# Research Journal

### Friday, May 2nd, 2014

I had a surprisingly fun time at URECA this past Wednesday. I started off the day very nervous and not looking forward to presenting my poster at all, but that changed fairly quickly. A couple of freshman physics majors saw my poster and started asking me a bunch of questions. It rewarding to have someone take a genuine interest in my project and it took a lot of stress away from me. After a presenting my poster to a few more people I was able to explore the hundreds of other posters in the ballroom. It was amazing to see the types of research other departments do. My favorite poster at URECA was James Dragan's poster on Adiabatic Rapid Passage. He presented his research in a clear and concise way, with a lot of enthusiasm. The event was a great experience and I look forward to doing more stuff like URECA.

### Friday, April 25th, 2014

I measured the voltage threshold of four different LEDs in the LTC. By plotting the voltage threshold vs 1/wavelength of the LEDs, Planck's constant can be found from the slope of the data.

I found Planck's constant to be 7.986*10^-34. This simple experiment yielded Planck's constant with a percent error of 20.6%, but there are some problems with it. The voltage threshold of an LED is an ambiguous value. It is commonly referred as the voltage at which the diode begins to emit light or when the current starts to rise exponentially. I know from previous measurements that the current through an LED is always exponential with a forward voltage. Also, the LED begins to emit light before the human eye can detect it.

### Friday, April 4th, 2014

Last Tuesday I measured the I-V curve of a red LED. The LED is rated for 50mA at a forward voltage of 2 Volts. I recorded the current over a range of 10.7 microAmps to 47.8 milliAmps. I plotted the data below along with the circuit I was using.

Unfortunately, the data did not come out as well as I had hoped. I believed the problem was because of one of the voltmeters was not giving accurate measurements. The jump in the data occurred when the Keithley 195A DMM I was using to record the voltage switched sensitivity scales. On Thursday I returned to my junior lab class and confirmed that the Keithley 195A was not giving an accurate reading for the voltage.

I was asked to further explain the circuit I was using. A DC power supply supplies 15V to the potentiometer. A potentiometer is a variable resister with three prongs being used to divide the voltage. The resistance across the outer prongs is constant at the max resistance. The resitance from an outer prong to the middle prong can be varied from zero to the max resistance. The voltage sent into the circuit is given by the formula:

Vout=VinRmax/(Rv+ Rmax)

Next, the variable resistor is being used to control the amount of current that can flow through the circuit. R1 and VR1 act as an ammeter to measure the current that will pass through the LED. The measurement is made this way because the ammeters we have do not have enough precision to measure very small currents. The voltage drop across a big resistor will be a lot larger than the current passing through it. For example a 1 MegaOhm resistor with 1 microAmp passing through it will have a voltage drop of 1 Volt. Next, the voltage is measured across both R1 and the LED. The voltage across the LED can be calculated by subtracting VR1 from VX. It might seem easier to measure the voltage across the LED directly, but this causes problems when the current passing through the LED is small. The resistance of the LED can be in the MegaOhm range for small currents and the voltmeters have a finite internal resistance of 10 MegaOhms. This will cause the current to split between the diode and voltmeter. Thus, the previously measured current will not be accurate. The problem is alleviated by putting the ammeter inside the VX to measure the true current passing through the LED.

### Sunday, March 30th, 2014

Borrowing ideas from my junior lab, I have created a set up that can measure various properties of a light emitting diode. This circuit can be seen below.

This circuit has two different ways to vary the current delivered to the LED. Both the potentiometer and variable resistor can be changed to alter the current. The resistor, R1, has two purposes in this circuit. It acts as a limiting resistor by preventing too much current from flowing through the LED when Vout is maximum and the variable resistor is at it's minimum. It also serves the purpose of an ammeter by measuring the voltage across it with the voltmeter, VR1. The current through the LED will be VR1/R1. The voltage across the diode LED will be VX-VR1.

The light from the LED is modulated by an optical chopper and its frequency will be sent to the lock-in amplifier. The photodiode measures the modulated light from the LED and the noise from other sources and sends the signal to the lock-in amplifier. The lock-in amplifier will send the signal from just the LED to an oscilloscope. Alternatively, an AC signal generator can be used to modulate the LED output instead of the optical chopper.

This setup will allow me to study properties of the LED such as the its efficiency at different currents or if it follows the inverse-square law. The capability of the lock-in can be explored too. The current of the can be reduced LED until the signal can no longer be detected or by moving the photodiode further away.

### Saturday, March 29th, 2014

Dr. Noé thought it would be a good idea to think of a project related to my electronics and instrumentation lab. The most recent unit that I have been studying in the junior lab involved diodes. As the LTC is an optics lab I thought it would be a great idea to study light emitting diodes. An ideal diode will only allow current to pass through it in one direction. Also, the diode will open up and current will only flow once the voltage has reached a minimum threshold. The minimum voltage for current to flow through a LED ranges from 1.2v to 4v depending on the semiconductor material of the diode. The maximum current a LED can sustain before being damaged ranges from 20mA to 50mA. Once the diode opens up, its I-V curve will increase exponentially.

In the LTC we connected a battery to a resistor and LED in series, but the LED failed to Light up. Connecting the LED directly to the battery without the resistor did light up the LED. So the question was how could we find out if there was a resistor built into the LED. Graphed below is the I-V curve for a 100 Ohm resistor, an LED, and the same resistor and diode in series.

For an ideal LED, the current would be shoot up instantly at its forward voltage, but in reality the I-V curve is exponential. I assumed the I-V curve of the LED was ideal in order to plot the I-V curve of the resistor and LED in series. In this case the LED has a forward voltage of 2v and can sustain 20mA before it will be damaged. For the series combination, the current will be zero until the voltage reaches 2v. Above 2v the voltage drop across the LED will remain constant at 2v and any additional voltage will drop across the resistor. I also calculated that the max voltage that the LED and resistor in series can sustain before damaging the LED is 4v. So the current through a LED with a built in resistor will increase much slower than just an LED and the series combination will be able to sustain a higher voltage before damaging the LED. The reason the LED with the resistor did not work in the LTC was because the resistor was too large. We used a 10k resistor and only a 2.5v battery, thus the current would be on the order of a microamp.

### Wednesday, March 19th, 2014

Last time I was at the laser teaching center I was shown a hollow cylindrical permanent magnet that belongs to a HeNe laser. The magnetic field of this magnet needs to be mapped and compared to a theoretical model. Andrew Koller had a similarly shaped magnet, but with thicker walls and a smaller diameter. The magnetic field strength of Andrew's magnet was recorded and found to agree with the predicted theoretical model. This can be seen in his paper in appendix C at the bottom. Since the geometries between the two magnetics are slightly different the field would have to be measured again. Unfortunately, the Gauss probe that would be used map the field of the HeNe laser magnet no longer available. Although, the Faraday effect could be used to calculate the strength of the magnetic field. Below, is one way an experiment could be set up.

The magnetic field can be calculated by placing the Terbium Gallium Garnet crystal along different points inside the magnet and seeing how much the polarization of light rotates.

### Sunday, March 9th, 2014

Last time I saw Dr. Noé, he brought up the idea of using a lock-in amplifier to explore the Faraday Effect or the properties of a photoelastic modulator. A lock-in amplifier is a powerful tool that can detect very small signals in the presence of large amounts of noise. A lock-in amplifier requires an input signal, V(t)input=V0sin(ωt+φ), and a reference signal, VR(t)= sin(Ωt). The product of these two signals yields Vinput(t)VR(t)= V0/2{cos[(ω-Ω)t+φ]-cos[(ω+Ω)t+φ]}. When the frequencies of the two signals are equal the result is a sinusoid offset by a DC level. When the frequencies of the input and reference signal do not match, the product of the two is a beat with an average voltage of zero. A graph of these two cases can be seen below.

The signal that is input into a lock-in amplifier contains the data of interest modulated at reference frequency, as well as, unwanted noise at various frequencies. The two signals are multiplied together inside the lock-in amplifier and then pass through a low-pass filter. The low-pass filter will attenuate any signal with a frequency greater than half of the reference signal's frequency. Ideally, only the component of the input signal with Ω=ω will remain. The output of the amplifier will be Vout=1/2V0cos(φ).

### Friday, February 21st, 2014

Today I set up the password for my linux account and downloaded the SSH Secure Shell on my pc. The SSH Secure Shell allows me to edit my webpage without having to go the the lab. I am going to create an ideas page to collect information on topics that I am interested in.

### Wednesday, February 19th, 2014

Dr. Noé setup my webpage today and showed me some of the basics to editing my webpage. I am familiar with navigating in linux, but I have never worked with HTML before. HTML stands for HyperText Markup Language and it is used to create webpages. The language seems straightforward and easy to use, so I should not have any problems picking it up.

### Friday, February 7th, 2014

I met with Dr. Noé for the first time today in the Laser Teaching Center. He explained how the LTC is not the typical research lab. Students get to explore what they're curious about in optics and design their own project based on their interests. This is a welcome change of pace from the structured physics lab courses offered at Stony Brook that offer little in the way of creativity. At first this challenge seemed overwhelming to me. How could I possibly choose just one topic to study when there are so many interesting and cool phenomena to choose from?

Dr. Noé went on to briefly explain what will be required of me as I proceed with research in the LTC. First, I will receive a research notebook at our next meeting; this will be used to document my observations in the lab. The notebook should be written neatly in pen and in a professional manner. I will also be required to create online journal entries after each visit to the lab. I can even write about anything interesting that I read or do pertaining to physics. Finally I will have to write a report and present a poster at the Research Celebration in late April. After Dr. Noé talked with me in his office he brought me out into the lab and showed me a few things. First, I was shown Casey and Nick's poster on helium-neon laser modes. I was able to relate to this project through a lab in my Waves and Optics course last fall. In this lab I measured the width of a Gaussian laser beam by blocking a varying portion of the beam with a razor blade and measuring the transmitted intensity. The laser beam was sent through a polarizer to reduce its intensity, but this had a serious unintended consequence: as the laser switched between different modes that had different polarizations the intensity after the polarizer fluctuated dramatically. Dr. Noé told me about a technique for circumventing this problem: Split the beam and record the intensity of both beams, one partially blocked by the razor blade and the other not blocked. The width could then be accurately determined from the ratio if these two intensities, in spite of the fluctuations. An alternative very simple "work around" that we could easily have done with no additional equipment .....

Another cool experiment Dr. Noé showed me involved sending linearly polarized light through sugar water and viewing it through a polarizer. As the polarizer was rotated the sugar water went through a whole range of colors. This is due to the chirality of the molecules. Chiral molecules have two chemically equivalent forms that can be left or right handed and they cannot be super imposed on eachother. Your hands are a macroscopic example of this. Due to the Chirality of the sugar water the polarized light is rotated an angle dependent on the wavelength of the light. Thus as you rotate the polarizer you can see different colors. Mara Anderson's report can be viewed to learn more.