Semiconductor light sources Outline

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1 Light soures Semiondutor light soures Outline Thermal (blakbody) radiation Light / matter interations & LEDs Lasers Robert R. MLeod, University of Colorado Pedrotti 3, Chapter 6 3

2 Blakbody light Blakbody soures Ideal inoherent soures Radiate energy, but unless T > 700 o K, emit very little visible radiation and thus appear blak. Plank s explanation of the blakbody spetrum in 900 was the beginning of the development of quantum mehanis. The radiane of a blakbody, L, does not depend on angle, and they are thus ideal Lambertian soures. 3 h 3 L W/m /Sr 5 h λkt λ e b λ Max, b µ T E σt 4 ( ) [ ] [ m K] W -8 W [ ], σ [ 4 ] m m K Plank s equation Wein s displaement law Stephan-Boltzman law for total emittane T7000 o K, λ Max.44 µm T5500 o K, λ Max.57 µm T4500 o K, λ Max.645 µm The surfae of the sun is roughly 5500 o K and radiates in the green. Humans at body temperature radiate in the infrared at about 9.5 µm. Robert R. MLeod, University of Colorado 4

3 Light / matter interation Interation of photons and matter aka Einstein oeffiients (Stimulated) absorption Photon inident, eletron in low energy state E E photon hν R St.Abs. B ( ν ) g E I N Photon absorbed, eletron in higher energy state, E E -E ~ h ν E E Simulated emission Photon inident, eletron in high energy state E E photon hν R St.Em. B ( ν ) g I N Photon emitted, eletron in low energy state. New photon idential to inident. E E E Spontaneous emission R Sp.Em. [ A ] N NO photon inident, eletron in high energy state Note: no I on right Photon emitted, eletron in low energy state. New photon diretion random. E E E E Robert R. MLeod, University of Colorado 5

4 Light / matter interation Appliation to light generation The rate onstant for stimulated emission and absorption are equal B B. Therefore, the relative rates of the two proesses is given by the population of the two states R R St.Em. St.Abs. B B ( ν ) g g I ( ν ) N In thermal equilibrium ( normal onditions ), the ratio of the population densities is given by their energy differene via the Boltzmann distribution N N I N N ( E E ) ( K T ) B e < So higher energy states are oupied, but less than lower energy states. At room temperature, the thermal energy, K B T ev and the energy of a green photon is E photon h / λ.4 / 0.5 µm.48 ev or N 0-4 N! Thus emission happens, but at a rate << less than absorption. Conlusion: In general, matter absorbs and does not amplify light (probably a good thing). Corollary: If we ould ahieve a population inversion in whih N > N, the opposite would be true. That is, the matter would at as an amplifier. LASER Light Amplifiation via Stimulated Emission of Radiation N In ontrast, an LED reates N in exess of the equilibrium. These exited states radiate via spontaneous emission. 6 Robert R. MLeod, University of Colorado

Light / matter interation Fluoresene and phosphoresene Energy diagram http://miro.magnet.fsu.

html Intersystem rossing Singlet Absorption Emission Absorption Emission Triplet Spin states Singlet ground

5 Light / matter interation Fluoresene and phosphoresene Energy diagram blonski/lightandolor/index.html Intersystem rossing Singlet Absorption Emission Absorption Emission Triplet Spin states Singlet ground Singlet exited Triplet exited Fluoresene spetra & mirosopy appliation niques/fluoresene/anatomy/fluoromir oanatomy.html Robert R. MLeod, University of Colorado 7

Light / matter interation LEDs Simplified juntion struture Lensed pakage http://hyperphysis.phy-astr.gsu.edu/hbase/eletroni/led.html http://www.bio-enviro.om/struture-of-an-led.

Enapsulant and lens is often inluded in pakage to inrease TIR angle and to onentrate radiation into forward diretion. Bandwih measured in 0 s of nm.

6 Light / matter interation LEDs Simplified juntion struture Lensed pakage Output is from juntion is ~isotropi. Only fration of photons not trapped by total internal refletion an esape. Enapsulant and lens is often inluded in pakage to inrease TIR angle and to onentrate radiation into forward diretion. Bandwih measured in 0 s of nm. By varying material, enter wavelength now ranges from infrared to near ultraviolet. Unpolarized. Can typially be modulated in a few ns. More effiient than inandesent bulbs and possibly fluoresent lamps. Organi LED displays beoming important. Robert R. MLeod, University of Colorado 8

Laser Basis of the laser Energy in to reate population inversion Mirror Mirror Filter Transverse modes Photons Gain medium d Effetive avity length in index Gain Cavity gain and loss Cavity

7 Laser Basis of the laser Energy in to reate population inversion Mirror Mirror Filter Transverse modes Photons Gain medium d Effetive avity length in index Gain Cavity gain and loss Cavity longitudinal modes Filter transmission ( N ) ( N ) ( ) d Bandwih of rounrip gain d N d d Loss ν [ Hz] Resonant modes require d N λ N / ν Linewih related to avity loss ν [ Hz] Can be periodi and/or polarization sensitive ν [ Hz] Output spetrum Robert R. MLeod, University of Colorado Frequeny quite sensitive to d and thus temperature of avity. ν [ Hz] 9

8 Laser Lasers vs. lamps Spetrum Laser Inandesent lamp Effiieny Wall-plug effiieny is optial power out to eletrial power in. As high as 60% for diode lasers, 30% for diode-pumped solid states. Luminous effiieny is power in visible spetrum / eletrial power in. As low as % for inandesent bulbs. Filters an redue spetral wih but dramatially redue effiieny. Brightness L φ [ W] ( π w [ m] ) M [ ] φ λ ( ) 0 n w rad n M 0 [W/m /Sr] for a perfet W laser at λ um in air. π λ 0 L φ [ W] ( filament area [ m ]) 4π [ Sr] ( ) 0 5 [W/m /Sr] for a bulb with a mm filament emitting W Robert R. MLeod, University of Colorado 0

Laser Laser diodes Heterostruture laser diode Transistor outline pakage www.mellesgriot.om/produts/lasers/lsrdiode.asp www.eio.om/repairfaq/sam/laserfil.

9 Laser Laser diodes Heterostruture laser diode Transistor outline pakage Output is typially anamorphi (elliptial), astigmati (diff urvature), single transverse mode, multi-longitudinal mode, polarized. Typial output faet x3 µm. Ciruit diagram Photodiode integrated into pakage for power monitoring and feedbak ontrol. Thermal ontrol with a thermo-eletri (Peltier) ooler is also ommon. Robert R. MLeod, University of Colorado

Laser Typial red LD spes Temperamental beasts Threshold urrent gain has overome loss in the avity and the devie is operating as a laser, not an LED. Varies between and within models.

10 Laser Typial red LD spes Temperamental beasts Threshold urrent gain has overome loss in the avity and the devie is operating as a laser, not an LED. Varies between and within models. Max rated power faet (mirror) damage ours above this optial output power. Temperature dependene LDs are less effiient as their temperature inreases. Thus as they hange temperature, the bias urrent must be modified to keep the power onstant (and not blow it up). Range of urrent operation between the threshold and the urrent whih ahieves the max rated power. Both depend on temperature This range an be a small fration (0%) of the threshold urrent How to blow up a LD: Reverse urrent or urrent transients, partiularly during on/off Exessive optial power blows faets. Robert R. MLeod, University of Colorado

Line Radiative Transfer

Line Radiative Transfer http://www.v.nrao.edu/ourse/astr534/ineradxfer.html ine Radiative Transfer Einstein Coeffiients We used armor's equation to estimate the spontaneous emission oeffiients A U for À reombination lines. A