Optical Conductivity and Dielectric Response of an Organic Aminopyridine NLO Single Crystal

Journal of Minerals & Materials Characterization & Engineering, Vol. 10, No.13, pp.1225-1231, 201 1

1225

Optical Conductivity and Dielectr ic Response of an Organic

Aminopyridine NLO Single C rystal

T. Arumanayagam, P. Murugakoothan*

PG and Research Department of Physics, Pachiyappa’s College, Chennai, India.

*Corresponding author: [email protected]

Abstract

This paper explores the correlation of electro-optical properties with dielectric properties of

an organic single crystal. The optical constants of the organic aminopyridine single crystal

have been studied. The second harmonic generation efficiency of the grown crystal, based

on powder measurement, is 2.9 times higher than that of KDP. The real and imaginary part

of the complex refractive index and dielectric constant of the crystal were determined. The

optical and electrical conductivity of the grown crystal were studied.

Keywords: Nonlinear optical materials, Optical conductivity, Dielectric Properties,

1. INTRODUCTION

In recent years, researchers devoted much attention to nonlinear photonic crystals as their

use in photonic band gap materials for controlling and molding the flow of light. The growth

of research in nonlinear optics (NLO) is closely linked to the rapid technological advances

that have occurred in related fields such as ultra-fast phenomena, optical communication and

optical storage devices [1]. Organic nonlinear optical crystals which possess a good second

harmonic generation efficiency due to their high optical band gap and low dielectric constant

are in rich demand in optical storage devices, colour display units and optical

communication systems etc [2]. It has been already reported that the pyridinium acceptor

shows large second harmonic nonlinearity [3]. Recently optical properties of aminopyridine

complexes and its suitability for the optoelectronic devices fabrications were reported [4, 5].

In the present work, we have made an analysis of electro-optical properties correlated with

dielectric properties of 2-aminopyridine 4-aminobenzoate single crystal.

2. EXPERIMENTS

The high optical quality 2-aminopyridine 4-aminobenzoate (APAB) single crystal was

grown by slow evaporation technique. The compound was prepared by the reaction of

1226 T. Arumanayagam, P. Murugakoot han Vol.10, No.13

2-aminopyridine (20 milli mol) and 4-aminobenzoic acid (10 milli mol) in (1:1) water

methanol. The solution w as h e ated with reflux about the temperature of 50 ˚C for 3 hours. A

good optical quality single crystal of APAB was harvested after a period of 20 days and is

shown in Figure 1(a ). The unit cell dimension of APAB crystal was identified by single

crystal X-ray diffraction analysis using Enraf Nonius FR 590 diffractometer with Mo Kα

(λ=0.7170 Å) radiation. The cut and polished 2 mm thickness crystal was subjected for

transmittance studies by using Model 1601 spectro-photometer. The second harmonic

generati o n (SHG) efficienc y of APAB was m easured by Kurtz and Perr y powder technique.

The same thickness samples were used for the analysis of dielectric measurements for

various frequencies and temperature using a HIOKI 3532-50 LCR HITESTER.

Fig. 1(a). Photograph of as grown single crystal.

(b). UV-Vis-NIR transmittance spectra of APAB crystal.

3. RESULT AND DISCUSSION

The grown crystals were subjected to single crystal X-ray diffraction analysis. The crystal

belongs to the triclinic crystallographic system. The obtained latt ice paramet er values a re in

good agreement with the reported literature values [6].

3.1. Linear and Nonlinear Optical Studies

Optical transm itt ance ran ge an d tr anspar enc y cut off wav elen gth of the c r ystal are i mport ant

factors for optical applications. Figure 1(b) which shows that there is no transmission up to

367 nm and it reveals that the crystal absorbs the entire UV region and transmit the visible

and NIR region. The optical band gap energy of the grown cr ystal was calculated using the

Vol.10, No.11 Optical Conductivity and Dielectric 1227

formula Eg = 1240/ λ (nm) in eV, where λ is the lower cut off wavelength (367 nm). The

band gap of the APAB crystal is found to be 3.37 eV. As this crystal exhibits wide

transmission range, starting from 367 nm onwards, it can be used for optical applications

including the second harmonic generation of Nd: YAG laser of fundamental wavelength

λ=1064 nm. It is also interesting to note from the spectrum that this crystal absorbs UV

radiati on and henc e APAB cr ystal can be used as an effect ive UV sh elter [ 7]. The measured

transmittance (T) data was used to calculate the absorption coefficient (α) from the

following relation,

α = (1)

where t is the thickness of the crystal. The dependence of absorption coefficient on photon

energy was analyzed in the absorption regions to obtain the detailed information about the

energ y band gap of the c rystal. Fi gure 2(a) shows the variation of absorption coefficient (α)

as a function of photon energy at room temperature. From the graph it is evident that the

absorption coefficient varies from 6.5-16.4 cm-1 with increasing photon energy of

3.2-3.5 eV. The optical constants such as the refractive index (n) and the extinction

coeffi cien t (k ) have al so been es ti m ated us in g the for mu la as repo rted ea rli er [ 8]. Figure 2 (b)

shows the plot of refractive index as a function of wavelength and it could be noticed that

the refractive index decreases abruptly as the wavelength increases and gets saturated

beyond the wavelength of 450 nm. The refractive index of grown APAB crystal for longer

wavelen gth ( vi si ble re gio n ) was cal cul at ed t o be 2.36. From th e calcu lat ed values o f n and k,

the real and imaginary functions of dielect rics wer e also det ermined . The complex dielectric

constant (ε = εr+εi) char a ct eri z es t he o pt ical pro pe rt ies of t he crystals an d is calculated using

the expressions as report ed earl ier [9]. The real and i m aginary parts of t he di el ectri c con st ant

of the grown crystal were determined and shown in Figure 3(a). From the graph it is clear

that both real and imaginary part of dielectric constant increases with increase of photon

energy. The real part of the dielectric constant increases linearly with higher value than the

imaginary part. The lower value of dielectric constant with wide band gap of APAB crystal

suggests the suitability of optoelectronic devices.

The optical conductivity is one of the powerful tools for studying the electronic states in

materials. The frequency dependence of dielectric reflects the fact that a material’s

polarization does not respond instantaneously to an applied field. For this reason, dielectric

constant is often treated as complex function of the frequenc y of the applied field. A perfect

dielect ric is a material that has no conductivit y. However the grown crystals associated with

low dielectric loss inhibit the propagation of electromagnetic energy which aided

conductivity. According to the one component or anomalous Drude model, both the carrier

relax at ion time and its effective mass of the charges are assumed to be as the functions of

photon frequency (ω). But for alternative, multi component model, the real part of the

optical conductivity (σ) of the crystal was calculated using the following relations [10, 11]:

σ = Im (ε) (2)

1228 T. Arumanayagam, P. Murugakoot han Vol.10, No.13

where the value of Im (ε) is given by,

Im (ε) = . (3)

where μr is the relative permeability. For most crystalline materials μr is very close to 1 at

optical frequencies. On substuting the value of Im (ε) in Eq. (2)

σ = (4)

where c is the velocity of light. The plot between the optical conductivity against photon

energy (hν) was depicted in Figure 3(b). The spectrum indicates that the optical conductance

increases with the increase of photon energy.

Fig. 2(a). Plot of absorption coefficient against photon energy.

(b). Plot of refractive index against wavelength for APAB crystal.

For the second ha rmonic generation efficiency, a fundamental beam of wavelength 1064 nm

with a pulse duration of 10 ns and frequenc y repetition of 10 Hz from Q-switched Nd:YAG

laser was used as the source and passed through the powder sample [12]. The SHG behavior

was confirmed from the output of the laser beam which had bright green emission

(λ=532 nm) from the powder sample. The second harmonic signal of 32 mV was obtained

for an input energy of 31 mJ/pulse, while the standard KDP crystal gives a SHG signal of 11

mV for the same input energy. It shows that the SHG efficiency of APAB is 2.9 times that

of standard NLO material, KDP.

2.8 3.0 3.2 3.4 3.6 3.8 4.0

4

8

12

16

400600800 1000

0

20

40

60

80

100

120

(a)

α

(cm

-1

)

hν (eV)

(b)

Refractive index, n

wavelength (nm )

Vol.10, No.11 Optical Conductivity and Dielectric 1229

Fig. 3(a). Plot of εr and εi as a function of photon energy of APAB crystal.

3(b). Optical conductivity against photon energy of APAB crystal

3.2. Dielectric Studies

The dielectric properties are correlated with electro-optic properties of the crystals [13].

The dielectric constant (εr) was calculated by using the relation, εr = Ct/Aε0, where ε0 is

the permittivity of the free space, C is the capacitance and A is the area of cross section of

the sample. Figure 4(a) and 4(b) shows the plot between the calculated dielectric constant

and dielectric loss with respect to frequency for APAB crystal, respectively. The graph

reveals that the dielectric constant has higher value at lower frequencies and almost

constant at higher frequencies (beyond 100 kHz), calculated to be 32.5. The magnitude of

dielectric constant depends on degree of polarization and the charge displacement in

crystal. The decrease in dielectric constants at higher frequencies is attributed to the

absence of space charge polarization near the grain boundary interface [14, 15]. For a

material to be a potential candidate for NLO applications, dielectric loss (tan δ) must also

be kept as low as possible. From the graph, it is clear that the APAB crystal exhibit very

low dielectric loss at high frequencies and can be used for NLO applications effectively

[16].

The crystal was subjected to an external electric field. Then, generally, a redistribution of

charges occurs and currents are induced. The ac conductivit y of the samples was calculated

using the formula [17];

σac

= ε

0εr

ωtanδ

(5)

where ω be the angular frequency (ω = 2πν). Figure 5, shows the variations of ac

conductivity of APAB crystal with various frequencies. The conductivity is almost zero up

0.000

0.002

0.004

0.006

0.008

2.2 2.42.6 2.8 3.0 3.2

0

100

200

300

400

500

600

2.0 2.4 2.8 3.2 3.6

0

5

10

15

20

25

Im aginary dielectric function,

ε

i

Real dielectric functio n,

ε

r

(a)

ε

i

ε

r

Photon energy (eV )

(b)

Optical condu ctivity (x10

8

Ω

−1

m

-1

)

hν (eV)

1230 T. Arumanayagam, P. Murugakoot han Vol.10, No.13

to 10 kHz and then increases with the increase of frequency. The low value of electrical

conductivity is the effect of decrease in mobility of the charge carriers due to ionic size,

which leads to the change in electronic band structure. At higher frequency the ac

conductivity increases sharply. It reveals that the electrical conductivity is proportional to

mobility and carrier concentrations through the well known relation σ = ndeμe, where μe is

the mobility of electron and nd is the number density of electron. Thus the optical

conductivity of the grown APAB crystal increases by increased with increase in applied

energy.

Fig. 4(a). Variation of dielectric constant with frequency.

(b). Variation of dielectric loss with frequency.

1234567

20

40

60

80

100

120

140

1234567

0. 0

0. 5

1. 0

1. 5

2. 0

2. 5

(a)

35

o

C

100

o

C

Dielectric constant

log f

( b)

Dielectric loss

log f

1 2 3 4 5 6

0

15

30

45

60

75 35

oC

100 oC

Electrical conductivity (x10

-6

Ω

−1

m

-1

)

log f

Vol.10, No.11 Optical Conductivity and Dielectric 1231

Fig. 5. Plot of electrical conductivity versus frequency of APAB crystal.

1232 T. Arumanayagam, P. Murugakoot han Vol.10, No.13

3. CONCLUSION

Under the reaction of 2-aminopyridine and 4-aminobenzoic acid with the ratio 2:1, high

quality nonlinear optical crystal was grown using slow evaporation technique. The optical

transmittance, band gap and optical constants of the grown crystal were studied. The

complex dielectric functions of the APAB crystal were studied. The SHG efficienc y of the

grown APAB crystal is 2.9 times that of KDP. The optical and electr ical conductivi t y of the

sample was studied and it reveals that the conductivity increases with increase of photon

energy. The dielectric constant of the grown crystal is calculated to be 32.5 at higher

frequen cy. The high optical transmittance and band gap with low dielectric constant and

low dielectric loss suggest that the APAB crystal could be used in nonlinear optoelectronic

devices.

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