Title

1. Title Light Detectors

2. 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

3. Characteristics • Intensity range • Response time -effect of detector time constant • Price

4. 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

5. 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

6. Thermal Detectors • Calorimeter

7. Thermal Detectors

8. Thermal Detectors • Bolometer consists of N thermocouples in series

9. 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

10. Golay Cell

11. 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

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

13. Photodiode

14. Photodiode

15. 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

16. Photodiode

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

18. Photovoltaic detector • When illuminated generates electron-hole pairs

19. Photodiode

20. Photo Emissive Detectors • Depend on external photoeffect • Photocathode is of low work function

21. 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

22. 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

23. Photon Counting

24. 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

25. Streak Camera

26. 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.

27. 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.

28. 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

29. 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.

30. 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.

31. 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?)

32. 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).

33. 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.

34. 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.

35. 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.