Quasi-Phase-Matched Second-Harmonic Generation in a Periodic-Lens Sequence Waveguide with a Relatively Wide Wavelength-Tuned Width
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The phase-matching characteristic of a quasi-phase-matched second-harmonic generation in a periodic-lens sequence waveguide on z -face KTiOPO 4 (potassium titanyl phosphate) crystal was obtained by measuring, as a function of input optical wavelength, the intensity of frequency-doubled radiation. The fabricated waveguide is a kind of segmented domain-inverted waveguide of which grating pitch is 4 µ m. The phase-matching was observed in the fundamental wavelength region of 850-860 nm. The half-width of the peak around 854 nm is about 0.3 nm. The broadening of phase-matching in wavelength was explained using geometrical optics.Keywords:
Waveguide
Potassium titanyl phosphate
For many years, potassium titanyl phosphate (KTP) has been a well known crystal for frequency doubling of Nd:YAG lasers (1.064 μm). The large nonlinear coefficient of KTP crystal together with the wide temperature width, large acceptance angle, and very low absorption loss make KTP a promising candidate for high conversion efficiency. One problem with the use of KTP is that phase matching at 1.064 μm is possible only by Type II angle tuning, so that walk-off of the ordinary and extraordinary beams inside the crystal limits its utility in situations involving high finesse resonators. Recently however, Garmash et al. [1] have reported that Type II 90° noncritical phase-matching is possible with an a-cut KTP at 1.079 μm, thus enhancing the possibilities for intracavity cw frequency doubling. Indeed by following this lead, we have achieved 85% nonlinear conversion efficiency for doubling of 1.079 μm radiation to 0.54 μm.
Potassium titanyl phosphate
Crystal (programming language)
Frequency Conversion
Harmonic
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Conversion efficiency of 85% has been achieved in cw second-harmonic generation from 1.08 to 0.54 microm with a potassium titanyl phosphate crystal inside an external ring cavity. An absolute comparison between the experimental data and a simple theory is made and shows good agreement.
Potassium titanyl phosphate
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Frequency Conversion
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Summary form only given. In summary, a beam propagation (BPM) model of SHG in nonlinear waveguides has been developed and shown to simulate quasi phase matched (QPM) SHG in periodically segmented KTiOPO/sub 4/ (KTP) waveguides.
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Type-II second-harmonic generation was investigated in channel waveguides fabricated in potassium titanyl phosphate (KTiOPO4) crystals by ion exchange. The birefringence of the guided-wave structure was used to provide phasematching. A green (509 nm) output power of 0.25 mW was produced by frequency doubling the 1018-nm output of a titanium:sapphire laser; this power is 200 times greater than what would be obtained in the corresponding bulk interaction. Phasematching for second-harmonic generation was found to have broad tolerances with respect to temperature and wavelength. Blue light at 483 nm was produced by sum-frequency mixing of 1064-nm and 883-nm light.
Potassium titanyl phosphate
Harmonic
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The phasematching characteristics of bulk periodically-poled KTP (potassium titanyl phosphate), fabricated by electric-field poling, are measured for first-order quasi- phasematched second-harmonic generation and compared to theory.
Potassium titanyl phosphate
Poling
Quasi-phase-matching
Optical materials
Frequency Conversion
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Summary form only given. Recently, we achieved backward second-harmonic generation (SHG) in periodically-poled bulk LiNbO/sub 3/ (PPLN). We have improved the conversion efficiency by about an order of magnitude by using long laser pulses. Here, we report our new results of backward SHG by using periodically-poled potassium titanyl phosphate (PPKTP) waveguide. More importantly, we report our results on first observation of backward third-harmonic generation (THG) due to completely quasi-phase-matched cascaded SHG and sum-frequency generation (SFG) in PPKTP waveguide. This is the first time to achieve backward effective THG where both of SHG and SFG are quasi-phase-matched. Previously, we observed backward THG in PPLN by cascading SHG and SFG. However, only one of these two constituent processes is quasi-phase-matched.
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The spectral phase-matching bandwidths in LiIO3, LiNbO3, CDA, and KDP (type I and type II) for second harmonic generation (SHG) at 1.06 μm have been accurately measured using a tunable line-narrowed Nd : glass laser. The present results disagree with previously published data. Mechanisms such as sum-frequency generation which could contribute to the discrepancy are discussed and demonstrated in a 90° phase-matched deuterated CDA crystal.
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Frequency Conversion
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Optical waveguides can be formed in potassium titanyl phosphate (KTiOPO4, KTP) crystals by an ion-exchange process. Waveguides were fabricated in KTP substrates by exchanging rubidium ions for potassium ions. The resulting refractive index profile was measured as a function of diffusion time and temperature. Based on these characterization measurements, the phase-matching properties of the planar waveguide for guided-wave second-harmonic generation were modeled. Second-harmonic generation of blue/green light was observed at wavelengths shorter than the bulk phase-matching limit, in good agreement with the results of numerical modeling.
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Rubidium
Waveguide
Characterization
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Nonlinear second-harmonic generation (SHG)microscopy has become a commonly used technique for investigating interfacial phenomena(Kemnitz et al., 1986; Shen, 1989) and imaging biological samples.(Moreaux et al., 2000) Different non-centrosymmetric nanometric light sources have been recently studied in this context, e.g. organic nanocrystals.(Shen et al., 2001; Treussart et al., 2003) For those systems, resonant optical interaction leads to an enhancement of the nonlinear response but also to parasitic effect that is detrimental for practical applications, namely photobleaching due to two-photon residual absorption.(Patterson et al., 2000) Conversely, inorganic non-centrosymmetric materials with far-off resonance interaction avoid this limitation.(Johnson et al., 2002; Long et al., 2007) Recent achievements have been obtained using KNbO3 nanowires as a tunable source for sub-wavelength optical microscopy(Nakayama et al., 2007) and Fe(IO3)3 nanocrystallites as promising new SHG-active particles with potential application in biology.(Bonacina et al., 2007) However, either the dimensions of the used crystals are still of the order of themicrometer along one axis,(Nakayama et al., 2007) or the corresponding bulk material is not easily grown,(Bonacina et al., 2007) so that the crystal characteristics are not directly available. A complementary approach consists in considering a well-known SHG-active bulk material and investigating its properties in nanoparticle form. Different materials have been considered, e.g. BaTiO3 (Hsieh et al., 2009) and ZnO (Kachynski et al., 2008). In this view, we were among the pioneers in this domain, considering the well-known KTP material. Potassium titanyl phosphate (KTiOPO4, KTP) is a widely used nonlinear crystal.(Zumsteg et al., 1976) Studies on this material have focused on the optimized growth of large-size single crystals, which have found numerous applications in laser technology for efficient frequency conversion.(Driscoll et al., 1986) Here we show that KTP particles of nanometric size (nano-KTPs) are an attractive material for SHGmicroscopy. Under femtosecond excitation and in ambient conditions, a single nano-KTP with a size around 60 nm independently determined with atomic force microscopy (AFM), generates a perfectly stable blinking-free second-harmonic signal which can be easily detected in the photon-counting regime. Furthermore, we demonstrate that this single nanocrystal can 7
Potassium titanyl phosphate
Second-harmonic imaging microscopy
Photobleaching
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We generated 115 mJ of OPO energy at 2 μm from a two-crystal, two-pass, walkoff- compensated KTP pumped multimode at 1.06-μm. The phase matching angle for degeneracy in the X-Z plane of flux grown Chinese KTP is found to be 50.25 ± 0.25°, smaller than previously reported for KTP.
Potassium titanyl phosphate
Crystal (programming language)
Optical parametric amplifier
Optical Pumping
Degeneracy (biology)
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