High power blue lasers for white light generation. Marco Rossetti Workshop: Smart Lighting Friday, , EPFL

Transcription

1 High power blue lasers for white light generation Marco Rossetti Workshop: Smart Lighting Friday, , EPFL

2 Solid state lighting LEDs have conquered most of the lighting empire. But lasers may strike back! 2 12-Nov-15

3 Some examples RGB laser projectors 3 12-Nov-15

4 Some examples 4 12-Nov-15

5 Some examples BMW i8 laser-based headlights Hanafi et Erdl., Laser light, BMW ek-711, Nov-15

6 Why lasers? Because they are cool! but also 6 12-Nov-15

7 Outline LD vs LED: handbook of semiconductor physics GaN-based LDs: technology and performance White light generation 7 12-Nov-15

8 Output power (mw) LED guide for dummies p-type active n-type J sublinear Drive current (A) 8 12-Nov-15

9 Output power (mw) LED guide for dummies 10 1 Auger 0.1 radiative J An + Bn 2 + Cn Shockley-Read-Hall 1E-4 1E Drive current (A) 9 12-Nov-15

10 EQE (arb. unit) Efficiency droop in a LED 7 Best peak EQEs up to 70-80% Commecial LEDs: 30-50% droop Current density (A/cm 2 ) <20% Nov-15

11 In a laser p-type gain medium laser beam n-type mirror mirror absorption stimulated emission dn dt = J qd An Bn2 Cn 3 Gs ds = (G α)s + βbn2 dt E E G n n 0 α = α m + α i Nov-15

12 In a laser p-type gain medium laser beam mirror n-type mirror dn dt = J qd An Bn2 Cn 3 Gs ds = (G α)s + βbn2 dt P out η d = η ext η inj G n n 0 α = α m + α i lasing threshold G = α J Nov-15

13 In a laser p-type gain medium laser beam mirror n-type mirror dn dt = J qd An Bn2 Cn 3 Gs ds = (G α)s + βbn2 dt Efficiency droop free! η ext η inj G n n 0 α = α m + α i lasing threshold G = α J Nov-15

14 Extraction efficiency (%) Extraction efficiency (%) Extraction efficiency LED trend Laser η ext = α m α m + α i resonator length mirror reflectivity optical loss optimized epitaxy year o NiAu semitransparent contact o Flip chip o Thin film flip chip o Patterned sapphire substrate Optical loss i (cm -1 ) Nov-15

15 GaN-based laser diodes Epitaxy on GaN substrate Standard fabrication o Optical lithography o Wet and dry etchings o Thin film deposition bar cleaving Nov-15

16 Tailoring the emission wavelength QW structure GaN barrier engineering of band diagrams InGaN 8% indium content 15% Nov-15

17 Power (mw) EQE (%) EQE (%) Violet laser performance single lateral mode Current (ma) laser diode LED E Current density (ka/cm 2 ) peak WPE = 20% Lasers exceed LEDs Current (ma) Nov-15

18 Power (W) WPE (%) Flux (lm) Luminous efficacy (lm/w) Great expectations Going multimode estimate of power and flux 60W Laser pump Current (A) White source Current (A) Nov-15

19 Radiance (W / sr cm 2 ) When dimension counts More LDs per unit surface Radiance several orders of magnitude higher for LDs LD LED Device footprint (mm 2 ) LD very attractive for specialty applications needing compact and high radiance light sources Nov-15

20 Intensity (a.u.) White light generation with LDs Pure Play RGB laser Wavelength (nm) Nov-15

21 Intensity (a.u.) Intensity (a.u.) White light generation with LDs blue laser & yellow phosphor violet laser & RGB phosphor 1.2 GaN laser GaN laser Wavelength (nm) Wavelength (nm) Lower conversion loss LD more challenging Light diffusers needed Highest laser efficiency Best CRI Downconversion loss Nov-15

22 Laser diode speckle noise light dephasers violet + RGB Nov-15

23 Color anisotropy (pure blue LD) directional laser transmitted + scattered S. P. DenBaars, UCSB, Santa Barbara isotropic phosphor emission undesirable blue light emission Sandia & Corning light mixers required violet + RGB Nov-15

24 Conclusion cool compact efficient high radiance low droop Nov-15

25 EXALOS broad-band modules Nov-15

26 26 12-Nov-15

27 27 12-Nov-15

28 Intensity (a.u.) laser pump Increasing the emission wavelength CCD camera Increasing the indium content or QW thickness High indium content: lower composition uniformity/higher disorder broadening Wavelength (nm) dispersed gain lower efficiency Nov-15

29 Increasing the emission wavelength Impact of p-type growth on active region p-type: Tg ~ 1000C p-type active region: Tg ~ 700C active n-type requires low temperture p-type layers! laser pump CCD camera p-type growth 450 nm 450 nm In-rich region may get «burned» during p-gan growth Nov-15

30 Laser diode speckle noise Toshiba Lighting & Technology Corporation, J. Light & Vis. Env. Vol.37, No.2 & 3, 2013 light dephasers violet + RGB Nov-15

31 Design flexibility & remote phosphors phosphors in contact with LED chip Temperature degradation LD die remote phosphor Nov-15 LD specially suited for remote phosphor configurations! Meneghini et al. University of Padova