our cantilever is coated fully. Also, they show that blackened area should be maximum 70%. It is reported that above 70% blackened area photothermal efficiency will remarkably decrease. If we follow the description, maybe it will be possible to get higher amplitude.

First Vibration in Liquid Environment

failIf you decide to work in liquid environment with your atomic force microscope, you will need some adjustment. First, you will change the position of your camera to see your cantilever in liquid since you know Snell's Law. It is like nightmare! It's too hard to adjust it to find a hundred micrometer cantilever. Second, in liquid environment, the resonance frequency of your cantilever will decrease and some of them will not work such as your favourite cantilever you use in air. Third, if you use diether piezo excitation, maybe your piezo will be small to work in liquid. Or if you use photothermal excitation, maybe the power of your laser will not be enough. You will try to find solutions however %95 will not work. You will feel your stress in your blood in this time interval. However, trust me, when you see a peak in liquid environment for the first time, you will feel like you have won the war!

The most important problem in this stage was my PS3 lasers. Although there is no excessive amount of heat and output voltage is not high and also I wear anti static grounding strap, PS3 excitation laser broke down itself every day. Since it is happening every day, we thought that the problem could be the driver circuit. By using oscilloscope we monitored output voltage and saw that output is changing continuously. The problem was excessive amount of heat on op-amp. We need to spread out heat and by adding suitable cooler we did it.

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Since focusing your lasers in liquid is a bit harder, you need find a solution. Actually, focusing the detection laser is easy, its focus is changing a little since the angle between incidence laser and interface is 90 degree. However, focusing excitation laser is not due to its angle. In the picture, you see our flipped AFM head to focusing it easily by reflecting lasers on papers.

We tried long cantilevers however we could not see any reasonable oscillation peak and also nice graph due to their low resonance frequency. Secondly, by using short cantilever, since their resonance frequency is about 4 times higher than longer one, we caught nice graph in liquid environment.

fc20In graph, oscillation amplitude is 0.11V and center frequency is about 163 kHz while Ft=2.314V, Fn=-0.023V and Fl=-0.005V. We have roughly 32.5 kHz FWHM value.

Photothermal Excitation for Atomic Force Microscopy

In NanoMagnetics Instruments, I'm improving my instrumentation skills by trying to scan a surface with AFM (Atomic Force Microscope) which we are modifying. This AFM is working with diether piezo excitation. However, with this method, you couldn't scan properly sensitive samples such as biological samples. The good news is you can use photothermal excitation. Different from the other method, you use a powerful laser and focus it on the back of the cantilever at some frequency e.g. 150 kHz. Therefore, your cantilever starts to oscillate and you have more sensitive AFM.

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To modify our AFM, we cut 80x60 mm iron sheet and folded up it for appropriate angle. The iron sheet is screwed to left hand side of the AFM head. Thorlabs positioner is screwed to the iron sheet. We found useful cylindrical plastic tube and fixed our PS3 laser diode on it with adhesive after some retouching. The laser with the tube is screwed to Thorlabs positioner. It was really hard to focus violet laser on the cantilever. We need to get closer and closer to focus it. In PS3 laser diode, the focal length of violet and red laser is not close each other to use both of them in a single laser diode head for our application. With only PS3 laser diode, it was impossible to make a photothermal excitation experiment for AFM. What we need is two different laser diode; one of them is red and the other is violet laser of PS3 laser diode.

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The first result was awesome. When I saw this spectrum on my screen, I feel some excitement. The difference between diether piezo excitation graph (left) and photothermal excitation graph (right) is obvious. However, due to plastic tube, our rms value was decreasing. We need to spread out heat. I draw a new aluminium tube in SolidWorks and after manufacturing, it worked!

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Below, you see scanning of 10μmx10μm surface in air. During scanning, rms value is about 1.000V, scanning speed=20μm/s, oscillation amplitude=0.450V and ExtInP=600.

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Next, we will move on modifying our system to try scanning a surface in liquid environment. After succeed it, I will scan my own blood!

A CLOOSER LOOK @ PS3 LASER DIODE

It’s been said that there are two types of researchers; those who have blown a laser diode and those who will.

~Doug Hodgson and Bill Olsen

In an experiment we work on, we are using the laser diode of the PlayStation 3. As a brief introduction, the working principle of the laser diodes are based on p-n junctions. The wavelength of them can be adjusted by changing the ratio of composition. For instance, the laser diode material, InP is used in optical communication and if you increase the amount of In, the emission wavelength will increase. They run with %10-%50 efficiency. They are affected by changing temperatures. Most importantly, they are extremely sensitive to electro-static discharge. We can consider the laser diodes as boxes stacked on top of each other. At the top, there is a laser chip, under it heat spreader, then thermal electric cooler and lastly Cu/CuW base due to its high thermal conductivity.

The length between red laser and infra-red laser was longer than we expected. We tried to do experiment with the violet laser. But before then we want to measure the length between each laser.

laserdiodeFirstly, I measured the diameter of the laser diode as a reference. By using optical microscope, I took some picture close enough to determine the length between violet and red laser. Then by using Matlab Imtool with the references of measured laser diode diameter, the length between each laser is calculated over the taken pictures.

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What we observed easily is that the distance between red and violet laser is 13 micrometer and violet-IR laser distance is 125 micrometer.

References:
*https://www.rp-photonics.com/laser_diodes.html
*http://repairfaq.cis.upenn.edu/Misc/Blu-ray/site1/diode.html

EXPERIMENT: THE MICHELSON INTERFEROMETER

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It's my first optical setup!

Basically, an interferometer is used as a device that can be classified one of the application of interference. A light beam created by a light source, firstly splits into two different beam thanks to a beam splitter. Than, this two different beam go along two different path and eventually each of the beam returns by reflecting two different mirror. They recombines and finally arrive some screen or detector.
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This configuration of the interferometer is called Michelson Interferometer and invited by Albert Abraham Michelson in 19th century. There are many applications by using interferometer such as the measurement of the wavelength, distance, rotations and searching for gravitational waves.

If we want to represent this two beam mathematically, for the beam goes through L_1 is
E_1=E_xsin(kL_1+wt)

and the other beam is
E_2=E_ysin(kL_2+wt)

and our output beam is
E_3=E_1+E_2

We can calculate the intensity for output beam by using I=E^2
I=E_3^2
=[E_xsin(kL_1+wt)+E_ysin(kL_2+wt)]^2
=E_x^2sin^2(kL_1+wt)+E_y^2sin^2(kL_2+wt)+2E_xE_ysin(kL_1+wt)sin(kL_2+wt)

Remind that 2sin\alpha sin\beta=cos(\alpha-\beta)-cos(\alpha+\beta)
Therefore,
I= \frac{1}{2} (E_x^2+E_y^2+E_x E_y cos(k \Delta L)
- \frac{1}{2}E_x^2 cos(2kL_1+2wt)
- \frac{1}{2} E_y^2 cos(2kL_2+2wt)
-E_xE_ycos[k(L_1+L_2)+2wt]

Note that the function of sin oscillate between +1 and -1, and their averages are zero. Hence,
I=\frac{1}{2}(E_x^2+E_y^2)+E_xE_y cos(k \Delta L)

In order to calculate the beam divergence, it is used:
 v\cong \frac{\Delta w}{\Delta L}

and to calculate the wavelength, by assuming the speed of light is known, we use the following simple and fateful formula:
 \lambda = \frac{2\Delta d}{n}

References:
*A manuel sent by the instructor
*https://www.rp-photonics.com/interferometers.html?s=ak

1 year in 1 video

Starting to physics education in Middle East Technical University was my dream. After I got enough point to be a physics student in METU, there was one more step. I spent a year in School of Foreign Languages (we say briefly "preparation") to learn English and pass the proficiency exam. In METU, since all of the departments provide education with the language of English, before beginning our departmants we spent our one year in this school. This is our honeymoon! We enjoy the university by learning just English.

Here's a video I record during my preparation year. The video is consisting of 2-3 seconds I take almost every day. It's a brief summary of my first year in the university. Enjoy!