4th Asian Forum for Accelerators and Detectors(AFAD’13), Feb. 25-26, Novosibirsk, Russia

Ultrafast electron diffraction and microscopy using femtosecond RF gun

Jinfeng Yang

Osaka University, Japan

W. E. King, et al., JAP 97, 111101 (2005)

Structural dynamics in matter

An ultrafast measurement with the resolution of 100 fs & sub-Angstrom

has long been in goal for the scientists.

Ultrafast phenomena and dynamics in materials, i.e.

Phase transition in solid state,

Chemical reactions in molecules,Charge transfer in biology,

are occurred on femtosecond time scales over atomic spatial dimensions.

1) Ultrafast X-ray diffraction/image

Picosecond X-ray pulses from SR & femtosecond X-ray pulses from FEL or laserplasmas acceleration have been used.

→ big experiment/measurement,large energy deposited → large damage to samples.

2) Ultrafast electron diffraction (UED)

A fs laser pulse is used as pump, while a fs or ps e- bunch is used as probe.Almost UED facilities use keV DC electron DC guns. Very compact!Recently, the time resolution has been achieved to 100 fs using RF gun.

but no spatial resolution!

3) Ultrafast electron microscopy (UEM)

UEM can observe the dynamics of structural transformation in nanometer (evenatomic) spatial dimensions.

⇒ The resolution of UEM has achieved to 10 ns and 10 nm!

Ultrafast detection techniquesfor study of ultrafast phenomena and dynamics

Principle of ultrafast electron microscopy

： short electron pulse as probe

： short laser pulse as pump

Combination of pump-probe measurement with electron microscopy technique!

Δt=0

Δt>0

Δt = tl - te <0

Electron images Electron images at different timeat different time

Δt=0

W. E. King, et al., JAP 97, 111101(2005)

UEM

Short e- pulse

Phenomena & dynamics

Recent UEMs

•electron bunches with ~10 ns at 108 e-s/pulse•beam energy of ~100keV

(limited by the max. DC field:~10 MV/m)

photocathode DC gun in UEMs

Resolution in UEM is limited to10 ns & 10 nm(B. W. Reed, Workshop on UESDM, 2012)

λ= 211nm10 ns(FWHM)

λ= 1064 or 355nm12 ns FWHM

108 e-/bunch

e-s

B. W. Reed, Workshop on UESDM, 2012Problem using DC gun: Strong space-charge

force in low-energy e- bunch.Problem using DC gun: Strong space-charge

force in low-energy e- bunch.

Space-charge effect in recent UEM

Δt=300fs → 4ps during the 40cm transport of 30keV e- beam

B. J. Siwick et al., JAP 92, 1643(2002)

Bunch length increase due to space-charge force

mvrtNett0

22

2

0 2 επ+Δ≈Δ

1) Increase of bunch length during beam transport

2) Increase of energy spread during beam transport

ΔE/E→ 3x10-3 during the beam transport

1010--33

It is difficult to generate an fs electron beam in keV energy region!

⇒ The solution of the problems is to increase the beam energy!

⇒ The solution of the problems is to increase the beam energy!

Why use RF gun in UEM?

Can RF gun be used for UEM in fs time region?RF gun is a useful source in UED with fs time resolution!

A ~100 fs MeV electron bunch with 107-8 e-s/pulse

Resolution of 100 fs & 10 nm can be achieved?

What kind efforts and challenges are needed?

To answer the questions, we developed a prototype of time-resolved EM using RF gun.

photocathode DC gun

~keV electron bunches with ~10 ns

Single-shot imaging using 108 e-s/pulse

Resolution in UEMs: 10 ns & 10 nm

(B. W. Reed, Workshop on UESDM, 2012)

Recently, UEMs

a great technique for study of structural dynamics!

Ultrafast electron diffractionusing RF gun

Electron energy: 1～3 MeVTime resolution: 100 fs

What photocathode RF electron gun?

RF source

PhotocathodeCu, Mg

Femtosecond laser

2856MHz、~MW

fs, λ=266nm

RF cavities (Usually 1.5-cell)electric field: ~100MV/m

Femtosecond e- bunch1~3 MeV, 107-108 e-/bunch

x10 higher than DC electric field

Recently, the RF gun has been successfully used/proposed in UED facilities at BNL, SLAC, UCLA, Tsinghua Univ. , Osaka Univ.,DESY, Shanghai Jiaotong Univ., KAERI, …

Space-charge effect is reduced!

RF gun based MeV UED at Osaka Univ.

Diffraction

ApertureΦ0.3mm 

4.5x107 e/pulse

Scintillator  

EMCCD

RF gun

Probe 3ω:257nm,90fs,1kHz, 6mW

Pump 2ω:385nm,90fs, 10Hz, ＜40mJ/cm2

3MeV100 fs e‐ pulse

10Hz

apertureΦ0.3mm diffractionSi crystal

sample

P.Lens D.Lens

Probe 3ω

Pump 2ω(Φ600μm,

18.2o,p-pol )

1‐5MW@4μs,10Hz2856MHz

[1-10]

<10-8Pa

C.Lens Solenoid

Φ3.0mm

Φ300, 350μm

Sample

use of electron optical lenses as like in electron microscopy

Area: 5x5cmCsI (Tl) scintillator

Picture of UED system at Osaka Univ.

CCD

Probe laser

RF gun

CLDL

PLS

Pump laser

use of electron optical lenses, therefore, compact.

S.L

2.5 m

Single-shot measurement

1 shot (0.1s)

Si

single crystal SiThickness: 180nme‐ energy: 3MeV

Y. Murooka, et al., APL, 98, 251903 (2011)

1 shot (0.1s)

single crystal AuThickness: 15nme‐ energy: 3MeV

Au

A good-quality electron diffraction pattern can be observed with 105 e-s/bunch!

Laser heating and melting dynamics of single crystal Au

Time-resolved measurement

Electron pulse3 MeV, 100fs, 105e-/bunch

Pump laser pulse385nm,90fs, 40mJ/cm2

after melting

16 ps

3 ps

-4 ps

200

Ultrafast electron microscopyusing RF gun

Electron beam energy: 1～3 MeVTime resolution: 100 fsSpatial resolution: 10 nm

Femtosecond RF electron gun for UEM

Bunch length： ≤100 fsBeam energy： 1~3 MeVEmittance： 0.1 mm-mradEnergy spread： 10-4 (10-5 for challenge)e- number： 107-8 e-’s/bunch (1~10 pC)

The expected beam parameters:

10cm fs laser pulse

RF

fs electron pulse in MeV

eVEkin 26.0≅

Femtosecondphotocathode electron gun

Femtosecondphotocathode electron gun

Electron energy： 1~3 MeVBunch length： �100 fsEmittance： < 0.1mm-mradEnergy spread： 10-4 (10-5 for challenge)Charge： 107~108e-’s/pulse

Challenge !Challenge !

Time resolution: 100 fsSpatial resolution: 10 nmTime resolution: 100 fsSpatial resolution: 10 nm

Prototype of MeV UEM

FemtosecondLaser

Femtosecondelectron beam

electron optical lenses

RF gun

imagemeas.

Prototype of Time-resolved MeV electron microscopy

Sol. lens

Cond. lens

Obj. lens

Proj. lens

Interm. lens

Prototype of MeV UEM in Osaka Univ.（height: 3m, diameter: 0.7m）

The prototype was constructed at the end of Oct. 2012.

Prototype of Time-resolved MeV electron microscopy

FemtosecondLaser

Femtosecondelectron beamFemtosecondelectron beam

Design of 2T objective lens

f0 = 5 mm for 2MeV e-

Cs (Cc) ~ 4 mm

70 cm (D) x 35 cm (H)

Thin Polymer mirror (5µm)ORCA-R2 CCD

•Illu. Vol.(<50µm)• Bright• High resol. (Column Structure)

• Tough (for High Eng. Xray)

• Large: 5x5cm2

CsI (Tl) scintillator (Hamamatsu)

X, e

MeV electron

Detection of MeV electron images

The detection system was successfully used in UED measurement.(single-shot measurement with 105 e-s/pulse)

Single-shot

10 pulses

MeV electron diffractions observed with 10 pulses and single-shot measurements

Electron beam (2.3 MeV)

diff. plane(BFP)

Imaging plane(Selector diaphragm)

Projectorlens

Objective Lens

Sample： Au single crystal, ~15 nmElectron charge： ~100 fC/pulse

Specimen

First MeV electron diffraction in UEM prototype

A goodA good--quality MeV ED was observed!quality MeV ED was observed!

SingleSingle--shot measurement is available!shot measurement is available!

21

Observed images of Au film

First MeV electron imaging in UEM prototype

Images of Au film

Observation #1

Exposure time：2s Exposure time：60s

X385 x650 x1200

x650 x1200

10μm

10μm

38nm/pixel

38nm/pixel

Observation #2

Electron beam (2.3 MeV)

Interm.image planeProjector

lens

Specimen

Final image screen

BFP(Obj. Aper.)

Objective Lens

Sample: Au single crystal, 15 nmElectrons at screen: ~10 fC/pulse 83μm

38 μm

x50

Magnifications & spatial resolution

Electron beam (~MeV)

BFP

Interm.image plane

(Obj. Aper.)

Projectorlens

Objective Lens

Sample： Cu grid, 1000 meshElectron charge： ~10 fC/pulse

Specimen

Final image screen

StandardCu grid

(1000mesh)

�測例① X650 x1700

5 μm 30 nm/pixel75 nm/pixel10μm

X200

σ～300nm

σ～300nm

25μm

5.5 μm

Femtosecondelectron beam

Objective lens

Specimen

Intermediate lens

Projector lens

CL-1

CL-2

Solenoid lens

RF gun

Spatial resolution300 nm (present)

10nm (next step)

<1nm (in future)

New science and technology using UEM

Structural observation and imaging in “real space” with atomic-scale spatial resolution using high-energy electron beam.

Imaging Technologyイメ�ジング テクノロジ�

Electron beam（1～3MeV）

Observations of fundamental dynamic processes in matter occurring on femto-second time scales over atomic spatial dimensions.

UltrafastObservation超高速現象の�察

Electron pulse（probe）

Pump laser pulse

Methods

TargetsProtein Structural Dynamicsタンパク質構造ダイナミクス

•Protein structural dynamics•Macromolecular structure•Reveal of functioning processes•New technologies and applications in medical biology.

Making Molecular Movie分子運動の可視化：－新しい科�－

•Observation of single molecule motion.•Ultrafast chemical reactions•Solvation dynamics•Discovery of transition states and reaction intermediates.

Nano-technology/scienceナノテクノロジ��サイエンス

•Transformation dynamics of novel nano-scale materials.•Creation of new functional materials and devices for nanotechnology.

Compact high-voltage electron microscopy(dream TEM)

Phase transitions

Novel ultrafast technique at atomic level(dream UEM)

Conclusion

However, great efforts and many challenges are required:

reduce further the emittance (<0.1 μm) and energy spread (10-5 or less),

increase the beam brightness,

improve the stabilities on the charge and energy,

develop a detection of very electron with MeV energy, and so on.

Both RF gun based UED and UEM systems have been constructed at Osaka University.

In UED, single-shot and time-resolved measurements have been succeeded.

In UEM, the MeV electron diffraction and imaging experiment was carried out.

Both experiments suggest that RF gun is very useful for ultrafast MeV electron diffraction and is also expected to be used in ultrafast electron microscopy.

Acknowledgments

Co-workers at Osaka University:N. Naruse, Y. Murooka, K. TanimuraK. Kan, T. Kondoh, Y. Yoshida

Collaborators:J. Urakawa(KEK), T. Takatomi(KEK), R. Kuroda(AIST)

25

Present work was supported by Grant-in-Aid for Scientific Research from JSPS!

10m

3m

Standard 3-MeV TEMat Osaka Univ.

Next TEM: “Dream TEM”

Compact High-VoltageTransmission Electron Microscopy

•high-voltage TEM function(nm or sub-nm, MeV)

+•time-resolved function

(femtosecond)

Next TEM

3m

Dream TEM!

Weight: 140t

Photocathode RF gunPhotocathode RF gun

20cm

Electron energy： 1~3 MeVBunch length： 100 fsEmittance： 0.1 mm-mradEnergy spread： 10-4~10-5

Bunch charge： 107~108 e-

RF gun based ultrafast electron diffraction at Osaka Univ.

4.5x107 e/pulse

Scintillator  

CCD

Femtosecond e‐ beam from RF gun

Probe 3ω:257nm,90fs,1kHz, 6mW

Pump 2ω:385nm,90fs, 10Hz, ＜40mJ/cm2

3MeV100 fs e‐ pulse

10Hz

apertureΦ0.3mm diffractionSi crystal

sample

P.Lens D.Lens

Probe 3ω

Pump 2ω(Φ600μm,

18.2o,p-pol )

[1-10]

<10-8PaC.Lens

Solenoid

• Single-shot measurementusing 3 MeV fs e- bunch with 105 e-/bunch

1 shot (0.1s)

Si

1 shot (0.1s)

Au

•Time-resolved measurementlaser melting dynamics in Au film

200

Electron pulse（probe）

Pump laser pulse

after melting

16 ps

3 ps

-4 ps

Bunch length： �100 fs at 1~3 MevEmittance： < 0.1mm-mradCharge： 107~108e-’s/pulse

RF gun is very useful for ultrafast electron diffraction!

(height: 3m, diameter: 0.7m)

Prototype of Time-resolved MeV electron microscopy at Osaka Univ.

FemtosecondLaser

Femtosecondelectron beamFemtosecondelectron beam

FemtosecondLaser

Femtosecondelectron beam

electron optical lenses

RF gun

imagemeas.

�MeV electron diffraction

�MeV electron imaging

The prototype of time-resolved TEM was constructed at the end of Oct. 2012.Both the MeV electron diffraction and imaging experiment was carried out.