Light Detectors

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Title

• Light Detectors

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Characteristics

• Sensitivity
• Accuracy
• Spectral Relative Response(R(?))
• Absolute Sensitivity(S(?))
• Signal-to-noise ratio
• --Noise equivalent input power
• http//www.electron-tubes.co.uk/pmts/pmt_select.ht
  ml

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Characteristics

• Intensity range
• Response time
• -effect of detector time constant
• Price

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Types of Detectors

• Light Detectors can be classified int
• Thermal Detectors
• --changes the temperature dependent
  properties of detectors
• --wavelength independent sensitivity
• --sensitivity depends on detector parameters
• --heat capacitance
• --thermal losses

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Thermal Detectors

• Time constant of detector depends ratio of heat
  capacitance and thermal losses
• ? H/G
• where Hheat
  capacity
• 
  Gthermal losses
• --Sensitive to small values of G
• --time constant of detector limits the
  frequency of detector

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Thermal Detectors

• Calorimeter

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Thermal Detectors
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Thermal Detectors

• Bolometer
• ?consists of N thermocouples in series

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• Limitations
• Input impedance of the amplifier should be larger
  than R for a change in current
• Current through bolometer should be kept Constant
  
• Temperature rise due to joules heating limits
  the maximum current through bolometer

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Golay Cell
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Direct Photo detectors

• Direct Photo detectors are based on
• ? spectral based on emission of
  photoelectrons
• ? changes in conductivity of semiconductors
• ? voltage generated by the internal photo
  effect
• ?spectral response depends on work function
  or
• band gap

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Photodiode

• Doped semiconductors
• Can be either photovoltaic or photoconductive
• P-n junction when irradiated generates
  photovoltage
• Photoconductive elements change their internal
  resistance

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Photodiode
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Photodiode
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Photodiode

• Absorption coefficient is spectral dependent
• Should be operated at low temperature in order to
  minimize thermal excitation of electrons
• ?For ?lt 10 micrometers liquid nitrogen
• For ?gt 10 micrometers liquid helium
• add figure 4.81 and 4.82

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Photodiode
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Photoconductive diodes

• When illuminated its electric resistance
  decreases
• Time constant is dependent on diffusion time of
  electrons

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Photovoltaic detector

• When illuminated generates electron-hole pairs

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Photodiode
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Photo Emissive Detectors

• Depend on external photoeffect
• Photocathode is of low work function

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Photo multiplier Tubes

• Used in detection of low light levels
• Overcomes noise limitation by using dynodes
• Amplification factor depends on accelerating
  voltage U, incident angle,dynode material

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Photo multiplier Tubes

• Noise sources are
• Photomultiplier dark current
• Noise of the incoming radiation
• Shot noise and johnson noise caused by
  fluctuations of the amplication
• Noise of the load resistor

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Photon Counting
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Streak Camera

• Definition

?The streak camera is a device which measures
ultra-fast light phenomena and delivers intensity
vs. time vs. position (or wavelength) information
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Streak Camera
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Streak Camera

• Since the deflection sensitivity can be as high
  as 100 volts/cm, it can be seen that a drive
  pulse with rise time of 2000 volts/ns gives rise
  to a time base of 50 ps/cm. (The maximum
  deflection speed is approximately the speed of
  light.)
• The readout system typically an image
  intensified CCD camera can clearly resolve 100
  microns or less, giving an overall time
  resolution of 1 ps, or less.

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Streak Camera

• The streak image can contain spatial information.
  In a typical application the spatial information
  could be spectra, so the image shows
  intensity/time information over a spectral range
  of interest.

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Streak Camera

• Time resolved spectroscopy
• ?When used in combination with a spectroscope,
  time variation of the incident light intensity
  with respect to wavelength can be measured

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Why do we need a Streak Camera

• Time-resolved spectroscopy, fluorescence,
  absorption and Raman scattering are all extremely
  important techniques needed to understand many
  chemical, biological and physical processes.
• Fundamental processes caused by excited
  molecules, such as energy transfer, proton
  transfer and vibrational relaxation, occur on an
  ultrafast time scale.

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Why do we need a Streak Camera

• Time-resolved spectroscopy using streak
  technology is capable of capturing spectra of
  such fast processes in their transitional states
• studying their dynamic behavior with temporal
  resolutions ranging in the nanosecond to
  sub-picosecond domain.

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Streak Camera

• Parameters
• ?Slit width and read out pixel
• ?Tube Spatial Resolution
• ?Magnification and Deflection Speed
• ?Chromatic Aberration and Space Charge
• Limitation
• ?Scale Effects (Small is Beautiful?)

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Streak Camera

• Features
• Simultaneous measurement of light intensity on
  both the temporal and spatial axis (wavelength
  axis)By positioning a multi-channel spectroscope
  in front of the slit (for the incident light) of
  the streak camera, the spatial axis is reckoned
  for the wavelength axis. This enables changes in
  the light intensity on the various wavelengths to
  be measured (time-resolved spectroscopy).

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Streak Camera

• Superb temporal resolution of less than 0.2
  psThe streak camera boasts a superb maximum
  temporal resolution of 0.2 ps. This value of 0.2
  ps corresponds to the time it takes for light to
  advance a mere 0.06 mm.
• Handles anything from single event phenomena to
  high-repetition phenomena in the GHz rangeA wide
  range of phenomena can be measured simply by
  replacing the modular sweep unit.

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Streak Camera

• Measurement ranges from X-rays to the near
  infrared raysA streak tube (detector) can be
  selected to match any wave-length range from
  X-rays to near infrared rays.
• Ultra-high sensitivity (single photoelectron can
  be detected)The streak tube converts light into
  electrons, and then multiply it electrically. By
  this, it can measure faint light phenomena not to
  be seen by the human eyes. This enables
  monitoring of extremely faint light even single
  photoelectron can be detected.

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Streak Camera

• Dedicated readout systemA dedicated readout
  system is available which allows images recorded
  by a streak camera (streak images) to be
  displayed on video monitor and analyzed in real
  time. This enables the data to be analyzed
  immediately without the delay of film processing.