Apertureless Scanning Near-field Optical Microscopy: a comparison between homodyne and heterodyne

approaches

Journal Club Presentation – March 26th, 2007

Suraj Bramhavar

Lewis Gomez et al.,

J. Opt. Soc. Am. B, Vol. 23, No. 5, 823-833 (2006).

Outline

• Background– SNOM, ASNOM

• Problems– Background suppression– Interferometric effects– Possible solutions

• Heterodyne vs. Homodyne ASNOM– Experimental Results– Conclusions

Near-Field Optical Techniques

a)Aperture probe (SNOM) – Evanescent waves from tapered fiber probe are used either to illuminate sample or couple near-field light from sample into fiber

b)Apertureless probe (ASNOM) – Small (sub-wavelength) tip scatters near-field variations into far field

Pictures courtesy of --- Hecht et al. (2000)

ASNOM

Tip scatters both illuminated near field of sample (a) and incident far field (b)

Pictures courtesy of Hecht et al. (2000), Greffet et al. (1997)

ASNOM

• Advantages– Far field illumination and detection

allows for use of conventional optics– High resolution achievable through

smaller tip fabrication

• Drawbacks– Reflection from surface creates strong

background – Background field causes interference

effects that are hard to suppress

ASNOM

• Possible solutions– Fluorescent active centers at tip

extremity– Local tip field enhancement at apex– Tip-modulation harmonics– Heterodyne configuration

Eb = Background light scattered from sample

Et = Light elastically scattered by near-field interaction of tip and evanescent field from sample

Theory – Homodyne ASNOM

)cos(222

bttbtb EEEEI

)cos(222

bttbtb EEEEI

**tbtb EEEEI

bttbt EEEI cos22

After tip modulation and lock-in detection

Homodyne ASNOM

Aubert et al. (2003)

• Measurement includes subtle mix of both field intensity (1) or complex field amplitude (2)

• Small variation in sample leads to change in background field (Eb , ϕb)

• Determines which term dominates measurement

bttbt EEEI cos22

(1) (2)

Theory – Heterodyne ASNOM

trrt

brrb

tbtb

t

r

b

rtbrtb

tEE

tEE

EE

E

E

E

EEEEEEI

cos2

cos2

cos2

2

2

2

***

(1)

(2)

(3)

(4)

(5)

(6)

• (1 , 2) – Not time varying

• (3 , 4) – Time varying at tip modulation frequency

• (5) – Time varying at beat frequency (Δω). Used to align interferometer

Theory – Heterodyne ASNOM trrt tEEI cos2)6(

...

cos3cos

cos2cos

coscos

3

2

1

trrt

trrt

trrt

tEtEA

φφΔωtEΩtEA

tEtEA

With tip modulation

Ai = Fourier term weights

Ω = Tip modulation frequency

Pure amplitude (Et) and phase (ϕt) information can be extracted through lock-in detection

2

tnsfrequencie in lock

Experimental Setup

a) Reflection-mode backscattered heterodyne setup

b) Heterodyne setup for evanescent illumination of tip-sample through total internal reflection

Results - Nanowells• Reflection mode configuration used

• Nanowells fabricated using nanoimprint lithography method

• Well diameter = 500nm

• Well spacing = 800nm (center to center)

• Well depth = 450nm

SEM

AFM

ASNOM (Ω)

ASNOM (2Ω)

Results - Nanowells

ASNOM (2Ω – Δω) ASNOM (2Ω – Δω)

• Heterodyne measurements using p-polarized incident field shows improved contrast with no fringes (a)

• Contrast fades with s-polarized incident field (b)

Simulation - Nanowells• FDTD simulation run on nanowell array with same properties as experimental configuration

• Simulations used to calculate both magnitude (a,c) and normal component (b,d) of electric field at sample surface

• Calculations made using both p-polarized (a,b) and s-polarized (c,d) incident light• Results show strong normal components surrounding nanowells for p-polarized incident light, but not for s-polarized incident light

Results – Approach Curves• ASNOM experiments performed on evanescent waves generated in prism (n = 1.5) by total internal reflection

• Measurements made as function of distance between tip and surface

pdz

eEzE 0)(

Amplitude

pdz

eEzI2

20)(

Intensity

21

222 sin2 airp

p

nnd

Under current experimental configurations –- dp ≈ 144 nmIf true electric field amplitude is being measured by

amplitude channel of lock-in, approach curve should reveal correct value for dp

z = tip-sample distance

dp = penetration depth

Results – Approach Curves

(a) Ω – Δω (b)

Ω

Heterodyne approach curve (a) gives correct penetration depth: dp ≈ 145 + 5 nm

Homodyne approach curve (b) gives incorrect penetration depth: dp ≈ 65 + 5 nm

• Homodyne measurement describes subtle mix of intensity and complex field amplitude

• Dominant value is dependant on sample surface

Results - Waveguide• ASNOM experiment was repeated with 1.55μm laser light launched into integrated waveguide instead of prism

• AFM tip scanned over top of waveguide scattering evanescent field generated from within the guide

AFM ASNOM (Ω)

ASNOM (Ω-Δω)

ASNOM (Ω-Δω)

• Homodyne measurement (b) results in convoluted mixture of both complex amplitude and intensity

• Heterodyne measurement shows true amplitude (c) and phase (d) of laser light

• Wavefront of guided field clearly visible

• Reference field enhances total optical power at photodetector improving SNR and allowing for use of GaAs photodiode instead of PMT

Conclusions

• Problems with homodyne ASNOM measurements were demonstrated

• Significant background suppression was achieved with heterodyne technique

• True amplitude and phase information detected with sub-wavelength resolution and improved SNR

• Heterodyne-homodyne comparison demonstrated on nanowells as well as integrated waveguide

Questions?