and Other Devices

(gratings treated elsewhere)

edited 2/6/01

chem 454

Optical Interference occurs when two light beams are superimposed

• light of different wavelengths will not participate in interference
  • of course a beam with many wavelegths can have interference at each wavelength
  • it's easier to analyze intereference devices by focusing on monochromatic light
• The simplest interference occurs as light passes through a thin film or thin coating
  • about 10% of the light normally reflects from the face of the coating
  • of the 90% transmitted, about 10% will be reflected at the interface at the bottom of the thnk film
  • thse two reflected beams will travel in the same direction
  • these two beams become superimposed
    • the second component has had to travel further
    • as a result it is out of phase with the first reflected light
• If the two beams are in phase they will undergo constructive interference
  • the two waves will add, forming a wave of amplitude larger than either componenet
  • the result is a brighter image (more reflection) than either wave by itself
    • in fact, since Intensity is proportional to the square of the amplitude, the signal could be twice as intense as the sum of the two separate beams.
  • this condition occurs if the second path exceeds the first by a multiple of the wavelength of the light
    • this requires the film thickness to be half the wavelength (since light travels down and then back through the film)
    • (actually, this involves the wavelength in the medium. THis will, in turn, depend on the refractive index of the medium)
• If the two beams are 180 degrees out of phase we get complete destructive interference
  • if the two components are of equal amplitude, the produce a sine wave of amplitude zero (no light)
  • the result is no reflection (and by default 100% transmission)
    • this occurs if the film thickness is half the wavelength of light
• If the two beams are out of phase (but not 180 degrees) we will get partial interference and an intermediate result.

Applications

• Optial Coatings: camera and binocular lenses are often coated with a quarter wave thckness of a material like MgF2 or SiO2
  • this reduces the reflection at each lens surface
  • it increases the light transmission
  • it also eliminates stray light (strong lights might otherwise show up as a series of bright spots elsewhere in the image)
    • you can often see this when driving at light if you wear glasses or in photographs taken looking into the sun.
  • of course this is most effective at one wavelength, typically set at the middle of the visible range. It is still helpul (although less effective) throughout the spectrum.
• Interference filters are often made with multiple coatings to enhance the effect
  • a transmission filter can be ddesigned to transmit only a very narrow spectral region (perhaps 0.5 -1.5 nm)
  • some filters are desgined to transmit all but a specfici wavelength of light (e.g., to block one line from a Hg vapor lamp.)
  • some operate as wavelength specific reflection devices
    • actually the same filter can act as a narrow pass filter in transmission and a band rekjection filter by reflection.
  • If you titl the filter, you effectively change the path length by 1/cosine of the angle.
    • this shifts the wavelength range of the device slightly (a useful trick)
    • You can also buy wedge coating of varying thickness-- the wavelength transmitted chages throughout the device
• Dichroic Mirrors are basically designed to have extremely efficient reflection
  • they are built as described above
  • they can often reflect over 99.999% of the light
  • again, they work best at a sprecific wavelength
  • they are esental for many lasers
• Dichroic Paints and Inks
  • If you make a film with multiple thin layers it has the properties above
  • if reflects light better at one waavelength region
  • if you break the film into tiny flackes you can add it to an ink or paint
    • if will be deposited and generally alighn itself flat on the surface
    • there will be some variation in angle and hence shome shifts in color
    • you may see surface sparkle with a few slightly different colros
    • the color shifts as you tint the object or as you walk past it
      • some automobiles use such paints for greater visual effect
      • the denomiation (large 20 or 10 in lower right) on new US currancy is printed in a dichroic ink to fight couterfitting
        • the color shifts from green to black as you view a legitimate bill
        • a photocopy is made at a fixed angle and its number will not show this effect

The Fabry-Perot Etalon

• This is basically a thin film with a highly reflective coating on the top and bottom surface
• We now focus our attention on two beams
  • light passing through the top reflection surface, amking its way into the film
  • the part of that light that reflects twice off the bottom and top reflective coatings
• When those two beams are in phase we get constructive interference and enhanced intensity
  • actually, the beam (trapped between the reflectors) does this repeatedly
  • this is comparable to many thin layers in sequence
  • only light of a specific wavelength will avoid destructive interference
• the Etalon transmits light of a specific wavelength only (very narrow band)
  • We can choose the wavelength by the spacing of the reflective coatings
    • some devices actually have an air gap between two mirrors
    • others are of fixed thickness and therfore fixed wavelength

• There is now a tunable solid state filter based on the Fabry Perot Etalon
  • a thin layer of material is coated front and back with a reflective layer
  • the material is one that changes its refraactive index if an electrical field is applied
    • remember, the wavelength of light shifts with refractive index
    • frequency stays constant, but speed shifts (hence wavelength changes)
  • changing the voltage changes the wavelength of the device
  • this permits simple control of wavelength with an electrical signal
    • this also permits fabrication of very tiny devices if desired
    • (it's much harder to make tiny devices that must rely on moving components to change wavelength)