Third-harmonic generation by cascading second-order nonlinear processes in a cerium-doped KTiOPO4 crystal

We have achieved effective third-harmonic generation (THG) by cascading second-harmonic and sum-frequency generation in a single cerium-doped KTiOPO (4) crystal by using femtosecond laser pulses at 1.32 microm. A conversion efficiency of 0.17% was achieved with an average pump power of 35 mW.Furthermore, a unique quadratic power dependence for this type of THG was confirmed. We developed a theory and obtained an analytical solution for the THG.The solution exactly describes the measured characteristics of the THG.     

Efficient third-harmonic generation in partly periodically poled KTiOPO(4) crystal

A partly periodically poled KTiOPO(4) (KTP) crystal has been designed to integrate quasi-phase-matched second-harmonic generation (QPM SHG) with sum-frequency generation in one crystal for generating a third-harmonic beam. The highest conversion efficiencies of 45% and 3% have been achieved in our experiments for QPM SHG and third-harmonic generation, respectively, by use of picosecond laser pulses at 1.327 microm . We have also discovered that periodically poled KTP has slightly different indices (n(z)) from bulk KTP.     

Multimegawatt relativistic harmonic gyrotron traveling-wave tubeamplifier experiments

The first multimegawatt (4 MW, η=8%) harmonic (ω=sΩ_c, s=2,3) relativistic gyrotron traveling-wave tube (gyro-twt) amplifier experiment has been designed, built, and tested. Results from this experimental setup, including the first ever reported third-harmonic gyro-twt results, are presented. Operation frequency is 17.1 GHz. Detailed phase measurements are also presented. The electron beam source is SNOMAD-II, a solid-state nonlinear magnetic accelerator driver with nominal parameters of 400 kV and 350 A. The flat-top pulsewidth is 30 ns. The electron beam is focused using a Pierce geometry and then imparted with transverse momentum using a bifilar helical wiggler magnet. The imparted beam pitch is a α≡β_⊥/β_∥≈1. Experimental operation involving both a second-harmonic interaction with the TE₂₁ mode and a third-harmonic interaction with the TE ₃₁ mode, both at 17 GHz, has been characterized. The third-harmonic interaction resulted in 4-MW output power and 50-dB single-pass gain, with an efficiency of up to ~8% (for 115-A beam current). The best measured phase stability of the TE₃₁ amplified pulse was ±10° over a 9-ns period. The phase stability was limited because the maximum RF power was attained when operating far from wiggler resonance. The second harmonic, TE₂₁ had a peak amplified power of 2 MW corresponding to 40 dB single-pass gain and 4% efficiency. The second-harmonic interaction showed stronger superradiant emission than the third-harmonic interaction. Characterizations of the second- and third-harmonic gyro-twt experiments presented here include measurement of far-field radiation patterns, gain and phase versus interaction length, phase stability, and output power versus input power     

Efficient second-harmonic generation in birefringently phase-matched GaAs/Al(2)O(3) waveguides

We report efficient second-harmonic generation of femtosecond pulses in birefringently phase-matched GaAs/Al(2)O(3) waveguides pumped at 2.01mum. By use of pump pulses of ~200-fs duration and type I interaction, practical second-harmonic average powers of up to ~650muW were obtained, with an average input power of ~50muW. Waveguides of four different widths and two different lengths were investigated, and a normalized conversion efficiency of greater than 1000%W(-1)cm(-2) was obtained for a 1-mm waveguide. Measurements of pump and second-harmonic spectra provided clear evidence of phase matching and depletion of the pump spectrum. The measured bandwidth of the second harmonic was ~1.3nm. From the measurements of transmitted pump power at the phase-matching wavelength, pump depletions of more than 80% were recorded.     

Efficient energy conversion for cubic third-harmonic generation that is phase matched in KTiOPO(4)

We demonstrate, for the first time to our knowledge, that efficient third-harmonic generation can be achieved with a cubic contribution much larger than the quadratic processes. An energy-conversion efficiency of 2.4% is achieved for cubic third-harmonic generation that is phase matched along the x axis of a KTiOPO(4) crystal by use of a picosecond fundamental laser emitting at 1618 nm. The associated cascading processes are only 10% of the pure cubic interaction, which is very suitable for study of the specific quantum optical correlations. Calculations of the third-harmonic generation conversion efficiency with respect to group-velocity dispersion and to the longitudinal Gaussian beam profile account well for our experimental results.     

Defective photonic crystals with greatly enhanced second-harmonic generation

A one-dimensional defective photonic crystal structure is proposed with the aim of studying its nonlinear optical properties. In such a structure, extremely enhanced second-harmonic generation with an efficiency ~5 orders of magnitude higher than that of ordinary films is demonstrated in a numerical simulation. Extraordinary phase conditions of the process in such a structure were explored, and efficient forward and backward second-harmonic generation could be achieved simultaneously. The mechanism of the enhancement is the high field intensity and efficient wave coupling introduced by light localization in defect states, from which many other nonlinear processes can also benefit.     

Third-harmonic generation in a general two-component quasi-periodic optical superlattice

The quasi-periodic optical superlattice is a promising material for use in optical frequency conversion. We propose a method for designing a quasi-periodic structure for efficient third-harmonic generation (THG) at any given wavelength. With this method we have made a LiTaO(3) sample in which 27% THG at 0.48microm was achieved, together with a series of highly efficient multiwavelength second-harmonic generation outputs. The result is in good agreement with the theoretical prediction.     

Phase-conjugate backward stimulated emission from a two-photon-pumped lasing medium

Optical phase conjugation of the two-photon-pumped frequency upconversion backward stimulated emission of a new dye solution has been demonstrated. When the 1064-nm pump intensity reaches a certain threshold value, a highly directional and phase-conjugate backward stimulated emission at ~616-nm wavelength can be observed. After backward passage through an aberration plate placed in the input pump beam path, the introduced aberration influence (1.6-1.8 mrad) can be entirely compensated by backward stimulated emission with a final beam divergence of only 0.23 mrad. The net conversion efficiency from the absorbed IR pump energy to the backward visible stimulated emission energy can reach 10%.     

Three-level operation of a diode-bar-pumped Yb:S-FAP laser

We present an all solid-state Yb:S-FAP laser system running on the three-level laser transition at 985 nm. The pump source was a high fill-factor laser diode bar, with the output reformatted using a two-mirror beamshaping system to produce a rectangular pump beam that focused to a square spot. A nearly on-axis multipassing system was used to obtain four pump passes through a 1.6 mm Yb:S-FAP laser crystal. Gain-switched three-level laser output was achieved with an efficiency of 4.3% with respect to incident pump power. Electro-optic Q-switching produced 0.12 mJ pulses for a pump pulse energy of 11 mJ. Intra-cavity second-harmonic generation yielded a maximum pulse energy at 492.5 nm of 12 μJ.     

Highly efficient third-harmonic generation with electro-optically Q-switched diode-end-pumped Nd:YVO4 slab laser

We have achieved efficient third-harmonic generation (THG) with an electro-optically Q-switched diode-end-pumped slab laser by cascading second-harmonic and sum-frequency generation in a lithium triborate (LBO) crystal. The high conversion efficiency, short pulse length and high pulse energy is the characteristic of the output 355 nm light. An average power of 11.1 W at a repetition rate of 10 kHz was achieved. The pulse energy is 1.1 mJ and the pulse length is 5 ns. The peak power of pulse is 0.22 MW. The conversion efficiency from 1064 nm to 355 nm reached 44.4% which is to our knowledge the highest conversion efficiency. Furthermore, the 355 nm light is near TEM₀₀ mode. The beam quality is M² < 1.5. In this paper, the experimental setup, results and the factors which can affect the conversion efficiency are discussed.     

A Doubly Resonant Photonic-Crystal Microcavity for Second-Harmonic Generation

A two-dimensional, photonic-crystal microcavity is proposed in order to achieve efficient second-harmonic generation in the third optical communication window. The simultaneous resonance conditions of the pump beam at the fundamental frequency and second-harmonic field generated inside the structure with a defect provides a considerable enhancement of the conversion efficiency. In the configuration we propose, a transverse-magnetic (TM) polarized beam resonating at the fundamental frequency generates a second-harmonic field corresponding to a transverse-electric (TE) polarized resonant mode. The design of this doubly resonant microcavity is carried out by a linear analysis to search for the resonance frequencies and calculate their field distributions. The nonlinear analysis of the second-harmonic generation is performed using a dispersive finite-difference time-domain code.     

Phase-matched third-harmonic generation in highly germanium-doped fiber

Phase-matched third-harmonic generation is demonstrated in a germanium-doped optical fiber. Green light at 514.4 nm is generated in an LP(03) mode when a pump field at ~1543.3 nm is launched into the fiber in the fundamental LP(01) mode. The phase matching is achieved for a particular combination of the germanium doping concentration and the fiber core diameter.     

Methods for increasing the efficiency of nonlinear optical interactions in nanostructured semiconductors

Methods for increasing the efficiency of the optical second-and third-harmonic generation in gallium phosphide and silicon nanostructures formed by electrochemical etching of crystalline semiconductors are discussed. The efficiency of nonlinear optical interactions can be increased by using phase matching in anisotropic nanostructured semiconductors that exhibit form birefringence or by increasing the local field, as in scattering in macroporous semiconductors. The efficiencies of third-harmonic generation in porous silicon and of second-harmonic generation in porous gallium phosphide are found to increase by more than an order of magnitude.     

Active formation of an antiguide structure in a photorefractive polymer for enhanced second-harmonic generation

An antiguide structure for enhanced second-harmonic generation was actively constructed in a photorefractive polymer by use of a pump beam. Irradiation of a pump beam enhanced second-harmonic power, and blocking the pump returned the power to the initial value. The electric-field dependence of the degree of enhancement of the second-harmonic power confirmed that the antiguide structure was constructed through a photorefractive-index change in the medium. The photorefractive-index change accompanied molecular reorientation induced by the pump-generated space charges. The thermo-optic effect on formation of the structure is also discussed.     

Continuum generation of the third-harmonic pulse generated by an intense femtosecond IR laser pulse in air

The continuum generation by intense femtosecond IR laser pulses focused in air including the effect of third-harmonic generation is investigated. We have used a theoretical model that includes the full spatio-temporal dynamics of both the fundamental and the third-harmonic pulses. Results of our numerical calculations show that a two-color filamentation effect occurs, in which the third-harmonic conversion efficiency remains almost constant over the whole filament length. It is found that this effect is rather independent of the wavelength of the input beam and the focal geometry. During the filamentation process the third-harmonic pulse itself generates a broad continuum, which can even overlap with the continuum of the fundamental pulse for the longer pump wavelengths. In consequence, the continuum generation generated by intense IR laser pulses is further extended into the UV. PACS 42.65.Jx; 42.65.Ky; 52.35.Mw     

Effect of Air Gap on Dual-Tripler Broadband Third-Harmonic Generation

We experimentally study the effect of the air gap on conversion efficiency and the spectrum of generated third-harmonic pulses in the dual-tripler broadband third-harmonic generation scheme. The experimental results are in good agreement with predictions that the 4-cm air gap is equivalent to a full cycle of phase mismatch among the three interacting pulses （i.e. the fundamental, second-harmonic and third-harmonic pulse）. The experimental results also show that the spectrum of the third-harmonic pulse is sensitive to the air gap. We also point out that the air gap effect can be ignored when the dual-tripler system is located in 1000 Pa atmosphere. These results will guide the design of the broadband third-harmonic generation system in high power lasers.     

Third-harmonic generation in relative-phase-controlled two-color laser field

We experimentally demonstrate third-harmonic generation (THG) by intense femtosecond laser pulse at a central wavelength of ??800?nm superposed by its second harmonic in air. The third-harmonic signal shows a periodic modulation with a period of ??0.67?fs when the delay between fundamental and second-harmonic wave is continuously changed. The periodic modulation of THG can be attributed to the interference of third-harmonic signals generated from a direct THG channel (3??=??+??+??) and a four-wave mixing (FWM) channel (3??=2??+2?????). With high pump intensity, the fitting of the measured TH spectrum as a function of delay implies that the pump pulse undergoes a strong pulse splitting and self-phase modulation at the focus.     

Time Character of Nanosecond Phase-Conjugate Signals in an Erythrosin B-Doped Polymer Film

The generating mechanisms of nanosecond phase-conjugate (PC) signal and the time delay of a backward pump beam, at which the maximum diffraction efficiency of the PC signal can be obtained, are investigated using degenerate four-wave mixing in an erythrosin B-doped polyvinyl alcohol film. The influence of the coherence time of a pulse source on the measurement of the mechanisms of the PC signal generation is also examined. The population gratings due to saturable absorption of dye molecules mainly contribute to the PC signal generation rather than thermal gratings. The maximum diffraction efficiency is obtained at zero time delay of the backward pump beam, at which the coherence peak is observed.     

A novel quasi-phase-matching frequency doubling technique

A new quasi-phase-matching technique for efficient second-harmonic generation is reported. It is based on the spatial periodic modulation of the light intensity along the propagation direction, rather than the conventional spatial periodic modulation of the nonlinear optical coefficients. It can be realized by using a novel dual-channel waveguide frequency doubler structure for the desired light intensity distribution. This dual-channel waveguide device has major advantages including very small beam size, high light intensity within long nonlinear-waveguide interaction length, highly efficient second-harmonic generation, ease in fabrication of the nonlinear channel waveguides without any spatially periodic poling, and low waveguide propagation losses. The new quasi-phase-matching technique can also be applied to third-harmonic generation and other nonlinear optics processes.     

Efficient high-power frequency doubling of distributed Bragg reflector tapered laser radiation in a periodically poled MgO-doped lithium niobate planar waveguide

We report on efficient single-pass, high-power second-harmonic generation in a periodically poled MgO-doped LiNbO3 planar waveguide using a distributed Bragg reflector tapered diode laser as a pump source. A coupling efficiency into the planar waveguide of 73% was realized, and 1.07 W of visible laser light at 532 nm was generated. Corresponding optical and electro-optical conversion efficiencies of 26% and 8.4%, respectively, were achieved. Good agreement between the experimental data and the theoretical predictions was observed.