Apertureless Scanning Near-field Optical Microscopy: a comparison between homodyne and heterodyne approaches Journal Club Presentation – March 26 th, 2007 Suraj Bramhavar Lewis Gomez et al., J. Opt. Soc. Am. B, Vol. 23, No. 5, (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

E b = Background light scattered from sample E t = Light elastically scattered by near-field interaction of tip and evanescent field from sample Theory – Homodyne ASNOM 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 (E b, ϕ b ) Determines which term dominates measurement (1)(2)

Theory – Heterodyne ASNOM (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 With tip modulation A i = Fourier term weights Ω = Tip modulation frequency Pure amplitude (E t ) and phase (ϕ t ) information can be extracted through lock-in detection

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 AFMASNOM (Ω) ASNOM (2Ω)

Results - Nanowells 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 AmplitudeIntensity Under current experimental configurations –- d p ≈ 144 nm If true electric field amplitude is being measured by amplitude channel of lock-in, approach curve should reveal correct value for d p z = tip-sample distance d p = penetration depth

Results – Approach Curves (a) Ω – Δω (b) Ω Heterodyne approach curve (a) gives correct penetration depth: d p ≈ nm Homodyne approach curve (b) gives incorrect penetration depth: d p ≈ 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 AFMASNOM (Ω) 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?