Say it with Microwaves:The SLU Microwave Optics

Symposium

Daniel W. KoonSt. Lawrence University Physics Dept.

dkoon@stlawu.edu

To find this talk online, Google the phrase, “Say it with microwaves”.

It is the only hit. (Updated version: next week)

Contents

• The SLU MW Optics Symposium [SLU-MOS]– History– What it is– Role in Physics Curriculum

• Improvements to the MW Lab– Novel prism design– Detector calibration– New experiments

ZooAveLa Garita, CR.

History of SLU-MOS

• MW Equipment purchased with US National Science Foundation grant, 1989.

• Symposium launched shortly afterwards.

• SLU-MOS forms part of sophomore (2nd) year physics lab curriculum.

Zoo AveLa Garita, CR

SLU-MOS: What is it?

• Students spend 2-3 weeks on Microwave Optics experiments

• Individualized experiments• Shared basic equipment, physics

• Students present their “research” as physicists do:

• Formal written lab report• Formal oral presentation to other students,

faculty.

MOS as ‘bridge’ within SLU Physics major

• Lab curriculum– Bridge between week-long,

“cookbook” experiments to longer, experiments requiring student design

• Communications skills– Bridge between University-wide First-

Year Program and Physics Dept. Senior Thesis defense

• Modern Physics lecture course– What does it mean to test the wave-

like properties of electrons, photons, X-rays, gamma rays, etc.?

Church, ca. 1693 Ujarras, CR

Impact of MOS on Department

• Improvement in senior theses (both written and oral versions)

• Better awareness of need to integrate teaching communications skills within major

• 2 weeks of experiments sacrificed for communications workshops during semester

• Workshop on formal lab reports• Workshop on oral presentation

Zoo Ave La Garita, CR

Pasco Scientific Microwave Optics System

• Common hardware– 10.5 GHz source, detector– Mounting hardware, incl.

goniometer.– Aluminum “mirrors”/blocks– “Half-silvered” mirrors

• Experiment-specific hardware– Bragg cube– Prism form– Polarizers– Dielectric slab Pasco Scientific’s Advanced

Microwave Optics System. WA 9316. www.pasco.com.

Standard Pasco experiments in Microwave Optics

Physical Optics Other phenomena

Standing waves Polarization (Malus’ law)

Double slit Fiber optics

Lloyd’s mirror Refraction (Snell’s law)

Fabry-Perot interferometer Brewster’s angle

Michelson interferometer

Bragg diffraction

Microwave Optics prisms

Pasco Scientific’s prism form. Just add polystyrene pellets. User is restricted to only two values of incident angle, one of which produces total internal reflection. www.pasco.com

Variable-angle prism form. After Albiol, F., Navas, S. and Andres, M. V., Am. J. Phys. 61, 165–169 (1993). Pellets inserted into gray region. Pellet spills a problem.

Microwave Optics prisms: The hemicylindrical prism

Hemicylindrical prism for investigating Snell’s Law. Unlike Pasco’s prism form, allows for a large range of incident angles. Provides for ease of use relative to the variable-angle prism form of Albiol et al, and allows for the use of other materials. Can be made of solid material (paraffin at right), or as form to be filled with pellets (cardboard at right).

Applications: Hemicylindrical MW Prism

• “Microwave Tunneling”. [Albiol et al., 1993]– Frustrated Total Internal Reflection.– Optical analogue for Quantum

Mechanical Tunneling.

• Snell’s Law.– Measure t for a variety of i.

– Plot sint vs sini. Slope = 1/n.

Coati (pizote), Monteverde

Applications: Hemicylindrical MW Prism

Calibrating microwave detectors• Detector measures arbitrary signal:

– not E-field magnitude, E– not intensity, IE2

– not a power of either I or E.• [despite Combes, P. F., Graftenil, J., and Santereau, J. F.,

Microwave Components, Devices and Active Circuits (Wiley, New York, 1987), cited in Albiol et al., op cit.]

– At best, a power law with a variable exponent. (curved line in a log-log plot)

Calibrating microwave detectorsMicrowave Intensity vs Signal for Detector 411

0.0001

0.001

0.01

0.1

1

0.01 0.1 1 10 100

Signal x Multiplier (Volt)

Inte

ns

ity

(a

rb. u

nit

s)

Collected Data Calibration Data

Ciesla, Stephen, “Beer’s Law”, SLU-MOS 2005, unpublished.

Calibrating microwave detectors

• For many experiments, this is not a problem.– Double-slit interference– Fabry-Perot interferometer– Bragg scattering– Snell’s law

• For others, it is:– Single-slit diffraction

– Beer’s law for absorptionxeIxI

aII

0

20

)(

sinsinc)(

Malus’ Law

20 cos)( II

• Intensity of MWs passing through tilted polarizer.

• Measured intensity when MW emitter, detector tilted relative to each other. Provides a way of calibrating MW detector.

Malus’ Law for calibration

Beer’s Law: Attenuation of MWs in water

Ciesla, Stephen, “Beer’s Law”, SLU-MOS 2005, unpublished.

Applications of MW detector calibration via Malus’ Law

• Beer’s Law.• Expand Pasco’s Fiber optic

experiment to test whether wave profile is Gaussian.

• Expand Pasco’s “Brewster’s Angle” experiment to test Fresnel Equations for reflection at incident angles, i.

Barva, Heredia

New experiments

• Beer’s Law. – Measure attenuation of intensity as microwaves travel

through suspended wet paper towels.

• Microwave Lenses.– Interference clearly visible on top of geometric optics.

• Beat patterns. – Typical f ≤ 10MHz out of 10.5GHz.

• Multiple thin-film interference.– Multiple-dielectric stack shows effects of both

geometrical optics, physical optics.

New Experiments:Beat patterns

•10.53GHz typical

•15MHz variation among sources

•2MHz “tunability” of each source.

New experiments:Multiple thin-film interference

Conclusions

• A symposium in Microwave Optics serves as opportunity to develop student– research skills– oral and written communications skills

• Pasco MW Optics equipment can be improved via– Hemicylindrical prism– Detector calibration à la Malus

• Other MW Optics experiments can be added to standard repertoire associated with this equipment.