Mid infrared-terahertz double Mid infrared-terahertz double resonance experiments at resonance experiments at Jefferson Lab – a progress Jefferson Lab – a progress

report. report. Harvey Rutt, University of Southampton

Michael Klopf, Jefferson Lab

The ‘JLab Team’!

SR User Meeting, 14th September 2007

What are we trying to do?What are we trying to do?THz

JLab CSR source

Mid IRGlowbar(initially)

Sample

Detect the influence of

this….

On this

Easy

NoveltyNovelty

Fully frequency multiplexed in the MIRFully frequency multiplexed in the MIR We should see double resonance We should see double resonance

wherever it occurs in the MIRwherever it occurs in the MIR CSR source has the power to do broad CSR source has the power to do broad

band excitation; PRF changeableband excitation; PRF changeable pS excitationpS excitation Precise, absolute phase informationPrecise, absolute phase information

Reminder….Reminder….THz

JLab CSR source

Mid IRGlowbar(initially)

Sample

Detect the influence of

this….

On this

Easy

In reality:In reality:

THzMIR

1 10 100 10000

1

2

3

4

5

6

7

8

9

10

Frequency (THz)

100 MHz 100 pC 150 x 150 mr

0.1 ps 0.3 ps 1.0 ps

0.1 1 10

Wa

tts/cm

-1

Frequency (cm-1)

840 W

540 W

54 W

The CSR SourceThe CSR Source

MHzN

F1497

Major interests74.9 MHz37.4 MHz18.7 MHz9.4 MHz4.7 MHz

Why? – gases.Why? – gases.

(Actually it wont work for CO2!The molecule must have a permanent

dipole moment.)

THz THz

Really a trial run for what follows.

But of some interest in measuring rotational relaxation rates, and possibly

for some specialist spectroscopicapplications.

Why - biomoleculesWhy - biomolecules

Amide (& other)bands are

structure dependent.

Myoglobin structure – ex WikipediaMyoglobin structure – ex Wikipedia

‘THz mode’

N-H The large scale motion of the‘THz mode’ modulates the

environment of the N-Hgroup.

Can we detect this?

A few little practical issues:A few little practical issues:

You need a special MIR You need a special MIR detectordetector

Best possible D* - as ever.Best possible D* - as ever. Crucially, Crucially, fastfast Normal 77K MCTs are typically~1MHzNormal 77K MCTs are typically~1MHz Our PRF is MHz to tens of MHz – and we Our PRF is MHz to tens of MHz – and we

would like low, stable phase shift.would like low, stable phase shift. VIGO TEC cooled, ~150MHz, VIGO TEC cooled, ~150MHz,

D* >2.5*10D* >2.5*1099cmHzcmHz1/21/2WW-1-1

Block THz with n-Ge; BaFBlock THz with n-Ge; BaF22 grid polarizer to grid polarizer to reduce residual fringes & further reduce reduce residual fringes & further reduce THz.THz.

You really need a good You really need a good purge….purge….

In the MIR, water(1300-1900cm-1)

and CO2

(~2340cm-1)are also serious

problems.

1m of air

Purge problems!Purge problems!

Window material choices.Window material choices.

Diamond; expensive in this apertureDiamond; expensive in this aperture High resistivity oxygen free ‘FZ’ High resistivity oxygen free ‘FZ’

silicon; low THz loss, some mid-IR silicon; low THz loss, some mid-IR loss but acceptableloss but acceptable

High index leads to extremely strong High index leads to extremely strong FP fringesFP fringes

Wedge angles very largeWedge angles very large Brewster angle the only solution.Brewster angle the only solution.

Doesn’t look that difficult!Doesn’t look that difficult!

Trickys gas cell:Trickys gas cell:

Conical bore to reduce Conical bore to reduce unsupported window area.unsupported window area.

Signal processingSignal processing

The signal is frequency multiplexed.The signal is frequency multiplexed. The ‘normal’ absorption spectrum is, as The ‘normal’ absorption spectrum is, as

usual in the low kHz region.usual in the low kHz region. The ‘interaction’ spectrum occurs as The ‘interaction’ spectrum occurs as

side-bands on an RF carrier at the FEL side-bands on an RF carrier at the FEL PRFPRF

In-phase (absolute!) is the prompt signalIn-phase (absolute!) is the prompt signal In quadrature is a ‘thermal’ signal In quadrature is a ‘thermal’ signal

How?How?

MIR Detector

FTIR SpectrometerNicolet

RF Phase sensitive detectorSR844

kHz region data

MHz region data

‘Normal’ FTIR spectrum

Phase referencefrom

accelerator

In Phase In quadratureAnti-Aliasing

Filter Anti-AliasingFilter

Data acquisition

‘Interaction’ spectrum

MIR BeamPolarize

r

THz block

Trivial – ground loops…….Trivial – ground loops…….

MIR Detector

FTIR SpectrometerNicolet

RF Phase sensitive detectorSR844

kHz region data

MHz region data

‘Normal’ FTIR spectrum

Phase referencefrom

accelerator

In Phase In quadrature

MIR Beam

Anti-AliasingFilter Anti-Aliasing

Filter

Data acquisition

All cables RG58to RG223 & new

BNC

Isolation amp

RFtransformer

HP Filter

Ferrite

Low jitterfibre optic

60dB better; maybe 15dB to go…………….

MIR BeamPolarize

r

THz block

‘Interaction’ spectrum

Future work.Future work.

Make it work!Make it work! GasesGases Small proteins etcSmall proteins etc Time resolved version using the MIR Time resolved version using the MIR

FELFEL Narrow band filter the THzNarrow band filter the THz Or use a THz FELOr use a THz FEL

Acknowledgements.Acknowledgements.

The entire JLab team!The entire JLab team! Kevin Jordan for extraordinary efforts in Kevin Jordan for extraordinary efforts in

improvising purge boxes!improvising purge boxes! Tom Powers and Pavel Evtushenko for Tom Powers and Pavel Evtushenko for

instrumentation helpinstrumentation help Gwyn Williams & George Neil for making it Gwyn Williams & George Neil for making it

possiblepossible Ken Frampton in the ECS workshop for Ken Frampton in the ECS workshop for

making very awkward shaped bits of making very awkward shaped bits of stainless steel.stainless steel.