Near-field Optical Microscopy

Transcription

1 Near-field Optical Microscopy R. Fernandez, X. Wang, N. Li, K. Parker, and A. La Rosa Physics Department Portland State University Portland, Oregon Near-Field SPIE Optics Microscopy East 2005 Group PSU Near-Field Microscopy Group Boston, MA

2 Conventional (Far-Field ) Optical Microscopy Optical Microscope Robust and reliable High throughput Non-invasive Low Cost BUT Limited Lateral Resolution Near-Field SPIE Optics Microscopy East 2005 Group PSU Near-Field Microscopy Group Boston, MA

3 Conventional (Far( Far-FieldField ) Optical Microscopy L E N S Diffraction limited resolution /2 ~ 250 nm S A M P L E

4 Near-Field Scanning Optical Microscopy Surpassing the diffraction limited barrier PROBE (NSOM) Optical fiber Metal 50 nm

5 Surpassing the diffraction limited barrier Aperture smaller than /2 Near-Field Scanning Optical Microscopy (NSOM) 50 nm

6 50 nm Near-Field Scanning Optical Microscopy Surpassing the diffraction limited barrier Requirement: To scan near the surface 10 nm (NSOM) Scan direction

7 FOURIER OPTICS y x E (x,y,0) (field is known at z = 0 ) E(x,y,z) =? z 2 { E B } n c 2 2 t 2 2 { E B E (r,t) = E(r)e -t r =(x,y,z) = (x,z) } 0 E(x,0) [A f (0)]e i2f. x df x df y where f =(f x, f y ) n E z [A f ] + (2) 2 2 [A f ] = 0, where 2 = (n /) 2 -/ f / 2 E(x,z) [A f (z)]e i2f. x df x df y, E 0. z

8 e i k.r f k k 2 = (2n / ) 2 = const E(x,0) [A f (0) e i2 z ] e i2f. x df x df y / f / < R Propagating modes + [A f (0) e -2 z ] e i2f. x df x df y / f / > R Propagating modes region / f / < R Evanescent modes region / f / > R Evanescent modes Where: 2 = (n / ) 2 -/ f / 2 and 2 = / f / 2 -(n /) 2 f y R R = n / 0 f x

9 Resolution and Finite Entrance Pupil k sample image lens focal plane

10 A : evanescent mode B : propagating mode x R1 =n 1/ o R2 =n 2/ o c B A z R1 R2

11 NSOM PROBE Fabrication by the heat-pulling method 1 mm 100 mm core 5 m cladding 10 m

12 0 PZT Plate X - Y 1.5 Translator Plate 1.75" Stepper motor Plate 0.9" X X - Y- Y 0.75" 0.5"

13 Need of probe-sample distance regulation Lateral scanning Scan direction Probe How to avoid crashing!!! Probe-sample distance How to automatically control the probe s vertical position? Near-Field SPIE Optics Microscopy East 2005 Group PSU Near-Field Microscopy Group Boston, MA

14 STRATEGY Probe is set to perform lateral oscillation (by an external device.) It is observed that the amplitude A of the lateral oscillations are damped when the probe s tip is in the proximity (~20 nm) to the sample s surface. Approach Curves Tip approaches the sample A(z) z Normalized A (z) Hydrophobic sample Hydrophilic sample S A M P L E (10) (20) (30) (40) Probe-sample distance Z (nm) Amplitude of lateral oscillations A decreases with distance z Near-Field SPIE Optics Microscopy East 2005 Group PSU Near-Field Microscopy Group Boston, MA

15 Damping contamination layer (hypothesis) Contamination Layer 20 nm TF V d A S A M P L E z Shear-force mechanism Hypothesis: The damping effect of the contamination layer should produce a distance dependence on the probe s amplitude of oscillations. That is: A = A(z) Near-Field SPIE Optics Microscopy East 2005 Group PSU Near-Field Microscopy Group Boston, MA

16 Shear-force detection Electrical implementation V z I Current amplifier Lock-in #1 Shear force signal V Signal generator TF GND Ref Substrate XY scanner

17 Contamination layer Optical implementation Dithering piezo Contamination layer S A M P L E Shear-force detection a = a o cos(f R t) A (z) Z Approaching Curve A(z) Amplitude (a.u.) signal generator Magnitude Phase Tip-sample distance ( nm ) reference Lock-in amplifier Phase ( degrees ) signal Z Amplitude of oscillations A = a Q Sample dither piezo laser beam diffraction pattern photodetector

18 Implementing probe-sample distance regulation in NSOM TF A V d Sample Feedback Circuit Set point Y Piezoelectric Correcting V z voltage Voltage divider X Near-Field SPIE Optics Microscopy East 2005 Group PSU Near-Field Microscopy Group Boston, MA

19 Experimental setup: Freq Resolved Contrast Photodiode IR lens lock-in Silicon sample IR reference NSOM probe IR = 1.15 m vis Infrared LASER (CW) Amplitude modulated light Acousto modulator LASER vis =512 nm Near-Field SPIE Optics Microscopy East 2005 Group PSU Near-Field Microscopy Group Boston, MA

20 exp(-t /) T i m e : carrier lifetime AS A FUNCTION OF TIME At low frequencies: f < (1 / ) ON OFF Silicon wafer IR (cw) (x,y) full signal f Amplitude modulated ) Transmitted infrared signal At high frequencies: f > (1 / ) AS A FUNCTION OF FREQUENCY ON OFF f th time Threshold Frequency f th smaller signal Modulation frequency

21 Frequency Resolved Contrast Charge Carrier Dynamics in Silicon 100 Hz 1 KHz 20 KHz Optical excitation frequency Near-Field images taken over the same region (7.5 m 2 ) To be compared with standard (far field) techniques: Resolution > 1 mm Near-Field SPIE Optics Microscopy East 2005 Group PSU Near-Field Microscopy Group Boston, MA

22 Carrier Dynamics in Silicon Near-field measurement -NSOM Far-field measurement Laser/Microwave Lifettech-88, Semitex Co. 100 Hz 1 KHz Optical excitation frequency For a threshold frequency of f th ~ 100 Hz: = 1 / f th ) ~ 1.5 ms! Near-Field SPIE Optics Microscopy East 2005 Group PSU Near-Field Microscopy Group Boston, MA