We report on transiently phase matched second harmonic generation in an on-chip lithium niobate (LN) microresonator fabricated by femtosecond laser direct writing followed by focused ion beam milling. We demonstrate a normalized conversion efficiency of 1.1×10<sup>-3</sup>/mW in the LN microdisk with a diameter of ~102 μm and a thickness of ~700 nm.
We report on fabrication of microresonators of high quality (high-Q) factors in both glass and crystalline materials by femtosecond laser 3D micromachining. Based on this novel approach, we obtained high-Q microresonators of non-in-plane geometries in glass materials such as fused silica and Nd: glass and demonstrated lasing at a pump power as low as 69 microwatts. We also fabricated on-chip microresonators of sub-100 μm diameters in crystalline materials including calcium fluoride and lithium niobate, and demonstrated efficient second harmonic generation using the high-Q lithium niobate microresonator. Furthermore, femtosecond laser 3D micromachining allows direct integration of the microresonators with other functional microcomponents, such as a microfluidic mixer and a microheater, leading to compact microdevices with enhanced functionalities. Our technique opens new avenues for fabricating high-Q microresonators with either 2D or 3D geometries on various types of dielectric materials.
Based on the spontaneous four wave mixing in micro/nano-fiber (MNF), we report the generation of quantum-correlated
photon pairs. The wavelengths of the signal and idler photons are in the 1310 nm and 851 nm bands, respectively. The
measured ratio between the coincidence and accidental coincidence rates of signal and idler photons is up to 530.
Moreover, we characterize the spectral property of the signal photons in the wavelength range of 1270-1610 nm. The
results reveal that the bandwidth of the photon pairs is much greater than the theoretically expected value due to the
inhomogeneity of the MNF; while the spectrum of Raman scattering in MNF is different from that in conventional
optical fibers and photonic crystal fibers, which may originate from the heating used for fabricating the MNF. Our
investigation shows that the MNF is a promising candidate for developing the sources of quantum light in micro- or
nanometer-scales, and the spectral property of photon pairs can be used to non-invasively test the diameter and
homogeneity of the MNF.
We report on fabrication of three-dimensional (3D) high-quality (Q) whispering-gallery-mode microcavities by femtosecond laser micromachining. The main fabrication procedures include the formation of on-chip freestanding microdisk through selective material removal by femtosecond laser pulses, followed by surface smoothing processes (CO<sub>2</sub> laser reflow for amorphous glass and focused ion beam (FIB) sidewall milling for crystalline materials) to improve the Q factors. Fused silica microcavities with 3D geometries are demonstrated with Q factors exceeding 106. A microcavity laser based on Nd:glass has been fabricated, showing a threshold as low as 69μW via free space continuous-wave optical excitation at the room temperature. CaF<sub>2</sub> crystalline microcavities with Q factor of ~4.2×10<sup>4</sup> have also been demonstrated. This technique allows us to fabricate 3D high-Q microcavities in various transparent materials such as glass and crystals, which will benefit a broad spectrum of applications such as nonlinear optics, quantum optics, and bio-sensing.