Restructuring the Physics 234 Course to Include Nanoscale Investigations

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Restructuring thePhysics 234 Course to Include
Nanoscale Investigations

• Stephanie Barker and Kurt Vandervoort
• Funding for this project was provided by the
  National Science Foundation Nanotechnology
  Undergraduate Education Program Award 0406533.

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Purpose of the Project

• To develop modules to introduce atomic force
  microscope (AFM) applications into the Physics
  234 course.
• To investigate surfaces at the microscopic level
  to reveal properties which account for
  macroscopic-scale phenomena in light.
• To introduce and familiarize students with
  research-grade equipment at an introductory level
  as important career preparation.
• To explore interesting engineering applications
  of nanotechnology.

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• Existing Course Lab Structure
• Experiments
• Data Analysis
• A.C. Circuits
• Microwave Optics
• Geometric Optics
• Physical Optics
• Spectroscopy
• Speed of Light
• Michelson Interferometer

• Proposed Revisions to Lab Structure
• Experiements
• Data Analysis
• A.C. Circuits
• Geometric Optics
• Physical Optics
• Spectroscopy
• Microwave Optics
• Speed of Light
• Michelson Interferometer
• Appendix A

Proposed revisions reflect the need to present
physics concepts in an order that introduce AFM
applications in the proper context. Modifed
Lab Modules An Appendix was added as a basic
reference for the standard operation of the AFM
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Geometric Optics Module

• Existing Objectives
• To observe the interaction of light with prisms,
  mirrors and lenses
• To measure refraction, reflection, critical and
  Brewsters angles
• To verify the laws of reflection/refraction and
  the lens makers equation
• Additional AFM Module Objectives
• To visually examine rough and smooth gold plated
  slides to verify specular or diffuse reflection
• To observe the microscopic surface topography of
  these slides
• Learning Enhancements
• Students will be able to directly confirm
  criteria that define the limit for geometric
  optics by distinguishing the microscopic origin
  of specular and diffuse reflection.

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Gold Plated Slides Exhibiting Specular and
Diffuse Reflection
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Microscopic Image of Speculary Reflective Slide
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Cross-Section of Specularly Reflective surface

• Surface feature widths and lengths 0.5 µm or
  500 nm
• Surface feature heights 10 nm
• Surface feature heights are significantly less
  than the wavelengths of visible light (400-700 nm)

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Microscopic Image of Diffuse Reflective Slide
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Cross-Section of DiffuseReflective Surface

• Surface feature widths and lengths 20 µm or
  20000 nm
• Surface feature heights 2000 nm
• Surface feature dimensions much larger than the
  wavelengths of visible light (400-700 nm)

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Physical Optics Module

• Existing Objectives
• To observe the basis for the wave theory of light
• To study the diffraction and interference of
  light
• To calculate the wavelength of light
• Additional AFM Module Objectives
• To visually examine the surface of an iridescent
  butterfly wing
• To observe the microscopic surface topography of
  the wing
• To observe the microscopic surface topography of
  a compact disc
• Learning Enhancements
• Students will be able to see direct applications
  of physical optics in both natural and industrial
  materials.

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AFM Image of Morpho Butterfly Wing
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Cross-section of Butterfly Wing
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Effects of Thin-Layer Interference

• The bright, shifting colors of a butterfly wing
  are due to interference which occurs in a series
  of thin layers on the surface of the wing.
• These structures can cause constructive
  interference for certain wavelengths of visible
  light, so that some colors seem more brilliant
  than usual.
• The colors may change as you (or the butterfly)
  change position, and the interference becomes
  visible at different angles of view.

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Interference in Thin Layers

• The film layer has thickness t and index of
  refraction n gt nair
• The wavelength ?n of light in the film layer is
• ?n ?/n
• Ray B travels a distance 2t further than Ray A
  before the waves recombine in the air above the
  film and interfere
• Ray A has an additional 180 degree phase shift
  following reflection

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Condition for Constructive Interference in Thin
Films

• If 2t ?n /2, then rays A and B recombine in
  phase, and constructive interference occurs, so
• 4nt ?
• where n is the index of refraction of the film, m
  is the order of interference, and ? is the
  wavelength of light in air.

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CD Exhibiting the Effects of a Reflective
Diffraction Grating
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AFM Image of a Compact Disc
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Cross-section of Compact Disc

• Size of surface features are on the order of the
  wavelength of visible light. Height of surface
  bumps is between 120 and 130 nm.

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Physics of a Compact Disc

• The bumps that were imaged by the AFM are
  variations in a thin polycarbonate layer. As the
  CD is read a laser is focused onto the region
  of these bumps.
• When the laser spot encounters a bump, half of
  the area of the spot covers the bump, and half
  covers the flat area surrounding the bump. The
  waves that are reflected from these two different
  heights destructively interfere.
• The condition for destructive interference
  depends on the wavelength of the laser light in
  the polycarbonate layer.

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Using Destructive Interference to Read a Compact
Disc
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• The condition for destructive interference
  between two waves is such that the total
  pathlength differs by a distance that is ½ the
  wavelength.
• In this case, the laser light is emitted from the
  same location, and the bump is the only change in
  pathlength that the waves encounter. The waves
  that encounter the flat areas travel a distance
  further than those encountering the bumps. This
  extra distance is equal to twice the height of
  the bump (2h).
• This difference in pathlength must be equal to ½
  wavelength for destructive interference, so
• ? 2h ½ ?, or h ?/4

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Expected Height of Bumps in Polycarbonate Layer

• ?0 wavelength of laser (in air) 780 nm
• ? wavelength in polycarbonate layer
• n index of refraction for polycarbonate layer
• 1.56
• ? ?0/n 500 nm
• ?/4 125 nm
• The cross-section of the CD scan does show
  surface feature heights that are near this value.

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Spectroscopy Module

• Existing Objectives
• To observe the effects of a multiple-slit
  diffraction grating on the polychromatic light
  emitted from gas spectra tube
• To understand how spectroscopy can be used to
  find the characteristic spectrum of a gas, and
  furthermore identify each element present.
• Additional AFM Module Objectives
• To view a microscopic image of the diffraction
  grating used and compare its actual features with
  any original assumptions about the construction
  of the grating
• Learning Enhancements
• Students will be able to closer observe the
  results of intricate machining involved in the
  application of nanoscale technology.
• Students will be introduced to the microscopic
  topography of a blazed diffraction grating.

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Image of a Multiple-SlitDiffraction Grating

• The grating is not actually a series of slits,
  but a series of angled grooves. Th size of these
  features is on the order of the wavelength of
  light.

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Microwave Optics Module

• Existing Objectives
• To gain some familiarity with microwave
  techniques and equipment.
• (Optional) To show that microwaves, like light,
  are transversely polarized electromagnetic waves.
• Additional AFM Module Objectives
• To determine the blaze angle for a standard
  diffraction grating by analyzing the
  cross-section of an AFM image.
• To observe the double-slit interference pattern
  for microwaves.
• To observe the effects of a macroscopic blazed
  diffraction grating on the diffraction envelope.
• Learning Enhancements
• Students will experience the advantages of a
  blazed diffraction on the macroscopic scale.

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Blazed Diffraction Grating Cross-section

• The height and width of the grooves can be used
  to determine the shallower angle, which is the
  blaze angle.
• Average groove spacing as measured by AFM is 1600
  nm.
• This result is within 5 of the nominal spacing,
  considering 600 lines/mm.
• The blaze angle is measured to be 23o, which is
  within 10 of the manufacturers specification.

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Blazed Diffraction Gratings

• By blazing the grating the diffraction envelope
  can be shifted so that the maximum intensity
  occurs for higher-order maximum (mgt1) of the
  interference pattern.
• Blaze condition
• sin-1(n sin ?B) ?B ?m

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Setup for the Microwave Experiment
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The Macroscopic Diffraction Grating
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Results for the Microwave Experiment(Slit width
4 cm Slit separation 6 cm)
White data points No diffraction grating
used Black data points Macroscopic diffraction
grating used

• The intensity maximum of the diffraction envelope
  is shifted to the m -1 position.

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Overview of Appendix ABasic OperationInstructio
ns for the AFM

• Includes background theory of atomic force
  microscopy
• Gives a detailed explanation of the functions of
  the software used to perform a scan with the AFM,
  including an index of the icons.
• Includes the step-by-step procedure for
  configuring the scanning parameters and operating
  the instrument
• Explains several methods of analysis for an
  image, including the 3D Image, Histogram, and
  Dimensional Analysis functions.