Download friday_presentation1

Title
Light Detectors
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.html
Characteristics
 Intensity range
 Response time
-effect of detector time constant
 Price
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
Thermal Detectors
 Time constant of detector depends ratio of
heat capacitance and thermal losses
= H/G
where H=heat capacity
G=thermal losses
--Sensitive to small values of G
--time constant of detector limits the frequency of detector
Thermal Detectors
 Calorimeter
Thermal Detectors
Thermal Detectors
 Bolometer
consists of N thermocouples in series
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 joule’s heating limits the
maximum current through bolometer
Golay Cell
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
Photodiode
 Doped semiconductors
 Can be either photovoltaic or
photoconductive
 P-n junction when irradiated generates
photovoltage
 Photoconductive elements change their
internal resistance
Photodiode
Photodiode
Photodiode
 Absorption coefficient is spectral dependent
 Should be operated at low temperature in order to
minimize thermal excitation of electrons
For < 10 micrometers– liquid nitrogen
For > 10 micrometers– liquid helium
add figure 4.81 and 4.82
Photodiode
Photoconductive diodes
 When illuminated its electric resistance
decreases
 Time constant is dependent on diffusion time
of electrons
Photovoltaic detector
 When illuminated generates electron-hole pairs
Photodiode
Photo Emissive Detectors
 Depend on external photoeffect
 Photocathode is of low work function
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
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
Photon Counting
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
Streak Camera
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.
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.
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
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.
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.
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?)
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).
Streak Camera
 Superb temporal resolution of less than 0.2 ps
The 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 range
A wide range of phenomena can be measured
simply by replacing the modular sweep unit.
Streak Camera
 Measurement ranges from X-rays to the near
infrared rays
A 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.
Streak Camera
 Dedicated readout system
A 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.