Wavelength-tunable Femtosecond Laser Research Articles

X-photon laser direct write 3D nanolithography

ABSTRACT Multiphoton photopolymerisation (MPP), also known as 3D nanoprinting, was studied using a wavelength-tunable femtosecond laser. The possibility of using any colour of the spectrum from 500 to 1200 nm with a fixed pulse width of 100 fs revealed an interplay of photophysical mechanisms more delicate than just two-photon photopolymerisation. An effective order of absorption, i.e. the X-photon absorption, as well as optimal exposure conditions were assessed for photosensitised and pure SZ2080 pre-polymer. The tunability of wavelength greatly influenced the dynamic fabrication window (DFW), optimised conditions resulting in a 10-fold increase. Furthermore, a non-trivial energy deposition by X-photon absorption was noted with an onset of a strong lateral size increase at longer wavelengths and can be understood as due to reaching epsilon-near-zero conditions. Such a control over the voxel aspect ratio and, consequently, the photopolymerised volume, may boost 3D nanoprinting efficiency. Overall, the results reveal wavelength being an important degree of freedom to tailor the MPP process and, if optimised, benefiting broad applications in areas of micro-optics, nanophotonic devices, metamaterials and tissue engineering.

Multiphoton Absorption in Gallium Nitride and Silicon Carbide Photodiodes: Applications for Single-Event Effects Tests

We report on the investigation of multiphoton absorption processes in gallium nitride and silicon carbide photodiodes. Up to three-photon absorption mechanisms were evidenced on both materials, thanks to a wavelength-tunable femtosecond laser chain. These results are applied to single event effect investigations on a commercial gallium nitride high electron mobility transistor. We could trigger single event transients by focusing the beam throughout the silicon substrate and collect charge that was photogenerated in the gallium nitride layer by a three-photon absorption process.

Broadband angular dispersion compensation for digital micromirror devices.

In this Letter, we present a compact broadband angular dispersion compensation method for digital micromirror devices (DMDs) and ultrashort pulse lasers, which effectively extends the conventional single-wavelength compensation design to a wide wavelength range of 300 nm. First, a parametric model was developed for the dispersion compensation unit, consisting of a transmission grating and a 4f telescope sub-unit, to guide the selection of components and parameter optimization for broadband applications. In the experiments, we designed a single slit-based metrology system to measure and quantify the compensated angular dispersion of a Ti:sapphire femtosecond laser with a pulse width of 75 fs. The results indicate that our method can reduce the angular dispersion to 0.04°, i.e., pulse widening less than 20 fs, over a wavelength range of 750-1050 nm. To demonstrate this, the DMD system was used as a multi-wavelength beam shaper to reconstruct a wavefront that contains the "CUHK" pattern and the results confirmed its ability to provide effective broadband angular dispersion compensation. This means the DMD can be used in different applications that employ a broadband light source, e.g., wavelength tunable femtosecond laser, attosecond laser, supercontinuum laser, and multi-color LED.

Theoretical simulations of the soliton self-frequency shift of mid-infrared femtosecond pulses in step-index tellurite optical fibers: broadband tunability and high efficiency

Broadband tunable ultrafast lasers using mid-infrared (MIR) fibers operating at a 3-5 µm atmospheric transmission window are attractive sources because of their numerous applications. Tellurite fibers possess the merits of large linear and nonlinear refractive indices, sufficient chemical stability, and wide transparency range up to ∼5 µm; also, they are highly suitable for high efficiency MIR ultrafast fiber laser sources based on soliton self-frequency shift (SSFS). We numerically simulate SSFS of MIR femtosecond pulses in step-index tellurite optical fibers. A femtosecond erbium-doped fluoride fiber laser at 3.5 µm is employed as the pump source. Parameters including the peak power of the input pulse and nonlinear fiber length are optimized for high efficiency broadband tunable MIR ultrafast laser performance. Our results show that a high-efficiency 3.5-6 µm wavelength-tunable femtosecond laser can be realized by employing SSFS in a 22-cm-long segment of tellurite step-index fiber pumped by femtosecond pulses with 10-300 kW peak powers at 3.5 µm. Ultra-high energy ratios of the most redshifted solitons to the input pulses of >50% are obtained across the 3.5-5 µm tuning range. The presented numerical study provides valuable guidance for SSFS of MIR femtosecond pulses in step-index tellurite fibers and is valuable for future high efficiency wavelength-tunable MIR ultrafast fiber laser development.

Accurate manipulation of optogenetic proteins with wavelength tunable femtosecond laser system

Photoactivated proteins controlled by optogenetic tools have broad application prospects in cell biology, neuroscience, and brain science. However, due to the narrow excitation wavelength width and the inflexibility of spatiotemporal operations, conventional sources such as visible light severely limit the further application of optogenetics. In this work, a femtosecond laser-operated system based on the optogenetic application was designed to address these limitations. The interaction between the photoreceptor and its partner protein can be triggered by a wavelength-tunable femtosecond laser. The results indicated that this process can be used to accurately manipulate optogenetic proteins in cells, which met spectral flexibility (700–1040 nm) and operational flexibility in time and space (a single cell to multiple cells). To demonstrate the practical applications of this process, the apoptotic signaling pathway of cancer cells was taken as an example. We believe that this wavelength-tunable femtosecond laser system will promote the development of optogenetics, making optics and even physics more powerful tools in biology.

Terahertz Wave Generation From Noble Gas Plasmas Induced by a Wavelength-Tunable Femtosecond Laser

We investigated the effects of pump wavelength on terahertz wave generation via two-color laser-induced plasma in various gas targets including nitrogen and noble gases. The terahertz energy was measured as functions of pump wavelength, input pulse energy, gas species, and pressure. The results suggest that the plasmas in the heavier gases induced by relatively longer wavelength lasers are more likely to generate higher energy terahertz waves. However, the existence of phase slippage in the neutral gases causes distinct fluctuations in the terahertz energy as the gas pressure increases. Meanwhile, the terahertz waves produced in the heavier gas are more likely to saturate at relatively higher pressures or incident optical powers. We propose an ionization rate formula associated with the gas species and pump wavelength to theoretically explain the validity of the experimental results. As a result, the terahertz pulse energy approaches a maximum value of 0.099 μJ per pulse in xenon with a 1500-nm excitation laser, and the terahertz wave to optical pulse energy conversion efficiency reaches 5.6 × 10–3, which is an order of magnitude higher than that of a conventional 800-nm pump laser.

Exploring Coupled Plasmonic Nanostructures in the Near Field by Photoemission Electron Microscopy.

The extraordinary optical properties of coupled plasmonic nanostructures make these materials potentially useful in many applications; thus, they have received enormous attention in basic and applied research. Coupled plasmon modes have been characterized predominantly using far-field spectroscopy. In near-field spectroscopy, the spectral response of local field enhancement in coupled plasmonic nanostructures remains largely unexplored, especially experimentally. Here, we investigate the coupled gold dolmen nanostructures in the near field using photoemission electron microscopy, with wavelength-tunable femtosecond laser pulses as an excitation source. The spatial evolution of near-field mapping of an individual dolmen structure with the excitation wavelength was successfully obtained. In the near field, we spatially resolved an anti-bonding mode and a bonding mode as the result of plasmon hybridization. Additionally, the quadrupole plasmon mode that could be involved in the formation of a Fano resonance was also revealed by spatially resolved near-field spectra, but it only contributed little to the total near-field enhancement. On the basis of these findings, we obtained a better understanding of the near-field properties of coupled plasmonic nanostructures, where the plasmon hybridization and the plasmonic Fano resonance were mixed.

Damage and its mechanism to the visible wavelength filter irradiated by femtosecond laser

The mechanism of damage to the optical coatings of filters induced by femtosecond laser at different wavelengths was studied.A near infrared wavelength tunable femtosecond laser was used to rediate the visible wavelength filter from backside.The wavelengths were selected in the transitional region of the filter pass band and the corresponding threholds of average power density were obtained.Based on the avalanche ionization process,the relationship between damage threshold and distribution of interference field was investigated.The theoretical results can predict the trend of thresholds.The results indicate that the damage behavior of a coating irradiated by femtosecond laser pulses exhibits an overall behavior.The damage threshold decreases with the increase of transient residual energy in the multi-layer structure of the coating.

Wavelength tunable femtosecond laser based on short Er3+/Yb3+ codoped phosphate fiber

A wavelength tunable laser system mode-locked by nonlinear polarization evolution based on a 3 cm-long homemade Er3+/Yb3+ codoped phosphate fiber has been reported. By simply adjusting the polarization controllers, the central wavelength of the mode-locked spectrum can be tuned over 1537.1–1563.3 nm continuously. Moreover, 264-fs pulse with 3-dB spectral width of 39.6 nm and peak power of 7.8 kW at a 7.55 MHz repetition rate is generated directly from the all-fiber ring cavity.

Relation between Two-Photon Absorption and Dipolar Properties in a Series of Fluorenyl-Based Chromophores with Electron Donating or Electron Withdrawing Substituents

We investigate two-photon absorption (2PA) in a series of fluorenyl-based 9,9-diethyl-2-ethynyl-7-((4-R-phenyl)ethynyl)-9,9a-dihydro-4aH-fluorene chromophores with R being various electron donating (ED) and electron withdrawing (EW) groups. We use wavelength-tunable femtosecond laser pulses to measure the 2PA cross sections in the lowest dipole-allowed transition and show that the substituents with stronger ED or EW character enhance the peak 2PA cross section (up to σ(2) ∼ 60-80 GM) while the neutral substituents lead to smaller cross sections, σ(2) < 10 GM. We apply two-level approximation to establish a quantitative relation between the 2PA in the pure electronic transition (0-0) and the corresponding change of the permanent electric dipole moment upon the excitation (Δμ). This relation is elucidated by comparing Δμ values obtained from the 2PA measurements with quantum-chemical calculations and with measurements of solvatochromic shifts in a series of solvents. We show that the calculated Δμ correlate well with the values obtained from the 2PA spectroscopy. The Δμ values obtained from the solvatochromic shifts agree well with the above two methods for the chromophores with neutral or weak EW or ED substituents. On the other hand, stronger EW or ED end groups give much larger Stokes shifts, which lead to an overestimation of the Δμ values. We tentatively attribute this effect to the excitation-induced electronic density change occurring predominantly at the substituent side of the molecule, which causes the effective point dipole associated with the Δμ to interact more strongly with the surrounding solvent.

Periodic nanoripples in the surface and subsurface layers in ZnO irradiated by femtosecond laser pulses

We present the formation of periodic ripples in ZnO crystal irradiated by a wavelength-tunable femtosecond laser. The results indicate that in the surface thin layer, the periods change from 0.1 lambda to lambda with laser fluences and pulse numbers, and in the subsurface layer the periods are always lambda/2n, where n is the refractive index. The formation processes and mechanisms are also discussed.

Multiphoton excitation characteristics of cellular fluorophores of human skin in vivo

In vivo multiphoton tomography with a wavelength-tunable femtosecond laser has been performed to investigate the autofluorescence intensity of major endogenous fluorophores of human skin in dependence on the excitation wavelength. In high-resolution multiphoton images of different skin layers, clear trends were found for fluorophores like keratin, NAD(P)H, melanin as well as for the elastin and collagen networks. The analysis of the measurements is supplemented by additional measurements of fluorescence lifetime imaging and signal-decay curves by time-correlated single-photon counting.

Optical Breakdown of 6H SiC Induced by Wavelength-Tunable Femtosecond Laser Pulses

The damage mechanisms and micromachining of 6H SiC are studied by using femtosecond laser pulses at wavelengths between near infrared (NIR) and near ultraviolet (NUV) delivered from an optical parametric amplifier (OPA). Our experimental results indicate that high quality microstructures can be fabricated in SiC crystals. On the basis of the dependence of the ablated area and the laser pulse energy, the threshold fluence of SiC is found to increase with the incident laser wavelength in the visible region, while it remains almost constant for the NIR laser. For the NIR laser pulses, both photoionization and impact ionization play important roles in electronic excitation, while for visible lasers, photoionization plays a more important role.