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UNIT – IV Advanced Physics
Syllabus: Lasers - Spontaneous emission, Stimulated emission, Population inversion, Solid state (Ruby) laser, Gas (He-Ne) laser, Semiconductor laser(Ga-As), applications of laser. Holography -Principle, Recording, Reproduction and applications. Optical fibers - Structure of optical fiber, Types of optical fibers, Numerical aperture, Fiber optics in communications and its advantages. Optoelectronic devices - Qualitative treatment of photo diode, LED, LCD and Solar cell and its applications. Introduction to nanotechnology - Nanomaterials, electrical and mechanical properties, applications in electronics, computer and medicine. ============================================================================= ========== LASERs A LASER ( L ight A mplification by S timulated E mission of R adiation) is an optical source that emits photons in a coherent beam.. The process of particle transfer from normal state corresponding to minimum energy of the system to a higher energy state is termed as excitation and the particle itself is said to be excited. In this process the absorption of energy from the external field takes place. The time during which a particle can exist in the ground state is unlimited. On the other hand, the particle can remain in the excited state for a limited time known as lifetime. The lifetime of the excited hydrogen atom is of the order of 10-8sec. Their exist, some excited states in which the lifetime is
10-8 sec. These states are called as metastable. The basic principle involved in laser action is the phenomenon of stimulated emission. There are three kinds of electromagnetic radiations between two energy levels E1 and E2 in an atom.
i) Induced absorption :- If the atom is initially in the lower state E1 it can be
raised to E2 by absorption of a photon of energy E2- E1 = h F 0 6 E. This is called
induced absorption.
After being in the excited state, the particle returns to the ground state.
ii) Spontaneous emission:- If the atom is initially in the upper state E2, it can
drop to E1 by emitting of a photon of energy h F 0 6 E. This process is known as
Spontaneous emission.
The spontaneous emission depends on the type of the particle and type of transition but is independent of outside circumstances. The waves coincide neither in wavelength nor in phase. Thus the radiation is incoherent and has a broad spectrum. The rate of spontaneous emission is proportional to the number of atoms in the excited state. iii) Stimulated emission:- If an atom is already in the excited state of energy level E2 whose ground level energy is E1, at this moment, a
photon of energy h (^) F 0 6 E= E2- E1 is incident on the excited atom, the incident
photon stimulates a similar
photon from the excited atom. Now the atom returns to the ground state.
This type of emission is called as stimulated emission. It is coherent with the stimulating incident radiation. It has the same frequency & phase as the incident radiation. The rate of stimulated emission depends both on the intensity of external field and also on the number of atoms in the excited state. Differences between spontaneous and stimulated emission: Spontaneous emission Stimulated emission
- Transition occurs from a higher energy level to a lower energy level.
Transition also occurs from higher energy level to lower energy level.
- No incident photon is required Photon whose energy is equal to the difference of two energy levels is required.
- Single photon is emitted. Two photons with same energy are emitted.
- The energy of emitted photon is equal to the energy difference of two levels.
The energy of the emitted photons is double the energy of stimulated photons.
- This was postulated by Bohr. This was postulated by Einstein.
Population Inversion : The number of particles N2, i.e., population of higher energy level is less than the population N1 of lower energy level. Making the number of particles N2 more in higher energy level than the number of particles N1 in lower energy level (N2 > N1) is called as population inversion or inverted population. A system in which population inversion is achieved is called an active system. The method of raising the particle from lower energy state to higher energy state is called as pumping. A more common method of pumping is optical pumping. Characteristics of a laser : i) The light is coherent with all the waves exactly in phase with each other. ii) Laser beam hardly diverges. i.e., The laser rays are almost parallel. iii) The beam is nearly monochromatic. iv) The laser beam is extremely intense. The beam can produce a temperature of 104 oC at a focused point.
Principle of operation of lasers: Consider a group of atoms all in the same excited state. A passing photon may cause stimulated emission in one of these atoms. This results in the emission of two photons. Each of these photons may cause induced emission in two other excited atoms. This process may continue in a chain reaction. The result will be an intense beam of photons moving in the same direction and all are coherent.
Ruby laser: The first successful laser utilized a ruby rod. Construction :- A ruby is a crystal of aluminium oxide Al2O3 in which some aluminium atoms are replaced by chromium
The collisions of the He & Ne atoms with the electrons from the discharge excite (or pump) the helium & neon atoms to metastable states. Some of the excited He atoms transfer their energy to ground state Ne atoms by collisions. Thus He atoms help in achieving a population inversion in the Ne atoms. When a Ne atom passes spontaneously from the metastable state at 20.66eV to state at 18.70eV, it emits photon. This photon travels through the gas mixture, and if it is moving parallel to the axis of the tube, is reflected back & forth by the mirror ends until it stimulates an excited Ne atom and causes it to emit a fresh photon in phase with the stimulating photon. This stimulated transition from 20.66eV level to 18.70eV level is the laser transition. This process is continued and a beam of coherent radiation builds up in the tube. When the beam becomes sufficiently intense, a portion of it escapes through the partially – silvered end. The excitation of He & Ne atoms occur all the time, unlike the pulsed excitation in ruby laser, the He-Ne laser operates continuously. Semiconductor (GaAs) laser: Among the semiconductors there are direct band gap semiconductors and indirect band gap semiconductors (Germanium & Silicon). GaAs (Gallium Arsenide) is a direct band gap semiconductor and hence it is used to make light emitting diodes and lasers. The wavelength of the emitted light depends on the band gap of the material. Construction:- In GaAs diode laser, the active medium is a P-N junction diode made from crystalline Gallium Arsenide. The P-N junction layer is very thin. Electric current is applied to the crystal platelet through a strip electrode fixed to its upper surface. At the junction, the sides through which emitted light is coming out are well polished. Working:- A population inversion is obtained by injecting electrons across the junction from the n- doped region to the p-region by means of a forward bias voltage. Particularly when a relatively large current of the order of 10^4 amp/ cm^2 is passed through the junction to provide excitation, the direct recombination process is taking place efficiently. Further the emitted photons increase the rate of recombination of injected electrons from the n-region by electric current
and holes in p-region by inducing more recombination. Thus more no of photons are produced. The wavelength of the emitted radiation depends upon the concentration of donor and acceptor atoms in GaAs. The efficiency of laser emission increases when we cool the GaAs diode. When cooled to 20K, GaAs laser has delivered an output of more than 2 watts of continuous power. Uses of laser: In consumer electronics , telecommunications, and data communications, lasers are used as the transmitters in optical communications over optical fiber and free space. They are used to store and retrieve data from compact discs and DVDs, as well as magneto-optical discs. Laser lighting displays (pictured) accompany many music concerts. In science , lasers are employed in a wide variety of interferometric techniques, and for Raman spectroscopy. Other uses include atmospheric remote sensing, and investigation of nonlinear optics phenomena. Holographic techniques employing lasers also contribute to a number of measurement techniques. Lasers have also been used aboard scientific spacecraft.
In medicine , the laser scalpel is used for laser vision correction and other surgical techniques. Lasers are also used for dermatological procedures including removal of tattoos, birthmarks, and hair. In industry , laser cutting is used to cut steel and other metals. Laser line levels are used in surveying and construction. Lasers are also used for guidance for aircraft. Lasers are used in certain types of thermonuclear fusion reactors. In law enforcement the most widely known use of lasers is for lidar to detect the speed of vehicles. Military uses of lasers include use as target designators for other weapons; their use as directed-energy weapons is currently under research.
Holography
Introduction:-
- (^) It is the process of image construction by recording and reconstruction by means of interference techniques without the aid of lenses.
- In holography along with the amplitude, phase is also recorded by interference pattern.
- It is based on the principle of wave front reconstruction. Recording and reconstruction of a hologram: The process of making a hologram is called recording or construction and the method of displaying the hologram is termed reconstruction. Recording – A low power helium-neon laser is used as the light source because it emits coherent light. The light beam is spread and then divided into two beams by a beam splitter. One beam of light is called the illumination beam and it illuminates the object to be holographed. The object, in turn reflects part of the illuminating beam on to the film. This beam is called the object beam. The second beam of light leaving the beam splitter is called the reference beam. It is redirected to the film by front surface mirror. Optical interference between the reference beam and the object beam, due to the superposition of the light waves, produces a series of intensity fringes that can be recorded on standard photographic film. These fringes form a type of diffraction grating on the film, which is called the hologram.
Reconstruction – In the reconstruction process, the hologram is illuminated by the reference beam. The hologram acts as a diffraction grating and diffracts the reference beam. During diffraction the recorded wave front will be reconstructed. We can see the three dimensional image of the object just viewing through a window. The reconstruction beam need not be a laser light. We can use ordinary sodium light.
Applications of Holography:
- The most important applications of holography has been in interferometry.
- It can also be used for character recognition.
- This could be used to identify finger prints etc.,
- Holography techniques have been employed to produce gratings.
- (^) Holograms can also be used for data storage devices and hence are of much use in computer technology.
- In the production of photographic masks used to produce microelectronic circuits.
- Industrial applications of holography include NDT (non-destructive testing) techniques to find flaws in structural parts.
F 0 6 62 = 90^0. When the angle of incidence exceeds the critical angle (i.e., when^ F 0 6 61 >^ F 0 6 6c), there is no refracted ray and we have
what is known as total internal reflection (TIR). Definition:- An optical fiber is a hair thin cylindrical tube of glass or any transparent dielectric medium for transmitting light. It is designed to guide light waves along the length of the fiber with the help of successive total internal reflections from sidewalls of the fiber.
Construction : - An optical fiber consists, in general, the following three regions: i) Innermost region, which is the light guiding region, called core. It is made of pure Silicon dioxide (SiO2)
ii) Core region is surrounded by a middle region called Cladding. The refractive index of cladding (n2 F 0 B B1.45) is always lower than that
of the core (n1 F 0 B B1.465). The cladding is usually pure silica while the core is usually silica doped with germanium.
iii) The outermost region is called sheath (made of Plastic or Teflon). The sheath protects the core and cladding from moisture, abrasion contamination and to give mechanical strength to fiber. Basic purpose of cladding is to confine the light to the core. As the light falling on core and striking core cladding interface at an angle greater than the critical angle will be reflected back to the core. Main function of the optical fiber is to accept maximum light and transmit the same with minimum attenuation or reduction.
Propagation of light through optical fiber:
Consider the figure, which is a section of the optical fiber. The refractive indices of the core and the cladding are n1 and n2 respectively. The fiber is normally in air (n0=1) but could also be in a medium of refractive index n0. The axis of the cylindrical structure is the optical axis. A ray is incident at an angle ‘ i ‘at the entrance face and refracted into the core. It then strikes the core-cladding interface at a certain angle. If this angle exceeds the critical angle, it is totally reflected and strikes the interface on the other side of the axis. Here it is again totally reflected. This process is repeated till the ray emerges out of the fiber at the other end. The ray is thus guided by total internal reflection. The angle of incidence at the entrance face for which the ray strikes the core-cladding interface at the critical angle is called the Acceptance angle or cut-off angle. The ray is guided for all the angles of incidence smaller than the acceptance angle at the entrance face. However, if the angle of incidence at the interface is less than the critical angle, both reflection and refraction takes place. Due to refraction at each incidence on the interface, the light beam dies off over a certain distance. There is no guidance. Types of optical fibers: [In an optical fiber light travels as an electromagnetic wave and all the waves moving in directions above the critical angle will be trapped in the fiber due to TIR. However all such waves do not propagate through the fiber, and only certain ray directions are allowed for propagations. These allowed directions correspond to modes of fiber. The light ray paths along which the waves are in phase inside the fiber are called modes. Number of modes, a fiber can support depends on the ratio d/ (^) F 0 6 Cwhere d is the diameter of the core and (^) F 0 6 Cis the wavelength of the wave transmitted.]
Optical fibers are in general of two types: i) Single Mode Fiber (SMF) and ii) Multi Mode Fiber (MMF) A single mode fiber has a smaller core diameter (<10 (^) F 0 6 Dm) and can support only one mode of propagation.
For intercity cabling (i.e., for long distances) and highest speed Single mode fibers are used. Advantages: Low loss, data rate upto 40 GB/Sec. Multi mode fiber (MMF) has a large core diameter (50 F 0 6 Dm to 100 F 0 6 Dm) and can support a large number of modes. For short distances MMF’s are used. Multi mode fibers are further classified on the basis of index profile. [Index profile is a graph of refractive index (along X-axis) and distance from the core (along Y-axis)]. Index profile of a MMF can be either step index (SI) type or graded index (GRIN) type. Index profile of SMF is usually a step index type.
Single mode step index fiber – It consists of a very fine thin core (of radius ‘a’) of uniform refractive index surrounded by a cladding (of radius ‘b’) of refractive index lower than that of the core. Since the refractive index abruptly changes at the core-cladding
boundary, it is known as step index fiber. A typical SMF has a core diameter of 4 (^) F 0 6 Dm. Light travels along a single path.
Multi mode step index fiber – It is similar to the single mode step index fiber with the exception that it has a large diameter (~ 100 F 0 6 Dm). Core diameter is very large as compared to the wavelength of transmitted light. Light moves in zigzag paths along MMF. Typical
structure along with profile of step MMF is shown in figure.
Graded index fiber (GRIN) – A GRIN is a multi mode fiber, which has concentric layers of refractive indices, which means that the refractive index of the core varies with distance from the fibers axis. i.e., it has high value at the center and falls of rapidly as the radial distance increases from the axis. In case of GRIN fibers, acceptance angel and numerical aperture diminish with increase of radial distance from the axis.
Acceptance angle – It may be defined as the maximum angle that a light ray can have relative to the axis of the fiber and propagates down the fiber. Greater the diameter of the core larger the acceptance angle. Numerical Aperture (NA): Definition:- The Sine of the acceptance angle of an optical fiber is known as Numerical Aperture. Or The light gathering capacity of an optical fiber is know as Numerical Aperture. Consider an optical fiber through which light is being sent. The end at which light enters is called launching end. Let the refractive indices of the core and cladding be n1 and n2 respectively; n2 < n1. Let the refractive index of the medium k through which light is launched be n0. Let the light beam enter at an angle i to the axis of the fiber. The ray gets refracted at an angle r and strikes the core-cladding
interface at an angle F 0 6 6. If F 0 6 6is greater than the critical angle C, light will undergo total internal reflection. The ray will also undergo total
internal reflection at interface as n1 > n2. It means that so long as (^) F 0 6 6is greater than critical angle C, light will stay within the fiber.
Let us now calculate the incident angle i for which F 0 6 6F 0 A 3C such that light rebounds within the fiber.
Applying Snell’s law to the launching face of the fiber, we get
this current and is amplified. Then this message is put into signal converter to get it either in video or audio form depending upon the nature of the original message.
Applications & Uses:
- As fiber communication has large bandwidth, the system is capable of handling a large number of channels.
- It is widely used in defense services because high privacy is maintained.
- (^) These are used for the transmission of digital data such as that generated by computers.
- The military applications include communications; command and control links on ships and aircraft’s, data links for satellite earth stations.
- It is used for signaling purposes.
- Fiber sensors are used to measure temperature, pressure, rotary & linear position and liquid levels.
- It is used in cable television, space vehicles, ships & submarine cable.
- It has wide applications in security and alarm systems, electronic instrumentation systems, industrial automation and pressure control.
- To study interior of lungs and the other parts of the body that cannot be viewed directly.
Light Emitting Diode (LED) – It is a specially made forward biased p-n junction diode, which emits visible light when energized. Principle : Energy is needed to create an electron –hole pair, and this energy is released when an electron & a hole recombine. In silicon & germanium the recombination energy is absorbed by the crystal as heat, but in certain other semi conductors like gallium Arsenide, a photon is emitted when recombination occurs. This is the basis of the LED.
Theory : When a junction diode is forward biased, electrons from n-side and holes from p-side move towards the depletion region and they recombine. During this process, energy is released. If Eg is the semiconductor band gap, then the energy Eg = h (^) F 0 6 E= hc / (^) F 0 6 Cis emitted in the form of radiation. The corresponding emission wavelength is given by, (^) F 0 6 C= hc / Eg In case of Gallium Arsenide Phosphide (GaAsP), band gap Eg = 1.9eV. F 0 6 C= hc / Eg = [6.62 x 10-34 x 3 x 10^8 ] / [1.9 x 1.6 x 10-19] = 653.3nm^ (red) Construction: At first an n-type layer is grown on a substance & then a p-type layer is deposited on it by the process of diffusion. Metal contacts (anode) are made at the outer edge of the p-layer so that more upper surface is left for light to escape. For making cathode connections, a metal film (preferably gold) is coated at the bottom of the substance. This film also reflects as much light as possible to the surface of the device.
For a LED forward voltage is 1.2V and forward current is 20mA. Ga As P LED emits red light when forward bias. Uses: i) visible LED’s are used as solid-state indicator lamps and as light sources for numeric & alphanumeric displays. ii) Infrared LED’s are employed in optical fiber transmission.
Common materials for solar cells include silicon, gallium Arsenide (GaAs), indium Arsenide (InAs) & cadmium Arsenide (cdAs). The most common is Silicon. The maximum theoreticalefficiency of a solar cell depends on the band gap. For silicon, the maximum efficiency is 22%. Construction: A solar cell consists of a p-n junction diode made of Si. The p-n diode is packed in a can with glass window on top so that light may fall upon P and N type materials. The thickness of p-region & N-region is kept small to allow electrons & holes generated near the surface to diffuse to the junction before they recombine. A heavy doping of P and N regions is recommended to obtain a large photo voltage. A nickel-plated ring provided around the P-layer acts as +ve out put terminal and a metal contact at the bottom serves as –ve output terminal.
Working: When light radiation falls on a p-n junction diode, photons collide with valence electrons & impart them sufficient energy enabling them to leave their parent atoms. Thus electron-hole pairs are generated in both the P & N sides of the junction. These electrons and holes reach the depletion region ‘ W ‘ by diffusion [ fig(a)] and are separated by the strong barrier field existing there. However, the minority carrier electrons in P- side slide down the barrier potential to reach the N-side and the holes in the N-side move to the P-side [fig (b)]. Their flow constitutes the minority current, which is directly proportional to the illumination and also depends on the surface area being exposed to light.
The accumulation of electrons and holes on the two sides of the junction [fig©] gives rise to an open circuit voltage Voc which is a function of illumination. For Silicon Voc is 0.6V and Isc is 40 mA/Cm^2 Uses :- 1. These are used extensively in satellites and space vehicles to supply power to electronic and other equipments or to charge storage batteries.
- Used in power calculators & switches 3. Used to provide commercial electricity. INTRODUCTION TO NANOTECHNOLOGY The materials that have at least one of the three dimensions of about 1 to 10 nanometers are known as nono materials. The term nano is the factor 10-9 or one billionth. The properties of materials made of clusters of a few tens to few thousand nanoparticles are very much different than normal materials. The properties depend on the size of the nanoparticles consisting the materials. The synthesis of the building blocks of proteins can provide us an appropriate technology for making nano materials in the bulk form. Nanotechnology is molecular manufacturing of systems without violating any physical law.
Important properties of carbon nanotubes: The four most important properties of CNTs are, 1. Chemical reactivity, 2. Electrical conductivity, 3. Optical activity and 4. Mechanical strength. .1 (^) Chemical reactivity: The chemical reactivity of a CNT is more due to the curvature of its surface. CNT reactivity is directly related to the pi-orbital mismatch due to increased curvature. So a smaller diameter of nanotube results in increased reactivity.
.2 Electrical Conductivity: Depending on their chiral vector, carbon nanotubes with a smaller diameter are either semi-conducting or metallic. The differences in conducting properties are caused by the molecular structure that results in a different band structure and thus a different band gap. .3 Optical Activity: it is the property that depends on the size of the CNTs. For larger CNTs optical activity doesn’t exist. In optical devices CNTs play an important role. .4 Mechanical strength: CNTs have a very large Young’s modulus. The nanotube as a whole is very flexible because of the great length. Therefore, these compounds are potentially suitable for applications in composite materials that need anisotropic properties.
Applications of Nano materials :
- Carbon nanotubes are used for energy storage.
- CNTs can store liquid or gas in the inner cores through capillary effect. A CNT can store H2 gas.
- CNTs are used as capacitors.
- CNTs can be used as filed emitting devices in the applications as flat panel displays, gas discharge tubes in telecom networks, electron guns for electron microscopes and microwave amplifiers.
- A SWNT is used as a FET.
- Many carbon nanotube transitions can be coupled as a logical switch, which is the basic component of computers.
- (^) MWNTs are used in scanning probe instruments.
- The low density of nanotubes increases electrical conduction.
- Nanocrystaline materials may be used as potential recording media.
Medical applications:
- Implants and Prosthetics : The first field is implants and prosthetics. With the advent of new materials, and the synergy of nanotechnologies and biotechnologies, it could be possible to create artificial organs and implants that are more akin to the original, through cell growth on artificial scaffolds or biosynthetic coatings that increase biocompatibility and reduce rejection. These could include retinal, cochlear and neural implants, repair of damaged nerve cells, and replacements of damaged skin, tissue or bone.
- Diagnostics Using Sensors and Micro Electro Mechanical Systems (MEMS) : The second area is diagnostics. Within MEMS (Micro Electro Mechanical Systems), laboratory-on-a-chip technology for quicker diagnosis which requires less of the sample is being developed in conjunction with micro fluidics. In the medium term, it could be expected that general personal health monitors may be available. Developments in both genomics and nanotechnology are likely to enable sensors that can determine genetic make-up quickly and precisely, enhancing knowledge of people’s predisposition to genetic-related diseases.
- Drug Delivery Using Nano particles and Molecular Carriers : Finally, drug delivery is likely to benefit from the development of nanotechnology. With nanoparticles it is possible that drugs may be given better solubility, leading to better absorption. Also, drugs may be contained within a molecular carrier, either to protect them from stomach acids or to control the release of the drug to a specific targeted area, reducing the likelihood of side effects. Such drugs are already beginning pre-clinical or clinical trials, adhering to the
