Characterization Techniques for Photonics and Nano-materials
Subject: MSP 612 – Term Paper-I
Monday, 28 May, 2018
Register number: 172901001
1A. (i) – Average power of a laser can be defined as the rate of energy flow over a complete period or a cycle, whereas the peak power of a laser is the rate of energy flow in every pulse.
Hence, the quality of a pulsed laser is in terms of peak power since the laser output is off for some time and only a part of the output (pulse) has very high energy in that cycle, which when divided by its time gives us the peak power. In case of a CW laser, the energy is present in the complete cycle and hence its quality is decided by its average power.
(ii) – Burglar alarm can be constructed by giving an incident light constantly from one side of the door to a photodiode on the other side. Under reverse bias condition, photodiode can be used to convert photon energy into electrical energy. They can be used as light detectors, mobile brightness controllers, and as headlight dimmer in vehicles (this can be achieved using light dependent resistors).
1B. – A laser mainly consists of 3 components. They are active/lasing medium, external energy source and optical resonators.
External source – It may be any source, like electrical energy source for example, which triggers the ground state atoms to the excited energy state. They provide energy needed for population inversion and stimulated emission. Examples: Electrical discharges, flash lamps, or any other laser sources too.
Active medium – This is the main component of the laser where the lasing action takes place. The external source triggers the atoms of this active medium and the population inversion is achieved. This is the place where the spontaneous and stimulated emissions take place. The type of the laser is decided by the material used as the active medium. Examples: semiconductors, dyes, gases, solid materials like ruby.
Optical resonators – These are the set of mirrors placed parallel at both the ends of the laser. It is usually a combination of 100% and 99% reflecting mirrors which makes the spontaneously emitted photons to travel back and forth in order to stimulate other photons.
Process of photon amplification by stimulated emission – The photons emitted by the active medium due to the external energy are made to travel back to the active medium by the optical resonators. These photons trigger other atoms to the excited state, resulting in emission of another photon. This process repeats until the photons exit from the partially reflecting mirror. This process results in amplification of photons by stimulating other photons.
http://een.iust.ac.ir/profs/Behnam/MedEngPrinc/xray/X-ray_Prod_Tube_Gen-040902.pdfThe above diagram is the schematic of an X-ray generator.
An evacuated glass envelope is the medium where X-ray generation takes place, inside which there is a tungsten target (anode, made of copper) on which the incoming electron beam falls. Filament is a good thermionic emitter, hence it is used as cathode through which a current is applied from the high voltage source. This releases electrons due to thermionic emission. The electrons are accelerated by the voltage and moves towards the anode. At anode, 99% energy is converted into heat and only 1% into X-rays.
The characterization of nanomaterials is done by the X-ray diffraction method using the above setup as an X-ray diffractometer.
In order to understand the technique to characterize the nanomaterials, we should first understand Bragg’s law.
Let us consider a nanomaterial with interplanar spacing ‘d’. Let 2 beams from the X-ray source with wavelength ‘?’ get diffracted at 2 points in the nanoparticle as shown in the diagram below. Then, Bragg’s law says that if ‘?’ is the angle of diffraction, then for constructive interference of order ‘n’, n ?=2d sin (?).
http://wikipremed.com/image_science_archive_68/020700_68/187420_braggs_law_68.jpgThere are different types of X-ray diffraction techniques. To characterize nanomaterials, we use X-ray powdered diffraction.
This system consists of a source which generates monochromatic X-rays, a sample holder on which the powdered nanomaterial is placed, and a detector at the other end of the source. A thin film is wrapped around the sample cylindrically. When the X-rays are incident, since the particles are in the form of powder, each grain diffracts the X-rays at different Bragg angles, forming cone-like structures. When these cones intersect with film, we get the traces left with central spot corresponding to either 0 or 180 degrees. Using the distance between these lines and the central fringe from the unwrapped film, we can use Bragg’s law to characterize the nanomaterial.
1D – Fluorescence microscope works almost like a conventional microscope, except that it uses a high intensity light which excites the specimen and the specimen in turn emits a light of lower energy or a higher wavelength, which produces a magnified image.
Fluorescence: It is the phenomenon in which a sample absorbs light of certain wavelength and gets excited, and emits light of a lower energy or a higher wavelength than the incident light when it comes back to its ground state.
https://d32ogoqmya1dw8.cloudfront.net/images/microbelife/research_methods/microscopy/fluorescent_filters.jpgThe above diagram is a schematic of fluorescence microscope setup. The incident light is confined to a single selected wavelength (which matches the fluorescing material) using an excitation filter. This light is incident on the object to which a fluorescent specimen is attached. The specimen absorbs the incident light and the atoms in it gets excited and emits radiation back when it comes back to its ground state. The emitted light is of higher wavelength than the incident light, hence has lower energy. This fluorescent light from the specimen is separated from a brighter light in the emission filter in order to become visible to the human eye.
Scanning Electron Microscopy – It is a microscopic technique which uses an electron beam which excites the sample and the back scattered electrons reflected from the sample are imaged.
The below diagram is the schematic of a Scanning Electron microscope.
https://science.howstuffworks.com/scanning-electron-microscope2.htmThis setup is a closed vacuum chamber in order to prevent the electrons from encountering interference from the air particles. This setup consists of an electron gun which produces a high energy electron beam incident at the sample through an anode and lenses. The anode focuses and controls the direction of the electron beam. The lenses are used in order to produce detailed image by focusing the electron beam precisely. Then it falls on the sample. The sample is placed in a sample chamber since the chamber is sturdy and insulated from the external vibrations. The sample needs to be very still in order to perform this technique. The electron beam interacts with the sample surface, which leads the sample to emit secondary electrons of unique patterns. This secondary electrons are detected by the secondary detectors. The brightness level of the pattern on the monitor increases with the increase in number of secondary electrons reaching the secondary detector. The backscattered electrons from the sample are detected using a backscatter detector. From the information collected by the detectors, the image is formed in the monitor screen.
Atomic Force Microscopy – It is a technique which maps the surface of the nanomaterial and images its exact dimensions which a human eye can detect.
The below diagram is a schematic of an Atomic Force Microscope instrument.
https://www.aphys.kth.se/polopoly_fs/1.672822!/image/AFMprinciple2.jpgThe sample is kept on a piezoelectric XYZ scanner. A laser beam is constantly falling on a cantilever whose tip is made to touch the sample surface. A position sensitive photodetector is constantly detects the changes in the position of the laser beam reflected from the cantilever.
The tip of the cantilever is dragged on the sample surface. The laser beam reflected from the cantilever is constantly detected by the position sensitive photodetector. Whenever the tip of the cantilever encounters a change in the surface of the sample, there is a change in force resulting in change the position of the reflected laser beam and the position sensitive detector collects the height data and forms a 3-D map of the surface at each axis.
The role of piezoelectric scanner on which the sample is placed is to change the position in order to change the position of the sample by moving it. This is done in 2 ways. We can either move the sample keeping the tip stationary, or we can keep the sample stationary moving the tip.
Transmission Electron Microscope – It is a microscope which uses a transmitted electron beam to image the nanomaterial.
The below diagram is a schematic of the Transmission Electron Microscope.
http://hk-phy.org/atomic_world/tem/tem02_e.htmlThe transmission electron microscope is an instrument enclosed in a vacuum chamber in order to prevent the electron beam to interact with the air particle interference patterns.
The electron source consists of an electron gun which emits a high energy electron beam. This high energy electron beam is accelerated towards the sample due to the anode. The electromagnetic lens system consists of lenses and metal apertures in order to make the electrons pass through a small area, which makes it a well-defined energy beam. The lens focuses the beam and the metal removes the unwanted electrons, which together makes it an electron beam of well-defined energy. This beam falls on the sample, which transmits the beam. This transmitted beam forms an image at the imaging system. The imaging system consists of projection lens which again focuses the beam into the imaging plate. This imaging plate glows when electron beam hits it. The magnified image formed is similar to the principle of a projector.
Physical Vapour Deposition: This technique can be used to synthesize nanomaterials, or to form thin films. There are different methods under physical vapour deposition, in which thermal evaporation is the simplest.
http://www.nanoscience.gatech.edu/zlwang/research/pvd.htmlThe basic process is to sublimate a source material at high temperature in powdered form, and deposition of vapour in a defined temperature gives us the desired nanomaterial. The synthesis of nanomaterial is performed in a tube furnace where the temperature is very high. The substrate (made of materials like silicon wafer or sapphire) is placed as shown in the diagram, which collects the desired nanomaterial. Ends of the furnace tube consists of a cooling water constantly flowing in order to decrease the temperature gradient.
The system’s pressure is brought down initially using a pump. Then we introduce an inert gas which flows constantly in the system to increases the pressure. The temperature and pressure being constant vaporizes the source material and this results in deposition. When the vapour reaches the substrate, nanomaterial growth will occur. Then the system is brought back to the room temperature by the inert gas.
Chemical vapour deposition: It is the technique in which a non-volatile thin film is formed on a substrate due to the reaction of reactants (vapour phase chemicals).
The reactant gas is sent into a reaction chamber, decomposed and reacted at high temperature surface to form the thin film of nanoparticles.
The setup consists of a gas delivery system to supply precursors to the chamber where the deposition takes place. There is an energy source which provides heat energy to make the precursors react. There is a vacuum system to remove all the gases which are not required for the process. There is an exhaust system to remove the by-products.
https://www.slideshare.net/Tapan7777/chemical-vaour-deposition-physical-vapour-deposition-techniques?qid=d9de20e9-27ae-4d77-a98f-13b5dc5c536f&v=&b=&from_search=4The chemical vapour deposition process starts with a force applied to transport the reactants to deposition region. The reactants are then transported from main gas to the water surface by diffusion. The reactants start to adsorb at the water surface. Then the chemical decomposition takes place. Now the chemical by-products are separated from the surface, and are transported by diffusion back to the main gas from water surface. Then the by-products are transported away from the deposition region from the force convection.
Super-resolution microscopy is a technique used in order to overcome the limitations of conventional microscopy which is limited to the diffraction techniques. This technique is used to image the nanostructures beyond the diffraction limits.
Diffraction limit is because the exact convergence of the light rays is prevented which produces a spot which blurs the object into a finite size spot on the image.
There are different types of SRM, in which STED is one of the modern techniques.
STED microscopy (Stimulated Emission Depletion microscopy): It is a technique below super resolution microscopy which uses two synchronized laser pulses. The setup consists of a scanning stage on which the sample to be imaged is placed.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2835776/The first laser produces an excitation pulse which excites the fluorescent material. This produces a diffraction pattern of ordinary size, just like a conventional fluorescent microscope. This pulse is immediately followed by another pulse called the depletion pulse in order to achieve stimulated emission. The material comes back to the ground state even before the spontaneous fluorescence occur when it encounters another photon of a lesser energy, which matches the energy between the ground state and its excited state. Here the fluorescence remains unaffected due to the second laser pulse. This depletes the excited state fluorescent materials.
The STED should sharpen the excitation Point Spread Function (PSF). For that, the laser should have a pattern of no intensity at the focus and it should have certain intensity at the periphery. But this alone doesn’t overcome the diffraction limits. For that, we raise the power of the STED laser. At the focal point, the fluorescence is not very strongly affected when the depletion region expands due to the raise in STED laser power, since the intensity of the STED laser is zero at this point. The fluorescent signal can be observed in only a small region around that focal point, reducing the total effective point spread function. Then the imaging is done by scanning this effective PSF which is around the small region in the focal point.