Browsing by Author "Chien, R.-M."

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Hypersonic studies and refractive indices of CsTiOAsO4 and KTiOAsO4 single crystals
(1997-10-01) Tu, Chi-Shun; Yeh, Y.-L.; Schmidt, V. Hugo; Chien, R.-M.; Katiyar, R.S.; Guo, Ruqian; Guo, Ruyan; Bahalla, A.S.
The longitudinal Brillouin backscattering spectra along the [001] phonon direction have been measured as a function of temperature (24-896 °C) for both CsTiOAsO4 (CTA) and KTiOAsO4 (KTA). As the temperature approaches the ferroelectric transition temperature Tc, the acoustic phonon frequencies of both CTA and KTA show a strong softening (which reaches a minimum at Tc∼688°C for CTA). For both CTA and KTA, a broad damping peak was observed and can be attributed to the dynamic order-parameter fluctuations. This anomaly implies that an η2μ-type coupling is the dominant mechanism for the acoustic soft modes in CTA and KTA. Here η is the order parameter and μ is the strain. The refractive indices (nx,ny,nz) and Cauchy’s equations were obtained as a function of wavelength (0.4–1.7 μm) for KTA. The phase matching angles (θ,φ) of second harmonic generation for the fundamental wavelengths of 1.064 (type I) and 1.32 (types I and II) were also calculated.
Optical properties of RbTiOAsO4 single crystal
(1998) Tu, Chi-Shun; Yeh, Y.-L.; Katiyar, R. S.; Guo, Ruqian; Schmidt, V. Hugo; Chien, R.-M.; Guo, Ruyan; Bhalla, A.S.
Rubidium titanyl arsenate (RbTiOAsO4) belongs to the family of nonlinear optical crystals with the general formula M1+TiOX5+O4, where M = K, Rb, Tl, Cs and X = P, As. [1–6] The high damage threshold and broad angular acceptance have made such crystals attractive materials for frequency doubling of Nd-based lasers at λ=1.064 and 1.32 µm, and for optical parametric oscillators (OPO). In addition, the ion exchange properties also make them one of the best candidates for waveguide applications. Potassium titanyl phosphate, KTiOPO4 (KTP), is the most popular among such materials and has been used successfully in diﬀerent applications. However, the orthophosphate absorption at ∼4.3 and ∼3.5 µm in KTP severely limits the oscillator output power. In contrast, RTA has a broad infrared transparency (∼0.35-5.3 µm) and exhibits no overtone absorption between 3 and 5 µm. [4] This makes the RTA crystal a potential candidate for nonlinear optical applications. At room temperature, KTP-type crystals have an orthorhombic structure with non- centrosymmetric point group C2v (mm2) and space group Pna2 (Z=8). The crystal framework is a three-dimensional structure made from corner-linked TiO6 octahedra and PO4 tetrahedra. Four oxygen ions of the TiO6 belong to PO4 tetrahedral groups which link the TiO6 groups. In our earlier Raman results, a slight softening was exhibited by several LO and TO vibrational modes of RTA. [1,2] However, there is no typical soft mode observed in the lowfrequency modes of the Raman spectra. This motivated usto carryoutBrillouinscatteringmeasurements tolook for softening in the acoustic modes. We report here both the temperature-dependent acoustic phonon spectra and wavelength- dependent refractive indices. The Cauchy equations [n(λ) = A + B/λ2 + C/λ4] of nx, ny and nz are obtained. In particular, the ﬁrst direct evidence for acoustic phonon soft mode is presented.
Temperature and concentration dependences of Raman vibrational modes in Rb1-x(ND4)xD2AsO4 mixed crystals
(1997-02-01) Tu, Chi-Shun; Chien, R.-M.; Schmidt, V. Hugo
The Raman vibrations (10–103 cm−1) of A1, B2, and E symmetries along the [110] phonon direction have been measured as a function of temperature (80–293 K) in the mixed ferroelectric (FE) antiferroelectric system Rb1−x(ND4)xD2AsO4 (DRADA-x) for ammonium concentrations x=0, 0.10, and 0.28. With decreasing temperature, the B2 Raman shifts of RbD2AsO4 show both softening and hardening anomalies for four selected modes. In DRADA-0.10, the bending mode ν2(B2) (near 280 cm−1) of the AsO4 group exhibits a gradual softening down to T∼125 K and then has a rapid drop. This phenomenon is attributed to a quick development of long-range FE ordering. In DRADA-0.28, the in-plane bending mode δ(O-D) (B2) shows a softening near 220 K, whereas the bending mode ν2(B2) of the AsO4 group exhibits a hardening. We associate these anomalies with the onset of glass formation. A doublet with splitting of about 10 cm−1 was observed in the ν1 modes and was attributed to a slight deformation of the ν1 vibration due to AsO4 groups having different nearby ions (ND4 and Rb).
Temperature dependent Raman spectra of Rb1-x(ND4)xD2AsO4 mixed crystals
(1998) Tu, Chi-Shun; Gao, S.-S.; Jaw, R.-J.; Hwa, L.-G.; Schmidt, V. Hugo; Brandt, Dan; Chien, R.-M.
In the mixed ferroelectric (FE)-antiferroelectric (AFE) systemA1−x(ND4)xD2BO4 [A=Rb(orK,Cs)andB=As (or P)], there is competition between the FE and the AFE orderings, each characterized by speciﬁc conﬁgurations of the acid deuterons. [1–8] The random distribution of the Rb and ND4 ions is the main source to produce frustration which can increase local structural competition such that the long-range order of electric dipole disappears. Instead of a typical sharp FE or AFE phase transition, the phase coexistence becomes a characteristic in this type of mixed compounds. By a group theoretical analysis for the KDP-type structure (which contains two molecular units in a primitive unit cell); at zero wavevector, the vibrational modes in the tetragonal symmetry (space group I¯ 42d − D12 2d) can be decomposed into the following irreducible representations: Γvib = 4A1(R) + 5A2(Silent) + 6B1(R) + 6B2(R,IR) + 12E(R,IR). [9] The symmetry species A1, B1, B2 and E are Raman active. The situation in the mixed system D*RADA-x is more complicated than one in the parent crystals, because some Rb (or ND4) ions have been substituted by ND4 (or Rb) ions. In this case, the selection rule of the free AsO4 group is expected to be broken much easily than in the pure crystal. In the recent years, many measurements in D*RADAx system have been achieved on ferroelectric-side crystals x = 0.1, 0.10 and 0.28. [2–5] However, only a few experiments were done on antiferroelectric-side compounds (x ≥0.50).[6,7]A complete understanding for this mixed system is still lacking. This motivated us to carry out the polarized Raman scattering on D*RADA-0.55, 0.69 and 1.0. Here, we pay special attention to the stretching mode ν1 (near 755 cm−1) and the in-plane bending mode δ(O-D) (near 825 cm−1).