Detectors Class 1 : Background

1
DetectorsClass 1 Background

• Bob Kremens
• kremens_at_cis.rit.edu, 475-7286
• Don Figer

2
Book

• Rieke , Detection of Light From the Ultraviolet
  to the Submillimeter.
• Chap 1, 2 3, 4, 5, 6, 7, 9, 10, 11
• Radiometry solid state physics review
• Intrinsic photoconductors
• Extrinsic photoconductors
• Photodiodes, QWIP, STJ
• Amplifiers and readouts
• CCD, hybridized arrays
• Photoemissive
• Bolometers
• Coherent receivers
• Sub-millimeter

3
Class Expectations

• Attend classes, read material provided.
• Hand in homework. (30)
• Mid-term (35)
• Final (35)

4
Light preliminaries

• Light is the only thing we actually see eg.
  When I see you, I am actually seeing light
  reflected off you.
• Light is a transverse wave, whose origin is
  accelerating electrons, eg in the sun
• Accelerating electrons not only can produce
  light, but also radio waves, microwaves, x-rays.
  Grouped together as electromagnetic waves.
• Different types of electromagnetic waves differ
  in their frequency (and wavelength) light is
  just a small part of the electromagnetic spectrum
  with certain frequency range

5
Electromagnetic Waves

• Origin is accelerating electron how?
• Consider shaking a charged rod back and forth in
  space
• i.e. You create a current ( moving charges)
  that varies in time. i.e. a changing electric
  field in space.
• A changing electric field creates a changing
  magnetic field, that in turn creates a changing
  electric field, that in turn
• i.e. a propagating disturbance

• The vibrating (oscillating) electric and
  magnetic fields regenerate each other - this is
  the electromagnetic (EM) wave.

• The EM wave moves outwards (emanates) from the
  vibrating charge.

6
Maxwells Equations
where E is the electric field, B is the
magnetic field, ? is the charge density, e is
the permittivity, and µ is the permeability of
the medium.
7
Electromagnetic Wave Velocity

• An electromagnetic wave travels at one constant
  speed through space. Why?
• Inherently due to wave nature (eg objects like
  spacecrafts can change speed, and go at different
  constant speeds) specifically, induction and
  energy conservation
• The strength of the induced fields depends on the
  rate of change of the field that created it.
• So, if light traveled slower, then its electric
  field would be changing slower, so would generate
  a weaker magnetic field, that in turn generates a
  weaker electric field, etc.wave dies out.
• Similarly, if light sped up, would get stronger
  fields, with ever-increasing energy.
• Both cases violate energy conservation.
• What is the critical speed at which mutual
  induction sustains itself? Maxwell calculated
  this 300 000 km/s c
• i.e. 3 x 108 m/s
• This is the speed in vacuum, and about the same
  in air. Slower in different media depending on
  n.

8
The Electromagnetic Spectrum

• In vacuum, all electromagnetic waves move at the
  same speed c, but differ in their frequency (and
  wavelength). Classified like

recall c f l

• Visible light 4.3 x 1014 Hz to 7 x 1014 Hz
• i.e. red is at the low-freq end of light (next
  lowest is infrared)
• violet is the high-freq end (next highest is
  ultraviolet)

and long-wavelength
short wavelength
9
The electromagnetic spectrum cont.

• frequency of wave freq of vibrating source.
• Applies to EM waves too, where source is
  oscillating electrons
• Note that EM waves are everywhere! Not just in
  air, but in interplanetary empty space -
  actually a dense sea of radiation. Vibrating
  electrons in sun put out EM waves of frequencies
  across the whole spectrum.
• Any body at any temperature other than absolute
  zero, have electrons that vibrate and (re-)emit
  in response to the EM radiation that permeates
  us, even if very low frequency.

10
Transparent materials

• When light goes through matter, electrons in the
  matter are forced to vibrate along with the
  light.
• Response of material depends on how close the
  forced vibration is to the natural frequency of
  the material. Same is true here with light.
• First note that visible light has very high freq
  (1014 Hz), so if charged object is to respond to
  this freq, it has to have very little inertia ie
  mass. An electron does have tiny mass!
• Transparent materials allow light to pass in
  straight lines

• Simple model of atom think of electrons attached
  to nucleus with springs. Light makes these
  springs oscillate.
• Different atoms/molecules have different
  spring strengths - so different natural
  frequencies.
• If this natural freq that of impinging light,
  resonance occurs i.e. vibrations of electrons
  build up to high amplitudes, electrons hold on to
  the energy for long times, while passing it to
  other atoms via collisions, finally transferred
  to heat. Not transparent.

11
Transparent materials cont.

• So materials that are opaque, or non-transparent,
  to visible light, have natural frequencies in the
  range of visible light. (see more soon)
• Glass is transparent its natural freqs are
  higher, in the ultraviolet range.
• So glass is not transparent to ultraviolet.
• But is transparent to lower freqs i.e. visible
  spectrum.
• What happens in this off-resonance case?
• Atoms are forced into vibration but at less
  amplitude, so dont hold on to the energy long
  enough to transfer much to other atoms through
  collisions. Less is transferred to heat instead
  vibrating electrons re-emit it as light at same
  frequency of the impinging light.
• Infrared waves frequencies lower than visible
  can force vibrations of atoms/molecules as well
  as electrons in glass. Increases internal energy
  and temperature of glass. Often called heat
  waves.

• Glass is transparent to visible, but not to uv
  nor infrared.

12
Opaque materials

• Have natural frequencies in the visible range, Eg
  books, you, tables, metals
• So, they absorb light without re-emitting it.
• Light energy goes into random kinetic energy ie
  heat.
• Usually, not all the frequencies in the visible
  light spectrum are resonant -
• those that arent, get reflected this
  gives the object color
• Some cases of interest
• Earths atmosphere transparent to some uv, all
  visible, some infrared. But is
• (thankfully) opaque to high uv.
• - the small amount of uv that does get through
  causes dangerous sunburn.
• - clouds are semi-transparent to uv, so can
  still get sunburnt on a cloudy day.
• Water transparent. This explains why objects
  look darker when wet
• Light is absorbed and re-emitted, bouncing
  around inside wet region each bounce loses some
  energy to material. So less light enters your eye
  looks darker.

13
Transparent materials cont.

• So light-transparent materials (like glass)
  have natural frequencies that dont coincide with
  those of light. The atoms re-emit after
  absorbing.
• This re-emission is time-delayed

• This leads to speed of light being different in
  different media
• In vacuum, its c
• In air, only slightly less than c
• In water, its 0.75c
• In glass, 0.67c (but depends on type of glass)

When light emerges back into air, it travels
again at original c
14
Radiometry

• E h?hc/?
• h6.626 x 10-34 Js
• ? in hertz
• ? is wavelength in meters
• c2.998 x 108 m/s

15
(nu-bar) represents wavenumber, the number of
wavelengths in 1 cm
16

17
EM Waves
1 eV 1.6 x 10-19 J
18
Questions

• Why in the sunlight is a black tar road hotter to
  the touch than a pane of window glass?
• Can you get sunburnt through a glass window?

19
Questions

• Why in the sunlight is a black tar road hotter to
  the touch than a pane of window glass?
• Sunlight is absorbed and turned to internal
  energy in the road surface, but transmitted
  through the glass to somewhere else.
• Can you get sunburnt through a glass window?
• Glass is opaque to ultraviolet light, so wont
  transmit it, so you wont get sunburnt (although
  you might get hot!).

20
http//www.edmundoptics.com/TechSupport/DisplayArt
icle.cfm?articleid259
UVA 400 nm - 320 nmUVB 320 nm - 290 nmUVC 290
nm - 100 nm
http//www.fda.gov/fdac/features/2000/400_sun.html
Sunburn is caused by a type of UV light known
as UVB. The thinking was if you prevent sunburn,
you'd prevent skin cancer. In recent years,
scientists have come to appreciate that UVA, may
be just as, or even more, important in causing
some skin disorders. Although experts still
believe that UVB is responsible for much of the
skin damage caused by sunlight UVA may be an
important factor in other types of sun damage.
Most sunscreens block UVB but fewer filter out
most of the UVA.
21
(No Transcript)
22
Answer 1 Yes, because any radio wave travels at
the speed of light. A radio wave is an
electromagnetic wave, like a low-freq light
wave. A sound wave, on the other hand, is
fundamentally different. A sound wave is a
mechanical disturbance propagated through a
material medium by material particles that
vibrate against one another. In air, the speed
of sound is about 340 m/s, about one millionth
the speed of a radio wave. Sound travels faster
in other media, but in no case at the speed of
light.
23
(No Transcript)
24
Answer 3 All bodies with any temperature at all
continually emit electromagnetic waves. The
frequency of these waves varies with temperature.
Lamp B is hot enough to emit visible light. Lamp
A is cooler, and the radiation it emits is too
low in frequency to be visibleit emits infrared
waves, which arent seen with the eye. You emit
waves as well. Even in a completely dark room
your waves are there. Your friends may not be
able to see you, but a rattlesnake can!
25
Visible Light (hand drawn using 7000 stars)
26
Galactic Center
27
COBE Image - IR
28
The Sky 408 MHz
cosmic radio waves are generated by high energy
electrons spiraling along magnetic fields.
pulsars
29
Ultra-Violet (IUE)
30
X-Rays
Map with X-Rays
31
Gamma-Rays at photon energies above 100 million
electron Volts
32
WMAP
Wilkinson Microwave Anisotropy Probe
33 GHz 9.1 mm
41 GHz 7.3 mm
23 GHz 13.0 mm
61 GHz 4.9 mm
94 GHz 3.2 mm
http//map.gsfc.nasa.gov/m_or.html
33
(No Transcript)
34
The Electromagnetic Spectrum
35
Radio microwave regions (3 kHz 300 GHz)
36
Synchrotron Radiation
37
Synchrotron X-ray source and uses at LBL
38
X-rays for photo-lithography
You can only focus light to a spot size depending
on the lights wavelength. So x-rays are
necessary for integrated-circuit applications
with structure a small fraction of a micron. 1
keV photons from a synchrotron 2 micron lines
over a base of 0.5 micron lines.
39
Blackbody Radiation

• A black body is a theoretical object that absorbs
  100 of the radiation that hits it. Therefore it
  reflects no radiation and appears perfectly black.

40
Blackbody

• In practice no material has been found to absorb
  all incoming radiation, but carbon in its
  graphite form absorbs all but about 3. It is
  also a perfect emitter of radiation. At a
  particular temperature the black body would emit
  the maximum amount of energy possible for that
  temperature. This value is known as the black
  body radiation. It would emit at every wavelength
  of light as it must be able to absorb every
  wavelength to be sure of absorbing all incoming
  radiation. The maximum wavelength emitted by a
  black body radiator is infinite. It also emits a
  definite amount of energy at each wavelength for
  a particular temperature, so standard blackbody
  curves can be drawn for each temperature, showing
  the energy radiated at each wavelength. All
  objects emit radiation above absolute zero.

41
The Maxwell-Boltzman Distribution
In the absence of collisions, molecules tend to
remain in the lowest energy state available.
Collisions can knock a molecule into a
higher-energy state. The higher the temperature,
the more this happens.
Low T
High T

• In equilibrium, the ratio of the populations of
  two states is
• exp(DE/kBT )
• As a result, higher-energy states are always less
  populated than the ground state, and absorption
  is stronger than stimulated emission.

42
Absorption Spontaneous Emission
Stimulated Emission
43
Einstein A and B coefficients

• In 1916, Einstein considered the various
  transition rates between molecular states (say, 0
  and 1) involving light of irradiance, I
• Absorption rate B01 N0 I
• Spontaneous emission rate A N1
• Stimulated emission rate B10 N1 I
• In equilibrium, the rate of upward transitions
  equals the rate of
• downward transitions
• B01 N0 I A N1 B10 N1 I
• Rearranging
• (B01 I ) / (A B10 I ) N1 / N0
  expDE/kBT

Recalling the Maxwell- Boltzmann Distribution
44
Einstein A and B coefficients and Blackbody
Radiation

• Now solve for the irradiance in (B01 I ) / (A
  B10 I ) exp-DE/kBT
• Rearrange to B01 I expDE/kBT A B10 I
• or I A / B01 expDE/kBT
  B10
• or I A/B10 / B01 /B10
  expDE/kBT 1
• Now, when T , I should also. As T ,
  expDE/kBT 1.
• So B01 B10 º B Coeff up coeff
  down!
• And I A/B / expDE/kBT 1
• Eliminating A/B and use DE hn

I(?,T)2?h?³/c²(exph?/kBT-1)
45
Plancks Equation
I(?,T)2?h?³/c²(exph?/kBT-1)
use ? c/? I? d? I? d?
h Plancks constant 6.6260755 x 10-34 Jsec k
Boltzmanns constant 1/380658 x 10-23 J/K c
3 x 108 m/sec T in Kelvin I spectral radiant
excitance f(?,T) Wcm-2 ?m-1
46
Note
A/B ??3
Remember A spontaneous rate B stimulated
rate
X-ray lasers hard to make (need lots of B).
47
Blackbody Emission

• The higher the temperature, the more the emission
  and the shorter the average wavelength.

48
Wien's Law Blackbody peak wavelength scales as
1/Temperature.

• ?T ? 3000 ?m K (2898 actually)

49
Stefan Boltzmann Law
P ?AT4
For blackbodies
? 5.67033 x 10-8 W/K4 m2
P ??AT4
For real objects
? emissivity e.g. Aluminum foil 0.02 Red
brick 0.9 soot 0.95
50
EM Spectrum
The Electromagnetic Spectrum
51
Atmospheric Transmission