INFRARED SYNCHROTRON RADIATION: SPECTROSCOPY AND MICROSCOPY
Stefano Lupi
Department of Physics University of Rome La Sapienza,
CNR-INFM COHERENTIA and SISSI@Elettra (EU-Italy)
Outlinen Electromagnetic Spectrum;n Infrared Synchrotron Radiation;n Experimental Apparatus: Michelson Interferometry+Infrared Microscopyn Solid-State Applications:a Superconduting Transition (THz and Far-IR Spectroscopy) Flux Gain1. Gap determination2. Spectral-Weight and penetration depthb Metal-to-Insulator Transitions (from THz to visible)
Brilliance Gain1. Mott-Hubbard Physics2. Peierls and e-ph Physicsn Geological Applications:1. Microscopic fluid inclusions Brilliance Gainn Chemistry:1. Adbsorption at solid surfaces Brilliance Gain§ Biology:1. Cellular absoption Brilliance Gainn Cultural Heritage
FIR MIR NIR
Phonons; Drude absorption;Gaps in superconductors;Molecular Rotations;
Molecular Vibrations Fingerpr ints forChemistry, Biology, And Geology
Molecular Overtones and Combinations bands;Excitons;Gaps in semiconductors
The “THzgap” , Collective Excitations in Macromolecules and exotic electronic materials
Electromagnetic Spectrum
IR Units: 200 cm-1=300 K= 25 meV = 50 µµµµm = 7 THz
Infrared History
1976 Meyer&Lagarde (LURE, Orsay) publish the first paper on IRSR
1981 Duncan and Yarwood observe at Daresbury the first IRSRemission
1985 The first IRSR spectrum (on N2O) is collected at Bessy(Berlin)
1986 The first beamline becomes operating at UVSOR (Japan)
1987 Beamline at Brookhaven (USA)
1992-94 Beamlines at Orsay (France), Lund (Sweden), Daresbury(GB)
1995 First international workshop on IRSR, Rome (Italy)
IRSR Chronology
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IRSR Beamlines in the World
Advantages of IRSR
ADVANTAGES:ADVANTAGES:
Diffraction Limited Spatial ResolutionDiffraction Limited Spatial Resolution
Better SignalBetter Signal--toto--NoiseNoise
Faster Data CollectionFaster Data Collection MICROSCOPY
BRILLIANCE
SPECTROSCOPY
BROADBAND SOURCE
LINEAR & CIRCULAR POLARIZATION
FAR-IR SOURCE PULSED EMISSION
Source
A Michelson Interferometer is
based on the interference effect among the two emwaves at the beam-
splitter site
Detector Measure of the figure of interferenceInterferogram
Mobile mirror M2
Fixed mirror M1
Beamsplitterx=0
Instrumentation I:Michelson Interferometer
Spectral Resolution∆ν∆ν∆ν∆ν~1/d (cm-1) ~ 0.001 cm-1 ~ 1µµµµeV
d
Interferogram
Spectrum
Interferogram
Spectrum
Interferogram
Spectrum
Mon
ocro
mat
icP
olic
rom
atic
IR S
ourc
e
Fourier Transform
x
x
x
ν
ν
ν
ν0
ν1 νn
Synchrotronbeam
Interferometer
IR detector
White light
X-Y microstages
SchwarzschildObjectives
Sample visualisation
Adjustable aperture
Adjustable aperture
Instrumentation IIInfrared Microscopy
Infrared Microscope---->Beam CondenserVisualize and measure small and/or no-homogenous sample (size<100 µm)
with a high spatial resolution
Bruker-Hyperion
λλδ ≈=NA
61.0
In the IR spatial Resolution is determined by diffraction
For example with a 36x objective with NA=0.5, one obtains:
λλλλ
λλλλ
Production of IRSR
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Standard Bending radiation (emitted during the circular trajectory in the bending due to the constant B field)
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P (λλλλ) = 4.4 1014 x I x ΘΘΘΘH x bw x (ρ/ρ/ρ/ρ/λλλλ)1/3 photons s-1
I is the current in amperes,ΘΘΘΘH (rads) the horizontal collection angle,
bw the bandwidth in per cent, λλλλ the wavelength, and ρρρρ the radius of the bending
ΦΦΦΦV-NAT(mrad) = 1.66(1000 x λ (µm)/ ρ(m))1/3
at ALS for λ= 100 µm ΦΦΦΦV-NAT= 50 mrads
Very large emission angles SISSI: H=70 mrads; V=25 mrads
In the Far-Field approximation:
P = α α α α x I x γγγγ4ΘΘΘΘ2222/(1+ γ/(1+ γ/(1+ γ/(1+ γ2ΘΘΘΘ2222))))2222 photons s-1
I is the current in amperes,Θ(rads) the emission angle
(concentrated in ΘΘΘΘmax ~ 1/γγγγ ~10 mrads)
Edge Emission(emitted at the entrance (exit) of a bending magnet due to the rapid variation of the B field)
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IRSR FLUX
The IRSR flux and Brilliance depend only on:
-beam current-source size/emittance-extraction aperture-transmission optics
Instead scarcely depend on the machine energy
ElettraI= 300 mA
~(λλλλc/λλλλ)1/3
Increasing the Far-IR Flux:Coherent vs Incoherent Synchrotron Radiation
Flux:
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Far-IR X-rays
SR
CSR
incohvcohincoh IfNfNIII ))1(( 2ν+−=+=
2)cos()(= dzeznf zi
vθπ Bunch form factor
Diffraction due to chamber size
Production of Coherent Synchrotron RadiationTwo main methods
SISSI@Elettra: E. Karanzoulis, A. Perucchi and S.L.,2007
Low-αααα mode Needed to change the magnetic optics:
Only dedicated runs
Momentum compaction factor αααα : ∆∆∆∆p/p=αααα=σσσσ/LWhere σ is the bunch length and L is the length of the ideal trajectory inside the machine
IRIS@Bessy-II: U. Schade et al, PRL 2003
Low-e beam energyInjected the machine at low-E:Reducing life-time and stability
3/2E≈σ
IRSR BrillianceThe most important figure of merit for IRSR is the Brilliance
BSR
=d 2F
dθdϕASA
(photons/0.1%bw/cm2/str)
Where the Actual Source Area is an estimation of the dimension of the source at the exit port (Hulbert and Weber,92; A. Nucara, 1998)
Limiting NoiseSN
= 100A1/ 2D*
B(ν )∆νεt1 / 2ξWhere: A detector area, D* detectivity, B brilliance, ∆νbandwidth, ε etendue, t measuring time, ξ optical efficiency %N =
Brilliance gain at SISSI
S. Lupi et al, 2007
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Brilliance gain at SISSI
Experimental Techniques III
ωω
ω
Via Kramers-KronigTransformation
real and immaginary part of the optical response
functions εσcan be
obtained
Incident Radiation I0Incident Radiation I0
Reflection IR
Sample
Emission
Scattering
Transmittance ITAbsorbanceAbsorbance
Reflectivity, Transmittance, and Absorption R+T+A=1
Reflectivity experiments
Single crystals may be very small:
Reference:gold evaporated in
situReflectivity:
R = IRcrys/ IRgold
Kramers-Kronig transf.
optical conductivity
σ(ω)
Sample
Cryostat
Remotelycontrolled mirrors
Interferometer
Detector
Solid-State Applications ISuperconductivity
(FLUX GAIN)
Superconductivity today:
1. Looking for new materials• 1986: High-Tc cuprates• 1990: C60 fullerene• 2000: MgB2• 2003: NaxCoO2·3H2O• 2004: B-doped diamond
2. Understanding new materials
Infrared spectroscopy plays a role...
... because
Superconductivity is ruled by low-energy electrodynamics:
• Superconducting gap : THz-range• Spectral weight of condensate and penetration depth:THz-range
• Mediators of pairing (phonons, etc.): Far-infrared• Free-carrier conductivity above Tc: Infrared
Basic optics of Superconductors
[σ1(ω, T>Tc) - σ1(ω, T<Tc)] dω = ω2ps/8 = nse2/m*--> λ=c/ωps
Ferrel-Glover-Tinkham Rule
Superconducting gap observed if:•sample in the dirty-limit (2∆ < Γ) •Cooper pairs in s-wave symmetry
40x103
30
20
10
0
σ1(
ω) (
Ω−1
cm-1)
200150100500
ω (cm-1)
Normal State T = 0.9 Tc T = 0.6 Tc T = 0
Superconducting Gap
σ1sup(ω ) = ω 2
ps
8δ (ω ) + σ1
reg(ω )
1.000
0.995
0.990
0.985
0.980
Ref
lect
ance
100806040200
ω (cm-1)
Normal State T = 0.9 Tc T = 0.6 Tc T = 0
2∆ Drude absorption
Drude reflectance Γ
2∆
Minimum excitation energy:Cooper-pair breaking 2∆
Diamond: the “silicon” of the future?
Oppenheimer Diamond 254.7 carats
Takenouchi-Kawarada-Takano Diamond film : 0.7 carats
A diamond-based electronics?
nChemical Vapor Deposition films: less expensive and suitable for devices
nMechanically stable
nLarge-gap insulator (5.5 eV)nExcellent thermal conductivity (2 to 4 times that of Cu)
nStrong hole-doping already obtained
(nh >1022eh/cm3)
nElectron-doping comingn…and superconducting!(E.A. Ekimov et al., Nature 428, 542
(2004)).
THz Reflectivity of Superconducting Diamond
s-wave Dirty-Limit Regime; 2∆∆∆∆(2.6 K)=12±1 cm-1 2∆∆∆∆/kBTC=3.2 ± 0.5
1.05
1.00
3020100
1.00.80.60.40.20.0
T=2.6 K 3.4 K 4.6 K 7.2 K 15 K
8
6
4
2
01.00.50.0
∆ ∆ ∆ ∆ (c
m-1
)
T / TC
ω ω ω ω (THz)
Rs(
T)
/ Rn(
15K
)
ω ω ω ω (cm-1)
Mattis-Bardeen Model
ω Γ (T) : Rn (ω) = 1 - [8ωΓ(T)/ ωp2]1/2 ω 2∆(T) : Rs(ω) = 1
Peak at 2∆ in Rs/Rn
M. Ortolani, S. L. et al, PRL 2006
1.00
0.95
0.90
0.85
Ref
lect
ance
604020ω(cm-1)
T=2.6K 3.4 4.6 7.2 15
Tc=6.2 K
Absolute reflectivity and conductivity ofSuperconducting Diamond
2∆(0) = 12.5 cm-1 = 3 kBTc [BCS: 3.53]
Missing area
Penetration depth λ
λ(Τ) BCS-likeλ(0) = 1 µm(dirty limit)
M. Ortolani, S. L. et al, PRL 2007)
Which interaction is responsible for pairing?Electron-phonon coupling
The only optical phonons in diamond (~1300 cm-1=160 meV) is not IR-active, but Raman-activeThe peak in a2F(ω) indicates hole-phonon interaction.
Holes interact with a broad phonon branch centered around 140 meV:
A phonon branch appears at 70 meV with B-doping, which also interacts with the charges
α 2F(ω) ∝ W(ω ) = 12π
d 2
dω 2 [ωΓ(ω )]
α2F(ω) from theory
M. Ortolani, S. L. et al, PRL 2007
Anomalous-Drude model:
Superconducting CCa6
c-axis oriented policrystals Tc=11.4 K (G. Loupias et al, 2005)1.10
1.05
1.00
0.95
0.90
R(2
.6K
)/R
(15K
)
6 7 8 910
2 3 4 5 6 7 8 9100
ω (cm-1)
CaC6
Tc= 11.6 K
Graphite Incoherent RadiationCoherent Radiation
DiamondR(2.6 K)/R(10K)
Clean-limit regime Ca-Doped Graphite ------> No Optical Information (Gap, Penetration depth, …) about Superconducting State
M. Ortolani, S.L. et al, unpublished
CaAlSi (anisotropic superconductor)polarized gap measurements
exagonal plane c-axis
s-wave Dirty-Limit Regime both along the exagonal plane and along the c-axis:
2∆∆∆∆ab(0 K)=19±1 cm-1 2∆∆∆∆ab/kBTC=3.9 ± 0.22∆∆∆∆c(0 K)=23±1 cm-1 2∆∆∆∆c/kBTC=4.5 ± 0.2 S. Lupi et al, PRL 2007
Josephson Plasma Resonance along the c-axis of a High-Tc SC: La-Ba-Sr-Cu-O
JPR Edge at 14 cm-1 = 0.4 THz
M. Ortolani et al., Phys. Rev. B 2006
in-plane SC
in-plane SC
JPR
Tc = 36 K
Tc = 10 K
Solid-State Applications IIMetal-to-Insulator Transition
(BRILLIANCE GAIN)
Metal-to-Insulator Transition (MIT)
Theory of Metal to Insulator Transitions
High-Pressure IRSR Spectroscopy
Electron-Electron interaction and insulator-to-metal transition (MIT)
Electronic correlation: failure of band modelHubbard model
><↓↑
+ ++−=σ
σσij i
iiji nnUchcctH .).(
Pressure may increase t and therefore the t/U ratio inducing a MIT
t : hopping integral (strongly dependent on atomic distances)U : coulomb repulsion
U prevents double on-site occupancy. the opening of a gap in the
spectra of excitations is induced
Electron-Lattice interactionPeierls MIT
[from Gruner (88)]
A charge density wave (CDW) ground state develops in low-dimensional metals as a consequence of electron-phonon interaction.
Electron density, lattice distortion and single particle band in (a) the metallic state for T >TCDW and (b) in the CDW state at T = 0.The figure is for a one-dimensional, half-filled system. In this case a gap (D) opens in the single-particle spectra of excitations.
[3] G. Grüner, Density Waves in Solids, Addison Wesley (New York, 1994)
Pressure may increase t and reduce lattice distorsion inducing a MIT
IRSR Microscopy at high pressures
Force
Diamond
Gasket
Sample size<300µm
T=IT / I0
Op.D. = -ln(T) = αd
Rdiam/sam=IR / I0
Small optical troughput
Vanadium di-oxide VO2
T-dependent MIT (TMIT= 340 K) concomitant with a lattice modification from a low-T monoclinic (M1) to a high-T rutile (R) phase.
MIT Hubbard or Peierls mechanism?
Pressure dependent measurements may provide useful information
!"#$#
%&'(
)
Infrared measurements on VO2
Simultaneous measurements of reflected and transmitted intensity on a slab of VO2, 5
microns thick.
Iteratively from T and R the optical conductivity of VO2 has been obtained
A sudden increase is visible in the optical conductivity (especially in the MIR region)
above 10 GPa. A delocalized electronic state is therefore achieved in the monoclinic Peierls
state.
Spectral Weight (SW), obtained at low and high frequency shows a different behavior
below and above this pressure. E. Arcangeletti, et al PRL 2007
La0.96 Ca0.04MnO3-δδδδ
La0.8 Ca0.20MnO3
La0.75 Ca0.25MnO3
MIT transition of a manganite in the FIR
Pressure-induced MIT at 300 K
FAR-IR
A. Sacchetti et al., Phys. Rev. Lett. 2006 (sinbad@frascati)
In manganite high pressure measurements have shown that the MIT and the Curie transition are intrisically coupled
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Geological Applications: liquid inclusion in mineralsBrilliance gain
Liquid Inclusion in quarzite
Water Distribution
CO2 Distribution
3 µm
A. Perucchi, S.L. et al, 2006
Liquid Inclusion in quarzite
Infrared Spectra
Optical Image
3µm
Mapping with 3µµµµm x 3µµµµm (diffraction limited)
Chemistry
CO adsorbed molecules on Cu metallic surface
Hindered rotation
Vibration
G. Williams et al, PRL 1998
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Noise less than 0.01%
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How look the IR spectrum of a cell?
Medical and Biological Application of IR MicroscopyBrilliance gain
Lipids
Proteins
DNA,RNA
Vibration Frequencies correspond to finger-print for the molecule
IRSR spectrum of a single cell during mitosis
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P. Dumas et al, 2004
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Studying hair by IRSR microscopy
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P. Dumas et al, 2004
Collective (conformational) modes of macromolecules
Absorption coefficient
Organic macro-molecules must be in an acqueus environment to function, i.e. to undergo their conformational switching at THz frequencies
needed for high power THz intensity
Frequency (THz) Frequency (THz)
THz Single Strand and double strands DNA spectra
The coupling with sugar skeleton and with the complementary base induce a splitting of the conformational modes and the appearing of low-frequency (<1.5 THz) collective vibrations.
Possibility to detect conformational changing
A. Perucchi, S.L. et al, 2005
Application to Cultural HeritageStudy of the aging of an ancient paper
Intensity of the carbonyl group along a 10 µm fiber
Aging propagates along the fiber
Perspectives
1. Mid-IR and Far-IR imaging coupled with x-ray imaging for biomedical applications;
2. THz Imaging and spectroscopy at III° Generation machines;
3. Circular polarization and dichroism in the Far-IR coupled with High-Magnetic fields
4. fs pulsed THz radiation from IV° generation machine for no-linear time-resolved spectroscopy [email protected]
* %+,- .
P. Calvani and A. Nucara, L. Baldassarre
IRS Group, University of Rome “La Sapienza”
& CNR-INFM CoherentiaM. Ortolani and U. SchadeSynchrotron BESSY II Berlin
P. PostorinoHP Group, University of Rome “La
Sapienza”& CNR-INFM Coherentia
L. De GiorgiETH ZurichL. Malavasi
Dipartimento di Chimica, University of Pavia
M. MarsiCNRS-LPSU, France
V.A. SidorovInstitute for High Pressure Physics
Troitsk, Moscow Region RussiaP. Roy&P. Dumas
Synchrotron Soleil Paris (France)G.Williams
Jefferson Laboratory (USA)
and many others…….
The SISSI-ELETTRA group: A. Perucchi, R. Sopracase
D. Eichert, L. Vaccari, F. Morgera,
M. Kiskinova
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