Page 1

A hydrogen-bonded organic nonlinear

optical crystal for high-efficiency

terahertz generation and detection

Fabian D. J. Brunner,∗O-Pil Kwon,†Seong-Ji Kwon, Mojca Jazbinˇ sek,

Arno Schneider, and Peter G¨ unter

ETH Zurich, Institute of Quantum Electronics, Nonlinear Optics Laboratory, 8093 Zurich,

Switzerland

†Present address: Ajou University, Department of Molecular Science and Technology, Suwon

443-749, Korea

∗Corresponding author: nlo@phys.ethz.ch

Abstract:

hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene]malononitrile

by optical rectification of sub-picosecond laser pulses. We show that

OH1 crystals allow velocity-matched generation and detection of THz

frequencies in the whole range between 0.3 and 2.5THz for a pump laser

wavelength range from 1200 to 1460nm. OH1 crystals show a higher

figure of merit for THz generation and detection in the optimized range

compared to the benchmark inorganic semiconductor crystals ZnTe and

GaAs and the organic ionic salt crystal 4-N,N-dimethylamino-4′-N′-methyl

stilbazolium tosylate (DAST). The material shows a low THz absorption

coefficient α3in the range between 0.3 and 2.5THz, reaching values lower

than 0.2mm−1between 0.7 and 1.0THz. This is similar as in ZnTe and

GaAs, but much lower than in DAST in the respective optimum frequency

range. A peak THz electric field of 100kV/cm and a photon conversion

efficiency of 11 percent have been achieved at a pump pulse energy of 45µJ.

Broadband THz pulses have been generated in 2-[3-(4-

(OH1)

© 2008 Optical Society of America

OCIS codes: (040.2235) Far infrared or terahertz; (160.2100) Electro-optical materi-

als; (190.4710) Optical nonlinearities in organic materials; (160.4890) Organic materials;

(190.7110) Ultrafast nonlinear optics; (300.6495) Spectroscopy, terahertz.

References and links

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3. A. Schneider, M. Neis, M. Stillhart, B. Ruiz, R. U. A. Khan, and P. G¨ unter, “Generation of terahertz pulses

through optical rectification in organic DAST crystals: theory and experiment,” J. Opt. Soc. Am. B 23, 1822–

1835 (2006).

4. A. Schneider, M. Stillhart, and P. G¨ unter, “High efficiency generation and detection of terahertz pulses using

laser pulses at telecommunication wavelengths,” Opt. Express 14, 5376–5384 (2006).

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911–913 (2000).

6. M. Stillhart, A. Schneider, and P. G¨ unter, “Optical properties of 4-N,N-dimethylamino-4′-N′-methyl 2,4,6-trime-

thylbenzenesulfonate crystals at terahertz frequencies,” J. Opt. Soc. Am. B (to be published).

7. O-P. Kwon, S.-J. Kwon, M. Stillhart, M. Jazbinˇ sek, A. Schneider, V. Gramlich, and P. G¨ unter, “New organic

nonlinear optical verbenone-based triene crystal for terahertz applications,” Cryst. Growth Des. 7, 2517–2521

(2007).

#100370 - $15.00 USD Received 20 Aug 2008; revised 15 Sep 2008; accepted 25 Sep 2008; published 1 Oct 2008

(C) 2008 OSA13 October 2008 / Vol. 16, No. 21 / OPTICS EXPRESS 16496

Page 2

8. O-P. Kwon, S.-J. Kwon, M. Jazbinˇ sek, F. D. J. Brunner, J. I. Seo, Ch. Hunziker, A. Schneider, H. Yun, Y. S. Lee,

and P. G¨ unter, “Organic phenolic configurationally locked polyene single crystals for electro-optic and terahertz

wave applications,” Adv. Funct. Mater. (to be published).

9. Ch. Hunziker, S.-J. Kwon, H. Figi, F. Juvalta, O-P. Kwon, M. Jazbinˇ sek, and P. G¨ unter, “Configurationally

locked, phenolic polyene organic crystal OH1: linear and nonlinear optical properties,” J. Opt. Soc. Am. B (in

press).

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oftheorganicsalt4-N,N-dimethylamino-4′-N′-methylstilbazoliumtosylate,”Appl.Phys.Lett.69,13–15(1996).

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rectification and electro-optic sampling,” Appl. Phys. Lett. 69, 2321–2323 (1996).

12. A. Schneider, I. Biaggio, and P. G¨ unter, “Terahertz-induced lensing and its use for the detection of terahertz

pulses in a birefringent crystal,” Appl. Phys. Lett. 84, 2229–2231 (2004).

13. M. Fox, Optical properties of solids (Oxford University Press, New York, 2003).

14. Y. R. Shen, The Principles of Nonlinear Optics (John Wiley & Sons, New York, 1984).

15. A. Schneider, M. Stillhart, Z. Yang, F. Brunner, and P. G¨ unter, “Improved emission and coherent detection of

few-cycle terahertz transients using laser pulses at 1.5µm,” Proc. SPIE 6582, 658211 (2007).

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56, 130–131 (1966).

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19. M. Nagai, K. Tanaka, H. Ohtake, T. Bessho, T. Sugiura, T. Hirosumi, and M. Yoshida, “Generation and detection

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3974–3976 (2004).

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arbitrary biasing phase,” Appl. Opt. 45, 6598–6601 (2006).

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96, 139–204 (2005).

1.Introduction

In the recent years, the improvement of existing and the advent of novel sources and detectors

of THz radiation was driven by an increasing number of potential applications in many fields of

science and technology [1]. One field of research is the development of novel nonlinear optical

materials especially designed for efficient THz generation (through difference frequency gener-

ation or optical rectification) and detection (through electrooptic sampling). The requirements

for these materials are a high optical nonlinearity and a low dielectric constant, which results

in small Fresnel losses at the boundaries and, more important, allows velocity-matching [2].

Particularly among organic crystals, one can find materials that exhibit both of these desired

properties. The organic salt crystal 4-N,N-dimethylamino-4′-N′-methyl stilbazolium tosylate

(DAST) was proven to be an excellent THz emitter and detector [3, 4]. However, a transverse

optical phonon leads to a gap in the accessible THz spectrum around 1.1THz [5]. In a recent

work, the frequency and the oscillator strength of this resonance could be reduced by the use of

a different anion [6]. Nevertheless, the THz absorption remains relatively strong near the reso-

nance frequency. Instead of modifying the molecules of a salt crystal, one can circumvent the

problem of the ionic resonance by using a different class of organic materials, namely hydrogen

bonded organic crystals [7].

Recently, a very promising member of this class of materials, the configurationally

locked polyene crystal 2-[3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene]malononitri-

le (OH1) has been developed and its linear and nonlinear optical properties have been deter-

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mined [8, 9]. In this article, the linear optical properties of OH1 at THz frequencies are pre-

sented, and a detailed theoretical and experimental investigation of OH1 as a THz source and

detector is given, including the analysis of velocity-matching conditions.

2.Basic properties of OH1

2.1.

The OH1 molecule (see Fig. 1) shows a large dipole moment of µg= 3.44×10−29Cm and

contains an extended π-conjugated electron system, which leads to a large hyperpolarizabil-

ity of βz= 765×10−40m4V−1, as measured in chloroform solution [8]. The charge transfer

within the OH1 chromophore is induced by a phenolic (Ar−OH) electron donor and a di-

cyanomethylidene (C− −C(CN)2) electron acceptor [8]. In the crystalline phase, the material

has the point group symmetry mm2 with an acentric packing of four molecules per unit cell

with hydrogen bonds between the phenolic and the cyano group (i.e. Ar−OH···N− − −C) [8].

The angle between the charge transfer axis of the chromophores and the polar c-axis of the

crystal is 28◦, which results in a large macroscopic second-order nonlinear optical susceptibil-

ity of χ(2)

1.9µm [9]. OH1 single crystals grow as a-plates, i.e. with the largest surfaces perpendicular to

the a-axis [8].

Crystal structure

333= 240±20pm/V for second-harmonic generation at a fundamental wavelength of

CN

NC

HO

CH3

CH3

Fig. 1. The chemical structure of the molecule OH1.

2.2. Near infrared linear and nonlinear optical properties

Due to the orthorhombic symmetry of the crystal, the dielectric coordinate system (x1,x2,x3)

coincides with the crystallographic coordinate system (a,b,c). OH1 crystals are highly bire-

fringent due to a highly anisotropic linear polarizability of the chromophores and their acentric

packing (n3−n2>0.5 in the wavelength range between 0.6 and 2.2µm) [9]. The absorption co-

efficient for the b- and c-polarizations is below 0.3mm−1in the wavelength range between 680

and 1460nm [9]. The largest linear electrooptic coefficient r333= 52±7pm/V [9] at a wave-

length of 1300nm and a modulation frequency of 1kHz is similar to the largest electrooptic

coefficient r111= 53±6pm/V of DAST [10].

3.Terahertz spectroscopic measurements

3.1.Experiments

The linear optical properties of OH1 single crystals have been determined at THz frequencies

using THz time-domain spectroscopy (TTDS). The samples were OH1 a-plates with thick-

nesses of 0.956 and 0.365mm, respectively. The a-plates allowed the determination of the re-

fractive index n and the absorption coefficient α along the dielectric axes x2and x3, where n3

and α3are of main interest for THz applications (see Section 4).

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The source of the laser pulses for the generation and detection of the THz pulses was the

tunable output of an optical parametric generator/amplifier (OPG/OPA) (Quantronix, TOPAS)

pumped by a Ti:sapphire laser (Clark-MXR, CPA 2001). The pulses of the signal wave of

the OPG/OPA at the wavelengths used in these experiments had typically an energy of 40µJ

and a duration of 150fs full width at half maximum. The THz pulses have been generated by

optical rectification of the sub-picosecond near-infrared pulses in a nonlinear optical crystal

and detected by electrooptic sampling in a second nonlinear crystal [11]. In order to obtain the

refractive indices and the absorption coefficients of OH1 over the broadest possible frequency

range, we used two configurations of TTDS based on different nonlinear optical materials. The

first covered the ranges 0.3–0.8THz and 1.4–4THz with a laser wavelength of λ = 1400nm

using single crystals of DAST for both generation and detection [3, 12]. Anticipating the results

presented in section 5.1, an OH1 crystal served as THz source in the second configuration with

λ = 1460nm; for the detection, we used ZnTe with a frequency-doubled probe pulse [4]. The

spectrum in these measurements spanned continuously from 0.3 to 2.2THz.

0123

0

4

8

12

Frequency (THz)

Absorption coefficient (1/mm)

(a)(b)

0123

1000120014001600

2.2

2.3

2.4

Frequency (THz)

Refractive index

Wavelength (nm)

Fig. 2. (a) Absorption coefficient α3and (b) refractive index n3of THz waves polar-

ized along the c-axis. Diamonds: measured data. Solid lines: best fit to the measured

data using a Lorentz four-oscillator model [13] (see Eqs. (1)–(3) and Table 1). Dashed

line: optical group index ng,ccalculated with parameters from Ref. [9] as a function

of wavelength (upper scale).

3.2.Results

The results are presented in Figs. 2 and 3. We use a classical Lorentz multiple oscillator model

to describe the dispersion of the complex dielectric function ε(ω) = ε′(ω)+iε′′(ω), whose

real and imaginary parts are given by [13]

ε′(ω) = ε∞+

m

∑

j=1

ω2

jfj

j−ω2?2+γ2

jfjγjω

j−ω2?2+γ2

?

ω2

j−ω2?

?

ω2

ω2

jω2

(1a)

ε′′(ω) =

m

∑

j=1

?

ω2

jω2

,

(1b)

where ωjis the resonant angular frequency, γjis the damping parameter, and fjis the oscillator

strength of the jth oscillator. ε∞is the high frequency dielectric constant. The refractive index

#100370 - $15.00 USD Received 20 Aug 2008; revised 15 Sep 2008; accepted 25 Sep 2008; published 1 Oct 2008

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01234

0

5

10

15

Frequency (THz)

Absorption coefficient (1/mm)

01234

1.4

1.6

1.8

2.0

Frequency (THz)

Refractive index

(a) (b)

Fig. 3. (a) Absorption coefficient α2and (b) refractive index n2of THz waves polar-

ized along the b-axis. Diamonds: measured data. Solid lines: best fit to the measured

data using a Lorentz three-oscillator model [13] (see Eqs. (1)–(3) and Table 2).

n(ω) and the intensity absorption coefficient α(ω) can be obtained from ε′and ε′′by

n(ω) =

?

1

2

??

?

ε′(ω)2+ε′′(ω)2+ε′(ω)

?

(2)

α(ω) =2ω

c

1

2

??

ε′(ω)2+ε′′(ω)2−ε′(ω)

?

,

(3)

where c is the speed of light in vacuum.

The functions with the oscillator parameters shown in the Tables 1 and 2 are plotted in

the Figs. 2 and 3, respectively. They are in good agreement with the measured data. All the

deviations of the theoretical curves from the data points are within the experimental errors.

The features in the measured THz spectrum of the c-polarization can be described by four

Lorentzian oscillators, whose parameters are listed in Table 1. The absorption within the THz

frequency range between 0.7 and 1.0THz is very low (i.e. α3< 0.2mm−1) and remains lower

than 4mm−1in the whole range between 0.3 and 2.2THz. Due to the fourth resonance, which

is the strongest in the investigated spectral range, the transmission of the THz wave was below

the detection sensitivity for frequencies above 2.5THz.

The parameters of the Lorentz-model function for b-polarized waves as shown in Fig. 3 are

listed in Table 2. Here, we observed three main resonances. In the gaps at 1.7–1.9THz and

2.5–3.1THz, where no data points are given in Fig. 3, the absorption coefficient α2is above the

detection limit.

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Table 1. Parameters for the refractive index n3and the absorption coefficient α3

of OH1 in the Lorentz-model.a,b

Resonant frequency

νj= ωj/(2π)

(THz)

Damping

parameter γj

(THz)

Oscillator

strength fj

0.027±0.013

0.020±0.003

0.0175±0.0012

0.23±0.02

Oscillator j

1

2

3

4

0.368±0.007

0.595±0.004

1.467±0.006

2.85±0.03

0.18±0.15

0.26±0.06

0.97±0.11

3.06±0.23

aSee Eqs. (1)–(3).

bAn additional parameter used in the model calculation is the high frequency

refractive index n∞=√ε∞= 2.227±0.006.

Table 2. Parameters for the refractive index n2and the absorption coefficient

α2of OH1 in the Lorentz-model.a,b

Resonant frequency

νj= ωj/(2π)

(THz)

Damping

parameter γj

(THz)

Oscillator

strength fj

0.015±0.001

0.146±0.005

0.127±0.009

Oscillator j

1

2

3

0.820±0.002

1.772±0.006

2.64±0.02

0.34±0.03

1.84±0.09

1.8±0.2

aSee Eqs. (1)–(3).

bAn additional parameter used in the model calculation is the high frequency

refractive index n∞=√ε∞= 1.640±0.004.

4.OH1 for the generation and detection of THz waves: Theory

In the following, the dependence of the THz generation and detection efficiency using OH1

crystals on the laser wavelength and on the crystal thickness is discussed in terms of velocity-

matching of the optical and THz waves and their absorptions. For this discussion we use the

theory presented in Ref. [3], which is briefly summarized in the following.

THz pulses can be generated by optical rectification of sub-picosecond laser pulses. The

optical pulses give rise to a nonlinear optical polarization of the crystal at THz frequencies,

which acts as a source term in the nonlinear wave equation. The nonlinear optical susceptibility

for optical rectification χ(2)(ω,λ) is related to the linear electrooptical tensor r(ω,λ) at THz

angular frequencies ω as follows [14]:

riij(ω,λ) = −2χ(2)

jii(ω,λ)

n4

o,i(λ)

,

(4)

where no,i(λ) is the refractive index at the optical wavelength λ. In the following, we will omit

the tensor notation because we are interested here in noncritical type I interactions, where only

one tensor element χ(2)

sible in optical rectification). Using the plane wave and the non-depleted pump approximations,

the spectral amplitude of the electric field of the generated THz wave after the crystal length l

ijjof the nonlinear optical susceptibility is involved (note that i= j is pos-

#100370 - $15.00 USD Received 20 Aug 2008; revised 15 Sep 2008; accepted 25 Sep 2008; published 1 Oct 2008

(C) 2008 OSA13 October 2008 / Vol. 16, No. 21 / OPTICS EXPRESS 16501

Page 7

is given by [3]

|ETHz(ω)| =

??????

µ0χ(2)(ω,λ)ωI(ω)

?αT(ω)

no(λ)

?

c

ω

2

+αo(λ)

?

+i?nT(ω)+ng(λ)??

??????

lgen(ω,λ,l),

(5)

where µ0is the permeability of vacuum, I(ω) is the Fourier transform of the intensity of the

near-infrared pulses. nT(ω) and αT(ω) are the refractive index and the absorption coefficient at

the THz angular frequency ω, respectively. no(λ), ng(λ) and αo(λ) are the refractive index, the

group index and the absorption coefficient of the optical pulse with the central wavelength λ.

The dependence of |ETHz(ω)| on the crystal length l is given by the effective generation length

lgen[3]:

lgen(ω,λ,l) =

exp(−2αo(λ)l)+exp(−αT(ω)l)−2exp

?

−

?

αo(λ)+αT(ω)

?2

2

?

l

?

cos

?

πl

lc(ω,λ)

?

?αT(ω)

2

−αo(λ)

?2+

?

π

lc(ω,λ)

1/2

,

(6)

where the coherence length lcof THz generation by optical rectification can be written as [11]

lc(ω,λ) =

πc

ω??nT(ω)−ng(λ)??.

(7)

As one can see immediately from (6) and (7), for an efficient generation of THz pulses, the

refractive index of the THz wave has to be close to the optical group index of the optical beam

(velocity-matching), and the absorption should be low for both THz and near-infrared pulses.

In the limit of zero absorption, (6) simplifies to the following expression that is well-known

from other second order nonlinear optical processes [14]:

lgen(ω,λ,l) = sinc

?

πl

2lc(ω,λ)

?

l.

(8)

For each laser wavelength λ and THz frequency ν = ω/(2π), we can calculate the maximum

effective generation length lmax(ω,λ) and the optimum crystal length loptimum(ω,λ), which are

defined as follows:

lmax(ω,λ) := max

lmax(ω,λ).= lgen(ω,λ,loptimum).

l

lgen(ω,λ,l),

(9a)

(9b)

The detection of THz transients through electrooptic sampling is essentially the inverse pro-

cess of optical rectification, and the dependence of the sampling signal on the length of the

electrooptical crystal is the same as in (6). The sampling signal is proportional to the phase

shift ∆φ of the probe pulse induced by the THz electric field ETHz(ω) in the detection crystal

which is given by [3]:

∆φ(ω) =π

λn3

o(λ)r(ω,λ)A(ω)lgen(ω,λ,l)ETHz(ω),

(10)

where A(ω) is the normalized Fourier spectrum of the probe pulse amplitude.

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(C) 2008 OSA13 October 2008 / Vol. 16, No. 21 / OPTICS EXPRESS 16502

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From a contour plot of lmax(ω,λ), one can predict the range of the pump laser wavelengths

λ where a high conversion efficiency for a certain range of THz frequencies can be achieved.

Figure 4 shows loptimum(ω,λ) with the corresponding lmax(ω,λ) for THz generation and de-

tection exploiting the largest tensor element of the nonlinear optical susceptibility χ(2)

largest linear electrooptic coefficient r333, respectively. The refractive index and the absorption

in the THz range are taken from the four-oscillator Lorentz function (see Fig. 2), and the optical

data are taken from Ref. [9].

333and the

(b)(a)

0.5

0.5

1

1

2

2

2

4

4

4

6

6

6

8

8

8

10

10

0.5

1

1

1

1

1.5

2

2.5

3

3.5

4.5

Wavelength (μm)

Frequency (THz)

1.0 1.1 1.2 1.31.4 1.51.6

0.0

0.5

1.0

1.5

2.0

2.5

Wavelength (μm)

Frequency (THz)

1.0 1.1 1.21.3 1.4 1.5 1.6

0.0

0.5

1.0

1.5

2.0

2.5

Fig. 4. (a) Optimum OH1 crystal length loptimum(ω,λ) for the generation of THz

pulses (see Eq. (9b)); (b) the corresponding maximum effective generation length

lmax(ω,λ) (see Eq. (9a)). The values of the contour lines are in units of mm.

The highest conversion efficiency can be expected in the velocity-matching and transparency

range for THz frequencies below 2.2THz and optical wavelengths between 1200 and 1460nm.

The upper limit for the wavelength is given by the optical absorption. Above 2.2THz, the main

limiting factor is THz absorption.

Due to the very large birefringence n3−n2> 0.5 of OH1 in the THz as well as in the optical

range [9], the second largest coefficient χ(2)

THz pulses. Likewise, r223cannot be exploited in velocity-matched THz detection.

322cannot be used for velocity-matched generation of

4.1.Figure of merit for generation and detection of THz pulses

For the comparison of the performance of two THz systems using different electrooptic crys-

tals, one may define an overall figure of merit (FoM) for the generation and detection of THz

pulses [15]:

4n7

(1+no(λvm))2(1+ng(λvm))2,

FoM = b

o(λvm)r2

(11a)

b =

?

1

1/4

for standard electrooptic sampling in cubic crystals (e.g. ZnTe or GaAs),

for electrooptic sampling in birefringent crystals,

(11b)

where λvmis the velocity-matching wavelength. The overall efficiency of a THz system for the

generation and detection of the electric field at the frequency ν using the laser wavelength λ

is proportional to FoM×l2

max(2πν,λ). The equation (11) has been derived from (5) and (10)

#100370 - $15.00 USD Received 20 Aug 2008; revised 15 Sep 2008; accepted 25 Sep 2008; published 1 Oct 2008

(C) 2008 OSA13 October 2008 / Vol. 16, No. 21 / OPTICS EXPRESS 16503

Page 9

taking into account the Fresnel losses of the optical beam at the generation crystal and of the

THz beam at the generation and the detection crystals [15].

In Table 3, the figures of merit of OH1 and of the commonly used electrooptic crystals

DAST, ZnTe and GaAs are shown. Additionally, the THz absorption coefficient is given at the

typical frequency νpeakof the peak spectral amplitude of THz pulses generated through optical

rectification of 150fs laser pulses at the velocity-matching wavelength. OH1 shows the highest

figure of merit and a low THz absorption at the optimum conditions.

Table 3. Overall figure of merit (FoM) for generation and detection of THz pulses and other relevant

parameters of OH1 in comparison with commonly used electrooptic crystals.

λvma

(µm)

r

FoMb

((pm/V)2)

νpeakc

(THz)

α(νpeak)

(mm−1)

0.2

Material

no

2.16

ng

2.33

(pm/V) Refs.

OH1 1.35253001[9],

this work

[3, 5, 10]

[16, 17]

[18]–[21]

DAST

ZnTe

GaAs

1.5

0.8

1.4

2.13

2.85

3.40

2.26

3.23

3.61

47

4

1.3

4200

370

86

2

1

2

3–5

0.1

0.3

aλvmis the velocity-matching wavelength.

bsee Eq. (11).

cνpeakis the typical frequency of the peak spectral amplitude of THz pulses generated through optical

rectification of 150fs laser pulses at the velocity-matching wavelength.

5. OH1 for the generation and detection of THz waves: Experiments

5.1. Generation of THz pulses using OH1 crystals

We performed THz emission experiments using an unpolished 0.365mm thick a-plate of OH1.

The pump wavelength was 1460nm. The electric field of the generated THz pulse was meas-

ured by electrooptic sampling in a 0.5mm thick (110)-cut ZnTe crystal using a probe beam with

the second-harmonic wavelength (730nm) [4]. The wavelength was chosen to ensure nearly

velocity-matching in the ZnTe crystal. Although this is not the optimum wavelength for OH1,

one can see from Fig. 4(a) that loptimumis larger than our crystal length below 2.5THz. Hence,

velocity-mismatch and THz absorption do not affect the emitted amplitude significantly. Possi-

ble distortions of the measured THz transient, which may arise if the peak electric field exceeds

the linearity range of the detection, have been corrected using the algorithm from Ref. [22]. The

measured signal is plotted in Fig. 5. The emission spectrum is continuous and ranges from 0.3

to 3.0THz with a maximum at 1.3THz.

For a comparison, the same experiment has been performed with DAST crystals as a source

material under identical conditions. DAST has been chosen as a reference material for two

reasons. On the one hand, it has been demonstrated to be one of the most efficient THz emitter

materials so far; on the other hand, it is also very well velocity-matched at 1460nm [4]. Two

c-plates of DAST (thicknesses: 0.330 and 0.400mm, respectively) were used having an average

thickness which corresponds to the thickness of the OH1 crystal. The THz signals from the two

DAST crystals are averaged and plotted in Fig. 5.

The peak THz amplitude from OH1 exceeds that of DAST in both time- and frequency-

domain by 58 percent and 36 percent, respectively. A second advantage, that is relevant mainly

for spectroscopic applications, is that the spectrum from OH1 is continuous from 0.3 to 3THz,

incontrasttoDAST,whereatransverseopticalphononleadstoagapattheresonancefrequency

#100370 - $15.00 USD Received 20 Aug 2008; revised 15 Sep 2008; accepted 25 Sep 2008; published 1 Oct 2008

(C) 2008 OSA13 October 2008 / Vol. 16, No. 21 / OPTICS EXPRESS 16504

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–2 024

–4

0

4

Time (ps)

THz amplitude (a.u.)

01234

0.0

0.5

1.0

1.5

2.0

Frequency (THz)

Spectral amplitude (a.u.)

(a)(b)

Fig. 5. THz pulse emitted from OH1 exploiting χ(2)

(a) Time domain and (b) frequency domain signal. THz pulse emitted from DAST

exploiting χ(2)

details).

333and detected in ZnTe (red line).

111under identical conditions for comparison (black line; see text for

of 1.1THz. This fact is a direct consequence of the non-ionic nature of the OH1 crystal.

5.2.OH1 for both generation and detection of THz pulses

5.2.1.Pump energy dependence

OH1 crystals were also used for the detection of the THz pulses. Although standard electrooptic

sampling is not possible due to the large intrinsic birefringence of the crystals, we could use a

modified detection method based on the spatial phase-modulation of the probe beam induced

by the electric field of the THz wave through the linear electrooptic effect [12]. The spatial

phase-modulation leads to a focusing or defocusing of the probe beam depending on the sign of

the THz field. A fraction of the probe beam in its center after passing the detection crystal was

coupled into a multimode fiber with a core diameter of 62.5µm and its output was measured

with an InGaAs photodiode. The relative modulation m(E) = (W(E)/W(E = 0))−1 is pro-

portional to the THz electric field E for moderate modulations (i.e. m ? 0.5). As a reference,

we used the light that was not coupled into the fiber by measuring its backscattering from a

white screen with a second InGaAs photodiode. This reference was required to correct for the

pulse-to-pulse energy fluctuations of the probe beam and thus to enhance the signal-to-noise

ratio.

Figure 6 shows THz pulses generated and detected in two polished OH1 crystals (thicknesses

1.002 and 0.956mm, respectively) at an optical wavelength of 1300nm for several values of the

pump pulse energy. For this, the pump pulses were attenuated using different neutral density

filters. The maximum pump pulse energy of 45µJ leads to a maximum relative modulation

m(E) of 5.0, which is 3.6 times the maximum value reported to date (1.4 in DAST [4]). The

maximum observed defocusing of the probe beam reduced the measured diode signal to the

noise level. At the highest two pump pulse energies of 45 and 20µJ, the maximum defocusing

could also be observed in the reference. Thus, the ratio of the signal and the reference at the

maximum defocusing is smaller for the pump pulse energy of 20µJ than for 45µJ (see Fig. 6).

In the following, the conversion efficiency η = WTHz/Wpumpof the pulse energies is esti-

mated. We assume a Gaussian beam profile of the THz pulse. The electric field E in the center

#100370 - $15.00 USD Received 20 Aug 2008; revised 15 Sep 2008; accepted 25 Sep 2008; published 1 Oct 2008

(C) 2008 OSA13 October 2008 / Vol. 16, No. 21 / OPTICS EXPRESS 16505

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–2 0

Time (ps)

24

10–1

100

101

Relative modulation m(E) + 1

45 µJ

20 µJ

8 µJ

3 µJ

1 µJ

Fig. 6. THz pulses generated and detected in OH1 crystals using optical pulses at

a wavelength of 1300nm for different pump pulse energies. The ratio of the center

intensities of the probe beam with and without THz electric fieldW(E)/W(E = 0) =

m(E)+1 is plotted on a logarithmic scale as a function of time.

of the THz beam can be calculated from m(E) [12]:

E =

2λ

n3orlπm(E),

(12)

where l is the length of the detection crystal. We assume a diffraction limited THz beam profile

within the detection crystal; for the main frequency components near 1THz, this corresponds

to a full width at half maximum ρ0of the THz intensity of about 0.6mm. The THz pulse energy

can be calculated by integrating the intensity over the beam profile and the spectrum:

WTHz=ρ2

0πε0c

8ln2

?|E(ν)|2

T(ν)

dν,

(13)

where the intensity transmission coefficient T(ν) at the entrance of the detector crystal is in the

limit of zero absorption given by

T(ν) =

4nT(ν)

(1+nT(ν))2.

(14)

Since the relative modulation m(E) exceeds the linearity range of the detection for the

higher pump pulse energies by far, we calculated η for THz pulses generated by 3µJ pump

pulses. The peak electric field of this THz transient of Epeak= 8kV/cm is reached at a max-

imum value of the relative modulation of mmax= 0.46, i.e. the whole waveform is in the lin-

ear regime. Using (13)–(14), we calculated an energy conversion efficiency of η = 6×10−5,

which corresponds to a photon conversion efficiency ηphoton= ηc/(λ ¯ ν) of 1 percent, where

¯ ν =?|E(ν)|2νdν/?|E(ν)|2dν is the average generated THz frequency. Since the generated

measured for 3µJ to the pump pulse energy of 45µJ. This extrapolation yields a peak electric

field of 120kV/cm, an energy conversion efficiency of η = 1×10−3and a photon conversion

efficiency of 15 percent.

So far, the effect of two-photon absorption of the pump beam has not been taken into account.

We measured a two-photon absorption coefficient β3= 0.3cm/GW for c-polarized light with

a wavelength of 1300nm using the method described by Bechtel and Smith [23]. Two-photon

absorption of the pump beam is insignificant for the THz pulse generated with a pump pulse en-

ergy of 3µJ, where the peak intensity was Ipump, max=3GW/cm2. However, it becomes relevant

THz field is proportional to the pump pulse energy, we can extrapolate the THz electric field

#100370 - $15.00 USD Received 20 Aug 2008; revised 15 Sep 2008; accepted 25 Sep 2008; published 1 Oct 2008

(C) 2008 OSA13 October 2008 / Vol. 16, No. 21 / OPTICS EXPRESS 16506

Page 12

for the 45µJ pump pulse, where Ipump, max= 41GW/cm2. To estimate the effect of two-photon

absorption on the THz conversion efficiency, we averaged the pump pulse energy—which is

proportional to the THz electric field—over the crystal length, and obtained a reduction of η

by a factor of 0.74, resulting in a photon conversion efficiency of 11 percent for the 45µJ pump

pulse.

5.2.2. Pump wavelength dependence

We investigated the wavelength dependence of the THz spectra generated and detected using

OH1 crystals in the velocity-matching and transparency range between 1200 and 1460nm. The

intensity of the pump beam has been attenuated to obtain a THz signal in the linear detection

regime (m ≤ 0.5). Figure 7(a) shows the normalized spectra obtained with a 1.002mm thick

source crystal and a 0.956mm thick detector crystal. For all these wavelengths, the spectral

amplitudes reach their maxima near 1.3THz. Below 1.4THz, all spectra normalized to their

maximum values are almost identical. The spectral amplitude is reduced around the resonance

near 1.5THz (oscillator 3, see Table 1), however it is still large enough for spectroscopic ap-

plications. This is in contrast to DAST crystals, for which the absorption at the resonance fre-

quency of 1.1THz is more than 10 times larger than that of OH1 at 1.5THz. The position and

the amplitude of the second local maximum in the normalized spectra depends on the pump

wavelength. A calculation of the spectra according to the theory from [3] leads to the same

result (see Fig. 7(b)). The plane-wave approximation has been used in the calculation for both

the optical and the THz wave. However, the diffraction of the THz beam due to the finite pump

beam diameter decreases the spectral amplitudes at low frequencies [3]. This explains the dis-

crepancy between the measured and the calculated spectra below about 0.8THz.

Although all the experiments were carried out in a box purged with dry air, there was some

residual humidity. The additional features in the measured spectra compared with the calculated

ones stem from this residual water vapor, visible mainly at 1.1, 1.7 and 2.2THz [24].

0123

0.0

0.5

1.0

Frequency (THz)

Spectral amplitude (normalized)

(a)

(b)

0123

0.0

0.5

1.0

Frequency (THz)

Spectral amplitude (normalized)

1200 mm

1250 mm

1300 mm

1350 mm

1400 mm

1450 mm

Fig. 7. THz spectra generated and detected in 1mm thick OH1 crystals using different

wavelengths, normalized totheir maximum values.(a) Measurements;(b)calculation.

6. Conclusions

The refractive indices n2and n3and the absorption coefficients α2and α3of the organic crystal

OH1 have been measured in the spectral range between 0 and 4THz using THz time-domain

spectroscopy. Based on these results, we found that the largest element χ(2)

opticalsusceptibilitytensorcanbeexploitedforhighefficiencygenerationanddetectionofTHz

pulses, thanks to a low absorption α3for frequencies below 2.5THz and to velocity-matching

with a large range of pump laser wavelengths (1200–1460nm). The peak spectral amplitude

333of the nonlinear

#100370 - $15.00 USD Received 20 Aug 2008; revised 15 Sep 2008; accepted 25 Sep 2008; published 1 Oct 2008

(C) 2008 OSA 13 October 2008 / Vol. 16, No. 21 / OPTICS EXPRESS 16507

Page 13

from OH1 was found to be 36 percent larger than that from DAST under identical conditions.

For a pump pulse energy of 45µJ, we achieved a conversion efficiency of pulse energies of

7×10−4, corresponding to a photon conversion efficiency of 11 percent.

The results presented in this article are also valid for tunable narrowband THz generation

through difference frequency generation of two lasers with slightly different wavelengths. Dif-

ference frequency generation of frequencies around 1THz will be especially efficient due to

the very low absorption of OH1.

Acknowledgments

The authors thank J. Hajfler for his expert sample preparation. This work has been supported

by the Swiss National Science Foundation.

#100370 - $15.00 USD Received 20 Aug 2008; revised 15 Sep 2008; accepted 25 Sep 2008; published 1 Oct 2008

(C) 2008 OSA 13 October 2008 / Vol. 16, No. 21 / OPTICS EXPRESS 16508