Microresonator-based Kerr soliton microcombs are coherent light sources consisting of equally spaced and phase locked discrete optical frequency components, which are essential tools for practical applications in precision spectroscopy and data processing. While anomalous microresonator dispersion is mandatory for Kerr soliton microcomb formation, so far almost all dispersions are susceptible to manufacturing error and cannot be tuned once the microstructure is made. Moreover, microcomb formation in strongly Raman-active mediums like Lithium Niobate (LN) is challenging in the suppression of stimulated Raman scattering and mode crossing due to the existence of densely distributed multiple Whispering Galley Mode (WGM) families. Here, Kerr soliton microcombs were formed in a normal dispersion LN microdisk resonator by mode trimming. Despite that the fundamental WGM family is of normal dispersion and there are densely distributed high-order WGM families within the LN microdisk, high-Q square modes of anomalous dispersion and small mode volume are coherently formed by introducing weak perturbation for mode trimming. Under the optical pump of the square mode of 35-mW power, densely distributed WGM families are avoided to be excited, leading to the suppression of Raman scattering effects and mode crossing, and the formation of soliton microcomb with a spectrum spanning from 1450 nm to 1620 nm.
Mode trimming of suitable dispersion in a high-Q microresonator is of vital importance for lots of photonic applications ranging from Kerr soliton comb generation, single-frequency lasing to nonlinear frequency conversion. However, almost all the dispersion engineering and mode trimming cannot be tuned once the resonator structure is made. To overcome this difficulty, weak perturbation was introduced into a circular lithium niobate (LN) microdisk for the formation of polygon modes by a coupled tapered fiber. Various polygon modes with different spatial field distributions and star mode can be formed by tuning the pump wavelength and the coupling position. Thanks to the small spatial modal overlap between the polygon modes and the whispering gallery modes (WGMs), densely distributed WGM families were avoided to be excited under polygon-mode optical pump. Therefore, single-frequency/dual-wavelength microlasers have been demonstrated with narrow linewidths in weakly-perturbed erbium-doped LN microdisks, and Kerr soliton microcomb has been generated in the telecom band in a normal-dispersion LN microdisk by mode trimming through the excitation of the anomalous-dispersion polygon modes.
We demonstrate monolithically integrated on-chip single-frequency micro-disk lasers coupled with bus-waveguides fabricated on active lithium niobate platform by photolithography assisted chemo-mechanical etching. Benefiting from the high-Q factors, long cavity lengths of 1 mm for high absorption of pump light, and the elegant control on the modes formed within the large microdisk by breaking the centrosymmetry of the microdisk cavities, a microdisk laser with a narrow linewidth of 0.11MHz and a maximum output power of 62.1uW has been achieved at room temperature. Moreover, soliton optical frequency generation has been observed from square modes in weakly perturbed pure lithium niobate microdisks with a diameter of ~125um, a thickness of about 860nm and a wedge angle of 21 degrees. The solition optical frequency comb covered a bandwidth from 1450nm to 1650nm when pumped at 1542nm wavelength, suffering from the mode crossing of the dense whispering gallery modes.
Single-frequency microlasers and photonic generation of microwave signal with a fast tuning are highly in demand for lots of applications, including high-resolution spectroscopy, precision metrology, coherent communication, and so on. Design and fabrication of narrow-linewidth microlasers and low-phase-noise microwave signal are challenging. Here, we fabricated high-Q erbium ion doped lithium niobate microcavities for single frequency lasing through simultaneous excitation of high-Q polygon modes at both pump and laser wavelengths. Tunable single-mode microlasers with linewidth as narrow as 454 Hz was demonstrated. Moreover, photonic generation of low-phase-noise microwave signals were synthesized from dual-wavelength microlasers on single active LNOI microdisks. The dual-wavelength microlasers were generated from high-Q nearly degenerate polygon modes with spatial intensity distributions almost the same but a -phase difference. Due to the suppression of the gain competition, dual-wavelength lasing and in turn the low noise microwave source are stable. The phase noise of the microwave signal was measured to -123 dBc/Hz.
Single-frequency ultranarrow linewidth on-chip microlasers with a fast wavelength tunability play a game-changing role in a broad spectrum of applications ranging from coherent communication, light detection and ranging, to metrology and sensing. Design and fabrication of such light sources remain a challenge due to the difficulties in making a laser cavity that has an ultrahigh optical quality (Q) factor and supports only a single lasing frequency simultaneously. Here, we demonstrate a unique single-frequency ultranarrow linewidth lasing mechanism on an erbium ion-doped lithium niobate (LN) microdisk through simultaneous excitation of high-Q polygon modes at both pump and laser wavelengths. As the polygon modes are sparse within the optical gain bandwidth compared with the whispering gallery mode counterpart, while their Q factors (above 10 million) are even higher due to the significantly reduced scattering on their propagation paths, single-frequency lasing with a linewidth as narrow as 322 Hz is observed. The measured linewidth is three orders of magnitude narrower than the previous record in on-chip LN microlasers. Finally, enabled by the strong linear electro-optic effect of LN, real-time electro-optical tuning of the microlaser with a high tuning efficiency of ∼50 pm / 100 V is demonstrated.
We present our latest progresses on the development of integrated photonic devices as well as microfluidic chips of unprecedented characteristics and performances using femtosecond laser micromachining. We demonstrate ultra-high Q microresonators in lithium niobate on insulator (LNOI), on-chip micro-laser and waveguide amplifier, and high-throughput micro-chemical reactor. The achievements are the result of persistent effort on improving the precision and efficiency in ultrafast laser processing.
Eu doped Sr2Si5N8 red phosphors with different Eu concentration were synthesized by high-temperature solid phase method. The crystal structure, luminescence and the thermal quenching properties were investigated in detail. It was found that the thermal quenching property was largely dependent on the Eu concentration. Interestingly, the thermal quenching property for low concentration Eu doped Sr2Si5N8 was greatly improved as increasing Eu concentration. In order to understand the relevant mechanism, we examined the change of relative intensity and fluorescence lifetime for the two-peak emission in Sr2Si5N8: Eu. The result revealed that Eu distribution changed with increasing Eu concentration, which should be the mainly mechanism of the thermal quenching behavior for low concentration Eu doped Sr2Si5N8 phosphors.
Recently, low-loss (0.027 dB/cm) ridge waveguides have been demonstrated on lithium niobate on insulator (LNOI) by laser patterning followed by chemo-mechanical polishing. However, the fabricated waveguide supports multi-mode propagation due to the relatively large cross-sectional dimensions. Here, we report conversion of the multi-mode LNOI waveguides into single mode waveguides with a mode field size of ~2.5 μm with a cladding layer of Ta2O5. The propagation loss of the single mode waveguide is measured to be ~0.042 dB/cm. Most importantly, we show that this fabrication approach has allowed to fabricate meter-length long LNOI single mode waveguides of low propagation loss.
The realization of micro-disk resonators (MDRs) of high quality (Q) factors using lithium niobate on insulator (LNOI) as the substrate has spurred great interest in developing on-chip nanophotonic structures which hold the promise for efficient nonlinear wavelength conversion, fast electrooptic light modulation, and high density photonic integration. Here, we report on fabrication of crystalline lithium niobate microresonators with quality factors above 10^7 as measured around 770 nm wavelength, which is almost one order of magnitude higher than the state-of-the-art Q factors around the visible and near-infrared wavelengths reported so far. Our fabrication process includes four steps. First, a thin layer of chromium (Cr) was deposited on the surface of the LNOI by thermal evaporation coating. Subsequently, the Cr film on the LNOI sample was patterned into a circular disk using space-selective femtosecond laser direct writing. Next, the chemo-mechanical (CM) polishing process was performed to fabricate LN MDRs by a wafer polishing machine, the surface smoothness is greatly improved by the CM polishing process, leading to a significant increase of the Q factor. Finally, the fabricated structure was first immersed in Cr etching solution, and then underwent a chemical wet etching in a buffered hydrofluoric acid (HF) solution to partially remove the SiO_2 layer beneath the LN microdisk to produce the freestanding LN MDRs. We have also demonstrated nonlinear processes including second harmonic generation and Raman scattering in our LN MDRs.
Three-dimensional (3D) microfluidic structures provide new opportunities for developing novel functional lab-on-a-chip devices. In the past decade, the femtosecond laser direct writing has been developed to become a unique and powerful technology for straightforward fabrication of 3D microfluidic structures in glass. Herein we overview our efforts on femtosecond laser fabrication of three-dimensional microfluidic structures in glass and their lab-on-a-chip applications such as the creation of high-complexity of microfluidic devices and functionalization of microchannels.
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 1.1×10-3/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.
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 (CO2 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. CaF2 crystalline microcavities with Q factor of ~4.2×104 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.
Recently, hybrid integration of multifunctional micro-components for creating complex, intelligent micro/nano systems
has attracted significant attention. These micro-/nano-systems have important applications in a variety of areas, such as
healthcare, environment, communication, national security, and so on. However, fabrication of micro/nano systems
incorporated with different functions is still a challenging task, which generally requires fabrication of discrete microcomponents
beforehand followed by assembly and packaging procedures. Furthermore, current micro-/nano-fabrication
techniques are mainly based on the well-established planar lithographic approach, which suffer from severe issues in
producing three dimensional (3D) structures with complex geometries and arbitrary configurations. In recent years, the
rapid development of femtosecond laser machining technology has enabled 3D direct fabrication and integration of
multifunctional components, such as microfluidics, microoptics, micromechanics, microelectronics, etc., into single
substrates. In this invited talk, we present our recent progress in this active area. Particularly, we focus on fabrication of
3D micro- and nanofluidic devices and 3D high-Q microcavities in glass substrates by femtosecond laser direct writing.
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