High power blue lasers for white light generation

Workshop: Smart Lighting

Friday, 30.10.2015, EPFL

Marco Rossetti

2 12-Nov-15

Solid state lighting

LEDs have conquered most of the lighting empire.

But lasers may strike back!

3 12-Nov-15

Some examples

RGB laser projectors

4 12-Nov-15

Some examples

5 12-Nov-15

Some examples

BMW i8 laser-based headlights

Hanafi et Erdl., Laser light, BMW ek-711, 21.05.2015

6 12-Nov-15

Why lasers?

Because they are cool!

𝒃𝒖𝒕 𝒂𝒍𝒔𝒐 … …

7 12-Nov-15

Outline

LD vs LED: handbook of semiconductor physics

GaN-based LDs: technology

and performance White light generation

8 12-Nov-15

LED guide for dummies

0.00 0.02 0.04 0.06 0.08 0.100

1

2

3

4

5

6

7

8

9

10

Outp

ut

pow

er

(mW

)

Drive current (A)

sublinear

p-type

n-type

active

J

9 12-Nov-15

LED guide for dummies

1E-4 1E-3 0.01 0.1

0.01

0.1

1

10

Outp

ut

pow

er

(mW

)

Drive current (A)

𝑱 ∝ 𝑨𝒏 + 𝑩𝒏𝟐 + 𝑪𝒏𝟑

radiative

Auger

Shockley-Read-Hall

10 12-Nov-15

Efficiency droop in a LED

1 10 100 10000

1

2

3

4

5

6

7

EQ

E (

arb

. u

nit)

Current density (A/cm2)

droop

Best peak EQEs up to 70-80%

Commecial LEDs: 30-50%

<20%

11 12-Nov-15

In a laser

p-type

n-type

gain medium laser beam

mirror mirror

𝒅𝒏

𝒅𝒕=

𝑱

𝒒𝒅− 𝑨𝒏 − 𝑩𝒏𝟐 − 𝑪𝒏𝟑 − 𝑮𝒔

𝒅𝒔

𝒅𝒕= (𝑮 − 𝜶)𝒔 + 𝜷𝑩𝒏𝟐

E

absorption

E

stimulated emission

𝑮 ∝ 𝒏 − 𝒏𝟎

𝜶 = 𝜶𝒎 + 𝜶𝒊

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In a laser

p-type

n-type

gain medium

laser beam

mirror mirror

𝒅𝒏

𝒅𝒕=

𝑱

𝒒𝒅− 𝑨𝒏 − 𝑩𝒏𝟐 − 𝑪𝒏𝟑 − 𝑮𝒔

𝒅𝒔

𝒅𝒕= (𝑮 − 𝜶)𝒔 + 𝜷𝑩𝒏𝟐

𝑮 ∝ 𝒏 − 𝒏𝟎

𝜶 = 𝜶𝒎 + 𝜶𝒊

J

Pout

lasing threshold

𝑮 = 𝜶

𝜼𝒅 = 𝜼𝒆𝒙𝒕𝜼𝒊𝒏𝒋

13 12-Nov-15

In a laser

p-type

n-type

gain medium

laser beam

mirror mirror

J E

ffic

ien

cy

lasing threshold

𝑮 = 𝜶

droop free!

𝜼𝒆𝒙𝒕𝜼𝒊𝒏𝒋

𝒅𝒏

𝒅𝒕=

𝑱

𝒒𝒅− 𝑨𝒏 − 𝑩𝒏𝟐 − 𝑪𝒏𝟑 − 𝑮𝒔

𝒅𝒔

𝒅𝒕= (𝑮 − 𝜶)𝒔 + 𝜷𝑩𝒏𝟐

𝑮 ∝ 𝒏 − 𝒏𝟎

𝜶 = 𝜶𝒎 + 𝜶𝒊

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Extraction efficiency

1995 2000 2005 2010 2015

20

30

40

50

60

70

80

90

Extr

actio

n e

ffic

ien

cy (

%)

year

LED trend

𝜼𝒆𝒙𝒕 = 𝜶𝒎

𝜶𝒎 + 𝜶𝒊

resonator length

mirror reflectivity

optical loss

Laser

optimized epitaxy

5 10 15 20 25 30

20

30

40

50

60

70

80

90

Extr

action e

ffic

iency (

%)

Optical loss i (cm

-1)

o NiAu semitransparent contact

o Flip chip

o Thin film flip chip

o Patterned sapphire substrate

15 12-Nov-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

16 12-Nov-15

Tailoring the emission wavelength

GaN barrier

InGaN

QW structure

indium content 8% 15%

engineering of

band diagrams

17 12-Nov-15

0 50 100 150 200 250 300 350 4000

50

100

150

200

250

300

350

400

Pow

er

(mW

)

Current (mA)0 50 100 150 200 250 300 350 400

0

10

20

30

40

50

EQ

E (

%)

Current (mA)

single lateral mode

1E-3 0.01 0.1 1 100

10

20

30

40

50

laser diode

LED

EQ

E (

%)

Current density (kA/cm2)

Lasers exceed LEDs

Violet laser performance

peak WPE = 20%

18 12-Nov-15

Great expectations

0.0 0.5 1.0 1.5 2.00.0

0.5

1.0

1.5

2.0

2.5

Po

we

r (W

)

Current (A)

0

5

10

15

20

25

30

35

WP

E (

%)

Going multimode……

0.0 0.5 1.0 1.5 2.00

200

400

600

800

Flu

x (

lm)

Current (A)

0

20

40

60

80

100

Lu

min

ou

s e

ffic

acy (

lm/W

)

Laser pump White source

60W

estimate of power and flux

19 12-Nov-15

When dimension counts

More LDs per unit surface

Radiance several orders of

magnitude higher for LDs

0.1 110

1

102

103

104

105

106

107

108

LED

Radia

nce (

W /

sr

cm

2)

Device footprint (mm2)

LD

LD very attractive for specialty applications needing compact and high radiance light sources

20 12-Nov-15

White light generation with LDs

Pure Play RGB laser

400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

1.2

Inte

nsity (

a.u

.)

Wavelength (nm)

21 12-Nov-15

400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

1.2

Inte

nsity (

a.u

.)

Wavelength (nm)

White light generation with LDs

400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

1.2

Inte

nsity (

a.u

.)

Wavelength (nm)

blue laser & yellow phosphor violet laser & RGB phosphor

Highest laser efficiency

Best CRI

Downconversion loss

Lower conversion loss

LD more challenging

Light diffusers needed

GaN laser

GaN laser

22 12-Nov-15

Laser diode speckle noise

• light dephasers

• violet + RGB

23 12-Nov-15

Color anisotropy (pure blue LD)

directional laser

transmitted

+

scattered

isotropic phosphor

emission

S. P. DenBaars, UCSB, Santa Barbara

undesirable blue

light emission

Sandia & Corning

• light mixers required

• violet + RGB

24 12-Nov-15

Conclusion

low droop

compact

high radiance

efficient

cool

25 12-Nov-15

EXALOS broad-band modules

Confi

dentia

l

25

11/12/

2015

26 12-Nov-15

27 12-Nov-15

28 12-Nov-15

Increasing the emission wavelength

Increasing the indium content or QW thickness

High indium content: lower composition

uniformity/higher disorder

laser

pump CCD camera

400 450 5000.0

0.2

0.4

0.6

0.8

1.0

1.2

Inte

nsity (

a.u

.)

Wavelength (nm)

broadening

dispersed gain

lower efficiency

29 12-Nov-15

Increasing the emission wavelength

laser

pump CCD camera

active region: Tg ~ 700C

p-type: Tg ~ 1000C p-type

n-type

active

In-rich region may get «burned» during p-GaN growth

450 nm 450 nm

p-type growth

Impact of p-type growth on active region

requires low temperture

p-type layers!

30 12-Nov-15

Laser diode speckle noise

Toshiba Lighting & Technology Corporation,

J. Light & Vis. Env. Vol.37, No.2 & 3, 2013

• light dephasers

• violet + RGB

31 12-Nov-15

Design flexibility & remote phosphors

Meneghini et al.

University of Padova

Temperature degradation

LD specially suited for

remote phosphor

configurations!

phosphors in

contact with LED

chip

LD die

remote

phosphor