SEMICONDUCTOR LASERS EXCIMER LASER PH 0101 UNIT-3 / LECT - 4 SEMICONDUCTOR LASERS EXCIMER LASER UNIT III Lecture 4

SEMICONDUCTOR (Ga-As) LASERS Introduction : The semiconductor laser is today one of the most important types of lasers with its very important application in fiber optic communication. These lasers use semiconductors as the lasing medium and are characterized by specific advantages such as the capability of direct modulation in the gigahertz region, small size and low cost. UNIT III Lecture 4

Basic Mechanism : The basic mechanism responsible for light emission from a semiconductor is the recombination of electrons and holes at a p-n junction when a current is passed through a diode. There can be three interaction processes 1)An electron in the valence band can absorb the incident radiation and be excited to the conduction band leading to the generation of eletron-hole pair. UNIT III Lecture 4

To convert the amplifying medium into a laser Contd. 2) An electron can make a spontaneous transition in which it combines with a hole and in the process it emits radiation 3) A stimulated emission may occur in which the incident radiation stimulates an electron in the conduction band to make a transition to the valence band and in the process emit radiation. To convert the amplifying medium into a laser Optical feedback should be provided Done by cleaving or polishing the ends of the p-n junction diode at right angles to the junction. UNIT III Lecture 4

Contd. When a current is passed through a p-n junction under forward bias, the injected electrons and holes will increase the density of electrons in the conduction band. The stimulated emission rate will exceed the absorption rate and amplification will occur at some value of current due to holes in valence band. As the current is further increased, at threshold value of the current, the amplification will overcome the losses in the cavity and the laser will begin to emit coherent radiation. UNIT III Lecture 4

Simple structure (Homojunction) : The basic semiconductor laser structure in which the photons generated by the injection current travel to the edge mirrors and are reflected back into the active area. Photoelectron collisions take place and produce more photons, which continue to bounce back and forth between the two edge mirrors. This process eventually increases the number of generated photons until lasing takes place. The lasing will take place at particular wavelengths that are related to the length of the cavity. UNIT III Lecture 4

Basic semiconductor laser structure a) Side view, b) Projection Heterostructures UNIT III Lecture 4

UNIT III Lecture 4

Heterostructures : The heterostructure laser is a laser diode with more than single P and N layers. GaAs/AlGaAs is a heterojunction laser. The notations P+ and N+ and P- and N- indicate heavy doping and light doping respectively. The P-N structure consists of the two double layers, P+ - P- and N+ - N- . A thin layer of GaAs is placed at the junction, the active region. The substance is selected because the electron-hole recombinations are highly radiative. This increases the radiation efficiency. UNIT III Lecture 4

The P and N regions are lightly doped regions that have an index of refraction n2 less than n1 of the active region. These three layers, n2-n1-n2, form a light waveguide much like the optical fiber, so that the light generated is confined to the active region. UNIT III Lecture 4

Laser heterostructure (a) Schematic projection (b) Refractive index profile UNIT III Lecture 4

UNIT III Lecture 4

EXCIMER LASER : An excimer laser or exciplex laser is a form of ultraviolet chemical laser which is commonly used in eye surgery and semiconductor manufacturing. The term excimer is short for 'excited dimer', while exciplex is short for 'excited complex'. An excimer laser typically uses a combination of an inert gas (Argon, krypton, or xenon) and a reactive gas (fluorine or chlorine). Under the appropriate conditions of electrical stimulation, a pseudo-molecule called a dimer is created, which can only exist in an energised state and can give rise to laser light in the ultraviolet range UNIT III Lecture 4

Contd. Laser action in an excimer molecule occurs because it has a bound (associative) excited state, but a repulsive (dissociative) ground state. This is because noble gases such as xenon and krypton are highly inert and do not usually form chemical compounds. When in an excited state (induced by an electrical discharge or high-energy electron beams, which produce high energy pulses), they can form temporarily-bound molecules with themselves (dimers) or with halides (complexes) such as fluorine and chlorine. UNIT III Lecture 4

Contd. The excited compound can give up its excess energy by undergoing spontaneous or stimulated emission, resulting in a strongly-repulsive ground state molecule which very quickly (on the order of a picosecond) disassociates back into two unbound atoms. This forms a population inversion between the two states. Most "excimer" lasers are of the noble gas halide type, for which the term excimer is strictly speaking a misnomer (since a dimer refers to a molecule of two identical or similar parts): The correct but less commonly used name for such is exciplex laser. The wavelength of an excimer laser depends on the molecules used, and is usually in the ultraviolet region UNIT III Lecture 4

EXCIMER WAVELENGTH Excimer Wavelength ArF 193 nm KrF 248 nm XeBr 282 nm XeCl 308 nm XeF 351 nm CaF2 KrCl 222 nm Cl2 259 nm N2 337 nm Excimer lasers are usually operated with a pulse rate of around 100 Hz and a pulse duration of ~10 ns, although some operate as high as 8 kHz and 30 ns. UNIT III Lecture 4

All commercial excimer lasers employ the modules All commercial excimer lasers employ the modules. Laser light is generated in the laser cabinet. The electrical energy required by the laser to form laser pulses is generated by the high voltage supply. A gas supply and a vacuum pump are required to fill the laser with the appropriate laser gas mixture. The control computer is usually linked to the laser cabinet and high-voltage supply by a fiber optic network. The computer provides laser function user control. UNIT III Lecture 4

TYPICAL EXCIMER LASER CONFIGURATION UNIT III Lecture 4

USES : The UV light from an excimer laser is well absorbed by biological matter and organic compounds. Rather than burning or cutting material, the excimer laser adds enough energy to disrupt the molecular bonds of the surface tissue, which effectively disintegrates into the air in a tightly controlled manner through ablation rather than burning. Excimer lasers have the useful property that they can remove exceptionally fine layers of surface material with almost no heating or change to the remainder of the material which is left intact. UNIT III Lecture 4

These properties make them useful for surgery (particularly eye surgery), for lithography for semiconductor manufacturing, and for dermatological treatment. Excimer lasers are quite large and bulky devices, which is a disadvantage in their medical applications, although their size is rapidly decreasing with ongoing development. UNIT III Lecture 4

Solved Problem (1) : Calculate the wavelength of emission from GaAs semiconductor laser whose band gap energy is 1.44 ev (plank’s constant is 6.625 x 10-34 Js and charge of an electron is 1.6 x 10-19 C. Given data : Band gap energy Eg = 1.44 ev (or) 1.44 x 1.6 x 10-19 Joules. Solution : We know Band gap energy (Eg) = h (or) hc/ we can write  = hc/Eg  = 6.625 x 10-34 x 3 x 108) / (1.44 x 1.6 x 10-19) = 8.6263 x 10-7 m = 8626.3 x 10-10 m  wave length of GaAs laser = 8626.3 Ao UNIT III Lecture 4