Optical spectroscopy of Condensed MatterOptical spectroscopy of Condensed Matter ... Broadband IR...

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

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Principles

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


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THE ELECTROMAGNETIC SPECTRUM


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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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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


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Vibrational transitions (IR regime)

Electronic transition (Visible, UV)

ΔE ~ (1-1000)meV

INFRARED SPECTROSCOPY


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ℎ𝜈𝜈𝑖𝑖𝑖𝑖 ≠ ℎ𝜈𝜈𝑜𝑜𝑜𝑜𝑜𝑜 PHOTOLUMINESCENCE

ΔE ~ 1eV

RAMAN SCATTERING

ΔE ~ (1-100)meV


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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.,)


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SELECTION RULES – CO2

Optical Spectroscopy

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Experimental Realization

Optical Spectroscopy – Techniques

15 Slough Bus Station inspired by Herschel’s infrared experiment


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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


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Dispersive spectrometer Double beam infrared spectrometer


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Fourier Transform Spectrometer


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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)


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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.,


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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)


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ℎ𝜈𝜈𝑖𝑖𝑖𝑖 = ℎ𝜈𝜈𝑜𝑜𝑜𝑜𝑜𝑜 Reflection

Reflection, R(𝜈𝜈) = 𝐼𝐼𝑅𝑅(𝜈𝜈)𝐼𝐼0(𝜈𝜈)


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ℎ𝜈𝜈𝑖𝑖𝑖𝑖 = ℎ𝜈𝜈𝑜𝑜𝑜𝑜𝑜𝑜 Reflection

Reflection, R(𝜈𝜈) = 𝐼𝐼𝑅𝑅(𝜈𝜈)𝐼𝐼0(𝜈𝜈)

oxygen atom lanthanum atom

••

a

c

c

b


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Complex Optical functions

M. Dressel and G. Gruener, Electrodynamics of Solids (2002)


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Optical conductivity


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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!)


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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


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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

Magneto-Optical Spectroscopy at MagLab

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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!!)


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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)


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Spatially-resolved PL and Raman spectroscopy

Diameters of optical heads 23 mm > d ≥ 16 mm


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Spatially-resolved PL and Raman spectroscopy


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Spatially-resolved PL and Raman spectroscopy: Examples

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


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I. Probing phonons by Raman spectroscopy

Raman active modes: 7

2A1g + 5Eg = A1g + A1g + 5Eg

∥c ⊥c ⊥c


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II. Excitation Spectroscopy: PL vs Raman spectroscopy

532nm excitation

PL Raman


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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

Thank You! Komu Thirunavukkuarasu

[email protected]

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Optical spectroscopy of Condensed MatterOptical spectroscopy of Condensed Matter ... Broadband IR...

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