high resolution spectroscopy using a tunable thz synthesizer based on photomixing arnaud cuisset,...

HIGH RESOLUTION SPECTROSCOPY USING A

TUNABLE THz SYNTHESIZER BASED ON PHOTOMIXING

Arnaud Cuisset, Laboratoire de Physico-Chimie de l’Atmosphère, Maison de la Recherche en Environnement Industriel, Dunkerque France

66th International Symposium on Molecular SpectroscopyJune 23, 2011

Motivation

Development of spectrometers using optoelectronic sources dedicated to gas phase THz analysis of molecular systems with environmental interest

Line position and Line profile analysis of atmospheric compounds from their rotational signatures in the THz range

Detection, quantification and kinetic studies of pollutants in realistic media


CW-THz Photomixing(Hindle et al Comptes Rendus Phys. 9, pp. 262-275, (2008))


Spatial overlapping of two continuous lasers with THz

frequency separationV

Optoelectronic conversion of

the THz beatnote into photocurrent

THz wave emission via a

specially designed antenna

PTHz Vdc

2 (P1 P2)2Ra(1

THz

2 2)(1THz

2 Ra2C2)

The photomixer: A specific antenna connected toan interdigitated electrode array

on a LTG-GaAs substract

CW-THz Photomixing


100 1000

1E-5

1E-4

1E-3

0.01

0.1

1

Vertical configuration

Det

ecte

d vo

ltage

, V

Frequency, GHz

µW level

First version of photomixer without coating

Planar spiral antenna on ion irradiated InGaAs excited at 1550 nm

Horn antenna antireflection coating Planar spiral antenna on LTG G

aAs

with antireflection coating

A large tunability: 0.3 THz – 3.3 THz ( 10 cm-1 – 110 cm-1)

A good spectral purity: < 3 MHz ( 10-4 cm-1)

A THz output power limited from μW @ 1THz to nW @ 3 THz

An absolute THz frequency accuracy limited to 100 MHz using a commercial

wavelengthmeter. 3261.5 3261.6 3261.7 3261.8 3261.9 3262.0 3262.1

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

Tra

nsm

issi

on

, u.a

.

Fréquence (GHz)

HCN, 3.9mbar J=36T = 293KL = 100mm

1846.68 1846.72 1846.76 1846.80 1846.84

1000

2000

3000

5 1 4 - 4 2 3

6 0 6 - 5 1 5

Tran

smis

sio

n,

a.u

.

Frˇ quence, GHz

Photomixing : How does one obtain the THz frequency?


fTHz= f1-f2 mais f1(775 nm) ≈ f2(775 nm) ≈ 387 THz An accuracy of 1 MHz for the frequency of the THz radiation require the same

order of accuracy on the frequency of Ti:Sa laser (10-9 !!!)

Periodicity in time = periodicity in frequency

f0: Carrier-envelope offset frequency

Frequency metrology based on femtosecond lasers (Th. Hansch)

fsyn

DL2


CW-THz synthesizer 1st generation: :

2 ECDL are locked in phase on 2 lines of the frequency comb

☺ THz frequency accuracy and spectral purity are better than

100kHz

☹ The continuous tunability is reduced to the tens of MHz

CW-THz synthesizer 2nd generation: :The fixed base frequency is generated by locking DL0 and DL1 to a FC. The phase locked tuning of the synthesized frequency is performed by PLL3 locking DL2 to DL1. The optical beatnote between DL0 and DL2 is converted to THz radiation

☞ 300 MHz of continuous tunability

CW-THz Synthesizer based on a frequency comb(Mouret et al., Opt. Express, vol.17, pp. 22031, (2009))

fTHz = fsynt + n.frr

(with f0=-f1)

The CW-THz spectrometer


THz line position measurements on H2CO


In order to test the frequency metrology of the new version of the THz synthesizer, 61 new ground state rotational transitions of H2CO have been measured in the 1.1 – 1.7 THz

region.Parameters global fit fit CDMS (only) fit LPCA(only)*A(MHz) 281970.5731(128) 281970.5746(129) 281967.59(139)B (MHz) 38833.98730(32) 38833.98732(32) 38833.9769(105)C(MHz) 34004.24360(31) 34004.24367(32) 34004.2492( 92) DJ(MHz) 0.07032103( 67) 0.07032153( 71) 0.0703169(133) DJK (MHz) 1.3211015(163) 1.3211070(176) 1.32154( 37)DK(MHz) 19.39260(282) 19.39240(287) 17.64( 78)dl(MHz) -0.010437997(213) -0.010437975(218) -0.0104231(138)d2(KHz) -2.501429( 63) 2.501432( 64) -2.4964(113)HJ (mHz) 3.95( 48) 4.38( 52) -5.3(134)HJK (Hz) 7.446( 37) 7.458( 42) 0.0313(110)HKJ(KHz) 0.010748(201) 0.010850(206) -0.055( 33)HK (KHz) 4.019(301) 3.944(311) -0.223(101)h1 (Hz) 0.032348(155) 0.032349(159) 0.0410(180)h2 (Hz) 0.047869(133) 0.047894(135) 0.053( 37)h3 (Hz) 0.015917( 46) 0.015905( 46) -5.3(140)LJJK (mHz) 0.0926(183) -0.0988(224) -0.12( 87)LJK (mHz) -0.67(286) -1.65(296) -0.176(115)LKKJ (mHz) -1.9( 70) 0.3( 72) 0.386(266)LK (Hz) 3.3(112) 7.1(116) 0.76( 43)l2 (µHz) -0.352(108) -0.380(112) -9( 40)l3 (µHz) -0.404( 38) -0.401( 38) -6.8(140)l4 (µHz) -0.1307(136) -0.1252(143) -8.0( 52)RMS (MHz) 0.035890 MHz 0.033152 0.029183Number of lines 392 331 61

A new list of frequencies is available for the spectroscopic databases.

The molecular parameters in the ground state are fitted at the experimental accuracy, only the highest order terms are sensitive to the new measurements

94,6-84,5 94,5-84,4

244,21 - 234,20


THz line position measurements on H2CO

Recent measurements of rotational transitions in v4 and v6 excited states have been performed in the 0.7 – 1.5 THz frequency region by means of a THz propagation in a

mulipass cell (L=8.8m)

v4: Out of plane bending 1167.3 cm-1

v6: CH2 rocking 1249.1 cm-1

Goal: To characterize the vibrational dependence of the rotational constants providing in particularly the vibration-rotation constants.

Still in progress: 25 new THz lines have been measured with an average precision of 40 kHz

An average uncertainty of 240 kHz is obtained relatively to the predictions of Agnès Perrin


THz line profile analysis on CH3Cl

Large scans allowed a multifit procedure applied on K series of J → J + 1 transitions of CH3Cl

An accuracy lower than 3% is obtained on the broadening coefficients

J=31


THz line profile analysis on CH3Cl

Collisional analysis : Self, O2, N2

Studies from 0.84 THz up to 1.4 THz

Studies on the J and K dependency of the broadening coefficients

Work from Jdown = 31 up to Jdown= 50 and for K=0;1;2;3;4;5;6;9; and 12

K dependency of the self-broadening

Self pressure shift

0 2 4 6 8 10 12

335

340

345

350

355

360

365

370

375

Se

lf p

ress

ure

bro

ad

en

ing

(m

K.a

tm-1)

K

K dependency of the pressure Boadening for j=31 CH3Cl-O2 296 K

J=40->41

65

67

69

71

73

75

77

79

81

0 5 10 15 20 25 30 35 40

Ki=Kf

LW

, 10-

3 cm

-1 a

tm-1

expt [Blanquet_JMS93_O2]nu3calc [Bouanich_JMS93]

EXPT DUNK (1σ)

calc 5terms LR

calc 5 terms L&SR

Excellent agreement with theory

(collaboration with Besançon)


THz line recognition in methanol/ethanol gas phase mixture

The synthesizer is able to discriminate ethanol and methanol spectra from a mixture at THz frequency

The sensitivity of the measurement can be increased by implementing a frequency modulation scheme.

THz detection and quantification of OH and SH radicals produced in a cold plasma


OH is detected in a cold plasma produced by means of a

continuous flow of precursor excited in a RF discharge.

OH Hyperfine

components resolved in the 2,5 THz

region

FM has been used for the recording of

new SH transitions in the 1,3 – 1,6 THz region

2Π3/2: 5/2+ - 3/2-

RF off RF on

Conclusion

Photomixing technique is now an alternative solution to implement THz tunable solid source The photomixing technique has the greatest tuning range of any known coherent source in the THz region, and has a spectral purity inferior to the Doppler broadening limit. Therefore, the photomixing technique allow to generate a THz radiation very promising for gas phase spectroscopic applications:

• Line profil analysis up to high excited rotational levels• Accurate determination of molecular parameters from

THz frequency measurements.• Gas phase monitoring of stable and unstable

atmospheric species



Gaël Mouret Francis Hindle Sophie Eliet Robin Bocquet Mickael Guinet

high resolution spectroscopy using a tunable thz synthesizer based on photomixing arnaud cuisset,...

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