coherent thz pulses from the nsls sdl photo-injected ......coherent thz pulses from the nsls sdl...

Coherent THz Pulses from the NSLS SDL Photo-injected Linac and Applications in Materials Science

G. Lawrence (Larry) CarrPhoton SciencesBrookhaven National Lab

in collaboration with Y. Shen, X. Yang, Y. Hidaka, C.-C. Kao, J.B. Murphy, T. Watanabe and X.-J. Wang

Funding: U.S. Dep’t of Energy contract DE-AC02-98CH10886

Outline• What are coherent THz pulses? What are they used for?

• The NSLS Source Development Lab photo-injected linac and accelerator-based THz.

• Some possible applications of strong coherent THz pulses in condensed matter physics.

• Electro-optic sampling, jitter and strong E-field effects.

• Demonstration of strong-field effects in superconductors.

Talks I am missing at Brookhaven …Subject: NSLS2 seminar talksDate: Mon, 10 Jan 2011 17:44:02 -0500From: Cheng, Weixing <[email protected]>To: Photon Sciences Personnel <[email protected]>CC: Cheng, Weixing <[email protected]>, Krinsky, Samuel <[email protected]>

There will be seminar talks tomorrow and Wednesday.

1. Tuesday, January 11, 2011"Super KEK-B Status"Makoto Tobiyama, High Energy Accelerator Research Organization, KEKHosted by Sam Krinsky, Weixing Cheng1:30 PM, NSLS-II Seminar Room, Bldg. 817

2. Wednesday, January 12, 2011"Compact ERL Project Status at KEK"Takashi Obina, High Energy Accelerator Research Organization, KEKHosted by Sam Krinsky, Weixing Cheng1:30 PM, NSLS-II Seminar Room, Bldg. 817


• The NSLS Source Development Lab photo-injected linac and accelerator-based THz..




Coherent THz pulses• Single or few cycle EM waves having 1ps duration & spectral content in the

0.1 to 10 x1012 Hz range => spectral width is typically comparable to the frequency content.

• Produced with ultra-fast lasers (e.g. optical self-rectification in a non-linear material)or as radiation from ultra-short electron bunches.

• Allow coherent detection (of E-field) => magnitude and phase information −> direct connection to complex optical response

• Note that 1 THz is about:– 300 μm wavelength– 33 cm-1

– 4 meV– 50 K

• THz can sense: Intraband electronic transitions, acoustic phonons and soft modes, vibrational/rotational modes of (large) molecules, charge and spin density waves, magnons, cyclotron and spin resonance, pair-breaking in superconductors …

time

E-field “half-cycle” pulse

~<

Synchrotron radiation: E-field for a single electron

.e

δt ∼ ρ /2cγ3

log

Inte

nsity

log frequency

ωc ~ 2cγ3/ρ

time

Elec

tric

field

32~

γρδc

t

• Dipole bending magnet radiation: Monopolar current, expect ½ cycle E-field pulse

A “bunch” of 10 electrons

Time or Distance

Resulting radiated E-field for 10 electronsAt low frequencies (λ > bunch length), Erms scales as n => I ~ n2 (coherent enhancement)

Time or Distance

Coherent Enhancement:

Difficult to achieve in storage rings due to relatively long bunch length (cm’s and longer).

Linear accelerators are capable of producing much shorter bunches ( <100 fs) with number of electrons ~1010

( ) ( ) ( )[ ] ( )ωωω

ωω

ddIfNNN

ddI

clemultiparti1−+=

( ) ( )2

/ˆ∫∞

∞−

⋅= drrSef crni rωωwhere (Nodvick & Saxon)

Synchrotron radiation waveforms

.e

time

Elec

tric

field

• Transition radiation: emitted when electron’s Coulomb field crosses abrupt boundary between two different media, usually vacuum and a metal conductor.Bipolar current => Full-cycle E-field pulse

θ ~1/γ

θ ~1/γ

Metal

e

e

Shielding currents in metal respond totime-varying Coulomb field & radiate

e

• Dipole bending magnet radiation: Monopolar current, ½ cycle E-field pulse


• The NSLS Source Development Lab photo-injected linac as a source of coherent THz.




National Synchrotron Light SourceUser Facilities: 2.8 GeV X-ray and 0.8 GeV VUV/IR storage ringsSource Development Laboratory: 300 MeV photo-injected linac and free electron laser

SDL Linac & FEL

X-ray ring

VUV/IRring

10 m NISUS Wiggler (SDI)

The NSLS Source Development Lab Photo-injected Linac300 MeV

S-Band Linac (DARPA)

Ti:Sapphire Laser

BNL Photo-injector IV

Chicane Bunch Compressor

X.-J. Wang

Intense THz Pulses at the Source Development Lab

coherent THz transition radiation100 μJ per pulse (1017 photons) (could be 4X higher)

– single-cycle pulses– spectral content up to 2 THz

1 mm spot, would yield intensity of 3x1013 W/m2

Strong transient E-field … and B-field = E/c ~ 0.3T

photocathodee- gun

dipole chicanecompressor

~300 fs ~300MeVnC electron bunches

THz

Ti:Sapphire800 nm 150 fs pulses

266 nm8 ps~ 8 ps

“mono”~ 8 ps chirped

off-crestsection

~ 300 fs

THz Exp’t

X.-J. Wang, Y. Shen, J.B. Murphy, X.Xi, GLC et al

( )IE Ω= 3772 =108 V/m (MV/cm or 0.1V/Å)

A Comparison of Dipole and Transition Source ParametersSource Type Dipole bend E = 51 MeV

ρ = 1 m Transition E = 51 MeV

θrms (3λ/4πρ)1/3 60 mrad; (λ=1mm)35 mrad; (λ=200μm)

~1/γ 10 mrad

σrms (4λ2ρ/3π2)1/3 5mm; (λ=1mm) 2mm; (λ=200μm) ∼λγ 100mm; (λ=1mm)

20mm; (λ=200μm)

Zo(Rayeigh) (16ρ2λ/9π)1/3 83 mm; (λ=1mm)

48 mm; (λ=200μm) ∼λγ2 10m; (λ=1mm) 2m; (λ=200μm)

Multiple cycle?

Undulator(M. Gensch)

Slicing @ UVSOR(Bielewski et al)

Smith-Purcell

(Walsh & Kimmitt)

Intense sources of coherent THz pulses: Laser vs Accel. Strong fields => more than 10 μJ.

Thermal limits => absorbed power > 1W challenging for low T exp’ts..

Not shown: laser spectral range affected by NLO materials (5 to 10 THz difficult).

Not shown: highest energy laser-based THz pulses often have multiple cycles at higher frequencies(A. Leitenstorfer, U. Konstanz)

Accelerator-based sources offer better combination of pulse energy, rep. rate and spectral content for photoexcitation. Stronger fields even at low THz frequencies, single-cycle waveforms.

Plus, can combine THz and X-rays for unique THz pump

10 μJ

strong field effects

⊕ “slice”

⊕Lasers

⊕ SDL⊕ LCLS

⊕ JLab ERL

Laser Sources of Strong-Field THz via NLO

A. Sell, A. Leitenstorfer, and R. HuberOptics Letters 33, 2767 (2008).

Difference frequency generation in a solid

Several-cycle pulses: E ~ 100 MV/cm: useful for some science problems but not all

Strong field with less pulse energy due to smaller volume (diffraction limits spotsize)

5 to 15 THz difficult due to NLO absorption (phonons)

< 5 THz requires more pulse energy for strong fields.

Science Opportunities with Intense Single-Cycle THz PulsesSystems driven by large transient Electric and/or Magnetic Fields.• Superconductivity: dynamics of phase excitations

– how do vortices initially form at the instant J exceeds Jcritical?– how does SC state recover? (SC magnets, RF cavities)– disrupting phase order in cuprates

• Ferroelectricity:– maximum switching rate, domain wall velocity?

• Magnetism:– spin dynamics (precession, relaxation)

• Dielectrics:– onset of dielectric breakdown

• Structural distortions: soft modes, lattice / electron coupling– detailed atomic potentials (double well, flat bottom)?

• Semiconductors: high-field transport, band structure– dynamics of band-bending and internal fields

• Non-linear optics

Jsuperconducting film

Some phenomena could be probed with an ultra-fast x-ray pulse

Strong Transient E-Field: Ferroelectric Switching

Y-H Shin et al, Nature449 016165 (Oct 2007)

Calculations for PbTiO30.5 MV/cm field strength

Measurement idea:Strong transient E-field to induce domain wall motion on femto-second time scale to sense fundamental limits on switching

½ cycle pulse desirablefor driving polarization state, sense domains by microdiffraction?

• Idea:Use strong THz field to affect magnetization state of a thin film on < 10-12 s time scale.

• Ex situ approachUse propagating THz wave external to accelerator. Contrast study at SLAC/SPPS (Stöhr et al, Nature) where specimen was placed inside linac and directly exposed to electron beam. Need “half-cycle” type of THz waveform.

• Method:pre-saturate film, expose to THz field pulse, then perform post image analysis (SEMPA)

Strong Transient H-Field: Magnetization SwitchingH

THzFigure from C. H. Back, et al., Science 285, 864 (1999)

Stohr et alStanford / SLAC

Electromagnons in Multiferroic RMnO3Complex coupling of a lattice vibrational mode

(electric polarization) and a lattice spin wave (magnon) in a multiferroic oxide.

Experiment: Pump the electromagnon with few cycle THz pulse matched to mode frequency, measure lattice (diffraction) and magnetization (element-specific XMCD) as function of time

321

Frequency (THz)321

Frequency (THz)0 4 5

A. Pimenov et al, Nat. Phys. 2, 97 (2006) ; A. Sushkov, J. Phys: Cond. Mat 20, 434210 (2008); R. Valde´s Aguilar, Phys. Rev Lett. 102, 047203 (2009)

J.-H. Kimet al

Excitations within a potential well

Can THz waveform shape be controlled to drive system through and into specific quantum states?

Laser slicing (IMS/UVSOR). Variable field EM undulator? Smith-Purcell?

V distance

E-field

time

Superconductivity and DynamicsOrdered, coherent state of paired electrons (Cooper pairs) in a conductor.– electrons in a pair have opposite momentum and opposite spin.

– sea of paired electrons nsuper, behaves as kind of superfluid.

– gap Δ forms in the electronic (excitation) spectrum.

– coherent state can be described by a complex order parameter −> ψ = ψ0e iϕ ~ Δ e iϕ • amplitude ψ0 ∼ Δ is measure of gap and pair density• phase ϕ describes electrodynamics

– supercurrent density J ~ ∇ϕ– voltage (potential) difference φ ~ ∂ϕ /∂t

– gap revealed in electron (quasiparticle) tunneling, photoemission, THz spectroscopy.

Phase Excitations and Dynamics in Cuprates and Arrays

SC

AFIPG

metal

T

x

Pairing driven by spin interactions - occurs for temperature above TC? If so, have non-zero order parameter amplitude for T>TC, long range superconductivity results when phase ordering takes place.

Normally, unable to sense electronic scattering under the SC dome by spectroscopy (superfluid dominates)..

Use ultra-fast strong-field THz to disrupt phase, probe under the SC dome: THz to sense transport, x-ray pulse for stripes. Follow recovery (time-resolved).

Regular and random Josephson Junction arrays (weakly connected superconducting islands)

A model system for studying SC phase excitation and relaxation: THz pump and THz probe

Graphene

Graphene as a Non-linear Optical Material

For Ω = 2πx1012 rad/s (1 THz), T=300K, V=106 m/s => 8 kV/cm but, with electronic scattering at rate Γ, need factor of Γ/Ω more field => 50 to 200 kV/cm

S.A. Mikhailov, Eur. Phys. Lett. 79, 27002 (2007)

Graphene Nanoplatelet films are thick (0.35 μm), have large area (cm2) and show Dirac-particle behavior. What is electromagnetic response for a system of (nearly) mass-less particles moving at a “fixed” speed?

calculatedcurrentresponse (to AC field)

calculated strength of odd harmonics

THz Characterizion: Standard Electro-Optic MethodsCoherent detection setup for measuring THz waveforms using Pockels Effect: “THz Electro-Optic switch” (Zhang et al, Heinz et al)

( ) ( ) ( ) ( ) K+−+−+= 22

2

0 ½ oo ttdtEdttdt

dEtEtE

λ/4polarizer polarizing splitter

samplingpulse

ZnTeTHzE(t)

~300 fsE-field signal

Result: Detector signal gives instantaneous THz E-fieldWorks well for low jitter and high rep. rates.

Electro-optic material (ZnTe) acts as a “variable waveplate”

L

( )⎥⎦⎤

⎢⎣⎡ Δ+−⎟

⎠⎞⎜

⎝⎛ ttznE Elaser φωλ

π0

0

2cos~ ( ) ( )[ ]tEnLt THzE Δ⎟⎠⎞⎜

⎝⎛=Δ

0

2λ

πφwhere

THz Characterizion: Electro-Optic ImagingCoherent detection setup for measuring THz waveforms using Pockels Effect: “THz Electro-Optic switch” (Zhang et al, Heinz et al)

( ) ( ) ( ) ( ) K+−+−+= 22

2

0 ½ oo ttdtEdttdt

dEtEtE

λ/4polarizer polarizer

samplingpulse

ZnTeTHzE(t)

~300 fs

Result: Snapshot of instantaneous THz E-field.

Electro-optic material (ZnTe) acts as a “variable waveplate”

L

video camera

( )[ ]ttkzE Elaser ωφ −Δ+cos~ ( ) ( )[ ]tEnLt THzE Δ⎟⎠⎞⎜

⎝⎛=Δ

0

2λ

πφwhere

E-field along horizontal plane

Note: opposite sides are asymmetric, as shown

(radial polarization)

Measurement

Simulation/2(assumes stronger focus)

Temporal-spatial E-field profile of coherent radiation pulse @ 5X source demagnification

EO sampling of SDL Linac THz Pulses: Spatio-temporal Map

Jitter (~ 150 fs) limits ability toextract detailed waveforms & spectra.

Need a “single-shot” method

Single-Shot Electro-Optic Method

linearpolarizer

spectrometer witharray detector

Use chirped sampling laser to encode waveform’s entire time-dependence onto different wavelengths of laser in a single pulse. Avoids need for multiple sampling.[Jiang and Zhang, Appl. Phys. Lett. 72, 1945 (1998)].

linearpolarizer

λ/4

Setup for single-shot EO sensing of THz waveform

chirpedsamplingpulse

ZnTeTHz

Wavelength | Time

Single-Shot EO Sampling of SDL THz Pulse using Chirped Laser

775 780 785 790 795 8000

500

1000

1500 no THz

In

tens

ity [a

rb.]

Wavelength [nm]

-3 -2 -1 0 1 2 3 Time [ps]

775 780 785 790 795 8000

500

1000

1500 no THz THz ON

In

tens

ity [a

rb.]

Wavelength [nm]

-3 -2 -1 0 1 2 3 Time [ps]

peak is too high!

1x

2x

Single-Shot EO Sampling of SDL THz Pulse: Higher intensity

385 380 375 370

Frequency [THz]

Spe

ctra

l Int

ensi

ty [a

rb.]

775 780 785 790 795 800 805 810 815

No THz THz ON

Wavelength [nm]

Non-linear Optics with Strong THz Pulses at SDL“Simple” EO setup to observe time-dependent phase modulation (no initial laser chirp)

Without polarizers, sense only time-dependent components

( ) ( ) ( ) ( ) K+−+−+= 22

2

0 ½ oo ttdtEdttdt

dEtEtE

polarizer

Ti:S laserpulse

ZnTeTHzE(t)

L

tinst βωω +=2),( ttkxtx βωφ −−= → spectral shifting & chirpingso, if then

( )[ ]ttkzE Elaser ωφ −Δ+cos~ ( ) ( )[ ]tEnLt THzE Δ⎟⎠⎞⎜

⎝⎛=Δ

0

2λ

πφwhere

Y. Shen et al, Phys. Rev.Lett.(2007)

Calculated Phase Modulation Effects

Other details: Lensing from spatial variation of n(t) (time-dependent gradient index lens)

500 kV/cm field

0.5mm thick ZnTe

380 375 370 Frequency [THz]

Spec

tral I

nten

sity

[arb

.]

780 790 800 810 Wavelength [nm]

-1 0 1

-1.0

-0.5

0.0

0.5

1.0THz

Laser

Time [ps]

E-fields

Laserspectra

( ) K+⎥⎦

⎤⎢⎣

⎡⎟⎠⎞

⎜⎝⎛+⎥⎦

⎤⎢⎣⎡ −⎟

⎠⎞⎜

⎝⎛+= 2

2

20)( tdt

EdtdtdEEt THzTHz ηωηηφ

(a) No THz

(c) Δt=250fs

(d) Δt=500fs

(e) Δt=750fs

(f) Δt=900fs

(b) Δt=0

790 800780 810Wavelength (nm)

Measured Phase Modulation with SDL Linac Coherent THzElectro-optic measurements of SDL THz pulses.

35 μJ pulses, 2mm focus, 0.5mm ZnTe.

~ 130 fs (FWHM) unchirped laser sampling pulse, no polarization analysis.

Probably still a mixture of effectsphase modulation (2nd and 3rd order NLO)dynamic lensing that affects coupling into

spectrometer’s optical fiber.

Vary

delay

betw

een T

Hz an

d las

er pu

lse222

02 ynn α±=Δ THzErnn 41

30

4=Δ

α2 ~ 0.0023 for E =1 MV/cm f ~ 400 mm for d = 0.5mm

( )2220

2 1 ynn α±= dnf 21

α≈

Related Effects:Lensing due to parabolic refractive index gradient

EO Detection of Bunch Coulomb Field (inside linac)X. Yan et al (PRL ’00)I. Wilke et al (PRL ’02)H. Loos et al (PAC ‘03)

H. Loos et al

Full calculation with modulation effects

THz Compression of an Ultra-Fast Laser Pulse

Y. Shen et al, Phys. Rev. A (2010)

Set up

Results

165 fs laser pulse, compressed to 45 ps (factor of 3.7)

Superconductivity and DynamicsOrdered, coherent state of paired electrons (Cooper pairs) in a conductor.– electrons in a pair have opposite momentum and opposite spin.

– sea of paired electrons nsuper, behaves as kind of superfluid.

– gap Δ forms in the electronic (excitation) spectrum.

– coherent state can be described by a complex order parameter −> ψ = ψ0e iϕ ~ Δ e iϕ • amplitude ψ0 ∼ Δ is measure of gap and pair density• phase ϕ describes electrodynamics

– supercurrent density J ~ ∇ϕ– voltage (potential) difference φ ~ ∂ϕ /∂t

– gap revealed in electron (quasiparticle) tunneling, photoemission, THz spectroscopy.

Response of a Superconductor to a low ω THz Pulse

Current can potentially exceed JC

How does the superconducting state fail? How quickly? What controls the recovery?How can (or should) one measure the behavior?

( ) tdtEJt

gn ′′≅ ∫∞−

ωσ

Jc ~ 108 A/cm2

Incident THzFull-cyclepulse

Low ω response is almost purely inductiveE-field accelerates superfluid (current density J).

Model Calculation: Finite Difference Time Domain Technique• FDTD starts with discrete formulation of Maxwell’s equations. (K. Yee - ‘66)

• Dielectric response included through displacement.

• Solve numerically– Recursive convolution method for materials where loss is described by exponential damping

(e.g., Lorentzian) (Luebbers, Hunsberger and Kunz - ‘91).– Drude model dielectric response: describes a normal metal (τ small)

or a superconductor (τ large) – Successfully used for time-resolved THz spectroscopy (where ωp and/or τ are themselves time-

dependent). (Beard and Schmuttenmaer - ‘01)

( ) [ ] ( )τθτωχ τtp et −−= 12( ) ( )ωτ

τεωωσ

ip

−=

10

2

( ) ( ) ( ) ( ) ττχτεεε dtEtEtDt

∫ −+= ∞ 000

rrr

tDH

∂∂

=×∇r

r

tBE

∂∂

=×∇v

r

also B(t) as function of χm(τ)

frequency domain time domain

FDTD Calculation for Transmissionω << 1/τ

ω >> 1/τ

THz Optics for Low-Temperature Studies (Superconductors)THz extraction, electro-optic and small signal detection setup (motors, motors & motors)

Optical cryostat for 5.5K<T<350K

detectorfilter wheel

samplefilter wheel

wire gridpolarizers

THz detector

THz beamTi:sapphire laser

NbN: Transmitted Spectral Intensity versus E-Field Strength

5% ND THz Filter

increasing THz E-field strength

Detected THz rel. to incident

Broadband THz Detector

Incident THz (ω < ωg)

Low-pass filter

Can we detect any non-linear THz upconversion?

Summary & Outlook• Linac-based sources of coherent THz can serve as unique tools for probing ultra-fast behavior in

materials subjected to strong transient fields. – MV/cm E-field and nearly 1T magnetic field, ps or faster pulse.– Two of the U.S. Dep’t of Energy Basic Research Needs.

• Pulse repetition rate > 1kHz desirable for S/N, high charge and flexible bunch length.– Advantage for Energy Recover Linac (ERL) .– 2σ bunch lengths from ~3ps down to 30 fs would be good. (0.1 THz to 10 THz)– 100 pC or higher, especially for longer bunches (need more charge with long bunches for same E).

• Dipole bend and Transition radiation sources result in similar basic power and energy, but source geometry and extraction may favor one over the other.

– Simpler polarization and source size for dipole bend.– Large apparent source size for transition, but interesting radial polarization.– Note: expect to be able to use conventional optics to control waveform shape and polarization.

• A number of interesting science experiments can be envisioned using the source as a pump– Combined with UV, soft and hard x-rays for unique THz pump, spectroscopic or structural probe..– Shown that the phase of a thin film superconductor can be completely disrupted within a full-cycle pulse.

• Stability is Important for both the THz and for supporting optical systems (e.g., ultra-fast laser used in electro-optic sampling).

– Laser research community expects stability comparable to that in their own Lab.

Thank you for your attention

coherent thz pulses from the nsls sdl photo-injected ......coherent thz pulses from the nsls sdl...

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