Effects of laser pulse duration on the formation dynamics of laser-induced periodic nanostructures.

Formation dynamics of laser-induced periodic surface structures (LIPSSs) on the SiC substrates were described with varying pulse numbers and pulse duration. As the number of laser pulses increases, two significant transformations become evident in the progression of structural formations. First from surface roughening with nanoparticles to LIPSS with the period that is slightly shorter than the laser wavelength. Second it turns to LIPSS with a period less than half the laser wavelength. It is found that maintaining the crystallinity is the key to changing the structures. In the cases of longer pulse width than sub-nanoseconds, no LIPSS formations are observed or LSFL does not change to HSFL because the irradiated area is poly-crystallized.

>50 MW peak power, high brightness Nd:YAG/Cr4+:YAG microchip laser with unstable resonator.

We demonstrated a flat-convex unstable cavity Nd:YAG/Cr4+:YAG ceramic air-cooled microchip laser (MCL) generating a record 37.6 and 59.2 MW peak power pulses with an energy of 17.0 and 24.1 mJ and a width of 452 and 407 ps at 20 Hz by using a uniform power square and hexagon pump, respectively. For hexagon pump, the near field hexagon donut beam was changed in to a Bessel-like beam in far field, whose beam quality was estimated as 2nd moment M2 of 7.67. The brightness scale of unstable resonator MCL was achieved up to 88.9 TW/(sr·cm2) in contrast with flat-flat cavity MCL. However, the high intense center part of Bessel-like beam increased its brightness effectively more than 8 times, up to 736 TW/(sr·cm2).

High peak power Nd:YAG/Cr:YAG ceramic microchip laser with unstable resonator.

A doughnut mode microchip laser was demonstrated by introducing a monolithic ceramic Nd:YAG/Cr4+:YAG chip in an unstable resonator to deliver laser pulses with an energy of 13.2 mJ and a pulse width of 476 ps, corresponding to a record peak power of 27.7 MW. The laser beam quality was characterized by M2∼6 at 10 Hz repetition rate. No significant degradation or change of beam pattern, pulse width, and M2 was confirmed during energy scaling in the case of the unstable cavity, promising for further brightness improving. In comparison with a flat-flat cavity, pulse broadening and M2 increase was observed up to ∼1.2 ns and ∼10, respectively, during energy scaling up to 18 mJ due to the beam pattern degradation. The doughnut beam was observed to have an Airy disk at the focal point, which was suitable for laser induced breakdown in air. The measured breakdown threshold of doughnut beam was comparable to a near-Gaussian beam (M2=1.3).

Sub-nanosecond laser induced air-breakdown with giant-pulse duration tuned Nd:YAG ceramic micro-laser by cavity-length control.

We first demonstrated a continuously and widely giant-pulse duration tunable laser based on a short monolithic Nd:YAG/Cr:YAG ceramic by cavity-length control in 100 Hz operation. The tuning range of pulse duration τ was from 0.5 to 9 ns as keeping peak powers of over 0.5 MW up to 6 MW. The characteristics of the ceramic laser was discussed in detail such as cavity-length dependent beam pattern and divergence, pulse shape, pulse energy due to the transverse and longitudinal modes, and an elliptical polarization status. Laser induced breakdown in laboratory air was investigated as a function of τ in sub-nanosecond region using the developed laser. Air-breakdown threshold intensity Ith was measured using three different focusing conditions. We confirmed that 1) the measured Ith was almost constant at the longer τ than τCI named as limit-pulse-duration of cascade ionization (CI), 2) Ith had ~τ-2 scaling for τ < τCI, 3) the increase of Ith is not connected to a specific intensity level, and 4) τCI was not constant and depended on focusing conditions. These phenomena were discussed with considering temporal-spatial intensity of laser.

0.54 µm resolution two-photon interference with dispersion cancellation for quantum optical coherence tomography.

Quantum information technologies harness the intrinsic nature of quantum theory to beat the limitations of the classical methods for information processing and communication. Recently, the application of quantum features to metrology has attracted much attention. Quantum optical coherence tomography (QOCT), which utilizes two-photon interference between entangled photon pairs, is a promising approach to overcome the problem with optical coherence tomography (OCT): As the resolution of OCT becomes higher, degradation of the resolution due to dispersion within the medium becomes more critical. Here we report on the realization of 0.54 µm resolution two-photon interference, which surpasses the current record resolution 0.75 µm of low-coherence interference for OCT. In addition, the resolution for QOCT showed almost no change against the dispersion of a 1 mm thickness of water inserted in the optical path, whereas the resolution for OCT dramatically degrades. For this experiment, a highly-efficient chirped quasi-phase-matched lithium tantalate device was developed using a novel 'nano-electrode-poling' technique. The results presented here represent a breakthrough for the realization of quantum protocols, including QOCT, quantum clock synchronization, and more. Our work will open up possibilities for medical and biological applications.

Thermal characteristics of second harmonic generation by phase matched calorimetry.

We analyze a solution of the heat equation for second harmonic generation (SHG) with a focused Gaussian beam and simulate the temperature rise in SHG materials as a function of the second harmonic power and the focusing conditions. We also propose a quantitative value of the heat removal performance of SHG devices, referred to as the effective heat capacity Cα in phase matched calorimetry. We demonstrate the inverse relation between Cα and the focusing parameter ξ, and propose the universal quantity of the product of Cα and ξ for characterizing the thermal property of SHG devices. Finally, we discuss the strategy to manage thermal dephasing in SHG using the results from simulations.

Parasitic-light-suppressed quasi-phase-matched optical parametric oscillation device.

Nonlinear absorption - such as green-induced infrared absorption (GRIIRA) - increases the risk of the catastrophic damage during high peak- power wavelength conversion. We propose a novel concept to suppress parasitic green second-harmonic generation (SHG) in optical parametric oscillation (OPO) using specially engineered quasi-phase-matched (QPM) structures. This selective suppression was achieved by relative π-phase shift in only SHG not OPO. Compared with a periodic device, a parasitic-light-suppressed (PLS) QPM device produced smaller normalized conversion efficiency in green and maintained singly resonant OPO performance.

Noncollinear parametric fluorescence by chirped quasi-phase matching for monocycle temporal entanglement.

Quantum entanglement of two photons created by spontaneous parametric downconversion (SPDC) can be used to probe quantum optical phenomena during a single cycle of light. Harris [Opt. Express 98, 063602 (2007)] suggested using ultrabroad parametric fluorescenc generated from a quasi-phase-matched (QPM) device whose poling period is chirped. In the Harris's original proposal, it is assumed that the photons are collinearly generated and then spatially separated by frequency filtering Here, we alternatively propose using noncollinearly generated SPDC. In our numerical calculation, to achieve 1.2 cycle temporal correlation for a 532 nm pump laser, only 10% -chirped device is sufficien when noncollinear condition is applied, while a largely chirped (50%) device is required in collinear condition. We also experimentally demonstrate an octave-spanning (790-1610 nm) noncollinear parametric fluorescenc from a 10% chirped MgSLT crystal using both a superconducting nanowire single-photon detector and photomultiplier tube as photon detectors. The observed SPDC bandwidth is 194 THz, which is the largest width achieved to date for a chirped QPM device. From this experimental result, our numerical analysis predicts that the bi-photon can be compressed to 1.2 cycles with appropriate phase compensation.

Thermal performance in high power SHG characterized by phase-matched calorimetry.

We proposed a method to determine device quality in heat removal. Temperature change depending on SH power was analyzed by fitting with a new model to characterize heat removal performance of SHG modules, named as phase-matched calorimetry (PMC). The thermal disposal performance of SHG devices was improved by combination of metal housing and reduced crystal aperture. With a tight aperture, we demonstrated a 19 W single-pass 532-nm SHG at a conversion efficiency of 26.5% in a 10-mm-long PPMgSLT crystal without saturation.

Measurement of refractive index and thickness of transparent plate by dual-wavelength interference.

We developed an accurate and efficient method for measuring the refractive indices of a transparent plate by analyzing the transmitted intensity versus angle of incidence. By using two different wavelengths, we resolved the 2pi-ambiguity inherent to the phase measurement involving a thick medium, leading to independent determination of the absolute index of refraction and the thickness with a relative uncertainty of 10(-5). The validity and the accuracy of our method were confirmed with a standard reference material. Furthermore, our method is insensitive to environmental perturbations, and simple to implement, compared to the conventional index measurement methods providing similar accuracy.

Nondestructive quality evaluation of periodically poled lithium niobate crystals by diffraction.

Quasi-phase-matching devices are usually fabricated by electric field poling over photolithographically defined electrode patterns on ferroelectric crystal substrates. For the optimal nonlinear optical performance of such devices, the micro-poled domain structure must ensure good fidelity to the designed grating structure. We present a nondestructive diffraction method to evaluate the quality of periodically poled lithium niobate crystals, by utilizing index modulation caused by the internal field effects. Our proposed method is much simpler than the conventional second-harmonic generation experiment, and provides a fast, low-cost but accurate means for micro-poling quality evaluation.

Ultra-broadband optical parametric generation and simultaneous RGB generation in periodically poled lithium niobate.

We report on efficient collinear optical parametric generation (OPG) with gain band ranging from 1400 to 2600 nm in a 2 cm-long periodically poled lithium niobate (PPLN) crystal. Such an ultra-broad gain band was obtained by choosing the pump wavelength at 933 nm, at which the group-velocities of the signal and the idler match near the degeneracy point. High OPG efficiency was obtained by quasi-phase matching (QPM). The ultra-broadband OPG led to efficient collinear RGB generation from a single PPLN crystal at a fixed pump wavelength. The green and red beams were found to be originating from high-order QPM sum-frequency generation between the pump and selected frequencies in the OPG band, while the blue beam was high-order QPM second-harmonic generation of the pump.

Broadband optical parametric amplification at the communication band with periodically poled lithium niobate.

We report broadband optical parametric generation (OPG) in a single periodically poled lithium niobate crystal with a picosecond pump pulse at a fixed wavelength. We also demonstrate efficient optical parametric amplification of a broadband seed pulse within the quasi-phase-matched OPG band. The broad parametric gain band is attributed to group-velocity matching and degeneracy between the signal and idler, and the broad spectral width of the pumping source.