PROBING SOFT MATTER WITH OPTICAL TECHNIQUES

Transcription

1 PROBING SOFT MATTER WITH OPTICAL TECHNIQUES Irena Drevenšek Olenik Faculty of Mathematics and Physics, University of Ljubljana and J. Stefan Institute, Ljubljana, Slovenia

2 OUTLINE 1) Introduction 2) Some examples of techniques+investigations Polarization optical microscopy (POM) investigations of bulk liquid crystals (LCs), polymer-lc composites, cellulose fibers Dynamic light scattering (DLS) investigations of confined LC structures Optical sum-frequency generation (SFG) surface structures of guanosine derivatives 3) Conclusions

3 RESPONSE OF MATTER TO LIGHT D = ε 0 E + P Reflection and refraction: Optical imaging (microscopy), refractometry, ellipsometry, conoscopy, etc. Absorption: UV-VIS absorption spectroscopy, IR spectroscopy, linear and circular dicroism (CD), etc. Diffraction and scattering: Fraunhoffer and Fresnel diffraction, static and dynamic light scattering (Rayleigh scattering, Raman scattering, Brillouin scattering, etc.), speckle interferometry, etc. Fluorescence: Fluorescence imaging, fluorescence spectroscopy, fluoresence resonance energy transfer (FRET), etc. Non-linear response: Optical harmonic generation, optical parametric processes, self-phase modulation, selffocusing, optical wave-mixing, etc.

4 POLARIZATION OPTICAL MICROSCOPY (POM) Polarization optical microscopy (POM) image of a nematic liquid crystal.

5 POLARIZATION OPTICAL MICROSCOPY VERSUS USUAL MICROSCOPY POM is: contrast-enhancing extension of conventional optical microscopy, which is based on the use of polarized light aimed at investigations of optically anisotropic materials (minerals, ceramics, polymers, liquid crystalline compounds, and various biological assemblies) provides a possibility for qualitative as well as quantitative analysis of the structural anisotropy of the observed specimens Simple polarization optical microscope Special components: A: 1 st polarizing filter (Polarizer) B: 2 nd polarizing filter (Analyzer) C: Retardation plate (optional) Strain free objectives&condensors are needed (P, Pol)!!

6 EXAMPLE OF CONTRAST ENHANCEMENT Stripes of transparent adhesive tape on glass plate (no polarizers) Inserted between crossed polarizers. Inserted between parallel polarizers.

7 OPTICAL POLARIZATION + - B E Effect of polarizing filter ( polaroid ): Unpolarized light: Direction of E randomly changes in time and space. Linearly polarized light: Oscillation direction of E is constant. E Transmission configuration = perpendicular to polymer chains. (parallel to trasmission axis of the filter) (absorption is weak) E Extinction configuration = parallel to polymer chains (perpendicular to transmission axis of the filter) (absorption is strong)

8 OPTICALLY ISOTROPIC MATERIALS E Linearly polarized light E E A I out = 0 No light!!! I in P Optically isotropic material Unpolarized light If optically isotropic material (usual liquids, glass, NaCl salt crystal, etc...) is placed between two crossed polarizers, no light passes through the analyzer (material looks dark). D = ε 0 ε E = ε 0 E + P In optically isotropic medium dielectric response (described by ε) is independent of the direction of optical electric field. Consequently ε is a scalar property and refractive index n=(ε) 1/2 is independent of polarization properties of the light beam. Such a medium cannot modify polarization state of the optical beam.

9 OPTICALLY ANISOTROPIC MATERIALS Optical double refraction (birefringence) Monocrystal of calcite (hexagonal symmetry uniaxial ε) Unpolarized beam (n e -n o ) = = 0.17 E ordinary extraordinary ray ray n 1 n 2 Different refractive index for two orthogonal linear eigen-polarizations leads to different refraction angles of the associated beams. (called ordinary and extraordinary ray)

10 TRANFORMATION OF LIGHT POLARIZATION From linear to elliptical polarization: Elliptically polarized outgoing beam Phase difference attained between the two eigen-rays in birefringent materials: Df=(2pLn 1 /l)-(2pln 2 /l) causes transformation of linearly polarized light into elliptically polarized light. E L Optically birefringent medium Electric field vector E rotates (clock-wise or anti clock-wise) on an ellipse. Linearly polarized ingoing beam: Electric filed vector E oscillates along a fixed direction.

11 EFFECT ON POLARIZATION MICROSCOPY IMAGES Diascopic illumination E θ AP =90 o I out I in L A P Transformation from linear to elliptical polarization causes that part of the light intensity can escape through crossed polarizers! This is the light observed (detected) in polarization optical microscopy. The ratio (I out /I in ) depends on: orientation of the sample crystallographic axes with respect to polarization direction of the beam entering the sample phase difference φ attained in the sample between the two rays corresponding to two linear eigen-polarizations.

12 EFFECT OF SAMPLE ORIENTATION Example: Rotation of KH 2 PO 4 (KDP) monocrystal between crossed polarizers. Sample sides are parallel to crystallographic planes of the crystal. P A a) b) c) d) e) a, c, e: Polarization of light passing the polarizer (P) coincides with one of the eigenpolarizations of the medium for that reason polarization state is preserved and light cannot escape through analyzer (A). Therefore material looks black. b, d: Polarization of light passing the polarizer (P) does not coincide with one of the eigen-polarization of the medium so its polarization state is transformed from linear to elliptical and part of light escapes through analyzer. Material looks gray. Optical polarization microscopy can resolve orientation of crystallographic axes (studies of textures, grain structure and orientation, orientation of filaments, direction of internal stress, etc.)

13 COLOURS OF THE IMAGE Due to colour dispersion of optical refractive index, phase difference between the two eigenpolarization rays Df (also called retardation) induced by the medium depends also on wavelength of light λ, i.e. n 1 =f 1 (l), n 2 =f 2 (l), therefore Df=(2pLn 1 /l)-(2pln 2 /l) = Df(l)!! The ellipticity of the light emanating from the sample consequently depends on λ. Thin layer of liquid crystal Layers of adhesive tape Black regions: Eigenpolarizations coicide with directions of P&A. Because ellipticity determines, how much light can pass the analyzer, light components of different wavelengths emanate from the system with different intensities. Most vivid colours are observed for medium thick samples. For very thick samples variation of φ(λ) and the corresponding spectral modifications are so rapid that the resulting colour is gray. In very thin samples spectral modifications are too weak so they also look gray.

14 EXAMPLE 1 LIQUID CRYSTAL STRUCTURES heating cooling Solid (crystal) Liquid crystal (anisotropic) Usual liquid (isotropic) Example: pentyl-cianobiphenyl

15 INVESTIGATION OF PHASE TRANSITIONS in BULK LIQUID CRYSTALS Phase transition of 5CB from nematic to isotropic phase. P A During heating the material is transformed from optically anisotropic (nematic) phase to optically isotropic (usual liquid) phase. Due to the 1 st order type of transition, coexistance of both phases can be observed.

16 CONFINED LIQUID CRYSTALS POLYMER DISPERSED LIQUID CRYSTALS (PDLCs) light beam (UV) Photopolymerization of the prepolymer/lc mixture induces phase separation of the constituents. This process results in formation of liquid crystal droplets, embedded in a polymer matrix. PDLC HOLOGRAPHIC POLYMER DISPERSED LIQUID CRYSTALS (HPDLCs) Planes with LC droplets separated by planes of more or less pure polymer inhomogeneous phase separation SEM image

17 2D PHOTONIC CRYSTALS from HPDLCs Optical polarization microscopy image of the square lattice under crossed polarizers. Colored areas are LC-rich regions and dark areas are polymer-rich regions. The inset shows an overlay of the calculated intensity profile of the optical interference pattern ov UV light. Bright areas are regions of high intensity, in which polymer molecules accumulate. M. Fally, I. Drevensek-Olenik, M. A. Ellabban, P. K. Pranzas, J. Vollbrandt: Phys. Rev. Lett. 97, (2006).

18 TEMPERATURE INDUCED MODIFICATIONS 62.0 o C 68.5 o C 75.0 o C The magnified parts of the microscopy images correspond to the sample area of 40 x 40 µm 2. POM study reveals progressing of nematic-isotropic transition from LCpolymer interface (edge regions) towards central region of the LC domains. M Devetak, J Milavec, R A Rupp, B Yao, I Drevenšek-Olenik: J. Opt. A: Pure Appl. Opt. 11, (2009). J. Milavec, M. Devetak, J. Li, R.A. Rupp, B. Yao, I. Drevensek-Olenik: J. Opt. 12, (2010).

19 EXAMPLE 2 WETTING OF CELLULOSE FIBERS Art.:Vilmed, 84 g/m 2, water absorptivity 800 g/m 2 1 mm Dry fibers Fibers soaked with water

20 P A SORPTION KINETICS Nonexposed fibre Establishing contact with water. Plazma modified fibre

21 DYNAMIC LIGHT SCATTERING Droplets of nematic liquid crystal phase.

22 LIGHT SCATTERING WITH COHERENT SOURCE random refractive index variation n(r) Random diffraction pattern (speckle pattern ) Coherent radiation (e.g. laser) To observe speckle pattern, coherent illumination of the scattering medium is needed. All radiation is at least partially coherent. Longitudinal (temporal) coherence Longitudinal coherence length Transverse (spatial) coherence Transverse coherence length

23 EXPERIMENTAL RESTRICTIONS θ speckles θ=scattering angle To see speckle at 0 < θ < 2π, requires Laser light: Full spatial coherence ξ T = beam width Large longitudinal coherence: ξ T from 1 cm up to 1 km.

24 DYNAMIC LIGHT SCATTERING (DLS) Incident wave r 1 (t) r 1 (t+τ) r 2 (t) moving scattering objects produce temporal variations of local refractive index n=n(r,t). Consequently, intensity of specles fluctuates with time. Example: r 2 (t+τ) Dr= r 2 - r 1 Dr (t+τ)- Dr (t) ~ (l/sin(q /2)) relative phase coherence is lost Scattered waves q Count rate (khz) detector (specle size!!!) Brownian motion of macromolecules in solution t(ms)

25 PHOTON CORRELATION SPECTROSCOPY small size T c Autocorrelation function of scattered light intensity I at selected scattering angle θ (scattering wave vector q) is measured. G (2) (τ)= Operation is repeated for many different values of τ in the range 10-9 s < τ < 10 3 s (typical autocorrelator gives results for 256 values of τ).

26 CORRELATION FUNCTIONS Intensity correlation function G (2) (τ) Usually normalised function is measured. example of measured g (2) (t).

27 WHAT CAN BE INVESTIGATED by DLS? DLS detects fluctuations of refractive index of the medium: δn(r,t)= q,t)= δn(q,t)e iqr Laser H sample H V θ k r f k r i r q S = r k f r k i Maximum cross section for δn( n(q=q s ). g < Es (t')es (t' + t) > < E (t') >< E (t' + t) > Detector (I(t) E s (t) 2 ) measurement g (2) (t)=<i(t )I(t +t)>/<i> 2 =1+ α(g (1) (t)) 2 C e (t) = (1) ( t / τ l s s l Information on dynamic modes related to δn( n(q,t) on the time scale s. l ) β l S l

28 FLUCTUATIONS OF REFRACTIVE INDEX δn(r,t)=,t)= δn(q,t)e iqr The main challenge of DLS investigations is to deduce the origin of refractive index fluctuations δn(r,t) and to gain understanding on dynamic processes associated with them. Some phenomena, which can cause refractive index changes: thermaly induced density fluctuations of the medium translational and rotational motion of the scatterers mechanical stress/strain birefringence fluctuations...

29 ORIENTATIONAL FLUCTUATIONS OF LIQUID CRYSTALS (LCs) Liquid crystals: Exhibit strong optical birefringence. n(r) typical LC materials: Nematic director field n(r) is fluctuating by time this causes strong fluctuations of the local refractive index of the medium

30 ORIENTATIONAL FLUCTUATIONS and LIGHT SCATTERING Thermaly induced orientational fluctuations in a planarly aligned LC layer (D>>λ): n(r)=n 0 (r)+δn(r) δn(r)= )= δn(q)e iqr D are related to increase of the elastic deformation energy of the LC director field n(r): W d =(V/2) [ n 1 (q) (K 1 q 2 +K 3 q 2 )+ n 2 (q) (K 2 q 2 +K 3 q 2 ) ] q dw d /dn i =-γ i n i / t, i=1,2 kt Relaxation of the fluctuations : δn(q,t)=δn(q,0)e -t/ t/t 2 2 K i N Relaxation rate: (1/t ) (K/γ)q 2 kt n 0 1 q s 10-5 cm 2 /s

31 EFFECT OF CONFINEMENT ON FLUCTUATIONS A) Spherical droplets of radius R q min π/r, (1/t ) min (K/γ)R -2 q min π/d, (1/t ) min (K/γ)D -2 B) Thin planar layer of thickness D 1/τ q s =(q x,0,0) 1/τ? qr s For ellipsoidal droplets one expects a situation intermediate between A) and B) 1/τ q s =(0,0,q z ) q s D

32 TYPICAL EXAMPLE OF g (1) (t) FOR H-PDLCH sample VIS, Λ=0.78 µm 1.0 t slow =36 ms S slow = slow process t: ms S: g (1) (t) t=0.27 ms 0.8 S= fast process t: ms S > t (ms) Fit: g (1) (t)=a+b exp((-t/τ) S )+B slow exp( (-t/τ slow ) Sslow ) Two different orientational relaxation processes are detected

33 SLOW RELAXATION DIFFUSION OF THE AVERAGE LC DROPLET ORIENTATION <n(r)>. < H(V)V scattering, θ=20 o VIS Λ=0.78 µm UV-2B Λ=0.78 µm - Sensitive to imperfections of the LCpolymer interface and to interpore orientational coupling. g 1 (t) t (ms) (Quasi)periodic network results in band structure of the modes. M. Avsec, I. Drevensek-Olenik, A. Mertelj, S. Gorkhali, G. P. Crawford, M. Copic: Phys. Rev. Lett. 98, (2007).

34 FAST RELAXATION decay of the normal modes of nematic director field δn(q,t) θ=20 o θ=120 o - Signal from intrapore orientational fluctuations. g (1) (t) t (ms) - Dispersion is observed at large scattering angles relaxation time decreases with increasing scattering angle θ.

35 DISPERSION OF THE FAST MODE (sample( L=0.8 mm) y 1/τ f (khz) 1/τ f (khz) 14 q 12 s K g q 12 s II K g q y,min q z,min 0 0,0 5,0x10 6 1,0x10 7 1,5x10 7 2,0x10 7 s q (m -1 ) VIS SEM 1 mm q i,min» (p/ d i ) z d z» 250 nm d y» 600 nm Analysis of dispersion data reveals size and shape of the LC domains. 1) I. Drevensek-Olenik, M. E. Sousa, A. K. Fontecchio, G. P. Crawford, M. Copic: Phys. Rev. E, 69, (2004). 2) Dynamic processes in confined liquid crystals, M. Vilfan, I. Drevenšek Olenik, M. Copic: in "Time-resolved Spectroscopy in Complex Liquids - An Experimental Perspective", edited by R. Torre, p (Springer 2008).

36 OPTICAL SUM FREQUENCY GENERATION (SFG) Surface film of lipophilic guanosine derivative with 3 decanoyl tails on mica.

37 SECOND-ORDER ORDER OPTICAL NONLINEAR PROCESSES iω t i 1t i 2t i t ( E r e E r e ) ( E r e E r e ) 1 * ω 1 ω * ( ) ( ) ( ) ( ) ω E( r, t) = (1) P = ε 0χ E + ε 0χ (2) EE χ (1) w 1 w 2 Non-linear response χ (2) V 2w 1 2w 2 w 1 + w 2 w 1 - w 2 SHG SHG Sum frequency generation (SFG) Difference frequency generation (DFG) w = 0, Optical rectification (static voltage appears) Conversion ratio to a specific NLO frequency depends on frequency dependence of χ (2) and on interference phenomena (phase matching condition).

38 IR-Vis Sum frequency generation (SFG) The sample is illuminated with an infrared (IR) ω 1 beam, which is tuned in resonance with a selected molecular vibrational state (tunable source of fs IR radiation in the range 2-10 µm ( cm -1 )) and a visible (VIS) ω 2 beam of a nonresonant frequency. The SFG signal generated at ω 3 =ω 1 +ω 2 consequently appears in the VIS range. This is important because it is much easier to detect weak optical signals in the VIS than in the IR range (there is no photomultipliers for IR light!). SFG IR VIS SFG VIS IR Collinear experimental geometry Non-collinear experimental geometry χ (2) ( ω Macroscopic SFG susceptibility : 3, ω 2, ω 1 ) = ( ω RES A ω SFG is an intrisically surface sensitive analogue to the Raman+IR spectroscopy. RES 1 + A ) + iγ NR e iφ

39 IR-Vis SFG setup Arrangement for frequency-domain measurements (SFG spectroscopy) Part of setup with Langmuir film as a sample.

40 FREQUENCY DOMAIN MEASUREMENTS BY TEMPORALLY EXTENDED fs PULSES The VIS pulses are extended by spectral filtering. SFG ps fs The SFG signal is detected by spectrometer. For a selected IR wavelength the full spectral range of the SFG is around 200 cm -1, so several vibrational lines can be monitored synchronously. χ (2) ( ω 3, ω2, ω1) = ( ω RES A ω RES IR ) + iγ + A NR e iφ

41 EXAMPLE OF SFG STUDY Surface assembly of guanosine monophosphate Formation of G4-wires on solid surfaces guanosine

42 DEPOSITION FROM 0.01 c 0.02 wt% SOLUTIONS Mica substrates (NH 4 ) 2 5 -GMP, 0.01 wt% Na 2 5 -GMP, 0.01 wt% A network of 1D aggregates (wires) is observed. K 2 5 -GMP, 0.02 wt% The wires have preferential orientation along 3 crystallographic directions of the basal mica plane. Single wires can be long up to several micrometers. Between the wires one can find large patches of the adsorbed medium with a subnanometer thickness. Very similar structures are observed for NH 4, Na and K GMP. K. Kunstelj, F. Federiconi, L. Spindler, I. Drevenšek-Olenik: Colloids and Surfaces B: Biointerfaces 59, (2007).

43 DEPOSITION FROM c = 0.2 wt% SOLUTIONS AFM image area is 5µm x 5µm. (NH 4 ) 2 5 -GMP Na GMP Terrace-like structures with holes are formed, which extend down to the substrate. K GMP How similar/different are these structures?

44 GMP FILMS PREPARED FROM c = 0.2 wt% SOLUTIONS IR spectral region cm -1. Results obtained for 2 different polarization combinations of the SFG, VIS and IR beams. Na 2 5'-GMP CH, CH 2 vibrations K 2 5'-GMP SFG signal for s-polarized IR beam is very low! s p No epitaxial-like growth (?) Thin film structures give very similar SFG spectra

45 FILMS of DIFFERENT THICKNESSES Thick films of Na-GMP are much less ordered than thick films of NH4-GMP. K. Kunstelj, L. Spindler, F. Federiconi, M. Bonn, I. Drevensek-Olenik, M. Copic, Chem. Phys. Lett. 467, 159 (2008).

46 Time-domain SFG measurements A short VIS pulsed is delayed with respect to the short IR pulse. SFG intensity is measured as a function of delay time τ ( pump-probe technique). fs fs SFG Dephasing of the molecular vibration states is directly probed. χ (2) RES ( τ ) = τ / T2 e e iω RES τ S. Roke et al., Surface Science 593, 79 (2005) Time-domain SFG setup

47 Comparison Time-domain/Frequency domain Langmuir films of heptadecanoic acid on water B S L G SFG intensity (a.u.) SFG spectra of Langmuir HDA ssp IR frequency (cm -1 ) (b) 25mN/m 15mN/m 5mN/m SFG intensity (counts/s) (a) FID SFG of Langmuir HDA ssp 25 mn/m 5 mn/m Time delay (ps) 15 mn/m Frequency domain data Time domain data

48 PRESENT STUDY LB FILMS OF LIPOPHILIC GUANOSINE DERIVATIVES In the absence of alkali metal cations lipophilic guanosine derivatives dissolved in organic solvents self-assemble into ribbon-like linear structures: Due to their macroscopic polarity, ribbons A are very intereseting for applications in molecular electronic. For instance, nanocrystals obtained by drop coating exhibit diode-like (rectification) behaviour: U

49 SURFACE ALIGNMENT Research challenge: How to obtain an unidirectionally oriented surface structure of guanosine ribbons on a macroscopic area??? Possible solution = Use of Langmuir-Blodgett (LB) surface deposition technique. dg(c 10 ) 2 Derivative with two (decanoyl) tails G Monolayer of amphiphilic molecules on water surface. Substrate Transfer of monolayer onto substrate (vertical deposition method).

50 G-DERIVATIVE WITH TWO TAILS LB FILM SFG spectrum in the spectral region of cm -1 ssp polarization combination Azimuthal dependence of the SFG signal 1,00 0,75 0, SSP 0,25 j 0, ,00 0, ,50 0, , The observed strong azimuthal anisotropy can be explained: A) with molecular alignment, B) inhomogeneity of the surface coverage. M. Devetak, S. Masiero, S. Pieraccini, G.P. Spada, M. Copic, I. Drevenšek Olenik: Appl. Surf. Sci. 256, 2038 (2010).

51 CONCLUSIONS Optical techniques are very suitable for investigations of various properties of transparent materials (structural and dynamic properties, response to external stimuli, surfaces and interfaces, etc.). They are typically nondestructive and can be applied in-situ during different processes, with no need for special sample preparation (drying, vacuum, etc.) Use of modern laser systems provides spatial resolution considerably below the optical diffraction limit (~ 50 nm) and temporal resolution better than any other experimental technique (few fs). More info: Thank you for your attention