Rapid modulation of light waves using electric fields

Project Title: Rapid modulation of light waves using electric fields

Project Lead’s Name: Samir Bali

Email: balis@miamioh.edu

Phone: (513) 529-5635

Please Choose the Primary Affiliation: CAS

Are There Other Project Team Members?: No

Brief description of project: "We all know what light is: but it is not easy to tell what it is." - Samuel Johnson.

Starting with introductory-level physics in college, up to the graduate level, students are taught that the study of light is not about just the few colors that we can see but about understanding the physical laws which govern the electromagnetism spectrum. They learn that light is an electromagnetic wave made up of co-propagating electric and magnetic fields dancing around one another, which sustain the wave through vacuum, even over vast distances as in the case of starlight bathing our planet. 

Demonstrations that directly elucidate the electromagnetic character of light waves are an indispensable component of undergraduate and graduate physics labs. This proposal seeks funding to purchase electro-optic modulator (EOM) systems which are devices that enable students to electrically control the amplitude, polarization, and phase of the electric field vectors constituting the light wave, thereby gaining a deep understanding of the electromagnetic character of light. Further, students learn about important industrial applications of EOM's to high-speed imaging and data recording.

Does this project focus on graduate student education or graduate student life?: No

Describe the problem you are attempting to solve and your approach for solving that problem.: The concept of waves is the first point of departure from Newtonian physics (Newton's Laws, objects sliding down ramps, etc) in PHY161/162/191/192 (introductory physics for science & engineering and life-science majors at Miami University). Well-prepared high school students who are science majors in college know Newton's Laws well enough, but they have not had waves before, in the sense that none can write down the basic equation for a wave. We have good demonstrations available at Miami University to help students visualize moving mechanical waves (for example, using a "slinky"), propagating sound waves (for example, using vibrating tuning forks), but electromagnetic waves are difficult for students to directly visualize. A recurring conversation that I have with students year after year is reproduced below:

Student: Water molecules vibrate in a water wave, air molecules in a sound wave. Who's waving in a light wave?
Professor: The electric and magnetic field vectors.
Student: Oh.

The problem of visualizing electromagnetic waves is not restricted to intro-level students, but extends well into the senior and graduate classes.

My approach to solving the problem is to arrange for the students to have direct hands-on experience with an electro-optic modulator (EOM) system. An EOM is the only device on the market that exploits a small electric field to control all three "knobs" that define a monochromatic electromagnetic wave at any point - the amplitude which represents the strength of the wave at that point, the polarization which represents the direction of the electric field vector at that point, and the phase which represents the propagation delay of the wave up to that point in units of the optical wavelength. In its simplest avatar an EOM is a nonlinear crystal across which a small electric field is applied, which tunes the refractive index of the crystal in a controlled manner. This tunes or modulates the optical wavelength in the crystal and thus the phase delay of the light wave. Depending on the orientation of the crystal with respect to the direction of the applied electric field the phase modulation may be converted to a polarization modulation. Further, by sandwiching the crystal between two polarizers placed on either side the experimenter may convert the phase modulation of the light wave to an amplitude modulation.

Note that because it is straightforward to rapidly modulate the small electric field applied across the nonlinear crystal, students learn about important industrial applications of EOMs' such as high-speed imaging and data recording.

The criteria state that technology fee projects should benefit students in innovative and/or significant ways. How would you describe the innovation and/or significance of your project?: Of the many natural manifestations of energy (heat, light, sound, mechanical, etc) the electromagnetic field (or light) is the one most harnessed by humankind in current technology (cellphones, lasers, GPS, microwave ovens, etc), yet the hardest to visualize. From both a pedagogical as well as future employability point of view, it is imperative that our students be given the best possible opportunity to learn how to visualize the oscillating electric and magnetic field vectors that form the light wave.

Incorporating the electro-optical modulator (EOM) system into our undergraduate and graduate curriculum impacts the students significantly by allowing them a hands-on introduction to a vital cutting-edge tool. EOM's are used in a wide array of research areas in optical science, such as rapid light switching, high-speed imaging, wideband data recording and more recently in adaptive optics for astronomy.

The significance of impact of this proposed project may also be gauged from the fact that the EOMs' will be used to serve three different student groups:

i) The EOM system is compact and portable, hence ideally suited as a demonstration in our PHY161/162/191/192 lecture classes, and in our PHY293 lecture classes. The PHY161/162 class typically has ~300 students per year and the PHY191/192 class has ~400 students. The PHY293 class for physics and engineering physics sophomores enrolls ~20 students per year.
ii) I teach a lecture/laboratory course PHY441/541 (3 weekly hour-long lectures and 1 two-hour laboratory session) entitled "Optics and Laser Physics" to seniors and Masters'-level students. This course is taught every other year and has an enrollment of typically 10 students with a 70%/30% UG/G mix. In the lab, the students are divided into groups of three. There is one EOM system planned for each group, hence we are requesting three EOM systems. Two lecture periods will be used to teach the physics behind the EOM, and two lab sessions will be devoted to in-depth manipulation of the EOM.
iii) During the time the EOMs' are not used for lecture demonstrations, or in the lecture/lab class one EOM will be used as a cutting-edge research device to help train, on an average, 2 undergraduate researchers and 1 graduate researcher per semester on building nanoscale ratchets for cold atoms at microKelvin temperatures. Results obtained will be used in peer-reviewed journal articles with the students as co-authors, and in grant proposals submitted by the PI for federal funding such as NSF.

How will you assess the project?: The impact of the EOM system on education at Miami will be assessed as below for the three different student bodies it is intended to serve:
i) The ~700 undergraduates in STEM and medical/health sciences taking introductory physics in PHY161/162 and PHY191/192, and the 20 physics and engineering physics sophomores in PHY293 will be given an in-class 10 minute quiz on the next day of class which will test their understanding of the basics of how the EOM works.
ii) The impact on the 10 students taking the lecture/laboratory class PHY441/541 will assessed by using a before and after survey. The current students who have seen the EOM in the laboratory will be asked questions (answer choices ranging from strongly agree to strongly disagree) such as are you comfortable operating an EOM, could you explain in detail the process of how an EOM works to a high-school physics student, could you explain how an EOM works to an elementary student. Questions of this nature are intended to delve into the students' understanding of the device and how it is used. The assessment given after the first year of implementation will be utilized to assess the impact of the project on student learning.
iii) The impact on the research conducted by the two undergraduates and one graduate student is easily assessed by noting the number of refereed publications co-authored by these students that utilize the EOM in any way. (This particular impact has a slightly longer-term scope: A period of two years ought to be given to collect data for this particular statistic.)

Have you applied for and/or received Tech Fee awards in past years?: Yes

If funded, what results did you achieve?: Proposal TF14-004: Through this proposal we obtained a spatial light modulator (SLM) system. In the funded 2014 proposal we had pledged we would target the same three student bodies as mentioned in the answer to the previous question. 

Over the past two years since we got the award, 25-30 undergraduates have been given demonstrations, as promised, on how the SLM works. Four undergraduates (Jack Brinton BS 2017, Hong Cai BS 2017, Kefeng Jiang BS 2019, Linzhao Zhuo BS 2020) are currently performing research studies on the SLM. Initial work on setting up the SLM was performed by Andrew Hachtel (BS 2012, MS 2014), now doing his PhD at the Univ. of Maryland, College Park, and by Rey Ducay (MS 2015), now pursuing an MS in Statistics at MU.

The promise made in the original proposal that we will perform a live demo of the SLM in the intro-physics classes has not fully materialized yet, owing to the fact that the SLM (unlike the EOM requested in the current proposal) has turned out to be a remarkably complicated instrument. In Fall 2015 and 2016 I incorporated a 15-20 minute discussion of the SLM into my PHY 191 lecture for science & engineering majors, including pictures of the arbitrary optical wavefronts generated (in particular, of the optical vortex beam - that is, a beam for which the wavefront spirals around the direction of propagation resulting in a hole or vortex in the middle so that the cross-section of the beam looks like a donut). I set a few quiz questions based on this material but only for extra credit. By 2018 I hope to incorporate the SLM into the regular PHY191 curriculum and also disseminate lecture-slides and pictures to my fellow introductory physics instructors for their lectures. The EOM is a far simpler system than the SLM to discuss in the introductory physics class (since, for example, there are no spiral optical wavefronts as in the SLM, just a modulation of the electromagnetic wave's amplitude or phase or polarization).

Did you submit a final report?: Yes

What happens to the project in year two and beyond? Will there be any ongoing costs such as software or hardware maintenance, supplies, staffing, etc.? How will these be funded?: The EOM system, if used appropriately in similar fashion to other high-end equipment regularly used by the PI for demonstrations, education, and research training over the last 16 years at Miami, should remain in service for an indefinite time period. No ongoing costs are anticipated.

Budget: Hardware, Other

Hardware Title(s) & Vendor(s): Electro-optic Phase Modulator ($2566) & Driver ($3450) & Mount ($400); Vendor: Newport Corporation, USA

Hardware Costs: $6,416.00

Other (please explain): Total of 3 EOM systems (1 for each of 3 laser workstations in PHY441/541) + shipping $100

Other Costs: $12,932.00

What is the total budget amount requested?: $19,348.00