Definitions Flashcards
How is an image formed in the eye?
- Cornea - focusing action
- Iris/pupil - controls light entering the eye
- Crystalline Lens
- Retina - inverted image formed - two photoreceptors -> rod cells and cone cells.
What is a black body?
An idealised way to describe a system with thermal radiating energy for T > 0.
What is coherence?
The measurement of correlation between signal disturbances
What is the difference between spatial and temporal coherence?
Spatial coherence depends on emission area of source while temporal coherence depends on bandwidth of the source
What is diffraction?
When light deviates from the straight line of propagation, spreading along the image plane
What is microscopy?
A source is illuminated and collected by a lens then subsequently projected onto a sample.
What is bright field microscopy?
Interference in incoherent sources which is mostly limited to the focal plane.
What are the three potential limiting factors in microscopy?
- Diffraction limited
- Detector limited
- Aberrations
What is Rayleigh’s criterion?
When two point sources are resolvable when separated by at least the distance of the first Airy disc.
How does a compound microscope form an image?
- Objective lens - acts as aperture stop and entrance pupil forming a real inverted image
- Field stop - limits the extent of the largest object that can be viewed.
- Ocular - forms a magnified virtual image
- Real image formed on camera
What is an ocular?
A magnifier that views the intermediate image of the object. It provides a virtual image.
What is the depth of field?
The distance from the nearest object plane in focus to the farthest plane also simultaneously in focus.
What does the Fourier transform tell us?
How much of each spatial frequency is present.
What do high k values tell us?
Higher k-values provide better resolution.
What is a diffraction pattern a result of?
Interference of different scattering orders.
What is a transform lens?
A transform lens acts as an optical computer able to perform instant FT.
How do we get interference in the transform plane?
At least two different scattering orders must enter the lens to have interference in the transform plane.
Why do we use an objective lens?
Produces a magnified image of the sample.
To bring diffraction of object from infinite, to a comfortable distance producing a 2D FT of the object at that plane.
It limits resolution of a microscope by its ability to collect a large amount of light from the specimen.
Why do we use an imaging lens?
To project the diffraction pattern of the light distribution at the transform plane onto the image plane.
It diffracts the diffracted beam.
How many Fourier transform’s does a compound microscope perform?
Two consecutive FTs.
How would a perfect optical system form an image?
It would form an image as a result of the interference of all spatial frequencies.
What is the numerical aperture?
A parameter that best describes the microscopes objective. First introduced by Abbe to characterise the angular acceptance of the lens.
What is Abbes theory of light imaging?
When a sample is illuminated it is diffracted resulting in a zero order and higher order spatial frequency terms.
This results in a diffraction pattern which is a result of different scattering orders.
At least two different scattering orders must enter the transform lens for it to perofrm an instant FT.
It then transmits the entire diffraction pattern onto the image plane.
How can we reduce the contrast in an image?
By reducing the level of the constant background.
What is the rose criterion?
The contrast to noise ratio must be > 4-5 to be reliably observable.
What does the rose criterion set the upper limit of?
The signal levels required to observe objects.
What is the fundamental cut-off frequency in any system?
The nyquist frequency , 1/2 maximum sampling frequency.
What does the modulation transfer function represent?
The MTF represents how strongly the contrast is transferred as a function of spatial frequency
How can we get an experimental measure of the MTF?
Can obtain an experiemnt measure form the point-spread function.
What is the PSF a measure of?
The PSF measures the response of the optical system to a point source.
Describe an upright microscope.
The objective is attached to a nosepiece turret on the microscope body.
Describe an inverted microscope?
The stage is fixed at the microscope body.
What is traditional illumination?
Converging/diverging light reaches the sample resulting in uneven illumination.
What is Kohler illumination?
Introduce a field lens such that each point of the source image is projected as a collimated stream of light. Resulting in even illumination, images are formed free from contrast.
What does kohler illumination exploit?
It exploits a set of conjugate planes in an optical system.
What is a conjugate plane?
Each plane within a set is conjugate with the other plane: all of the planes in a set can be seen simultaneously when looking through a microscope.
How do we observe a field plane?
A field plane is observed in normal viewing mode using the eyepieces.
How do we observe an aperture plane?
An aperture plane is observed using diffraction mode which requires an eyepiece telescope
What does a field diaphragm do?
Changes the width of the light bundle.
What are field and condenser apertures?
Controls illumination.
Holes or openings in opaque masks that eliminate stray light entering the light path.
Describe an infinity corrected microscope.
In an infinity corrected system the image distance is set to infinity by adding a tube lens. The object is placed at a distance equal to the focal length of the objective lens.
How is a glass lens manufactured?
- Pressure process
- Diamond Grinding & Polishing
- Metrology Techniques
- Magneterheological finishing
What are the two types of aberrations?
Chromatic and Geometric aberrations
What is a chromatic aberration?
Abberations caused by the focal lengths dependance on colour/wavelength
What are the different types of geometric aberrations?
- Round aberrations
- defocus
-spherical aberration - Non-round aberrations
- coma
- astigmatism - Field Curvature
- Distortion
Why do we get aberrations in a simple lens?
A simple lens is formed by spherical surfaces. It has many aberrations due to manufacturing failures and dispersion in the lens material.
How do we overcome aberrations in a simple lens?
By using a compound/complex lens which is several lenses glued together with different refractive indices.
How does refractive index vary with wavelength?
n increases with decreasing wavelength for all glasses.
How can we correct for chromatic aberrations?
Using achromatic doublet/triplet - two/three lenses together with different colour dispersion properties.
What is a spherical aberration?
Spherical aberration arises due to using spherical lenses.
It occurs when parallel rays are incident at central locations in the lens are focused at different axial locations.
How can we correct for spherical aberrations?
- Introducing an aperture that limits the range of angles incident on the lens.
- use aspherical lenses
What is a coma aberration?
An off axis aberration causing point objects to look like comets.
This occurs when light hits the lens obliquely.
How can we solve for coma aberrations?
By accommodating the diameter of the object field on the lens to the size of the objective.
What is astigmatism?
Off axis aberrations due to asymmetric lens curvature.
Rays passing through horizontal/vertical diameters of the lens focus slightly at different planes. Resulting in non-uniform illumination and inconsistent contrast.
What is field curvature?
Whole image cannot be brought into focus on a flat surface.
What is distortion?
A non-linear magnification of the image from the centre to the periphery.
What are the two modern objectives?
A dry objective and immersion objective.
What is a dry objective?
Total internal reflection reduces the range of angles that can be collected by the objective.
What is an immersion objective?
The total internal reflection process doesn’t occur, leading to collection of a larger amount of rays.
What is photodetection?
Photodetection describes the interaction of light with matter or the photoelectric effect.
What are photovoltaic detectors?
Photovoltaic detectors exploit the photovoltaic effect which is the generation of a potential across an active region of a detector.
What is an example of a photovoltaic detector.
A p-n junction under photon irradiation. The electrical conductivity changes upon illumination if the photon energy is larger than the detector band gap.
What happens when we join together semiconductors of p and n doping in thermodynamic equilibrium?
Electrons from the n region diffuse into the p region and vice verse such that an electric field is formed creating a depletion zone with a small number of mobile carriers.
What happens when we apply a bias to a p-n diode?
Forward bias: width of the depletion zone shrinks
reverse bias: width of the depletion zone expands.
What happens to a p-n diode under illumination?
Under front illumination the light penetrates the photodiode bulk from the p side. This is because the p+ layer is much thinner than light absorption length.
Describe a p-i-n diode under bias.
The layer width is miximised by the intrinsic layer inserted between the p-type and n-type layers,
Describe the operation of a photodiode under reverse bias?
Photodiodes usually operate under reverse bias. This results in a significant increase of current under illumination resulting in a transient current flow of electrons and holes away from the depletion layer. When a potential is applied across the widened depletion region it results in zero current.
What are thermal detectors?
The energy of the absorbed light heats the substrate such that the dissipated energy causes some other physical parameter to change.
What is a bolometer?
A thermal detector. Power from an incident signal is absorbed heating a thermal mass. The thermal mass is connected to a reservoir of constant temperature through a link with thermal conductance.
What are the time domain characteristic properties?
- Sampling rate
- Frame size
- Block size
What are the frequency domain characteristic properties?
- Bandwidth
- Spectral lines
- Frequency resolution
What is the sampling rate?
The number of data samples acquired per second
What is the frame size?
The amount of time that data is collected to perform a FT
What is the block size?
The total number of data samples acquired during one frame.
What is the bandwidth?
The highest frequency that is captured in the FT, equal to half the sampling rate
What are spectral lines?
After the FT, the total number of frequency domain samples
What is frequency resolution?
The spacing between samples in the frequency domain.
How can we reproduce a signal well?
According to nyquist’s sampling theorem we must sample at least 2x higher than the maximum frequency of interest
What is aliasing?
An effect that causes different analogue signals to become indistinguishable when sampled.
How can we avoid aliasing?
By applying low pass fillers or anti-aliasing filters.
What is noise?
Any undesired signal which masks the signal being measured.
What is external noise?
Disturbances due to effects outside the system
What is internal noise?
All noise generated within the detection system itself.
What are the three main sources of noise in a detector?
- Radiation noise / quantum noise
- Detector-generated noise
- Electronics
What is radiation/quantum noise?
Variation in the arrival of photons.
When do we get the best performance?
If photon noise > detector & electronics noise
What are the types of detector generated noise?
- Shot noise - due to the discrete nature of photoelectron generation.
- 1/f noise
What are the types of electronics noise?
Johnson noise - due to random thermal motion of charge carriers in a resistive element
What is 1/f noise?
Flicker noise is not well understood. Generally associated with non-ohmic contacts of the device, surface state traps & dislocations.
When does shot photo current noise dominate?
When at high light intensity conditions.
When does background shot noise dominate?
When background light is comparable to source light
When does Johnson noise dominate?
Only dominates when under low light levels and when a small load resistor is used.
What are important figures of merit in a detector?
- SNR
- Responsivity
- Quantum Efficiency
- NEP
- Detectivity
- Linearity & Damage Threshold
- Dynamic Range
- Detector Speed.
What is the Signal to Noise Ratio?
The ratio between the signal measured and the noise.
What are the two regimes of the SNR?
- Quantum noise limit
- Small signal regime.
What is the quantum noise limit?
The fundamental quantum limit which cannot be overcome by any detection system.
What is the small signal regime?
The thermal noise limit of detection.
What is a quantum detector?
A detector in which absorbed photons directly create free carriers.
What is the quantum efficiency?
The ratio of the number of photodetection events to the number of photons incident, prior to ANY amplification.
What does the quantum efficiency quantify?
The probability that a photoelectron or signal or digital count is produced when the photon is incident in the detector.
What does gain reduce?
The circuit noise, but it cannot improve the shot noise.
What is the responsivity?
The ratio of the detector output to the intensity of input optical flux
When is the responsivity the most important figure of merit?
When taking into account
- the biasing of the detector and the gains & dynamic ranges of subsequent amplifier stages
- how much optical power is needed to overcome Johnson noise in the electrons
What is the Noise Equivalent Power?
The incident optical power required to produce a SNR = 1 (0dB) for a given bandwidth,
What is the detectivity?
The inverse of the NEP
What is linearity?
A key characteristic for good performance of a detector.
Photodetectors can be very linear.
What are some possible sources of non-linearity?
Non-linearity arises due to either excessive current density or lateral voltage drops in the thin epitaxial layer.
What is the dynamic range of a detector?
The logarithmic ratio between the largest and smallest values that can be detected.
Describes the SNR for the case when the signal is the loudest possible for the system.
What is the damage threshold?
When the saturation point is reached, the photodiode becomes more and more non-linear until the damage threshold point is reached.
At this point the photodiode is no longer able to turn excess in incident optical power into a photocurrent output and all the extra energy absorbed is converted to heat.
What is the time response of a photodetector?
The response of a photodetector is not instantaneous.
How do avalanche photodiodes operate?
They operate under high reverse bias in a p-n diode. Where free carriers acquire enough energy to excite other carriers through impact ionisation. Avalanche breakdown occurs in high electric fields. This magnifies the photocurrent in photodiode resulting in an intrinsic amplification.
What other modes can a APD operate in?
Geiger mode which allows for work in a single photon counting regime.
What is the typical structure of an APD?
It is a p-i-n that is under a very large reverse bias. Wide undoped intrinsic semiconductor region between p- and n-type semiconductor regions.
What are the electron and hole ionisation rates?
The number of ionising collisions per unit distance. It is an exponential dependence on threshold electric fields.
What are the figures of merit of an APD?
- Electron and hole ionisation rate
- Gain and responsivity.
What is the gain dependence in an ADP?
The gain of an ADP is a very strongly increasing function of bias voltage near breakdown. Therefore a strong dependence on temperature of operation.
What is the excess noise factor?
The noise current has an additional term beyond a simple scaling by a factor of G. The ratio of the actual noise to that which would exist if the multiplication process were noiseless.
What is the origin of the excess noise factor?
Stochastic nature of the multiplication process. Gain fluctuation produces excess noise, which becomes progressively worse as G increases.
What is a photomultiplier tube?
A device that is designed to increase the sensitivity of a photoemissive detector.
How does a photomultiplier tube operate?
When photons strike the detector an electron is emitted from a photosensitive cathode. Dynodes connected in series between the cathode and anode accelerate the electrons amplifying the signal.
What are the alternative PMT designs?
- Channeltron
- Single continuous dynode
What is a channeltron?
A glass tube coated on the inside by a conductive and an emissive layer. A single channel electron multiplier.
What is a single continuous dynode?
Much smaller than a PMT with similar detection characteristics. Array of them form microchannel plates.
Which noise dominates in a PMT?
Shot noise.
What is a charge transfer device?
A device that stores non-equilibrium charges created by light on the potential well of a metal-insulator-semiconductor (MIS) capacitor.
Subsequently the stored charges are transferred across the semiconductor substrate towards the readout electronics.
What are the two main charge transfer devices?
- Charge coupled devices (CCD)
- Complementary metal-oxide-semiconductor detectors (CMOS)
What are the designs of CCD and CMOS based on?
A MOS capacitor which is a transparent contact with a silicon oxide isolator deposited onto p-doped silicon and a n-doped channel formed under the contact.
What is the MOS equivalent circuit?
Two capacitors in series
(1) C(MOS) insulating layer
(2) C(pn) pn junction
the positive voltage is applied to the contact with two switches for reset and read.
What are the four steps of the MOS operational cycle?
- Device charging
- Integration
- Read-out
- Reset
Describe device charging in a MOS capacitor.
The mechanical shutter closes, then the read switch opens while the reset switch is closed. This allows short bursts of current to charge both C(MOS) and C(PN) capacitors to the same charge.
Describe integration in a MOS capacitor.
Both the mechinical shutter and reset switch open simultaneously. Photons reach the p-n junction, generating the carriers. These carriers are separated by intrinsic electric field inside the p-n junction.
How is CMOS readout?
CMOS readout is performed by MOSFETs grouped around each MOS capacitor. The voltage is sampled directly at each pixel. This is known as parallel readout.
What is the source follower?
The source follower transfers the voltage from the MOS capacitor to the read column when the select switch is activated.
How is CCD readout?
The entire array is read through a single amplifier. This is known as sequential readout.
Does a CCD or a CMOS have a higher fill factor?
A CCD has a higher fill factor and therefore better sensitivity.
How can we increase the optical fill factor in CCDs?
Using micro lenses.
What is a rolling shutter?
A rolling shutter will expose the frame line after line. The number of exposures equals the number of lines in the frame.
What is an electronic / global shutter?
An electronic (CCD) or global (CMOS) shutter allows exposure of the whole frame at the same time.
What are frame/interline transfer topologies?
Entire exposed image is transferred in rows into optically shielded storage areas, where it waits to be transferred pixel-by-pixel through the amplifier.
Describe an interline transfer scheme.
A global shutter is described by an interline transfer scheme where each column is repeated separately under opaque metal screen as vertical shift register. The transfer gate shifts the entire charge array into vertical registers in a single step.
What are the figures of merit for CCD and CMOS detectors?
- Full-well capacity
in primarily CCD
- Blooming
- Smear
What is the full-well capacity?
The largest charge a pixel can hold before saturation, resulting in the degradation of the signal.
What is blooming?
Blooming occurs when the charge of the pixel becomes saturated and starts to fill neighbouring pixels causing large areas of the image to appear white.
What is smear?
Pixels become saturated causing light to spill over into the vertical shift register while clocking out.
Why do we use frame/interline transfer topologies?
To reduce blurring as increasing the bandwidth results in an increases in noise.
What is a colour filter matrix?
Dye filters for each pixel.
What noise dominates in CCDs and CMOS cameras?
Shot noise
What is a photon transfer curve?
A log-log plot of variance versus signal level.
How can we polarise light?
- By reflection
- By transmission through birefringent material
What is a birefringent material?
A material with different refractive indices along different directions.
What is an ordinary ray?
Spherical secondary wavefronts
What are extraordinary rays?
Ellipsodial secondary wavefront.
How do rays propagate in a birefringent material and which one is used in polarisation microscopy?
- Incidence splitting
- Incidence perpendicular to the optical axis
- Incidence parallel to the optical axis.
- is most commonly used in polarisation microscopy.
What is incidence splitting?
Two separate linearly polarised rays emerge ordinary/extraordinary
What is incidence perpendicular to optical axis?
Two rays emerge together but there is a phase difference between both.
What is incidence parallel to optical axis?
Two rays emerge together and are in phase.
What is polarisation microscopy?
Polarisation microscopy is a direction-sensitive technique that exploits the unique ability of polarised light to interact with polarisable bonds of ordered molecules or unique directions of non-cubic crystals.
What is a compensator?
Birefringent material that introduces different phases between the two optical rays.
What is colour-based polarisation microscopy?
A technique where different areas of the sample are observed as showing different colours due to the different phase changes and their wavelength dependence.
When do we use amplitude and phase contrast mechanisms?
When imaging samples based on absorption and scattering of light.
What is (Zernike) phase contrast microscopy?
Zernike invented an optical microscope that could exploit phase contrast by modifiying the ampltidue and phase of a scattered wave by the sample.
Light is diffracted by the sample and the beams recombines in the image plane.
Zernike shows more contrast compared to bright field as bright field contrast based on absorption scattering.
What are the two key things required for (Zernike) phase contrast?
- Separate the zero-order and diffracted rays emerging from the specimen.
- Advance the phase and reduce amplitude of 0th order light. Maximise the amplitude differences between object and background.
How does differential interference contrast microscopy work?
The input light is polarised at 45 degrees to the Normarski prism. The beam is split into two and separated by a short distance. They are brought back together afterwards using another Nomarksi prism. Only light that has gained polarisation in recombination of rays can pass. Only differentials can pass.
Better spatial resolution than Zernike.
What is Dark field microscopy?
An alternative to phase contrast. It uses hollow cone illumination from a blocking aperture. It excludes undiffracted rays with the objective aperture. Such that regions not diffracted are dark and diffracted regions are brighter.
Results in a stronger contrast than bright field.
What is fluorescence microscopy?
Fluorescence microscopy is based on fluorescence which is when light is emitted at characteristic wavelengths of atoms/molecules via the transient promotion of electrons to a higher excited state by radiant energy of a stimulating source.
What makes the fluorescent molecules visible?
tagging them with a fluorescent dye.
What are the components of fluroescence microscopy.
- Bright light source
- Epi-illuminator
- Three sets of filters - excitation, dichromatic mirror, and emission/barrier.
What is the structure of a dichroic mirror?
Multilayer dielectric mirror of alternate layers of low n and high n transparent materials.
What is a confocal microscope?
Confocal microscopy ensures both illumination and detection are focused in the same spot.
The sample is illuminated with focused laser, where the laser spot is diffraction limited. Uses fluorescence mechanisms.
What is the optical knife?
When the detector pinhole used in confocal microscopy removes all emission not origination from the focal plane.
Pinhole size ~ Airy disk.
How can we beat the resolution limit?
1) Scanning near field optical microscope (SNOM)
working in the near-field regime
What is far field diffraction?
The diffraction pattern has no resemblance with the aperture.
What is near field diffraction?
The aperture structure is imprinted in the diffraction pattern.
What does the blocking aperture do?
It excludes undiffracted rays such that regions with diffraction are brighter and those without are dark.
How can we get hollow cone illumination?
Toroidal mirror and central beam stop
What is Fraunhofer?
Far field diffraction
What is Fresnel diffraction?
Near field diffraction
How does a SNOM operate?
By control of molecular states of fluorophores via stimulated emission depletion.
What is stimulated emission depletion?
The excited state can decay faster if stimulated by a second photon. The asymmetric overlap of two lasers leads to an effective excitation spot smaller than the diffraction limited resolution.
What are the key properties of x-rays and their two regimes?
Short wavelength and highly energetic. With hard X-RAY < 1 angstrom, E > 10keV and soft X-RAY ~ 1nm, E ~ 0.1 - 10keV.
What are the applications of x-rays?
x-rays can be used for imaging for medical diagnosis, for astronomical science and for probing materials?
What are the three methods for x-ray production?
1) directing an energetic beam of radiation (particles) at a target material
2) fast deceleration of high-speed particles - production of broad Bremsstrahlung radiation
3) synchrotron radiation
Describe synchrotron radiation
A toroidal emission of linearly accelerating charge. The electrons are moving in circular track at a precise speed close to the speed of light.
Why does conventional optics not work for x-rays?
- It is a dispersive media.
- It has two types of interactions
What is a dispersive media?
A dispersive medium has a refractive index which is frequency-dependent.
What are the two types of x-ray interactions?
1) dissipative absorption
2) non resonant scattering
What is dissipative absorption?
Photon excites an atomic transition and is absorbed. This can result in secondary electrons or re absorption.
What is non resonant scattering?
Photons causes some oscillation of electron charge clouds. This can oscillate to high frequencies such that the sample polarises in response to the transverse E-field in radiation. The electron cloud or dipole molecules may get in resonance.
What must our model of x-rays account for?
Multiple resonances, a damping force, and an additional electric field.
What are multiple resonances?
Associated with multiple oscillators with oscillation strength fi
What is a damping force?
Friction when atoms vibrate, heat.
Why do dense materials have additional electric field?
From movement of other atoms. Ions in molecules can be important at resonances.
What is anomalous dispersion?
Strong decrease of real part of n, coinciding with absorption bands.
What is plasma resonances in metals?
Full absorption for frequencies below the plasma frequency = 10^15 Hz
What are the alternatives for x-ray optics?
- Absorption elements
- Reflection optics
- Refraction optics
- Diffraction optics
How do we focus x-rays for reflection optics?
Using very large reflective mirrors and glass capillaries.
Why do we use very large reflective mirrors?
Strong focusing from multiple mirror surfaces.
How do we focus x-rays using refractive optics?
Using multiple concave lenses. Although we need to reduce the materials thickness. Refractive optics can get hot and suffer from beam damage.
How do we focus x-rays using diffractive optics?
By looking at Fresnel diffraction which introduces the obliquity factor.
What is the obliquity factor?
Secondary wavelets aren’t perfectly spherical, but they have a preferred direction, given by Kirchoff’s obliquity factor.
Why do we use an aperture diameter?
To control the number of Fresnel zones contributing to diffraction.
Describe zone plates for x-rays?
X-ray lenses exploit that even/odd fresnel zones tend to nullify each other. The x-rays are opaque to zones such that a zone plate consists of a series of alternating transmitting/opaque rings.
What is the main type of x-ray detector?
A scintillation detector.
How do scintillators operate?
When struck by an incoming particle the luminescent material absorbs its energy and re-emits it as light at a lower frequency.
Describe the detection of an x-ray using a scintillator.
An x-Rays passes through a thin aluminium coating. The x-ray is then absorbed by the scintillator. It is then converted into visible light.The visible light is reflected by the reflective layers. It is then transmitted to the photodetector. Finally the visible light is detected and converted into an electrical signal.
What happens in a scintillator if the excited state is meta stable?
The relaxation back down from the excited state to lower states is delayed: phorsphorescence
What are some desirable properties of a scintillator?
- High Z
- High Density
- High reflectivity
What are the possible x-ray matter interactions?
1) photoelectric absorption
2) Compton scattering
3) production of electron-positron pairs
Describe photoelectric absorption.
Electrons are ejected from orbitals around an atom and x-ray are absorbed in the interaction.
Describe Compton scattering.
Gamma rays cause scattering of electrons which gives lots of energy to the electron
Where does each x-ray interaction occur/dominate?
E < 100 keV : photoelectric absorption
E ~ 1 MeV : Compton effect
E»_space; 1 MeV : electron-positron pair production
What is a scintillator detector made up of?
Scintillator formed by Csl:Tl embedded inside reflective layers of aluminium. X-ray detectors are based on scintillators.
How can we achieve x-ray absorption spectroscopy?
By using scanning transmission x-ray microscopy.
The x-ray absorption increases as you cross the energy threshold needed for a specific element transition. Can acquire the spectroscopic image by scanning wavelength selected by a monochromatic by tilting both crystals.
How can we use x-rays for medical imaging?
Can use soft x-rays for projection imaging. Computed tomography records projection x-rays of the same thing from different directions. This allows for the computation of the 3D structure.
How can we image using electrons?
Electrons can be easily accelerated and focused for imaging.
What are the two different types of electron microscopes?
Scanning/Transmission Electron Microscope
- SEM
- TEM
What system do we operate SEM/TEM in?
Ultra high vacuum levels. Combination of turbo molecular and ion pump to achieve high vacuum.
What is the limiting factor for SEM performance?
Current density and divergence
What is the electron source?
A W-filament source is a thermionic emitter and the simplest source. The electric field brings emission from a sharp tip and focusses it using a converging lens. After the beam converges it diverges forming a cone.
How do electromagnetic lenses work?
They contain a coil of copper wires inside an iron pole piece. Current flowing through the wire creates a magnetic field, used to converge the beam.
What are the main aberrations electromagnetic lenses suffer from?
- Spherical aberration !!
- Chromatic aberration
- Astigmatism.
How can we correct for aberrations in an electromagnetic lens?
By using multipole lenses.
Reducing the spherical aberration allows larger probe angle and larger fraction of emission from the electron gun to be used.
Describe the substrate interactions in electron detection?
The substrate interacts with both electrons in solid and with nuclear charge. This interaction leads to the generation of a wide range of signals : electrons and photons over an interaction volume.
What does the interaction volume increase with?
Interaction volume increases with increasing energy and decreasing Z.
What are the detectors used in SEM?
- Evenhart-Thornley detector
- Cathodoluminescence spectroscopy
- Energy dispersive x-ray spectroscopy
-> measures the atomic composition of the TEM specimen - Auger spectroscopy
What is an Everhart-Thornley detector?
A scintillator that exploits cathodoluminscence.
Cathodoluminscence is the emission of visible light by the sample when the electron decays from the conduction band to the valence band after excitation.
What are the main components of a TEM system?
- Electron gun
- Condenser system
- Objective lens
- Projector system
How can we do electron detection in TEM?
1) fluorescent screen
- images recorded using photographic film
2) CCD and CMOS
- direct exposure to the electron beam may damage the CCD.
What is the main difference between an optical microscope and electron microscope?
Carrier : Light rays vs Electrons
Medium: Air vs Vacuum
Lenses: Glass vs Electromagnetic
Visibility: Direct vs Fluorescent screen/CCD.
Describe Auger spectroscopy
Measure characteristic energy of electrons generated in the above four chemical analysis
How does SEM operate?
The beam scans from left to right and top to bottom. This has a one-to-one correspondence between the rastering pattern on the specimen and the one to produce the image.
How do we get secondary electrons?
An inelastic event of primary electrons that result in the transfer of energy to an atom, lead to secondary electron emission.
How do we get back scattered electrons?
Electrons are scattered pseudo-elastically, changing direction and being remitted.
How does backscattering depend on the atomic number?
The back scatter probability rises rapidly with atomic number.
Describe the sample preparation for SEM?
They are bulk samples often with a thin layer of gold or carbon with careful selection of electron energy.
What is electron beam lithography?
A multi step nano fabrication process that follows these steps.
- Spin-coating
- E-beam exposure
- Chemical development
- Metal deposition
- Liftoff.
What are the conventional TEM imaging modes?
- Diffraction contrast (BF/DF)
-> BF: unscattered beam
-> DF: scattered beam - Phase contrast
- Selected area diffraction
How is the imaging mode condition chosen in TEM?
Chosen by the collection semi angle.
How do we limit the number of beams that contribute to the image?
By introducing an objective aperture in the diffraction pattern
How can we reduce the aberrations in dark field imaging?
By using rays along the optic axis
What is selected area diffraction operation mode in TEM?
Only electrons of a given area of the sample contribute to the diffraction pattern.
How could you identify a large aperture?
A polycrystalline sample with many spots forming rings
How could you identify a small area selected by a smaller aperture?
Only one crystal contributes to the diffraction
Why do electrons scatter?
Due to electrostatic interactions with atoms. Such that there is elastic scattering due to the average position of atoms in lattice.
What diffraction pattern does a polycrystalline textured material give?
One with incomplete rings or a series of arcs
What diffraction pattern does a amorphous material give?
Diffuse rings reflecting short range order
Describe phase contrast operation mode in TEM?
The selection of more than one beam is required as the beams interfere to form the image.
what is the point resolution?
The first time sin(𝜒) = 0. All k values below it have the same sign.
What is the information limit?
It is beyond the point resolution, but interpretation for those k is complex.
How can we tune the contract transfer function?
By defocusing delta f to tune CTF. this is known as the Scherzer focus.
