Optical spectroscopy of Condensed Matter
Maglab Users Summer School 21 May 2015
Komalavalli Thirunavukkuarasu
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OUTLINE
Principles of spectroscopy – What? Why? How? Experimental realization
Infrared (IR) spectroscopy Raman scattering and Photoluminescence
CW optical spectroscopy at Maglab Introduction to Lab Practicals
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Optical Spectroscopy – What?
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Study of interaction of matter with electromagnetic radiation
Spectrum : Response of matter to radiation as a function of energy
Photon (or light) has • Energy (wavelength or frequency) • Momentum • Polarization (helicity or spin) • Intensity (or flux) • Coherence length and time
Optical Spectroscopy – What?
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Wavelength (µm) Wavenumbers (cm-1)
Energy (meV)
Frequency (THz) Temperature (K)
THE ELECTROMAGNETIC SPECTRUM
𝜆𝜆−1 =𝑐𝑐𝜈𝜈
𝐸𝐸 = ℎ𝜈𝜈 = 𝑘𝑘𝐵𝐵𝑇𝑇 = eV 1 meV = 8.064 cm-1
Optical Spectroscopy – Why?
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Probe various structural, electronic and magnetic states/excitations in matter
Basov et al., Rev. Mod. Phys., 83, 471 (2011)
Optical Spectroscopy – How?
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Pressure
Temperature Electric and Magnetic Fields
Types of cw optical spectroscopy
ℎ𝜈𝜈𝑖𝑖𝑖𝑖 = ℎ𝜈𝜈𝑜𝑜𝑜𝑜𝑜𝑜 Reflection and Transmission
THz, IR, Visible, UV
ℎ𝜈𝜈𝑖𝑖𝑖𝑖 ≠ ℎ𝜈𝜈𝑜𝑜𝑜𝑜𝑜𝑜 Raman scattering
Photoluminescence Chemical doping
Optical Spectroscopy – How?
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Vibrational transitions (IR regime)
Electronic transition (Visible, UV)
ΔE ~ (1-1000)meV
INFRARED SPECTROSCOPY
Optical Spectroscopy – How?
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ℎ𝜈𝜈𝑖𝑖𝑖𝑖 ≠ ℎ𝜈𝜈𝑜𝑜𝑜𝑜𝑜𝑜 PHOTOLUMINESCENCE
ΔE ~ 1eV
RAMAN SCATTERING
ΔE ~ (1-100)meV
Optical Spectroscopy – How?
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SELECTION RULES IR activity: Induced dipole moment due to change in atomic position
Raman Activity: Induced dipole moment due to deformation of electron shell
Induced polarization 𝑃𝑃 = 𝛼𝛼𝐸𝐸 𝜕𝜕𝛼𝛼𝜕𝜕𝜕𝜕 0
≠ 0
𝜕𝜕𝜇𝜇𝜕𝜕𝜕𝜕 0
≠ 0
𝛼𝛼 – Polarizability E – Electric Field
𝜇𝜇 is dipole moment
𝜕𝜕 is the normal coordinate at the equilibrium configuration
RULE OF MUTUAL EXCLUSION In a centrosymmetric molecule (i.e., one with a center of inversion symmetry) a vibrational mode may be either IR active or Raman active but not both.
IR modes: “Asymmetrical” (Eu, A1u etc.,)
Raman modes: “Symmetrical” (Eg, A1g etc.,)
Optical Spectroscopy – Techniques
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What is special about IR spectroscopy? Broadband IR spectroscopy – Not a one
source/detector system! Brightness of sources Sensitivity of Detectors Optical components – All materials have
absorption in IR Vacuum needed for even RT measurements Delivery losses Working close to diffraction limit in FIR
Transmittance of various optical materials
Spectral response of various IR detectors
Optical Spectroscopy – Techniques
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Fourier Transform Spectrometer
Aperture
Mirror Scanner
Beam splitter
Sample compartment
sample
DTGS
MCT/InSb
Bolometer
Sources
Control Electronics
BRUKER IFS 66v/S
Polarizer
Range: 20-20000 cm-1 (600 GHz to 600 THz or 1 meV – 2.5 eV)
Optical Spectroscopy – Techniques
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ℎ𝜈𝜈𝑖𝑖𝑖𝑖 = ℎ𝜈𝜈𝑜𝑜𝑜𝑜𝑜𝑜
Transmission
Transmission, 𝑇𝑇(𝜈𝜈) = 𝐼𝐼𝑇𝑇(𝜈𝜈)𝐼𝐼0(𝜈𝜈)
𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴,𝐴𝐴 = − log10 𝑇𝑇(𝜈𝜈)
Materials : Insulators, Semiconductors, Thin films etc.,
Optical Spectroscopy – Techniques
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ℎ𝜈𝜈𝑖𝑖𝑖𝑖 = ℎ𝜈𝜈𝑜𝑜𝑜𝑜𝑜𝑜 Transmission
Transmission, 𝑇𝑇(𝜈𝜈) = 𝐼𝐼𝑇𝑇(𝜈𝜈)𝐼𝐼0(𝜈𝜈)
100 200 300 400 500 600 7000.00
0.05
0.10
0.15
Reference (I0) Sample (IT)
Inte
nsity
(arb
. uni
ts)
Frequency (cm-1)
C70 fullerene
100 200 300 400 500 600 7000.0
0.2
0.4
0.6
0.8
1.0
Tran
smitt
ance
Frequency (cm-1)
Optical Spectroscopy – Techniques
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ℎ𝜈𝜈𝑖𝑖𝑖𝑖 = ℎ𝜈𝜈𝑜𝑜𝑜𝑜𝑜𝑜 Reflection
Reflection, R(𝜈𝜈) = 𝐼𝐼𝑅𝑅(𝜈𝜈)𝐼𝐼0(𝜈𝜈)
Optical Spectroscopy – Techniques
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ℎ𝜈𝜈𝑖𝑖𝑖𝑖 = ℎ𝜈𝜈𝑜𝑜𝑜𝑜𝑜𝑜 Reflection
Reflection, R(𝜈𝜈) = 𝐼𝐼𝑅𝑅(𝜈𝜈)𝐼𝐼0(𝜈𝜈)
oxygen atom lanthanum atom
••
a
c
c
b
Optical Spectroscopy – Techniques
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Complex Optical functions
M. Dressel and G. Gruener, Electrodynamics of Solids (2002)
Optical Spectroscopy – Techniques
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RAMAN SPECTROSCOPY TIMELINE 1923 – Inelastic light scattering is predicted by A. Smekel 1928 – Landsberg and Mandelstam see unexpected frequency shifts in scattering from quartz 1928 – C.V. Raman and K.S. Krishnan see “feeble fluorescence” neat solvents 1930 – C.V. Raman wins Nobel Prize in Physics
1960 – Invention of laser made Raman spectroscopy more effective and meaningful 1964 – Townes, Basov and Prochorov win Nobel Prize in Physics for maser and laser (Here again, Ted Maiman was the first to have a working laser!)
Optical Spectroscopy – Techniques
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Czerny-Turner spectrometer for Raman scattering and PL
1. Deliver high quality beam Low loss, clean excitation beam
2. Efficiently scatter light 3. Efficiently collect scattered
light, thereby have increased sensitivity
Advantages of dispersive Raman spectrometer
Optical Spectroscopy – Techniques
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Raman scattering
Inelastic light scattering from correlated electrons, Devereaux and Hackl, RMP 79, 175 (2007)
Ca-doped YBCO
10-7
< 10-9
Only 1 in every 107-109 photons scatters inelastically Very low signal
Optical Spectroscopy @ MagLab
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FTIR Magneto-spectroscopy
SPECTROMETERS: Bruker Vertex 80v Bruker IFS 66v/s Bruker IFS 113
MAGNETS: 18 T SCMs 31 T-35 T Resistive Magnets 45 T (Noisy!!)
Optical Spectroscopy @ MagLab
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FTIR Magneto-spectroscopy: Examples
monolayer graphene exfoliated on to Si/SiO2 wafer substrates
cyclotron resonance (CR) transitions to and from the n = 0 Landau level (LL) in monolayer graphene.
B=18T
E. A. Henriksen et al., Phys. Rev. Letters 104, 067404 (2010)
Optical Spectroscopy @ MagLab
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Spatially-resolved PL and Raman spectroscopy
Diameters of optical heads 23 mm > d ≥ 16 mm
LAB PRACTICALS – Optical spectroscopy of Al2O3
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I. Probing phonons by Raman spectroscopy
Two Al2O3 groups per unit cell
Phonon modes (1st order, Γ-point):
Γ = A2u + Eu + 2A1g + 5Eg + 2A2u + 4Eu + 3A2g + 2A1u
acoust. Raman IR silent
LAB PRACTICALS – Optical spectroscopy of Al2O3
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I. Probing phonons by Raman spectroscopy
Raman active modes: 7
2A1g + 5Eg = A1g + A1g + 5Eg
∥c ⊥c ⊥c
LAB PRACTICALS – Optical spectroscopy of Al2O3
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II. Excitation Spectroscopy: PL vs Raman spectroscopy
532nm excitation
PL Raman
LAB PRACTICALS – Optical spectroscopy of Al2O3
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II. Excitation Spectroscopy: PL vs Raman spectroscopy
Cr+
R1 (694nm)
R2 (693nm)
Ruby PL (and laser !!!)
R1 R2
gree
n
blue
Cr+ E4
E3 E2
E1