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Nano Scale Optics with Nearfield Scanning Optical Microscopy (NSOM)
November 2, 2006 2ECE 695SNanoscale Optics with NSOM
Presentation Overview
Motivation for nearfield opticsIntroduction to NSOMWhat is NSOM today? What can you do with NSOM?
November 2, 2006 3ECE 695SNanoscale Optics with NSOM
Diffraction imposes a natural limit to available ‘resolution’with conventional optics.
‘Best’ microscopes can obtain ~ 250nm resolution: confocal microscopy
Oscillating Dipole:
Motivation of NFO“And God said, `Let there be light,' and there was
light. God saw that the light was good, and he separated
the light from the darkness.” Genesis 1:3,4
To probe the ‘nearfield’ – require to be within 100 nm of objectThe nearfield scanning optical microscope was developed as the device to do this.
3 2
1 2 2 cos ( )4
ikrr z
r
ikE p t er r
θπε
= −
2
3 2
1 1 sin ( )4
ikrz
r
ik kE p t er r rθ θ
πε
= − −
November 2, 2006 4ECE 695SNanoscale Optics with NSOM
Presentation Overview
Motivation for nearfield opticsIntroduction to NSOM What is NSOM today? What can you do with NSOM?
November 2, 2006 5ECE 695SNanoscale Optics with NSOM
NSOM Instrumentation / Idea
http://www.physics.ncsu.edu/optics/nsom/NSOMintro.html Nanonics Imaging Ltd
November 2, 2006 6ECE 695SNanoscale Optics with NSOM
Conception of an instrumentE.H. Synge, "A suggested method for extending the microscopic resolution into the ultramicroscopic region," Phil. Mag. 6, 356 (1928). E.H. Synge, "An application of piezoelectricity to microscopy," Phil. Mag. 13, 297 (1932).
Proof of Concept with microwavesE.A. Ash and G Nichols, Nature 237, 510 (1972).
Experiments in the visibleD.W. Pohl, W. Denk, and M. Lanz, APL 44, 651-3 (1984).A. Lewis, M. Isaacson, A. Harootunian, and A. Murray, Ultramicroscopy 13, 227 (1984).
History of NSOM
November 2, 2006 7ECE 695SNanoscale Optics with NSOM
required fabrication of a 10-nanometer aperture (much smaller than the light wavelength) in an opaque screen A specimen was scanned very close to the aperture As long as the specimen remained within a distance less than the aperture diameter, an image with a resolution of 10 nanometers could be generated. Technical difficulties to overcome:
fabricating the minute apertureachieving a sufficiently intense light sourcespecimen positioning at the nanometer scalemaintaining the aperture in close proximity to the specimen
Synge’s original proposal
November 2, 2006 8ECE 695SNanoscale Optics with NSOM
Implementation in Microwave region
Microwaves with a wavelength of 3 cmAperture diameter of 1.5 mmProbe scanned over a metal grating with periodic line features0.5 mm gaps and lines were resolvableSubwavelength resolution of ~ 1/16 lambda
http://www.olympusmicro.com/primer/techniques/nearfield/nearfieldintro.html
November 2, 2006 9ECE 695SNanoscale Optics with NSOM
The IBM researchers employed a metal-coated quartz crystal probe on which an aperture was fabricated at the tip, and designated the technique scanning near-field optical microscopy (SNOM). The Cornell group used electron-beam lithography to create apertures, smaller than 50 nanometers, in silicon and metal. The IBM team was able to claim the highest optical resolution of 25 nanometers, or one-twentieth of the 488-nanometer radiation wavelength, utilizing a test specimen consisting of a fine metal line grating.
Optical Implementation
November 2, 2006 10ECE 695SNanoscale Optics with NSOM
Illumination by the tip is probably the easiest to interpret, and gives the most signal. It requires a transparent sample, so can’t always be used.Reflection modes give less light, and are more dependent on the details of the probe tip, but allow one to study opaque samples.The illumination/collection mode provides a complement to the reflection modes, but the signal contains a large background from the light reflected without ever making it out of the probe tip. (This is removed by oscillating the tip vertically and measuring the signal at that frequency.)
NSOM modes of Operation
http://www.physics.ncsu.edu/optics/nsom/NSOMintro.html
November 2, 2006 11ECE 695SNanoscale Optics with NSOM
Apertureless NSOMPSTM
(Photon Scanning Tunneling Microscopy)
NSOM modes of Operation
November 2, 2006 12ECE 695SNanoscale Optics with NSOM
Comparison to far-field microscopy
Two fundamental differences between near-field and far-field (conventional) optical microscopy:
In NF microscopy the NF interaction region at a single instant is much smaller than the FF interaction area for conventional microscopyNF microscopy has a subwavelengthdistance between the radiation source and the sample while FF microscopy has a much larger distance
November 2, 2006 13ECE 695SNanoscale Optics with NSOM
Practically zero working distance and an extremely small depth of field.Extremely long scan times for high resolution images or large specimen areas.Very low transmissivity of apertures smaller than the incident light wavelength.Only features at the surface of specimens can be studied.Fiber optic probes are somewhat problematic for imaging soft materials due to their high spring constants, especially in shear-force mode
Limitations of near-field optical microsopy
November 2, 2006 14ECE 695SNanoscale Optics with NSOM
Presentation Overview
Motivation for nearfield opticsIntroduction to NSOMWhat is NSOM today? What can you do with NSOM?
November 2, 2006 15ECE 695SNanoscale Optics with NSOM
NSOM Today: Similar to Standard Scanning Probe Microscopy with an Optical Channel
MechanicalTranslation stage, piezoelectric scannerFeedback Control (Z distance)Anti-vibration optical table
ElectricalScanning drivers for piezoelectric scannerz distance control Amplifiers, Signal processors Software and Computer
OpticalLight source (lasers), Fiber, Mirrors, Lenses, ObjectivesPhoton detectors (Photon Multiplier Tube, Avalanche Photo Diode, Charge Coupled Devices)Probe to provide the window to the near-field
November 2, 2006 16ECE 695SNanoscale Optics with NSOM
Z Distance Control
Shear force
Normal forceScience, 257, 189-195 (1992).
Nanonics Imaging Ltd Witec GmbHVeeco Instruments
November 2, 2006 17ECE 695SNanoscale Optics with NSOM
NSOM Probes
Aperture typeTaped fiber, multiple-taped fiberCantilevered AFM/NSOM tips (Si3N4, SiO2)Other kinds, such as tetrahedral tip, Fluorescent tipMetal Coating Optional
Apertureless typeDielectrics, semiconductors or metalsOther kinds, such as nano-particle attached tip
Nanonics Imaging LtdWitec GmbH
November 2, 2006 18ECE 695SNanoscale Optics with NSOM
Probe Fabrication: Fiber probes
http://www.olympusmicro.com/primer/techniques/nearfield/nearfieldprobes.html
Lazarev, A., Rev. Sci. Inst., 74, 3679-3688 (2003).
November 2, 2006 19ECE 695SNanoscale Optics with NSOM
Probe Fabrication: Cantilevered probes
MircofabricationRubbing Focus ion beam millingSpecial techniques, such as solid electrolysis, self-terminated corrosion
November 2, 2006 20ECE 695SNanoscale Optics with NSOM
Mirco-fabrication: a batch process
App. Phy. Lett., 68, 3531-3533 (1996).
November 2, 2006 21ECE 695SNanoscale Optics with NSOM
Commercial NSOM:Nanonics MultiView 2000
Nanonics Imaging Ltd
APD
Laser
Probe
Scanner
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Commercial NSOM: Witec ALphaSNOM
Witec GmbHProbe Mounting
Aperture
Cantilevered probe
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Commercial NSOM: Veeco Aurora3
Veeco InstrumentsFeedback control
Fiber mounting
System schematic
November 2, 2006 24ECE 695SNanoscale Optics with NSOM
Nanonics System at Purdue
Nanonics MultiView 2000NSOM / AFMTuning Fork Feedback Control
Normal or Shear ForceAperture tips down to 50 nmAFM tips down to 30 nmRadiation Sources:
(532, 633, 785) nmSupercontinuum 480-1500+ nm
Nanonics Imaging Ltd
November 2, 2006 25ECE 695SNanoscale Optics with NSOM
APD
APD
Computer
HV Source
Lock-In Detector
Freq. Gen.
532nm Source
PID Controller
To Piezoelectrics
Filter
Filter
Obj. Lens
Obj. LensUpper Piezo
Lower Piezo
Fiber Tip
Tuning Fork
Prof. Shalaev’s group Internal to Purdue
November 2, 2006 26ECE 695SNanoscale Optics with NSOM
Purdue Homemade NSOM Developed from Commercial AFM
System Setup
User Interface
Scanning: micro stages, piezoelectic scanner and probe headImaging: optics with video monitor, white source and HeNe laser
Light detection: Photomultiplier tube (PMT), photon counterSignal Processing: AutoProbe Electronic Module (AEM), Proscan software
Prof. Xianfan Xu’s group Internal to Purdue
November 2, 2006 27ECE 695SNanoscale Optics with NSOM
Schematic of Purdue Home-made NSOM
Scanner
HeNe Laser
ProbeHead
VideoMonitor
WhiteSource
PMT
CCD
Photon Counter AEM
Computer
PSD
Diode20×Objective
Prism
BS
BS
AFM/NSOM Probe
Z scanner
X,Y scanner
Sample
Laser
Prof. Xianfan Xu’s group Internal to Purdue
November 2, 2006 28ECE 695SNanoscale Optics with NSOM
Presentation Overview
Motivation for nearfield opticsIntroduction to NSOMWhat is NSOM today? What can you do with NSOM?
November 2, 2006 29ECE 695SNanoscale Optics with NSOM
What Can you do with NSOM?
Ultrahigh resolution OPTICAL Imaging / Plamonic studiesSpectroscopy
Nearfield Surface Enhanced Raman SpectroscopyLocal Spectroscopy of Semiconductor DevicesNear-field Broadband Spectroscopy
Modification of SurfacesSubwavelength photolithographyUltra High Density data storageLaser Ablation
Nearfield femtosecond studies
November 2, 2006 30ECE 695SNanoscale Optics with NSOM
High Resolution Optical Imaging
SEM
AFM
NSOM
APD
Obj. Lens
83 nm Optical resolution seen with “100” nm tip
Prof. Shalaev’s group Internal to Purdue
November 2, 2006 31ECE 695SNanoscale Optics with NSOM
NF Surface Enhanced Raman Spectroscopy (SERS)
What is Raman spectroscopy?Incident light scattered by a molecule will scatter at different wavelengths than the incident light. Measuring the spectrum of such scattered light provides powerful a “fingerprinting” technique for identification.
What is surface enhanced Raman spectroscopy (SERS)?Typical Raman signals are rather weak. The electromagnetic fields carrying the fingerprinting information can be enhanced by many orders of magnitude.
Add in NSOM:The addition of nearfield optics to such techniques enables localization of illumination or collection of light. This provides the means to detect trace amounts that would otherwise be undetectable.
November 2, 2006 32ECE 695SNanoscale Optics with NSOM
Nearfield SERS of R6-G on a semi continuous metal film
SpectraSpacing:470 nm
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#6 #8
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AFM
ReflectionNSOM
Spectrometer
Prof. Shalaev’s group Internal to Purdue
November 2, 2006 33ECE 695SNanoscale Optics with NSOM
NF Spectroscopy:Quantum Device
Science 264, 1740 (1994)
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Spectrometer
YAG 1064 nm
Photonic Crystal Fibre with broadband output
Tapered Fibre Probe
Objective Lens 100x 0.75NA
Si CCD
Sample Scanningxyz piezo stage
Tuning Fork (for feedback control)
NF Spectroscopy: Broadband
Prof. Shalaev’s group Internal to Purdue
November 2, 2006 35ECE 695SNanoscale Optics with NSOM
Representative AFM
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NF Broadband Spectroscopy Local Transmittance
633 SMFprobe with ‘red’ Filter
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Laser Phys. Lett. 3, No. 1, 49–55 (2006)
Optics Letters, Vol.30, No.24, 3356 (2005)
Mapping Function:
T (x,y,λ) = I(x,y,λ)/Io(λ)
Prof. Shalaev’s group Internal to Purdue
November 2, 2006 36ECE 695SNanoscale Optics with NSOM
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Prof. Shalaev’s group Internal to Purdue
November 2, 2006 37ECE 695SNanoscale Optics with NSOM
Subwavelength Photolithography
Use NSOM to write patterns into a photoresistCan use aperture NSOM in illumination modeCan use apertureless NSOM utilizing two photon absorption at the metallic tip
November 2, 2006 38ECE 695SNanoscale Optics with NSOM
Ultrahigh Density Data Storage
Can use aperture NSOM in illumination mode to both write and read data on magneto-optical materialWhen writing, probe heats the medium under the probe. The domain under the write beam becomes magnetized opposite of the surrounding material.This varying domain of magnetization can then be read using the same probe, but with a lower output power.
Appl. Phys. Lett. 61, 142-144 (1992)
November 2, 2006 39ECE 695SNanoscale Optics with NSOM
Nearfield femtosecond studies
Pump – Probe techniquesNSOM allows this technique to employ spatial localization to measure carrier dynamics. (eg – in semiconductor devices)
Localized laser ablationCurrently in use for photomaskrepairNanolithography
Rev. Sci. Inst. 70, 2758 (1999)
J. Microscopy 194, 537 (1999)
November 2, 2006 40ECE 695SNanoscale Optics with NSOM
Other things done with NSOM
Fluorescent spectrum studies of biological cells, molecules and DNAAtom trapping and ManipulationLocal probing of luminescent devices Low temperature NSOMSurface plasmon / Local field enhancement
November 2, 2006 41ECE 695SNanoscale Optics with NSOM
Recap – Nearfield Scanning Optical Microscopy (NSOM)
NSOM is a relatively new technology that defeats the diffraction limit for optical measurements. Utilizes the near field portion of electromagnetism to window down to ~ 10 nm spatial resolution.NSOM instrumentation has progressively developed over the past 15 years, to the point where several different basic techniques are available as commercial instrumentsThe principles of NSOM are very much in play within the research community. It has many different applications ranging from biological studies to nanofabrication.Up to Date Near-Field Optics Research
NSOM Conference: NFO9 http://www.nfo9.org/September 2006, look for conference proceedings in the Journal of Microscopy, 2007
Quick and Easy ResourcesReview Articles:
R.C. Dunn. Chemical Reviews, 99 (1999) 2891.B. Hecht, B. Sick, U.P. Wild, V. Deckert, R. Zenobi, O.J.F. Martin, and D.W. Pohl. Journal of Chemical Physics, 112 (2000) 7761.
Website: Olympus Microscopy (nearfield section)http://www.olympusmicro.com/primer/techniques/nearfield/nearfieldhome.html