H. C. Guo

National University of Singapore, Singapore, Singapore

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Publications (12)20.35 Total impact

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    ABSTRACT: Using terahertz time-domain spectroscopy, we measured the frequency dependent complex dielectric response and conductivity of n -type single-crystal ZnO epilayers with different carrier concentrations over the frequency range from 0.1 to 3.0 THz. The measured complex dielectric response and conductivity are analyzed using Drude model.
    Journal of Applied Physics 03/2010; · 2.21 Impact Factor
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    ABSTRACT: Using terahertz time-domain spectroscopy, we measured the complex conductivity and dielectric function of n -type GaN with various carrier concentrations on sapphire substrate. The measured complex conductivity, which is due to the free carriers, is well fitted by simple Drude model. The contribution from the lattice vibration to the complex dielectric function increases with the decrease in free carrier concentration. A better fitting of the frequency-dependent complex dielectric response was obtained by considering both of the Drude and the classical damped oscillator model.
    Journal of Applied Physics 10/2009; · 2.21 Impact Factor
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    ABSTRACT: By using a multipulse femtosecond pump-probe technique, we achieved an in situ characterization of terahertz wave shaping in the periodically poled Mg : LiNbO <sub>3</sub> crystal. The analysis method, which includes fast-Fourier transform (frequency spectrum) and Morlet wavelet transform (time-frequency spectrum) that detailed information on the shaping of the terahertz waves, is demonstrated.
    Journal of Applied Physics 03/2009; · 2.21 Impact Factor
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    ABSTRACT: With the nonlinear diffraction concept, we present a diffractive study of optical parametric interactions in nonlinear photonic crystals. The nonlinear diffraction concept enables the design of complicated nonlinear photonic crystal structures in an intuitive way. We show that there are two basic linear sequences, the anti-stacking and the para-stacking sequences, existing in a one-dimensional structure; and we present the realization of multiple phase-matching resonances in the combination of the two basic sequences. The parameters affecting the structure factor of a two-dimensional nonlinear photonic crystal are investigated, which indicate that not only the Ewald construction but also the relative domain size determines two-dimensional nonlinear diffractions.
    Applied Physics B 02/2009; 98(2):253-259. · 1.78 Impact Factor
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    ABSTRACT: We report a general description of quasi-phase-matched parametric process in nonlinear photonic crystals (NLPC) by extending the conventional X-ray diffraction theory in solids. Under the virtual wave approximation, phase-matching resonance is equivalent to the diffraction of the scattered virtual wave. Hence a modified NLPC Ewald construction can be built up, which illustrates the nature of the accident for the diffraction of the virtual wave in NLPC and further reveals the complete set of diffractions of the virtual wave for both of the air-dielectric and dielectric-dielectric contacts. We show the two basic linear sequences, the anti-stacking and para-stacking linear sequences, in one-dimension (1D) NLPC and present a general rule for multiple phase-matching resonances in 1D NLPC. The parameters affecting the NLPC structure factor are investigated, which indicate that not only the Ewald construction but also the relative NLPC atom size together determine whether a diffraction of the virtual wave can occur in 2D NLPC. The results also show that 1D NLPC is a better choice than 2D NLPC for a single parametric process.
    02/2008;
  • H. C. Guo, W. M. Liu, S. H. Tang
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    ABSTRACT: The 5 mol % MgO:LiNbO3 ferroelectric single crystal has attracted much attention in terahertz (THz) generation and detection by parametric process or optical rectification. In this work, the dielectric properties of 5 mol % MgO:LiNbO3 ferroelectric single crystal in 0.2−2 THz frequency range has been investigated by using transmission-type THz time-domain spectroscopy. The complex refractive index and dielectric function are extracted from the measured transmittance and phase shift. The power absorption and dispersion relationship of the lowest branch of the phonon polariton are observed. The results fit well with the classical damped oscillator model, indicating that the far-infrared dielectric response of 5 mol % MgO:LiNbO3 is dominated by the lowest transverse optical mode with E(x,y) symmetry centered at 4.533 THz. The investigation presented in this work provides important considerations for optimizing THz devices in 5 mol % MgO:LiNbO3 ferroelectric single crystal.
    Journal of Applied Physics 08/2007; 102(3):033105-033105-4. · 2.21 Impact Factor
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    ABSTRACT: We report a compact all-solid-state mid-infrared optical parametric oscillator with wide tunability using periodically poled 5 mol % MgO-doped LiNbO3 with a multiple-channel structure design and pumped by a compact diode-pump solid-state laser operating at 1.064 μm. Wide tunability from 1.44 to 1.58 μm at the signal beam wavelength and from 3.28 to 4.11 μm at the idler beam wavelength was achieved by both varying the temperature and translating the crystal through the resonator and the pump beam with no realignment required. The tuning performance and its related factors were analyzed, and at the same time the output performance as well as the effect of mid-infrared absorption of idler beam was investigated.
    Journal of Applied Physics 06/2007; 101(11):113112-113112-5. · 2.21 Impact Factor
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    ABSTRACT: We describe the observation of quasi-phase-matched spontaneous parametric down-conversion in eee-geometry in periodically poled LiTaO3 crystals. For scattered light, the two-dimensional angular-frequency intensity distribution was studied. Several detuning curves were recorded, corresponding to high orders m=-2, -3, -4, -5 of quasi-phase matching. The measured periods of domain gratings agree with the data obtained by atomic-force microscopy for the etched crystal surfaces. The presence of both odd and even orders indicates that the lengths of positive and negative domains are unequal. To determine the mean duty cycle for regular domain gratings we propose comparing of the intensities of spontaneous parametric down-conversion in different orders of quasi-phase-matching.
    Applied Physics B 05/2006; 83(2):273-278. · 1.78 Impact Factor
  • H. C. Guo, Y. Q. Qin, S. H. Tang
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    ABSTRACT: Quasi-phase-matched parametric downconversion via cascaded optical nonlinearities in an aperiodically poled MgO:LiNbO3 superlattice was studied in theory and experiment. Due to the cascading effect and the abundant reciprocal structure in an aperiodic optical superlattice, multiple-wavelength parametric downconversion in a wide spectrum range can be obtained. Enhancement of the conversion efficiency and output stability through coupling of two nonlinear processes is demonstrated. The result also reveals that the cascaded parametric downconversion process can be used to efficiently downconvert the fundamental wavelength to a longer wavelength of the infrared region. The process can be additionally served as an efficient mechanism to enhance THz wave propagation in a nonlinear optical medium.
    Applied Physics Letters 10/2005; 87(16):161101-161101-3. · 3.52 Impact Factor
  • H. C. Guo, S. H. Tang
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    ABSTRACT: Quasi-phase-matched parametric downconversion via cascaded optical nonlinearities in an aperiodically poled MgO:LiNbO3 superlattice was studied in theory and experiment. Coupling equations reveal that multiple-wavelength parametric downconversion in a wide infrared spectrum range can occur in a single crystal. Enhancement of the conversion efficiency and output stability through coupling of two nonlinear processes is demonstrated. The result also reveals that cascaded parametric downconversion process can be used to efficiently downconvert the fundamental wavelength to longer wavelength of infrared region.
    Proc SPIE 01/2005;
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    ABSTRACT: The cascaded parametric processes, i.e. optical parametric oscillation (OPO) and difference frequency generation (DFG), can be used to induce efficient CW mid-infrared radiation at 4–5 µm in a single optical superlattice obtained by aperiodically poling a ferroelectric crystal. The superlattice consists of basic building blocks modulated by a spatial function. We numerically solve the coupling equations considering pump depletion and mid-IR absorption and realize the optimal condition for the cascaded frequency-down-conversion processes. Simultaneously, it is seen that the output performance is enhanced by the cascaded parametric design compared to the standard OPO configuration.
    Journal of Physics Condensed Matter 11/2004; 16(47):8465. · 2.22 Impact Factor