New Modalities and Opportunities New Modalities and Opportunities with Optical Spectroscopy and with Optical Spectroscopy and

MicroscopyMicroscopy Jung Y. Huang Jung Y. Huang 黃中垚黃中垚

Department of Photonics, Chiao Tung University

Hsinchu, Taiwan

http://www.jyhuang.idv.tw July 6, 2007

Optical spectroscopy discloses the electronic structure associated to a material, while microscopy reveals its real-space configuration. This talk presents an overview on modern optical spectroscopy and microscopy to elicit the ideas useful for the development of photonic science. Sum-frequency vibrational spectroscopy and multi-dimensional FTIR are selected as the illustrating examples to reveal the characteristics and unique opportunity to be bringing out. For optical microscopy, emphasis is focused on the possibility and principles that allow optical microscopy to be employed to probe into the nano world with light.

Current scientific research throughout the Current scientific research throughout the natural sciences aims at the exploration of the natural sciences aims at the exploration of the collectivity of structures with dimensions between collectivity of structures with dimensions between 1 and 100nm (1 and 100nm (建構奈米組件建構奈米組件 ).).

There is a strong demand for technologies There is a strong demand for technologies offering access to these dimensions, for offering access to these dimensions, for structuring (structuring ( 製造製造 )), , manipulating (manipulating ( 操控操控 )), or , or measuring (measuring ( 量測量測 )) at high resolution. at high resolution.

SPATIAL RESOLUTION VS. CHEMICAL SPATIAL RESOLUTION VS. CHEMICAL INFORMATIONINFORMATION

Rough estimates of the typical timescales Rough estimates of the typical timescales associated to the energies involved in associated to the energies involved in molecular systemsmolecular systems

Real-Space Configuration, Material Property Real-Space Configuration, Material Property (Electronic Structure ), and Structural Dynamics(Electronic Structure ), and Structural Dynamics

Vibrational Vibrational SpectroscopSpectroscopyy

Unique finger-printing capability of vibrational spectroscopy : highly localizedwell characterized by theory

Sum-frequency vibrational spectroscopy can be employed to reveal the interfacial molecular structure.

Material properties are strongly affected by the structure and Material properties are strongly affected by the structure and type of species on surface or at interfacetype of species on surface or at interface

Smart SurfaceSmart Surface

SFG:SFG: (2)eff = (2)

eff(bulk) + (2)s(surface)

In a medium with an inversion symmetry:

(2)eff(bulk) = 0, (2)

s (surface) 0

Sum-frequency vibrational spectroscopy (SFVS)Sum-frequency vibrational spectroscopy (SFVS)

Resonance can be employed to yield sensitivity to molecular species.

Apparatus of sum-frequency vibrational Apparatus of sum-frequency vibrational spectroscopy (SFVS)---Laser Systemspectroscopy (SFVS)---Laser System

Apparatus of sum-frequency vibrational spectroscopyApparatus of sum-frequency vibrational spectroscopy

Sum-frequency vibrational spectroscopy of a LPUV-Sum-frequency vibrational spectroscopy of a LPUV-defined aligning layer for liquid crystal molecules defined aligning layer for liquid crystal molecules

(2) 0 (2) 0 (2) 0{2 ( 0 ) [ ( 0 ) ( 90 )] 1}

0.01

zxx zxx zyyQ

LCP on a LPUV-defined LCP on a LPUV-defined alignment layeralignment layer

Q Q (1515cm-1)=0.46(1515cm-1)=0.46

LCP StructureLCP Structure

Improving LCP Alignment on a LPUV-defined Improving LCP Alignment on a LPUV-defined SurfaceSurface

Improving LC Alignment with a LCP Coupling Layer Improving LC Alignment with a LCP Coupling Layer on a LPUV-defined Surface on a LPUV-defined Surface

Surface interactions can be used to unwind the spontaneous helix, which , which yields a uniform FLC alignment withyields a uniform FLC alignment with Fast ResponseFast Response

BistabilityBistabilityWide Viewing AngleWide Viewing Angle

Tracking correlated motion of molecular fragments of LC materials: SSFLC and SSFLC and ncnc-ZnO doped SSFLC-ZnO doped SSFLC

FTIR Study of the Field-Induced FLC SwitchingFTIR Study of the Field-Induced FLC Switching

ΦΦ

Data Representation of Data Representation of 2D IR2D IR

Asynchron. plotSynchron. plot

2D IR Revealing Site Effect of Atomic Group 2D IR Revealing Site Effect of Atomic Group Attached to Different Location on a MoleculeAttached to Different Location on a Molecule

OO

O

O

O

CH3

CH3

CN

CH2

Synchron. plot

Asynchron. plot

Time-resolved FTIR for Time-resolved FTIR for Snapshot of Snapshot of Molecular DynamicsMolecular Dynamics

2D IR Snapshots of Molecular 2D IR Snapshots of Molecular DynamicsDynamics

Some real issues for optical microscopy at far field:Some real issues for optical microscopy at far field:

1. Increased transverse resolution

Rayleigh criterion Δr = λ / (2NA)

NA = numerical aperture = n sin θ

2. Increased longitudinal resolution

Rayleigh criterion Δz = 2 λ / (NA)2 (longitudinal

resolution typically lower than transverse)

3. Ability to image through scattering medium

Scattering leads to loss of contrast

Scattering gets worse at shorter wavelengths

New Modalities in Optical MicroscopyNew Modalities in Optical Microscopy

Current Methods for Increasing Spatial ResolutionCurrent Methods for Increasing Spatial Resolution

Microscope types:Microscope types:Widefield and Confocal

The best resolution that can be obtained by diffraction-limited (200 nm) optical techniques is coarser than the molecular level by two orders of magnitude (2 nm).

Twofold improvements in resolution (approximately 100 nm) can be obtained in either confocal (4Pi) or widefield (I5M) technologies.

Super resolution beyond this resolution enhancement has been demonstrated using either saturation absorption coupled with structured illumination or stimulated emission depletion (STED).

Current StatusCurrent Status

Nano-Optics Nano-Optics is the study of optical phenomena is the study of optical phenomena and techniques beyond the diffraction limitand techniques beyond the diffraction limit

NLO and Superresolution: Saturated Structured-Illumination Microscopy (SSIM)

• A structured light interacts with fine patterns in the sample and creates a moiré effect. The fine patterns that were previously below the Abbe-Rayleigh limit can now be visualized as a moiré version.

See: Mats G. L. Gustafsson, PNAS 102, 13081–13086 (2005)

ObjectStructured Light

Illuminated Object

Things Are Even Better by using Saturated Absorption (SSIM) Response of a

saturable absorber to a sine-wave intensity modulation

Here is what is happening in k-space

Typical Laboratory Result of SSIM

A field of 50-nm fluorescent beads: (a) imaged by conventional microscopy, (b) linear structured illumination, and (c) saturated structured illumination using illumination pulses with 5.3 mJ/cm2 energy density.

Mats G. L. Gustafsson, PNAS 102, 13081–13086 (2005)

NLO and Superresolution:NLO and Superresolution:Stimulated Emission Depletion (STED) Microscopy

Axial and transverse resolution better than 50 nm.Hell, Dyba, and Jakobs, Current Opinion in Neurobiology, 14:599, 2004.

STED Principle: an initial excitation pulse is focused on a spot. The spot is narrowed by a second, donut-shaped pulse that prompts all excited

fluorophores to STED. This leaves only the hole of the donut in an excited state, and only this narrow hole is detected as an emitted fluorescence. The light doing the turning off is diffraction limited, and so it cannot provide

any greater resolution alone. The trick is the saturated depletion, which helps to squeeze the spot down to a very small scale—in principle

infinitely.

The Abbe-Rayleigh Criteria Becomes:

Typical Laboratory Result of STED

Imaging neurofilaments in human neuroblastoma. (left) Sub region of the confocal image after linear deconvolution (LD); (right) the deconvolved STED image to reveal object structures that are below 30 nm.

The principle of PALM: A sparse subset of fluorescent

molecules attached to proteins of interest are activated with a brief laser pulse at =0.405 m and then imaged at =0.561 m. This process is repeated many times until the population of inactivated, unbleached molecules is depleted.

The location of each molecule is determined by fitting the expected PSF to the actual molecular image. Repeating with all molecules across all frames and summing the results yields a superresolution image.

Photoactivated Localization Microscopy (PALM)See: Eric Betzig, et al., SCIENCE 313, 1642 (2006)

Typical Result of PALMTypical Result of PALM

• PALM image of dEosFP-tagged cytochrome-c oxidase localized

within the matrix of mitochondria in a COS-7 cell is compared

to its corresponding TEM image.

Eric Betzig, et al., SCIENCE 313, 1642 (2006)

Probing into the nanoworld with femtosecond Probing into the nanoworld with femtosecond resolutionresolution

Heterodyne Interferometric SNOM

Lensed-fiber launched optical waveguide device under SNOM

Probing into the nanoworld with femtosecond Probing into the nanoworld with femtosecond resolutionresolution

Verify the distributions of the amplitude and phase of an optical field at nanometer scale by combining SNOM and heterodyne fiber interferometry

Signal intensities Is 110-12 W 1107 photons/sec are below the

noise floor of photodiode detectors. By interfering this signal with Iref

110-4 W , however, the signal at the detector is boosted to Is

110-8 W , which is well within the detection limits of photo detectors.

FFT of the complex field corresponds to a projection in a basis of plane waves

The spatial frequencies in the FFT spectrum are related to the propagation constants of the optical guided modes.

Topography

S

( ) [ ( )]zF k FFT S Cos

( )Cos

Tracking optical-field propagation in nanoworld

N=38

(a) Triple-Line-Defect

1

10

1600 1620 1640 1660 1680 1700

Wavelength (nm)

Ridge WG30

N=38

Tra

nsm

itta

nce

(ar

. un

.)

20%

Triple line defects 1mTriple line defects

GaAs

AlO

SiO2

Triple-Line Waveguide (provided by Prof. S. Y. Lin, RPI)

Nano-Optics Nano-Optics is the is the study of optical study of optical phenomena and phenomena and techniques beyond the techniques beyond the diffraction limitdiffraction limit

ConclusionConclusionss

Molecular vibrational spectroscopy is an effective technique to yield useful information about molecular structures and alignment.

New imaging modalities in optical microscopy have been developed to allow researchers probing into nano scale at the molecular level .

There are essentially no fundamental limit on how far we can go beyond the Abbe’s diffraction limit.