Multi-gigahertz femtosecond pulses from linear and nonlinear propagation of a phase-modulated laser.

We propose and demonstrate a non-mode-locking approach to generating multi-gigahertz repetition rate, femtosecond pulses in burst mode by shaping a continuous-wave (CW) seed laser in an all-fiber configuration. The seed laser at 1030 nm is first phase modulated and de-chirped to low-contrast, ∼2 ps pulses at a 17.5 GHz repetition rate, then carved to bursts at a 60 kHz repetition rate, and finally shaped to <2 ps clean pulses by a Mamyshev regenerator. This prepared high-quality picosecond source is further used to seed an Yb-doped fiber amplifier operating in the highly nonlinear regime, delivering output pulses at 23 nJ/pulse and $20\,\mathrm{\mu}$J/burst, compressible to ∼100 fs level. The system eliminates the need for mode-locked cavities and simplifies conventional ultrafast electro-optic combs to using only one phase modulator, while providing femtosecond pulses at multiple gigahertz repetition rate, enhanced pulse energy in burst mode and the potential of further power/energy scaling.

High-power nonlinear amplification of an ultrafast electro-optic frequency comb with flexible GHz repetition rate.

We report on an all-fiber 200 W widely tunable GHz electro-optic (EO) frequency comb operating in the nonlinear regime. The EO comb pulses at 1030 nm are initially pre-compressed to sub-2 ps, then power amplified to 2.5 W, and finally boosted to 200 W in a newly designed large-mode-area, Yb-doped photonic crystal fiber. Continuously tunable across 12-18 GHz, the picosecond pulses experience nonlinear propagation in the last amplifier, leading to output pulses compressible down to several hundreds of femtoseconds. To push our system deeper into nonlinear amplification regime, the pulse repetition rate is further reduced to 2 GHz, enabling significant spectral broadening at 200 W. Characterization reveals sub-200 fs duration after compression. The present EO-comb seeded nonlinear amplification system opens a new route to the development of high-power, tunable GHz-repetition-rate, femtosecond fiber lasers.

Electro-optic comb pumped optical parametric oscillator with flexible repetition rate at GHz level.

We present a gigahertz (GHz)-repetition-rate optical parametric oscillator (OPO) pumped by an electro-optic comb at 1.03 µm, delivering sub-picosecond signal pulses across 1.5-1.7 µm from a MgO-doped periodically poled LiNbO3 crystal. Using a pump power of 5 W at 14.2 GHz repetition rate, 378 mW of signal power is obtained at 1.52 µm from a subharmonic cavity, corresponding to a signal extraction efficiency of 7.6%. By cascading a Mach-Zehnder modulator, the pump pulse repetition rate can be divided by any integer number from one to 14, allowing the OPO to operate with a flexible repetition rate from 1 to 14.2 GHz. A strategy leading to quasi-continuous repetition rate tunability of the OPO is also discussed.

Robust 17 W single-pass second-harmonic-generation at 532 nm and relative-intensity-noise investigation.

We demonstrate a 17 W single-frequency, low-intensity-noise green source at 532 nm, by single-pass second-harmonic generation of a 50 W continuous-wave fiber laser in a 30 mm MgO-doped periodically-poled stoichiometric lithium tantalate crystal. The maximum conversion efficiency is about 37%. A nearly Gaussian beam (M2<1.15 at 15 W) and low wavefront distortion are obtained. The system shows stable behavior over 100 h of uninterrupted operation. The evolution of the relative-intensity-noise transfer from the fundamental to the second harmonic is theoretically and experimentally investigated with high resolution.

Multi-GHz repetition rate, femtosecond deep ultraviolet source in burst mode derived from an electro-optic comb.

We present a femtosecond, 11.48 GHz intra-burst repetition rate deep UV source at 258 nm based on forth-harmonic generation (FHG) of an electro-optic (EO) comb operating in burst mode. Second-harmonic generation (SHG) of the burst-mode EO comb in LiB3O5 (LBO) leads to 3.7 W average power and 242 fs root-mean-square pulse duration. A second stage of SHG is further performed using two separate ß-BaB2O4 (BBO) crystals, delivering deep UV pulses at 523 mW and 294 mW, with estimated pulse durations of half-ps and sub-300 fs, respectively. At divided pulse repetition rates of 5.7 GHz and 2.9 GHz, FHG is also demonstrated, highlighting the potential of flexible repetition rate operation at the GHz level.

Singly-resonant pulsed optical parametric oscillator based on orientation-patterned gallium phosphide.

We report a pulsed singly-resonant optical parametric oscillator (OPO) based on the new nonlinear crystal, orientation-patterned gallium phosphide (OP-GaP). Pumped by a Q-switched Nd:YAG laser at 1064 nm, and using a 40-mm-long OP-GaP crystal with a single grating period of Λ=16 µm, the OPO generates signal and idler output across 1.6-1.7 µm and 2.8-3.1 µm, respectively, under temperature tuning. For an average pump power of 4.8 W at 50 kHz pulse repetition rate, mid-infrared idler powers of up to ∼20 mW have been obtained at 2966 nm with high output stability. For pump pulses of ∼13 ns duration, the OPO generates ∼6 ns output signal pulses. From temperature-dependent wavelength tuning measurements at two different pump powers of 4.2 W and 1.2 W, a discrepancy of 11-17°C in the internal crystal temperature is estimated, implying that the OP-GaP sample suffers from increasing thermal effects at higher pump powers due to absorption.

EZH2 promotes migration and invasion of triple-negative breast cancer cells via regulating TIMP2-MMP-2/-9 pathway.

Triple-negative breast cancer (TNBC) has a higher potential for invasion and metastasis than other types of breast cancer. Enhancer of zeste homolog 2 (EZH2) is the catalytic core protein in the polycomb repressive complex 2 (PRC2), which catalyzes the trimethylation of histone H3 at lysine 27 (H3K27me3) and mediates gene silencing of the target genes that are involved in fundamental cellular processes, such as the cell fate decision, cell cycle regulation, senescence, cell differentiation, and cancer formation. A consistent association between TNBC metastasis and EZH2 has not been confirmed. The aim of this study was to investigate the role of EZH2 in the regulation of tissue inhibitor of metalloproteinase (TIMPs) and matrix metalloproteinases (MMPs) to promote metastasis of TNBC cells and to characterize the metastasis-associated genes regulated by EZH2 in TNBC cells. We found that high levels of EZH2 expression induce repression of TIMP2 transcription, leading to increased activity of MMP-2 and MMP-9 and thus to increased invasive activity of TNBC cells.

Optical parametric generation in orientation-patterned gallium phosphide.

We report an optical parametric generator (OPG) based on the new nonlinear material, orientation-patterned gallium phosphide (OP-GaP). Pumped by a Q-switched nanosecond Nd:YAG laser at 1064 nm with 25 kHz pulse repetition rate, the OPG can be tuned across 1721-1850 nm in the signal and 2504-2787 nm in the idler. Using a 40-mm-long crystal in single-pass configuration, we have generated a total average output power of up to ∼18 mW, with ∼5 mW of idler power at 2670 nm, for 2 W of input pump power. The OPG exhibits a passive stability in total output power better than 0.87% rms over 1 h, at a crystal temperature of 120°C, compared to 0.14% rms for the input pump. The output signal pulses, recorded at 1769 nm, have duration of 5.9 ns for input pump pulses of 9 ns. Temperature-dependent loss measurements for the pump polarization along the [100] axis in the OP-GaP crystal have also been performed, for the first time, indicating a drop in transmission from 28.8% at 50°C to 19.4% at 160°C.

Nanosecond difference-frequency generation in orientation-patterned gallium phosphide.

We report a tunable, single-pass, pulsed nanosecond difference-frequency generation (DFG) source based on the new semiconductor nonlinear material, orientation-patterned gallium phosphide (OP-GaP). The DFG source is realized by mixing the output signal of a nanosecond OPO tunable over 1723-1827 nm with the input pump pulses of the same OPO at 1064 nm in an OP-GaP crystal, resulting in tunable generation over 233 nm in the mid-infrared from 2548 to 2781 nm. Using a 40-mm-long crystal, we have produced ∼14 mW of average DFG output power at 2719 nm for a pump power of 5 W and signal power of 1 W at 80 kHz repetition rate. To the best of our knowledge, this is the first single-pass nanosecond DFG source based on OP-GaP. The DFG output beam has a TEM00 spatial mode profile and exhibits passive power stability better than 1.7% rms over 1.4 h at 2774 nm, compared to 1.6% and 0.1% rms for the signal and pump, respectively. The OP-GaP crystal is recorded to have a temperature acceptance bandwidth of 17.7°C.