Blue Lasers. Photonics and Optical Communications Zubin Bharucha

Transcription

1 Blue Lasers Photonics and Optical Communications Zubin

2 Advantages of blue lasers and Blue (GaN) LEDs are around 100 times brighter than conventional LEDs More efficient (energy-wise) than light bulbs Longer lifetime than light bulbs (5-10 years instead of 1 year) Very compact as compared to other light sources Robust GaN based laser diodes have lifetimes of hours LEDs

3 Why GaN? GaN has a direct bandgap which allows optical transmission Indirect bandgaps which forbid optical transmission lead to less efficient lasing Band gaps of various materials [1]

4 Blue Laser History (people involved) Jacques Pankove Discovered GaN and its potential to emit blue light, 1970s Prof. Akasaki Helped develop methods to deposit device quality GaN films, developed and demonstrated p and n-type doping for GaN and developed GaN based LEDs, 1980s and 1990s Shuji Nakamura Invented processes necessary for commercial applications in connection with blue lasers. Developed GaN LEDs and lasers for commercial production, 1990s to date

5 Construction of a Blue Laser Diode A laser diode consists of a stack of extremely thin and precisely grown semiconductor layers of different materials Electrons and holes follow the potential gradient of an applied voltage and recombine in the depletion region to emit blue light The energy of the released photons is the same as that of the energy bandgap [1] GaN Laser Diode construction [1] Working of a GaN laser diode

6 Applications of blue lasers New generation DVD Data storage Traffic lights Outdoor displays Car headlights Medical applications Color scanners Large TV displays High resolution printers Lighting devices (to replace the conventional bulb)

7 New generation optical storage Shorter wavelength of laser means larger number of data pits (larger capacity) The shift is from infrared lasers (780nm) for CDs with 680Mb capacity to red lasers (650nm) for DVDs with 4.7Gb capacity [3]

8 HD DVD Next generation: HD DVD with 15Gb capacity Uses blue laser of wavelength 390nm Higher capacity achieved by using shorter wavelength laser Media Laser diode CD-ROM infrared laser DVD red laser HD DVD blue laser Blue Laser Recording Marks on 15GB [2] Wavelength (λ) 780nm 650nm 390nm Pit length (µm) Storage capacity 680 MB 4.7 GB 15 GB Green Laser Recording Marks on 2.6GB [2]

9 Problems with Blue Lasers Lattice Mismatch The Problem GaN grown on sapphire which has 15% smaller lattice constant Leads to high defect density Cracking of layers when structures are cooled down after growth due to high difference in thermal expansions of the two materials GaN is ideal choice for substrate but this is still in research The Solution Akasaki proposed solution: developing AlN buffer layers Nakamura proposed solution: growth of GaAlN buffer layers Akasaki solution [1]

10 Problems with Blue Lasers High Growth Temperature The Problem Thermal convection inhibits growth due to high temperature at growth The Solution Nakamura proposed solution: development of a two-flow growth reactor in order to curb the inhibitive effects of convection currents (also famous as the core of a $600 million Japanese lawsuit between Nakamura and Nichia Chemical Industries)

11 Problems with Blue Lasers p- doping impossible The Problem p-n junctions needed to realize lasers and diodes Previous to Akasaki s work, p-doping of GaN was impossible The Solution Akasaki proposed solution: demonstration of a p-type material which was e-beam annealed Nakamura proposed solution: annealing in ammonia gas (NH 3 ) passivates the acceptors and solves the problem

12 References [1] Blue and white GaN light emitters and lasers for lighting, displays, data storage, and traffic signals - background and implications, Eurotechnology, Japan [2] orage/dvdram [3] efinition

13 How a Laser works (1) LASER Light Amplification by Stimulated Emission of Radiation In a laser, the emitted light is coherent, monochromatic and very directional In a flashlight, all of the atoms release their photons randomly In stimulated emission, photon emission is organized

14 How a Laser works (2) Light Amplification Any atom releases a photon that has a certain wavelength that is dependent on the energy difference between the excited state and the ground state If this photon (possessing a certain energy and phase) should encounter another atom that has an electron in the same excited state, stimulated emission can occur The first photon can stimulate or induce atomic emission such that the subsequent emitted photon (from the second atom) vibrates with the same frequency and direction as the incoming photon [5]

15 How a Laser works (3) Optical Resonance Mirrors at either end of cavity, one half-silvered Photons, with a very specific wavelength and phase, reflect off the mirrors to travel back and forth through the lasing medium Other electrons stimulated to make the energy jump and cause the emission of more photons of the same wavelength and phase Cascade effect results and soon a large number of photons of same wavelength and phase are emitted - lasing [4]