Tutorial · chr is top h.l ien au @ un io lde nb ur g.d e ...

Breaking the diffraction limit: Analysis of diode lasers by nearfield scanning

optical microscopy (NSOM)

Jens W. Tomm a and Christoph Lienau b

a MaxBornInstitut für Nichtlineare Optik und Kurzzeitspektroskopie Berlin, MaxBornStr. 2 A, D12489 Berlin, Germany

b Institut für Physik, Fk. V, Carl von Ossietzky Universität Oldenburg Ammerländer Heerstraße 114118, D26129 Oldenburg, Germany christoph.lienau@unioldenburg.de

WWW.BRIGHTER.EU Tutorial

1 WWW.BRIGHTER.EU Plenary Meeting at Tyndall, Cork 2008

Outline

1. Introduction NSOM = Nearfield Scanning Optical Microscopy Principles and opportunities Spatial resolution

2. Methodology Laser emission, spontaneous emission and PL

3. NOBIC = Nearfield Optical Beam Induced Current 4. Experimental setups and equipment 5. Analysis of waveguides and determination of mode profiles 6. VCSEL 7. Surface recombination velocity at facets 8. Defect creation during device operation 9. Summary 10. Acknowledgement


Introduction

10 µm 1 µm 100 nm 10 nm 1 nm 0.1 nm

Farfield Optical Microscopy

NearField NanoOptics

Semiconductor Nanostructures

...

Nanostructures and Nanoanalytical Tools

Optoelectronic devices


Scanning probe microscopy

Disc players for atoms

Compare: If 1 atom had the size of an orange, the cantilever would be 100 km long

Introduction


1ns

100 pm

1 nm

10 nm

100 nm

1 µm

10 µm

100 µm

10ps 100ps 1ps 100fs 1fs Time 10fs

Space

Electronic motion in semiconductor nanostructures

Quantum transport

Energy transport in polymers + large biomolecules

Physical phenomena happen on ultrashort length and time scales

Electronic motion in atoms + molecules

Nuclear motion in molecules

Surface plasmon dynamics in metallic nanostructures

Introduction


1ns

100 pm

1 nm

10 nm

100 nm

1 µm

10 µm

100 µm

10ps 100ps 1ps 100fs 1fs Time 10fs

Space

Ultrafast nanoscale physics: Imaging tools

Ultrafast nanooptics

Electron microscopy Cathodoluminescence

Ultrafast farfield optical microscopy

Introduction


Intensity Sum

Spot 2 Spot 1

x 0.61 / N.A. λ ∆ ≈ Rayleigh limit:

The resolution in optical microscopy is limited by the wavelength of light

Introduction


Breaking the resolution limit in microscopy Nearfield scanning optical microscopy

Diffractionunlimited resolution

Introduction


Spatial resolution in nearfield microscopy

Electromagneticfield distribution: superposition of monochromatic plane waves:

E E ik r i t = − 0 0 exp( ) exp( ) r r ω = + − E i k x k y ik z i t x y z 0 exp( ( )exp( )exp( ) ω

k k k k k x y z 0 0 2 2 2 = = + +

r

for given k k x y , : two solutions:

(a) propagating waves:

(b) evanescent waves:

k k k k lat x y = + < 2 2 0 k real z .

k k k k lat x y = + > 2 2 0

k imaginary z .

Consequence: Intensity of evanescent waves decreases exponentially with increasing z

How to get optical superresolution ?: ∆ ∆ k x x ≥ 1

Use evanescent modes!

Introduction


Diffraction by 1dimensional slit (Kirchhoff Approximation)

Introduction

Diffraction by 1dimensinal slit (Kirchhoff Approximation)


0 50 100 150 200 0.0

0.2

0.4

0.6

0.8

1.0

Diffraction by 1Dimensional Slit (Kirchhoff Approximation)

z =0 nm z = 10 nm z = 20 nm z = 40 nm z = 80 nm z = 160 nm

Powe

r Spectrum (a

.u.)

Lateral Wavevector k lat (1/µm) 200 100 0 100 200

100

80

60

40

20

0

λ = 800 nm d = 50 nm n r = 3.4

z Position (nm)

Lateral Position (nm)

Monochromatic plane wave (k 0 = 2πn r /λ λ = 800 nm) incident on slit (width d = 50 nm)

Propagating waves: k lat < k 0 = 2πn r /λ k z 2 = (k 0

2 k lat 2 ) > 0 k z real

Evanescent waves: k lat > k 0 = 2πn r /λ k z 2 = (k 0

2 k lat 2 ) < 0 k z imaginary

Diffraction by 1dimensinal slit (Kirchhoff Approximation)

Introduction


0 20 40

0

20

40 z = 0 nm

40 20 0

40

20

0 0

20

40

|E x (x,y,z)/E 0 |

z = 1 nm

40 20 0

40

20

0

0 20 40

0

20

40

y (nm)

z = 2 nm

0 20 40 40 20 0

40

20

0 0

20

40 z = 5 nm

0 20 40 40 20 0

40

20

0

0

20

40 z = 10 nm

0 20 40 40 20 0

40

20

0

0 20 40

x (nm) 40 20 0

40

20

0 0

20

40 z = 40 nm

Twodimensional field distribution E x (x,y) behind a r=30 nm aperture in a perfect metal film

Introduction


Aperturebased nearfield microscopy

+ excellent rejection of propagating waves

low transmission efficiency (10 4 10 3 for 100 nm apertures)

Introduction


Aperturebased nearfield microscopy

Introduction


Nearfield Reflectance Spectroscopy with Uncoated Fiber Probes

Illumination/Collection Mode

1000 500 0 0.0

0.2

0.4

0.6

0.8

1.0

Reflectance (a

.u.)

Distance (nm)

Transmission efficiency close to 1 Extremely high collection efficiency

Sensitive probing of nearfield reflectance

Resolution down to λ/5 (150 nm)

Experiment: F. Intonti et al, PRL 87, 076801 (2001), PRB 63, 075313 (2001) Theory: R. Müller and C. Lienau, Appl. Phys. Lett. 76, 3367 (2000).

Introduction


2 1 0 1 0

2

4

6

2

4

6

Propagation through Uncoated Fiber Probes

1.0 0.5 0.0

40fs

2 1 0 1 2 2

4

6

z exit

Y (µm)

Y (µm)

52fs

Z(µm

)

z = z exit

|Ê x | 2

/A 0 2

260nm

1 0 1 2

z = z exit +25nm

285nm

R. Müller Appl. Phys. Lett. 76, 3367 (2000) and J. Microscopy 202, 339 (2001).

Pulse propagation through uncoated fiber probes

2D modelling colorcode represents │E│ 2 A 0 2

Introduction


30 40 50 60 70 80 0.0

0.5

1.0

720 760 800 840 880 0.0

0.5

1.0

10 4 |Ê

x | 2 /A 0 2 z=z

exit +25nm z =z

exit

10 4 |Ê

x | 2 /A 0 2

(b)

Delay Time (fs)

(c)

Wavelength(nm)

Power Density (n

orm.)

0.50 0.25 0.00 0.25 0.50 0

1

2

10 0.1 0.001

Z exit (a)

Y(µm)

Z(µm

) 0.1 0.1

0

2

4

Y (µm) 0.1 0.1 Y (µm)

Pulse propagation through metalcoated fiber probes

R. Müller and C. Lienau Appl. Phys. Lett. 76, 3367 (2000) J. Microscopy 202, 339 (2001).

2D modelling colorcode represents

│E│ 2 A 0 2

│E│ 2 A 0 2 of a 10 fs Gaussian pulse at the 100 nm aperture

____ input pulse ____ output pulse

Introduction


Application: Raman spectroscopy of Carbon Nanotubes

A. Hartschuh et al., Phys. Rev. Lett. (2003)

Introduction


Nanophotoluminescence of single localized excitons

790 792 794 796 798 800 0.0

0.2

0.4

0.6

0.8

1.0

Intensity (a

.u.)

Wavelength (nm)

T = 15 K 500 nm

F. Intonti et al., Phys. Rev. Lett. 87, 076801 (2001).

Introduction


Apertureless Nearfield Scanning Optical Microscopy

+ Strong field enhancement (10 x) at ultrasharp metal tips + Spatial resolution limited by radius of curvature of the tip + Spatial resolution down to 10 nm (and beyond?)

Introduction


Outline


2. Methodology, Laser emission, spontaneous emission and PL 3. NOBIC = Nearfield Optical Beam Induced Current 4. Experimental setups and equipment 5. Analysis of waveguides and determination of mode profiles 6. VCSEL 7. Surface recombination velocity at facets 8. Defect creation during device operation 9. Summary 10. Acknowledgement


Analysis of laser emission

W. D. Herzog, M. S. Ünlü, B. B. Goldberg, G. H. Rhodes, and C. Harder, (1997). Appl. Phys. Lett. 65 688

RW laser ~10 nm 1 µm

2 µm 3 µm 4 µm

5 µm 6 µm 7 µm


Analysis of laser emission VCSEL

I=7 mA I=10 mA I=15 mA

VCSEL aperture (blue)

Emission intensity (color)

Van der Rhodes et al. Appl. Phys. Lett. 72,1811 (1998)


Analysis of laser emission

Such work at devices is extremely useful,

1. if confocal microscopy fails

2. if the fiber tip does not influence the emission

These statements hold in general for the application of NSOM !

if not, you waste your resources

unfortunately, this is the case when investigating lasing devices


What is better?

Spontaneous emission Photoluminescence Absorption (Photocurrent)

fiber tip

heat sink

Substrate ARcoating

cladding

waveguide DQW

heat sink

cladding lockin amplifier


What is better?

Spontaneous emission Photoluminescence Absorption (Photocurrent)

fiber tip

heat sink

Substrate ARcoating

cladding

waveguide DQW

heat sink



What is better?

Spontaneous emission Photoluminescence Absorption (Photocurrent) NOBIC

fiber tip

heat sink

Substrate ARcoating

cladding

waveguide DQW

heat sink



NOBIC

NOBIC signal (a.u.)

NOBIC = Nearfield Optical Beam Induced Current

Buratto et al. (AT&T Bell Lab.) Appl. Phys. Lett. 65, 2654 (1994)

Excitation at 633 nm (2 mW in, ~2 nW out)

NOBIC

A

B


Outline





Experimental setups and equipment


Fibertip distance control



Homebuilt Scanning Nearfield Optical Microscopes Setup with cryostat for

Measurements at 10 300 Kelvin

"Customers:" Uni Magdeburg, D KTH Stockholm, Sweden Forschungszentrum Jülich, D University of Arkansas, USA

G. Behme et al., Rev. Sci. Instrum. 68, 3458 (1997)



NSOM data

PCsignal (R) topography



Outline





Analysis of waveguides

Heat sink Substrate

LOC waveguide Cladding layer

Double quantum well

AR coating AR coating AR coating

Modulator (1.2 kHz)

AR coating

Excitation laser Lockin amplifier

Lockin amplifier

PC

Setup


Results obtained at single symmetric and asymmetric waveguides

resonant NSOM Photocurrent (Absorption)

Step INdex SIN

Large Optical Cavity LOC symmetric waveguide

Large Optical Cavity LOC asymmetric waveguide

0 1 2 3 4 5

potential

x (µm)

0 1 2 3 4 5

potential

x (µm)

0 1 2 3 4 5

potential

x (µm)



1 0 1 0.0

0.5

1.0

1.5

2.0

PC Signal (a.u.)

Tip Position (µm) 1 0 1 0.0

0.2

0.4

0.6

0.8

1.0

1.2

PC Signal (a.u.)


0.2

0.4

0.6

0.8

1.0

1.2

PC Signal (a.u.)

Tip Position (µm)

Step INdex SIN



1 0 1

m=2

m=1

m=0

Position (µm) 1 0 1

m=2

m=1

m=0

Position (µm)

1 0 1

Position (µm)



Comparison of the guided modes in a waveguide and the wavefunctions in a quantum well



Step INdex SIN



1 0 1 0.0

0.5

1.0

1.5

2.0

2.5

PC Signal (a.u.)


0.5

1.0

PC Signal (a.u.)


0.2

0.4

0.6

0.8

1.0

1.2

PC Signal (a.u.)

Tip Position (µm)

NSOM Photocurrent reveals mode structure of waveguides asymmetric waveguides have a specific NSOM Photocurrent signature



2 1 0 1 2 0,0

0,5

1,0

1,5

2,0

2,5

excitation photon energy: 2.8 eV

absolute value phase

NOBIC line scans from the symmetric LOC structure

Absolute Value,Φ, and E g

(a.u., °, and eV)

x (µm) 2 1 0 1 2

0,0

1,0

2,0

3,0

NOBIC line scans from the asymmetric LOC structure

(117+24) nm excitation photon energy: 2.8 eV

absolute value phase

Absolute value,

Φ, and E g

(a.u., °, and eV)

x (µm)

NOBIC contrast from the photocurrent phase for non resonant (surface) excitation



Single waveguides Resonant excitation (coupling into the

waveguide modes) NOBIC reveals mode structure of waveguides asymmetric waveguides have a specific NOBIC

signature Nonresonant excitation (pure carrierpair creation) detection of the QW within the waveguide

T. Guenther et al. „NearField Photocurrent Imaging of the Optical Mode Profiles of Semiconductor Laser Diodes“ Appl. Phys. Lett. 78 14631465 (2001).



2 0 2 4 6 8 10 1.4

1.6

1.8

2.0

2.2

Energy gap (e

V)

Local position in growth direction (µm)

Nanostacks interband cascade diode laser

n n p p + J. P. van der Ziel and W. T. Tsang, Appl. Phys. Lett. 41, 499501 (1982).

J. Ch. Garcia, E. Rosencher, Ph. Collot, N. Laurent, J. L. Guyaux, B. Vinter, and J. Nagle, Appl. Phys. Lett. 71, 37523754 (1997).

S. G. Patterson, G. S. Petrich, R. J. Ram, and L. A. Kolodziejski, Electron Lett. 35, 395 (1999).

C. Hanke, L. Korte, B. D. Acklin, M. Behringer, G. Herrmann, J. Luft, B. De Odorico, M. Marchiano, J. Wilhelmi, Proc. SPIE 3947, 5057(2000).

http://www.osramos.com/news/news_power.html

‘regular’ highpower diode laser array

Nanostack



1 2 3 4 5 6 7 8

1

2

3

4

5

6

7

8

y (µm)

x (µm)

Photoluminescence Photocurrent (Absorption) Device emission

Detailed analysis of Nano stacks by NSOMbased

spectroscopy



Results from Nanostacks with two vertically stacked asymmetric waveguides

0 2 4 6 8 10 0.0

0.2

0.4

0.6

0.8

1.0

x (µm)

Electroluminescence (a. u.)

0 2 4 6 8 10 0.0

0.2

0.4

0.6

0.8

1.0

x (µm)

Laser signal (a.u.)

NSOM Device Emission

Substrate duty cycle: 2x10 5 no thermal load



NSOM PL Emission

0 2 4 6 8 10 0.00

0.25

0.50

0.75

1.00

x (µm)

PL signal (a

. u.)

0 2 4 6 8 10 0.0

0.2

0.4

0.6

0.8

1.0

11 µW 160 nW 16 nW

x (µm)

PL signal (a

. u.)

excitation photon energy: 2.8 eV detection photon energy: 1.5 eV

excitation photon energy: 1.69 eV detection photon energy: 1.5 eV

PLsignal ~ δn



resonant and nonresonant NSOM Photocurrent (Absorption)

0 2 4 6 8 10 0.0

0.2

0.4

0.6

0.8

1.0

x (µm) PC Signal (a. u.)

0 2 4 6 8 10 0.0

0.2

0.4

0.6

0.8

1.0

x (µm)

PC signal (a. u.)

excitation photon energy: 1.69 eV excitation photon energy: 2.8 eV

Photocurrentsignal ~ δn×grad(V)


V. Malyarchuk et al. "Uniformity tests of individual segments of interband cascade diode laser nanostacks " J. Appl. Phys. 92, 27292733 (2002).


0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 10 5 10 4 10 3 10 2 10 1 10 0

onset of the Braggmirror absorption at about 1.62 eV

'barrier absorption' due to the Al 0.25 Ga 0.75 As barriers

'QW region' 'defect region'

VCSEL emission energy

Photocurre

nt (a.u.)

Photon energy (eV)

NOBIC at VCSELS


Outline





Reflection PL experiment uncoated fiber tip spatial resolution: 150 nm

Surface Recombination Velocity

v s

V. Malyarchuk, J. W. Tomm, V. Talalaev, Ch. Lienau, F. Rinner, and M. Baeumler Appl. Phys. Lett. 81 346 (2002).

L D


1 0 1 2 3 0

1

2

PL

x (µm)

y (µm)

0 0.5 1.0

1 0 1 2 3 0

1

2

PL

x (µm)

y (µm)

0 0.5 1.0

1 0 1 2 3 0

1

2

∆PL

x (µm)

y (µm)

0.06 0 0.06

1 0 1 2 3 0

1

2

∆PL

x (µm)

y (µm)

0.06 0 0.06

P = 10 µW P = 300 µW

QW PL collected through the fiber and detected by a single photon counting system

Lineleveled images



0.0 1.0 2.0 0.0

0.5

1.0

300 µW 100 µW 30 µW R

Signal (a.u.)

x (µm)



0.0 1.0 2.0 0.0

0.5

1.0

PL v s = 0.5 x 10 6 cm/s

v s = 1.0 x 10 6 cm/s

v s = 3.0 x 10 6 cm/s

PL (a.u.)

x (µm)

D = 15 cm 2 /s τ = 1.7 ns

D rec L D τ =

2 2

0 0 2 2 ( ) ( , ) 0 a rec

n D n n g x y x y

∂ ∂ ∂ ∂ τ

+ − + =

2 2 0 0

2 ( ) ( )

2 0 0

1 ( , ) 2

x x y y

g x y e σ

πσ

− + − −

=

x=0: 0

v s x

n D n x

∂ ∂ =

= ⋅

diffusion length L D and the product (v ) s rec τ ⋅ can be deduced


Example calculated for:


0.0 1.0 2.0 0.0

0.5

1.0

300 µW 100 µW 30 µW R

Signal (a.u.)

x (µm)

L D

v s *☺



0.0

0.5

1.0

1.5

2.0

2.5

1 10 100 1000 0.5

1.0

1.5

2.0

2.5

3.0 L D

(µm)

Power (µW)

τ QW (n

s)

Additional ps PLexperiment provides τ



0

10

20

30

1 10 100 1000 0.0

2.0

4.0

6.0

8.0 v s

(10 6

cm/s)

Power (µW)

D (cm 2 /s)


V. Malyarchuk, J. W. Tomm, V. Talalaev, Ch. Lienau, F. Rinner, and M.Baeumler Appl. Phys. Lett. 81 346 (2002).


Outline





1.0 1.5 2.0 2.5 3.0

10 2

10 1

10 0

10 1

10 2

10 3

10 4 PC FTIR Spectra Red LD OSRAM 510311

PC signal (a.u.)

Energy (eV)

Before Aging 9h Aging 30h Aging 55h Aging

0.0 0.2 0.4 0.6 0.8 0

20

40

60

80

100

120

140

160

180

200

0.0

0.5

1.0

1.5

2.0

2.5 Osram (BRIGHT) 51011+3

Pow

er (m

W)

Current (A)

0h 0.5h 1.5h 3h 9h 15h 31h 55h

Voltage (V

)

Defect creation during device operation

Result from the BRIGHT project


1.0 1.2 1.4 1.6 1.8

10 2

10 1

10 0

PC FTIR Spectra Red LD OSRAM 510311

PC signal (a.u.)

Energy (eV)

Before Aging 9h Aging 30h Aging 55h Aging

DefecttoBand Transitions?

Where are the ‘defects’ located?

Active region???

At 1.7 eV (730 nm), there is a threefold increase of the signal within 55 h of aging.



1. Laterally, i.e., where along the device…

0.0 0.2 0.4 0.6 0.00

0.05

0.10

0.15

0.20

Osram (BRIGHT) 51011+3

pristine device 3h 9h

LBIC signal (a.u.)

y (mm) LBIC (Laser Beam Induced Current) excited resonantly to the defects reveals them to be underneath the metalized emitter stripe.

Front view of the laser structure

substrate

epilayer sequence

active region



2. Vertically, i.e., where along the layer sequence …

aged area

‘Pristine’ reference area

Front view of the laser structure substrate

~ 1.5 µm



633 nm 730 nm topography

pristine device region

1000 nm 1000 nm 1000 nm

NOBICData: (Nearfield Optical Beam Induced Current)



0.0 0.5 1.0 1.5 2.0 10 2

10 1

10 0

10 1 633 nm 730 nm

aged region Signal (a

.u.)

x (µm)

633 nm 730 nm topography

aged device region

1000 nm 1000 nm 1000 nm



0.0 0.5 1.0 1.5 2.0

10 2

10 1

10 0

10 1

633 nm 730 nm

Signal (a.u.)

x (µm)

pristine region

0.0 0.5 1.0 1.5 2.0

10 2

10 1

10 0

10 1

633 nm 730 nm

aged region

Signal (a.u.)

x (µm) 1.5 µm 1.5 µm

There is a threefold increase of the 730 nm signal with respect to the 633 nmsignal 1.5 µm away from the active region (towards the heat sink), there is no photosensitivity.

The creation of deep levels takes place at a location that allows interaction with the laser emission.

There are additional 3 sets of data couples from different regions, which confirm the result.


Claus Ropers et al. Appl. Phys. Lett. 88, 133513 (2006).


NSOM = Nearfield Scanning Optical Microscopy Principles and opportunities Spatial resolution

Methodology, Laser emission, spontaneous emission and PL NOBIC = Nearfield Optical Beam Induced Current Experimental setups and equipment Analysis of waveguides and determination of mode profiles VCSEL Surface recombination velocity at facets Defect creation during device operation

Summary

NSOM is extremely useful if you need a spatial resolution beyond the diffraction limit.

NOBIC allows optical analysis at devices along growth direction.


• Christoph Lienau

• Alexander Richter, Tobias Günther, Viktor Malyarchuk, Roland Müller, Claus Ropers, Thomas Elsässer

• Martin Behringer, Johannes Luft, Peter Brick, Norbert Linder, Bernd Mayer, Martin Müller, Sönke Tautz, Wolfgang Schmid, Götz Erbert, Jürgen Sebastian, Siegfried Gramlich, Eberhard Richter, Heiko Kissel, Frank Brunner, Markus Weyers, Günter Tränkle

Acknowledgement


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