Zuo-Guang. Ye Advanced Dielectric Piezoelectric and Ferroelectric Materials: Synthesis, Characterisation and Applications

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Handbook of dielectric, piezoelectric and ferroelectric materials930

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Part VIII

Novel properties of ferroelectrics and

related materials

931

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31.1 Introduction

Second harmonic generation (SHG) is described schematically in Fig. 31.1(a).

A single pump wave, with fundamental frequency ω, is incident on a nonlinear

medium and generates a wave at the second frequency 2ω. Figure 31.1(b)

illustrates the sum frequency generation (SFG). Two pump waves at frequencies

and ω

incident on a nonlinear medium combine to generate a wave at

the sum frequency ω

= ω

+ ω

. Third harmonic generation (THG) is

obtained normally by two cascading χ

(2)

processes: SHG and SFG. Difference

frequency generation (DFG), shown in Fig. 31.1(c) is similar to sum frequency

generation, except that the two pump waves combine to produce a wave at

the difference frequency ω

= ω

– ω

. The latter process is the same as that

involved in optical parametric amplification and oscillation. Optical parametric

Novel physical effects in dielectric

superlattices and their applications

YQ QIN and S N ZHU, Nanjing University, China

(a) (b)

2ω

31.1

Schematic illustration of the frequency conversion and

parametric processes discussed in this chapter. (a) Second harmonic

generation (SGH). (b) Sum-frequency generation (SFG). (c)

Difference-frequency generation (DFG). (d) Optical parametric

process in which in pump photon of energy splits into two photons

of signal and idler energy.

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Handbook of dielectric, piezoelectric and ferroelectric materials934

generation (OPG) is illustrated schematically in Fig. 31.1(d). A photon of

frequency ω

is incident on a nonlinear medium and spontaneously splits

into two lower-frequency photons. These are called the signal photon of

frequency ω

and the idler photon of frequency ω

. This phenomenon is

somewhat analogous to spontaneous Raman or Brillouin scattering with the

exceptions that both particles created in the scattering are photons and not,

for example, phonons. Another nonlinear χ

(2)

process is optical parametric

amplification (OPA). A pump wave and a signal wave are incident on a

nonlinear medium. Through the DFG process, the signal wave is amplified,

and the idler wave is generated at the difference frequency. Basically the

gain of an OPA is modest even for high pump intensities. The net gain can

be increased by providing positive feedback of the signal. A convenient way

of achieving this is to place the nonlinear crystal in a two-mirror resonator.

When the optical parametric gain of the resonator exceeds its optical loss,

oscillation occurs. Such a device is called an optical parametric oscillator

(OPO). Above the threshold for oscillation, the signal and idler waves grow

dramatically. It is well known that phase matching (PM) is necessary for

efficient nonlinear processes (frequency conversion and parametric process).

Three basic phase matching techniques are angle phase matching (scalar and

vector), temperature phase matching and quasi-phase-matching (QPM).

Armstrong et al. (1962) were the first to suggest ways to achieve QPM. The

most common technique is described here. The nonlinear crystal is divided

into segments each with a coherence length long. Each segment is then

rotated relative to its neighbors by 180° about the axis of propagation. Such

a structure with sign changing of nonlinear susceptibility is called dielectric

superlattice (DSL) throughout this chapter.

Artificial superlattices are normally divided, according to their constituents,

into several categories: semiconductor, metallic, dielectric, magnetic, etc.

These new materials show many unusual properties, which are of fundamental

interest in physics and have potential applications in microelectronics. For

the past two decades, inspired by the success of the semiconductor superlattice

and quasi-phase-matching (QPM) technique, the dielectric superlattice (DSL)

has become a hot topic in material science and photoelectronics. It is expected

that the material can provide a new means to control and manipulate light

and ultrasound by means of its unique functions. As is well known, in dielectric

crystals, the most important physical processes are the propagation and

excitation of classical waves (optical and acoustic waves). The propagation

of classical waves in a DSL (classical system) is similar to the electron

motion in periodic potentials of crystal lattice (quantum system). Thus, some

ideas in solid-state electronics, for example, the reciprocal space, Brillouin

zone, dispersion relation, may be used in classical wave processes. On the

other hand, DSL may consist of two kinds of dielectric materials or of

dielectric and non-dielectric materials layer by layer alternatively, forming

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Novel physical effects in dielectric superlattices 935

so-called heterostructures. Most of them consist of the same kind of material,

such as single ferroelectric crystals, in which the modulated structure is the

ferroelectric domain. All physical properties associated with a third-rank

tensor in such a superlattice will be modulated with the domain, whereas

those associated with an even-rank tensor remain constant. It is the modulated

physical properties that make the material different from the homogeneous

single domain crystal, and specially favorable for applications in nonlinear

optics and ultrasound.

In this chapter, an overview will be given of the studies on DSLs for

nonlinear optical effects and acoustic devices. Since there are reviews of

progress in periodic poling methods for second harmonic generation (SHG)

(Houe and Townsend, 1995) and in QPM nonlinear interactions (Byer, 1997),

much has been focused on the work done in our laboratory, especially on

coupled nonlinear optical frequency conversion in quasi-periodic structures

and the excitation of high-frequency ultrasound.

Preparation of DSLs can be realized by the modulation of microstructures,

such as by the modulation of domain structures, phase structures, compositions,

and by the modulation of crystallographic orientations and heteroepitaxy

structures. The modern technology of crystal growth and micro-processing,

the assembly of small dielectric spheres, the atomic-layer-controlled-epitaxy

and heteroepitaxy (patterned growth by molecular beam epitaxy, metal–

organic chemical-vapor deposition), or the use of acousto-optical effect,

electro-optical effect or photorefractive effect make it possible to fabricate

one-, two-and three-dimensional (1D, 2D, and 3D) DSLs. In the following

sections we will describe the preparation of DSL by the modulation of

ferroelectric domains and by photorefractive effect.

31.2 Preparation of DSLs by modulation of

ferroelectric domains

As mentioned above, most of a dielectric superlattice is fabricated by

ferroelectric crystal, which is composed of 180° anti-parallel laminar domains.

A variety of techniques for controlling domain patterns in ferroelectric crystals,

either during or after growth, have been developed. Among them, growth

striation technique (Feng et al., 1980 and Ming et al., 1982), field poling

technique (Yamada et al., 1993), electron writing (Ito et al., 1991) and Corona

poling are mainly used for bulk ferroelectric crystals, while chemical diffusion

and substitution of impurities are used for waveguide materials (Webjorn et

al., 1989). In spite of making progress, many of these techniques remain

semi-empirical in which the mechanisms of polarization reversal are poorly

understood. Nevertheless, this does not prevent them from being practical

methods to fabricate the various ferroelectric superlattices for different

applications.

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Handbook of dielectric, piezoelectric and ferroelectric materials936

31.2.1 Growth striation technique

Fabrication of ferroelectric superlattice using the growth striation technique

was first accomplished by Feng et al. (1980) and Ming et al. (1982) in a

Czochralski system. The technique has been successfully used to grow LiNbO

(LN) (Feng et al., 1980 and Ming et al., 1982), LiTaO

(LT) (Feng et al.,

1986), Ba

NaNb

(BNN) (Xu et al., 1992), and TGS (triglycine sulphate)

(Wang and Qi, 1986) superlattices. Feisst and Koidl (1985) and some other

groups respectively, reported their works on fabricating LN superlattices

using similar growth methods.

In the method the melt is doped with impurity, such as yttrium, indium or

chromium for LN, with a concentration about 0.1 – 0.5 wt.% to control

domain structure. Ming et al. (1982) found that a temperature fluctuation

may be introduced into the solid–liquid interface, either through an off-axis

rotation or through applying an alternating electric current. The temperature

fluctuation causes a spatial modulation of the impurity or composition in

crystal along the growth direction. The effect can form a space charge

distribution, and in turn induce a local electric field in crystal. When cooling

through the Curie point, the field plays a key role of causing in situ and local

laminar domain. The modulated domain structure may automatically realize

during the cooling process of crystal. Obviously the domain structures and

the impurity distribution should have the equal period. The period or structure

parameter of domain may be adjusted by choosing suitable pulling rate and

rotation frequency or by changing the period of modulating electric current.

Figure 31.2 shows the measured temperature fluctuations at the solid–

liquid interface (Fig. 31.2 (a)) and the formed growth striations (Fig. 31.2

(b)) for LN crystal. Ming et al. (1982) demonstrated the one-to-one

correspondence between the growth striations and the laminar domain structure

(Fig. 31.2(c) and (d)). The relationship between solute fluctuation and the

ferroelectric domain structure has also been revealed with X-ray energy

dispersive spectrum analysis. Figure 31.3 is the measured result of a LN

crystal sample doped with yttrium. This figure shows domain walls are

always situated at the places where the gradient of the Y dopant concentrations

changes its sign from plus to minus or vice versa.

A significant progress was made by Magel et al. (1990), who used laser-

heated pedestal growth to prepare an LN single crystal fiber. A domain

pattern with 2–3.5µm period in the fiber of ~250µm diameter was achieved

by periodically modulating the heating power. The mechanism in the method

is similar to that in the growth striation technique. Jundt et al. (1991) used

a single crystal fiber with 1.24mm in length and a 3.47µm domain period for

a second harmonic green generation of 2W from a 4W, 1.064µm fundamental

source. It was the first report of an LN superlattice being operated at average

power at the watt level.

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Novel physical effects in dielectric superlattices 937

An advantage of the growth striation technique is that the sample prepared

has a large cross-section, which can avoid tight position alignment tolerance

and results in a higher output in optical applications. Another advantage of

the method is that it is easy to dope some laser active ions into the crystal

during growth for the design of a multifunction laser device. Lu et al. (1996c)

and Zheng et al. (1998) doped Nd

and Er

ions into the LN superlattice

crystals, respectively. The nonlinear optical properties of substrate crystal

and spectral properties of Nd

or Er

ions were combined in the same

superlattice. The spectral structure (including absorption and fluorescence

0.75 C

1 min

200 µm

(d)

(c)

(b)

(a)

31.2

Temperature fluctuations measured on the solid–liquid interface

(a) and the corresponding surface rotational growth striations in a LN

crystal (b), while rotational rate was changed suddenly from 4 to 13

rpm in the experiment, surface rotational growth striations (c) and

the corresponding interior laminar ferroelectric domain structures (d)

(after Ming

et al.

, 1982).

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Handbook of dielectric, piezoelectric and ferroelectric materials938

spectra) of these superlattices is generally similar to those of doped crystals

with single domain or glass fibers, verifying no obvious effect of domain

wall for the excitation properties of doping ions.

31.2.2 Chemical diffusion

In 1979, Miyazawa (1979) discovered that the diffusion of element titanium

(Ti) could give rise to the domain reversal on the +z surface of an LN crystal.

Later, it was confirmed that proton or ion exchange followed by heat treatment

could also produce domain reversal on the +z face of LN (Zhu Y.Y. et al.,

1995), and the –z faces of LT (Ahlfeldt, 1994) and KTiOPO

(KTP) (Vanherzeel

and Bierlein, 1992), respectively. Following these discoveries, the chemical

diffusion and impurity ion exchange were exploited to fabricate the periodical

domain reversal located within a few micrometers of the surfaces of the LN,

LT and KTP crystals. These two methods were appropriate for guided wave

interactions and surface acoustic wave devices. Their advantage is that the

metal mask, defined by lithography, is deposited on the surfaces prior to Ti

diffusion or proton exchange, which can lead to a well-defined domain period

when domain reversal occurs. The disadvantage is that the shape of the

reversed domain is either triangular (for Ti diffusion in LN) or semicircular

(for proton exchange in LT). The shapes of reversed domains are not ideal

for optical or acoustic applications; however, high conversion efficiencies

were still obtained in waveguide devices due to long effective interaction

length and tight confinement of beam in the geometry (Yamada et al. 1993).

Yttrium concentration

(relativevalue)

2.5

2.0

1.5

1.0

0.5

0 2 4 6 8 10 12 14 (µm)

Positive

domain

Negative

domain

Positive

domain

31.3

The yttrium concentration distribution and ferroelectric domain

structures in rotational striations. The yttrium concentration

measured using X-ray energy dispersive spectrum analysis, point by

point, along the modulation direction of domain. It is worth noting

the domain boundaries are situated at places where the gradient of

yttrium concentration changes its sign (after Ming

et al.

, 1982).

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Novel physical effects in dielectric superlattices 939

A possible explanation to the domain reversal mechanism of Ti diffusion

was given by Peuzin (1986) according to the earlier works of Thaniyavarn et

al. (1985), Tasson et al. (1976) and Ming et al. (1982). Ming and Tasson had

certified that an impurity concentration gradient in LN had the same poling

property as an electric field (equivalent field). This mechanism might apply

more generally for the chemical diffusion and heat treatment methods.

31.2.3 Electron beam writing

Electron beams have been used to induce a modulated domain structure in

some ferroelectric crystals by writing on the negative polarity surfaces of

these crystals directly. Keys et al. (1990) and Ito et al. (1991) first made

progress in LN, whereafter, Hsu and Gupta (1992) in LT and Gupta et al.

(1993) in KTP, respectively, using a scanning electron microscope (SEM). In

a typical experimental geometry, the +c surface of the crystal was coated

with 50–150 nm Au, Al, Cr or other metal film, and mounted on the SEM

sample stage. The electron beam was focused and scanned on the uncoated

–z face of the crystal. The acceleration voltage of the electron beam is

sample dependent. It is 20–25kV for a LN wafer with 0.5mm thickness. The

beam current and beam size fall on the surface of sample ranges within

several pA–several nA and from 0.3–0.5µm, respectively, depending on the

period of domain and scanning speed that is generally set from 0.02 to

0.1mm/s. The penetration depth of the electron into treated samples depends

on the electron energy and the nature of the material. It is estimated to be

about a few micrometers for most ferroelectric crystals. The domain reversal

can steadily extend across the sample wafer, under certain conditions, and

the thickness can reach to 1mm for LN crystal, which are several hundred

times greater than the electron penetration depth. This method provides a

domain wall perpendicular to the surface. The mechanism for the domain

reversal by the electron beam is not very clear at this moment. However, a

volume domain grating can be fabricated in the ferroelectric sample. LN, LT

and KTP superlattices with period 3–7µm were fabricated by the method

and highly efficient second harmonic green and blue lights were generated

by the bulk and waveguide experimental geometry, respectively. Mizuuchi

and Yamamoto (1994) reported that ion-beam writing induced the domain

reversal in a LN or LT crystal wafer, and therefore can also be used to

fabricate ferroelectric superlattices.

31.2.4 Electric field poling

Although electron and ion writing can be used in the fabrication of small

periodic domain gratings, it is in practice limited by the slow writing speed

and the complicated and expensive beam scan system. A definite goal for

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