infrared spectroscopy of solids at the nslsinfrared spectroscopy of solids at the nsls j. hwang, h....

NSLS workshop, Feb 2008

Infrared Spectroscopy of Solids at the NSLS

J. Hwang, H. Zhang, H. Tashiro, J. LaVeigne, D.H. Reitze, C.S. Stanton, and D.B. Tanner

University of FloridaR.P.S.M. Lobo

ESPCILaszlo MihalyStony BrookJiufeng Tu

City College of New York G.L. Carr

NSLS


OutlineOutline

• Measurements in the very far IR

– mm waves, terahertz

– Physics opportunities

– Needed technology

• Nonlinear far-IR spectroscopy

– Time-resolved (pump-probe)

• Metallic superconductors

• Magnetic semiconductors

– High E-field-strength opportunities


Infrared Synchrotron RadiationInfrared Synchrotron Radiation

• “White” source• High Brightness• Long wavelength flux

(the far-infrared)• Pulsed (100s of ps) Frequency (cm-1)

Energy (eV)

Pow

er /

BW

(µW

/ cm

-1)

10 100 1000 10000

0.001

0.01

0.1 1

0.1

1

10

1

0.01

0.001

Without light cone

With light cone Vis-UV

87 mWVis-UV87 mW

Near-IR52 mW

Near-IR52 mW

Mid-IR17 mWMid-IR17 mW

Far-IR1 mW

Far-IR1 mW


Physics in the far-IRPhysics in the far-IR

Important physics (“All of solid-state physics…” A.J. Heeger• Superconducting gaps• Antiferromagnetic and ferromagnetic resonance• Collective modes• Free-carrier dynamicsMaterials• Quasiparticles in HTSC• Gaps in LTSC• Heavy Fermions• CDW (TaSe2)• Organics (TMTSF2-X, BEDT-TTF2-X) (CDW, SDW, SC)• Dilute magnetic semiconductorsSpectroscopically difficult• Weak thermal sources for FTIR• High frequencies for microwave sources• THz generators not mature, limited in bandwidth• Synchrotron sources, therefore, have advantages.


U12IR Beamline on Dipole BendU12IR Beamline on Dipole Bend

orbit plane

Mirror 16 cm aperture

Mirrors 2 & 315 cm aperture removable

SS mirror1.1 cm aperturediamond window

10 cm apertureglass viewport

microwavedetector

far-infraredinterferometer

far-infrareddetector

ring chamber


Typtical BeamlinesTyptical Beamlines


Synchrotron & Thermal SourcesSynchrotron & Thermal Sources

10 100 10000.1

1

10

100

10 mm

Frequency (cm-1)

I Sync

/ I

Ther

mal

0.01 0.1

Synchrotron

2 mm

1.25 mm

0.5 mm

Energy (eV)

0 2 4 6 8 100.0

0.2

0.4

0.6

0.8

1.0

Hg - arc lamp

Inte

nsity

(nor

m.)

Aperture (mm)


Signal to noise comparisonSignal to noise comparison

Synchrotron radiation allows

low noise far-IR measurements.

IRSR is diffraction limited.

Beam noise has to be

taken into account for larger

samples.

(Note also the effect of aperture

diameter at low frequencies.)

but…

10 100 10000.95

1.00

1.05

Frequency (cm-1)

Frequency (cm-1)

0.95

1.00

1.05

0.5 mm0.5 mm0.5 mm0.5 mm

10 100 10000.95

1.00

1.05

100

% L

ines

100

% L

ines

0.95

1.00

1.05

ThermalThermalThermalThermal

ThermalThermalThermalThermal

SynchrotronSynchrotronSynchrotronSynchrotron

SynchrotronSynchrotronSynchrotronSynchrotron

10 mm10 mm10 mm10 mm


Synchrotron vs. UA-3 Hg arcSynchrotron vs. UA-3 Hg arc

Crossover between Hg arc lamp and synchrotron radiation at ~ 20 cm-1

0 20 40 60 80 1000

1

2

3

4

5

6

Inte

nsit

y (a

. u.)

Frequency (cm-1)

Comparison uses ½” aperture, lamellar grating interferometer


Notions about design requirements Notions about design requirements

1. Efficient transfer of light from ring to detector2. Spectroscopic approach3. SNR of detector 4. Sample temperature range5. Technical noise from ring6. Interference, standing waves


Notions about design requirements 2Notions about design requirements 2

1. Efficient transfer of light from ring to detector2. Spectroscopic approach3. SNR of detector 4. Sample temperature range5. Technical noise from ring6. Interference, standing waves

• With no sample, transfer 75% of light• FTS, then R-> KK or R+T or ellipsometery. Magnet,

pressure.• Best detector available, use 0.3 K• Get T < ν• Active bunch steering, active beam steering• Dump reflected beams, aim for low VSWR.


1971 State of the art1971 State of the art


High pass filter in detector?High pass filter in detector?


High pass filter in detector? 2High pass filter in detector? 2

Simulated spectrum:

• Mylar beamsplitter, with fringes

• Efficiency ε = 4RT

• S(ν) ~ ν2 (Rayleigh-Jeans)

• Long pass filter with 100 cm-1 cutoff

• α = Cν2

Simulation has much more low-

frequency energy than what is

observed.

(4x at 10 cm-1, 12x at 8 cm-1)


Need for lower temperaturesNeed for lower temperatures

• Take hν = kT. (1 cm-1 1.44 K)• Assume feature of interest is a mean-field gap

at 5 cm-1.• Then Tc = 2 K, taking 2∆ = 3.5Tc

• Thus, 200 mK is Tc/10.

• Options (low-cost to high-cost):– Attach sample at bottom of 3He cryostat for 0.3K

detector (e.g., Reedyk, Basov, others)– Build 3He fridge, one shot or recirculating, in Dewar

with optical windows.– Dil fridge. Cryogen free, easy operation for ~200k.


Beam JitterBeam Jitter

• Need to reduce technical noise from SR.• Seems to be due to beam jitter• Use quad photodiodes at conjugate image planes, feed

back to steering mirrors (e.g., LIGO)• Could use beam divider, measure incident intensity,

divide• Throughput requirements would not be met


0 5 10 15 20 250

20

40

60

80

100

120U12IRlow frequency spectral signal

Sig

nal [

arb.

]

Frequency [cm-1]

U12IR fringe pattern


“Echo” produced by dipole chamber

U12IR port

23.25degrees

electronorbit

191 cm radius

8 cmwidth

reflectionpoint

dipolechamber

• Need to absorb wall reflections

• Light reflected from wall enters interferometer with well-defined phase wrtprompt beam.

• Affected by beam motion, chamber temperature!

• Gives second signature in interferogram.

- 1 0 1 2 3

- 0 .0 2

0 .0 0

0 .0 2

0 .0 4

0 .0 6

Det

ecto

r S

igna

l

P a th D i f fe r e n c e [ c m ]


spectrometer& detector

sample

laserpulse

infraredpulse

synchrotron

1

2

3

1. Laser pulse creates photoexcitations in sample, which subsequently evolve with time.

2. After time ∆t, broadband (continuum) IR pulse arrives and is partially absorbed (or reflected) by excitations.

3. IR pulse analyzed with a spectrometer, extracting details of excitations at a time ∆t after their creation.

• Cycle repeats at high (50 MHz) repetition rate.

• Photoexcitation evolution determined by measuring at a variety of ∆t’s.

• Employs “conventional” spectroscopy using high-sensitivity (slow-response) detectors.

Time-resolved IRTime-resolved IR


Pump-probe with Laser & Synchrotron Pulses

Pump-probe with Laser & Synchrotron Pulses

spectrometer

synchrotron source

NSLS - VUV ringBrookhaven Nat'l Lab

mode-locked (pulsed) laser

synch.signal

Far-infrared(probe) pulse

fiber optic cabletransports pulsesto sample

sample

detector


• Coherent mode-locked laser (MIRA 900p) pumped by Verdi green laser

• Tunable (700-1000 nm)• Frequency doubling capable • ~ 800 mW average power• 2 ps pulses• PRF synchronized to 2x

NSLS 53 MHz RF system (VUV or X-ray)

• Pulse selection to match various synchrotron bunch patterns

• Optical fiber deliveryto beamline(s).

Synchronized Ti:sapphire laserSynchronized Ti:sapphire laser


Pulses at specimen locationPulses at specimen location

• Synchronized laser & storage ring pulses.

• Measured at sample location.

• Ge APD (near IR detector, ~1 ns response).

• Useful for locating “zero” delay point.Shown with 4 ns delay.

• Synchrotron pulse length ~300 ps(compressed mode)

• NSLS II: bunch length is 10 ps(ordinary mode)

140 160 180 200 220-2

0

2

4

6

8

10

Det

ecto

r si

gnal

Time [ns]

laser synchrotron

4 ns


• Multi-step process, with a range of timescales.

• Coupled system of excess quasiparticles and phonons, trapping and effective lifetime.– Rothwarf & Taylor

Photoexcitation / Relaxation in Superconductors

Photoexcitation / Relaxation in Superconductors

high energy (E~EF) electronicexcitations (quasiparticles)

hνννν

~ femtoseconds

low energy qp’s & EDebye phonons

paired electrons & thermal quasiparticles

~ picoseconds

excess ∆ qp’s excess 2∆ phonons1/ττττR

1/ττττB

phonon escape(at rate 1/τγ

)Bottleneck => ns


Far-infrared transmittanceFar-infrared transmittance

0 10 20 30 400

1

2

3Pb

4.78 K 5.5 K 6.5 K

Tran

smis

sion

[rel

. to

norm

al s

tate

]

Frequency [cm-1]

0 1 2 3 4 5

Photon Energy [meV]

Peak in Ts / Tn is a measure of the superconducting gap

Pb MoGe (at 2 K)


Energy gap shift from pair-breakingEnergy gap shift from pair-breaking

80 mW

20 mW

0 10 20 30 40-0.05

0.00

0.05

−δT /

T

Frequency [cm-1]

• Superconducting Pb film on sapphire.

• Experiment (solid circles) & theory fits (sold lines).

• Good agreement

• Gap shift (~ 0.7 cm-1 or 3%), sensed as change in far IR transmission.

• Density of excess q-p’s <2%, comparable to or less than thermal population

=> weak to moderate perturbation

NSLS U12 IR


Time–dependent relaxation measurements

Time–dependent relaxation measurements

-1 0 1 2 3 4 5 6

-10

0

10

20

Time [ ns ]

MoGe 16.5nm at 2.2K

Pho

to-in

duce

d IR

Sig

nal [

arb

. ] differential integrated

-1 0 1 2 3 4 5 6

0

5

10

15

Phot

o-in

duce

d IR

Sig

nal [

arb

. ]

Time [ ns ]

experiment fit

−+−+

−−=∆wttwerfttwAT 00

2

2

14

exp21

τττ

Differential Technique:

– pump-probe delay “dithered”.

– differential transmittance signal (spectral average) for a range of delay time.

– time-dependent relaxation of excess quasiparticles by integration.

Relaxation Behavior:

– convolution of simple exponential decay and Gaussian synchrotron pulse.

– decay time ~ 1 ns.

– time-resolution determined by synchrotron pulse width (~300 ps).


Temperature dependence of relaxation time

Temperature dependence of relaxation time

0.4 0.5 0.6 0.7 0.8 0.9 1.00.0

0.5

1.0

1.5

2.0

τ eff [

ns

]

T / Tc

8.3 nm 16.5 nm 33 nm

(1/ 2) (1 / )eff R Bγ γτ τ τ τ τ≈ + +

1.357510055033

1.877915045016.5

3.501053704208.3

ττττR(0)/ττττB(0)ττττB(0)[ps]

ττττR(0)[ps]

ττττγγγγ

[[[[ps]d

[nm]

( )3(0) (1 ) 2R cb kTτ λ π= +

( )2 20(0) 4 0B av

N Nτ π αΩ= ∆

S. B. Kaplan et al.

• Relaxation times for MoGe films:


• Apply magnetic field

• Field puts vortex cores in the superconductor

• These give another relaxation channel

• Expect that the relaxation time will decrease with increasing magnetic field

• Follows the area density of vortices?

Magnetic fieldMagnetic field


Relaxation timeRelaxation time

• Minimal field variation up to Hc2/2, a factor of two above that• Mechanism above that unclear. Gap closes with increased field; this

will shorten lifetime.


Semiconductor Photoexcitation and RelaxationSemiconductor Photoexcitation and Relaxation

startlight

excitatione-phopt coll.∆t < 1 ps

e-phacous coll.∆t ~ 100’s ps

recombination(radiative, defect)

∆t > 1 ns

Undoped semiconductors at low temperatures

time

conductionband

valenceband

Eg

• Far-IR sees the moble free carriers.

• What is the effect of magnetic ordering in DMS?


Decay ParametersDecay Parameters

• The decay follows

• where S is the measured temporal response, A is the measured signal magnitude, T is the probe pulse width, and τ is the effective lifetime.

')(2)'(

0

'

dteeT

AtS Tttt −−

+∞−

∫= τ

π


Very short, very intense pulsesVery short, very intense pulses

• From SDL (Shen et al PRL 2007).• Equivalent to 1 cycle of THz radiation• E = 0.7 MV/cm• Leads to very nonlinear phenomena

– RF current > jc in superconductors– Hole burning in inhomogeneously

broadened transitions– Optical rectification in nonlinear materials– Domain wall motion in ferroelectrics


SummarySummary

• Synchrotron is a useful source of FIR radiation–High brightness makes it superior to Hg arc in FIR

• Gives opportunities for very far-IR spectroscopy

• Time structure enables pump/probe studies of many materials.

• Unique feature: broad spectral range of probe: FIR—visible

–Thanks to DOE, grant DE-FG02-02ER45984 at UF and DE-AC02-98CH10886 at the NSLS

The end


DMS TransmittanceDMS Transmittance

• DMS film: InGaMnAs (50nm) / InGaAs (120nm) / InP(001) (0.4mm)

• ~7.5% Mn doping level

• Ferromagnetically ordered below 40 K


Photoinduced SpectroscopyPhotoinduced Spectroscopy

• -T/T 1d = DCd/(1+ω2/Γ2), where S = DCdΓ = ne2d/m

• The Drude fit predicts a carrier density n ~ 71015 cm-3 per laser pulse.

• An estimate of photoinduced carrier density direct from laser pulse is n ~ 51015 cm-3 .

infrared spectroscopy of solids at the nslsinfrared spectroscopy of solids at the nsls j. hwang, h....

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