Green Lasers

Hello!!!
Jus surfing thru the net i today found a phrase GREEN LASER..First i associated it with somethin like environment friendly laser...but reading on found it interesting so sharin it with you...


                Green Lasers Overcome Last Barrier to Brilliant displays



Solidstate lasers can produce light in red and blue parts of the visible spectrum, generating laser light in all colours except green. But recent research work suggests that this ‘green gap’ could be plugged. New techniques for growing laser diodes could soon make brilliant fullspectrum displays a reality.Plugging the green gap in the redgreen-blue triad needed for full-colour laser projection and display would help speed the introduction of laser projectors for televisions and movietheatres, which will display much richer colours than other systems, and tiny handheld projectors as in cellphones.High-power green diodes might be employed in such applications as DNA sequencing, industrial process control and underwater communications.

The familiar green laser pointers used by lecturers employ a complicated two-step process to generate light. Semiconductor lasers inside these devices emit infrared radiation having a wavelength of around 1060 nanometres.This radiation then pumps a crystal that oscillates at half this wavelength—530 nanometres, which corresponds to green part of the spectrum. The process is costly, inefficient and imprecise; the second crystal can heat up, altering the wavelength of the resultant green light. Lasers that generate green light directly would avoid this problem.


Multiquantum-well fundamentals

Fig 1

Fig.1 shows the scheme of a homojunction laser where a single p-n junction is formed from only a singlecrystal semiconductor material. There is a highly reflective surface at one end and a partially reflective surface at the other end. The p-n junction is forward-biased by an external voltage source. As electrons move through the junction, recombination occurs just as in an ordinary LED light emitter. To achieve laser action, it is necessary to contain photons within the laser medium and maintain the conditions for coherence. The optical cavity,as shown in Fig.1,is known as Fabry-Perot cavity and provides positive feedback of the photons by reflection at the mirrors at either end of the cavity emitting coherent light. However, the radiative properties of a laser may be improved greatly by using heterojunctions instead of homojunctions. A heterojunction is an interface between two adjoining singlecrystal semiconductors with different bandgap energies.

Recently, double-heterojunction lasers have been fabricated with an active layer as thin as 2 nm instead of

MQW

0.1 to 0.3 µm of conventional doubleheterojunction structure. These devices are known as quantum-well lasers.In  multiquantum well (MQW) lasers, a number of such quantum wells are provided. In MQWs corresponding to multiple active layers,layers separating the active regions are called barriers. Carrier motion normal to the active layer in these devices is restricted, resulting in quantisation of the kinetic energy into discrete energy levels for the carriers moving in that direction. Due to this reason, the thin active layer causes drastic changes in electronic and optical properties in comparison with a conventional double-heterojunction laser.MQW devices score over conventional double-heterojunction lasers due to their lower threshold currents, narrower line-widths, higher modulation speeds, lower frequency chirps and lesser temperature dependence.

Green gap problem 
Scientists have long been able to build semiconductor lasers that produce light in red parts of the spectrum. In
the last decade, they conquered the blue and violet sections as well. As they try to push these lasers in the green part of the spectrum, the amount of power produced drops drastically .In the new approach, an exceedingly smooth, nanometres-thin layer of indium gallium nitride (InGaN) is sandwiched between two layers of GaN, forming the so-called heterostructure or quantum well (MQW fig). By applying a suitable voltage, an electric field is set up perpendicular to these layers.The electric field drives electrons and holes—positively charged particles that are devoid of electrons—together within the InGaN layers. Inside the
narrow central active layers ,the electrons and holes recombine, annihilating one another and generating
photons with an energy precisely determined by the properties of the active semiconductor material (Fig.MQW).By increasing the indium concentration in the alloy,one can lower this energy,thereby increasing the wavelength of the light and changing its colour from violet to blue to green.But, there is a flaw—the more the indium in these active layers, the more the likelihood of it pooling into small ‘islands’ during manufacture. The islands can alter the wavelength of light—an unacceptable flaw in laser

On either side of the InGaN active layer is a GaN barrier layer. GaN layer acting as an optical waveguide above the substrate is doped with silicon impurities to produce an excess of electrons. On the other end, GaN is doped with magnesium to give it an excess of ‘holes’ or positive charges. It also acts as an optical waveguide.The total optical thickness of the cavity is about 23.5λ, where ‘λ’ is the designed wavelength.

Fig 4 Crystal plane of substrate

In LEDs, the photons leave the well almost immediately, perhaps rebounding once or twice before exiting the device or being absorbed in other layers. But in laser diodes, which produce coherent light, the photons stay largely confined within the trench.Two highly reflective mirrors—generally polished crystal surfaces at either end of it—recycle the photons back and forth inside, further stimulating electron-hole recombination. The laser light generated by this ‘stimulated emission’ process is a tight pencil beam of exceedingly pure colour.
Initially, the scientists tried to produce green lasers by depositing layers of semiconducting materials parallely with sapphire substrate’s ‘c’-plane, which is perpendicular to the crystal’s axis of hexagonal symmetry     (Fig. 4). Unfortunately, electrostatic forces and internal stresses between successive layers of positively charged gallium or indium ions and negatively charged nitrogen ions create strong electric fields perpendicular to the ‘c’-plane. These fields, which can reach up to 100 volts per micron, counteract the applied external voltage. They pull electrons away from holes, making it harder for them to recombine and produce
light. As a result, the electrons pile up at one side of the central active layer and holes at the other end, both reluctant to cross over and recombine. The problem worsens as the wavelength shifts towards green from the violet-blue spectrum. This effect is known as quantum-confined Stark effect. And as the current through the
diode increases, the greater number of charge carriers partially blocks the internal electric fields that keep electrons and holes apart. With these fields partially screened out, electrons and holes then recombine at higher energies, shifting the light towards the blue end of the spectrum.These problems are the main reason why green laser diodes and  high-efficiency green LEDs could not be made.
Solution to green gap problem
To f ind a solution to this problem, scientists replaced the sapphire substrate with a thin wafer of pure, crystalline GaN that was sliced along a larger crystal’s ‘m’-plane as shown in Fig. 4 and then polished. Diodes fabricated on these non-polar substrates do not encounter the problems of
conventional polar ‘c’-plane devices, because the troublesome fields caused by polarisation and internal stresses are much lower. Diodes grown on GaN also produce light more efficiently than one grown on sapphire because they suffer from far fewer crystalline defects, submicroscopic irregularities and mismatches at the interfaces between successive layers. Such defects act as centres where electrons and holes
recombine to produce unwanted heat instead of light. They can easily propagate upward through the successive diode layers during the growth process and reach the active layers. Diodes grown on non-polar GaN substrate can therefore produce much more light and less heat to dispose of.


Applications 
Future models that rely on green lasers for handheld projectors allow for much brighter and efficient displays, and also shrink the projectors enough to allow them to fit inside the cell phone. Inside the laser projector, red, blue and green lasers focus onto a single mirror about the size of a pinhead. As light bounces off the
reflector, the mirror assembly rapidly scans back and forth to project pixels one by one onto a screen or wall. No use of lens means the projector never needs to be focused. The projector has DVD-equivalent resolution of 848×480 pixels



Hope you find it interesting too!!!


References..
http://en.wikipedia.org/wiki/Laser_pointer#Green
http://en.wikipedia.org/wiki/Laser_pointer#Green