Fundamental Laser Research Articles

Minimization of the response time of a 20 W deep-ultraviolet light at 266 nm during intermittent operation

We demonstrated stable intermittent operation of a 266-nm picosecond pulsed light source with an average power of 20 W. The 266-nm beam, which had a maximum average power of 35.5 W, was generated by frequency conversion of a 1064-nm laser with an LiB3O5 crystal and a CsLiB6O10 (CLBO) crystal. The 1064-nm laser had a repetition rate of 600 kHz and an average power of 130 W and was capable of intermittent operation with an acousto-optic modulator in the fundamental laser section. By investigating the crystal temperature rise caused by the 266-nm light absorption in the CLBO crystal, we found that the crystal temperature rise caused by nonlinear absorption must be suppressed to achieve stable intermittent operation. The countermeasures allowed stable-intermittent operation at an average power of 20 W to be achieved, with a response time of 1.1 s for the 10%–90% rise conditions and a stability of 2%p-p for the average power fluctuation from 2 to 120 s. These results show that deep-ultraviolet picosecond pulses with an average power of 20 W can be used for industrial applications that require stable intermittent operation.

Temporal behavior of the high-power pulsed gas terahertz laser pumped by a fundamental mode TEA CO2 laser.

Optically pumped gas molecular terahertz (THz) lasers are promising for generating high-power and high-beam-quality coherent THz radiation. However, for pulsed gas THz lasers, the temporal behavior of the output THz pulse has rarely been investigated. In this study, the temporal behavior of a pulsed gas THz pumped by a fundamental-mode TEA CO2 laser has been presented for the first time both in simulation and experiment. A modified laser kinetics model based on the density matrix rate equation was used to simulate the temporal behavior and output pulse energy of a pulsed gas THz laser at different gas pressures. The results clearly show that the working gas pressure and pump pulse energy have critical influences on the output THz pulse shape. Three typical pulse shapes were obtained, and the THz pulse splitting caused by gain switching was quantitatively simulated and explained based on the laser dynamic process. Besides, with an incident pump pulse energy of 342 mJ, the maximum output THz pulse energy of 2.31 mJ was obtained at 385 µm, which corresponds to a photon conversion efficiency of approximately 56.1%, and to our knowledge, this is the highest efficiency for D2O gas THz laser. The experimental results agreed well with those of the numerical simulation for the entire working gas pressure range, indicating that our model is a powerful tool and paves the way for designing and optimizing high-power pulsed gas lasers.

Melt-Pool Dynamics and Microstructure of Mg Alloy WE43 under Laser Powder Bed Fusion Additive Manufacturing Conditions

Magnesium-based alloy WE43 is a state-of-the-art bioresorbable metallic implant material. There is a need for implants with both complex geometries to match the mechanical properties of bone and refined microstructure for controlled resorption. Additive manufacturing (AM) using laser powder bed fusion (LPBF) presents a viable fabrication method for implant applications, as it offers near-net-shape geometrical control, allows for geometry customization based on an individual patient, and fast cooling rates to achieve a refined microstructure. In this study, the laser–alloy interaction is investigated over a range of LPBF-relevant processing conditions to reveal melt-pool dynamics, pore formation, and the microstructure of laser-melted WE43. In situ X-ray imaging reveals distinct laser-induced vapor depression morphology regimes, with minimal pore formation at laser-scan speeds greater than 500 mm/s. Optical and electron microscopy of cross-sectioned laser tracks reveal three distinct microstructural regimes that can be controlled by adjusting laser-scan parameters: columnar, dendritic, and banded microstructures. These regimes are consistent with those predicted by the analytic solidification theory for conduction-mode welding, but not for keyhole-mode tracks. The results provide insight into the fundamental laser–material interactions of the WE43 alloy under AM-processing conditions and are critical for the successful implementation of LPBF-produced WE43 parts in biomedical applications.

Proposal for High-Energy Cutoff Extension of Optical Harmonics of Solid Materials Using the Example of a One-Dimensional ZnO Crystal.

We propose a novel approach based on the subcycle injection of carriers to extend the high-energy cutoff in solid-state high harmonics. The mechanism is first examined by employing the standard single-cell semiconductor Bloch equation (SC SBE) method for one-dimensional (1D) Mathieu potential model for ZnO subjected to the intense linearly polarized midinfrared laser field and extreme-ultraviolet pulse. Then, we use coupled solution of Maxwell propagation equation and SC SBE to propagate the fundamental laser field through the sample, and find that the high-harmonics pulse train from the entrance section of the sample can inject carriers to the conduction bands with attosecond timing, subsequently leading to a dramatic extension of high-energy cutoff in harmonics from the backside. We predict that for a peak intensity at 2×10^{11} W/cm^{2}, as a result of the self-seeding, the high-energy cutoff shifts from 20th (7.75eV) order to around 50th (19.38eV) order harmonics.

Diode-pumped, actively Q-switched Nd,La:CaNb2O6 self-Raman laser at 1,174 nm

Diode end-pumped Nd,La:CaNb2O6 self-Raman laser with acousto-optic Q-switching was successfully demonstrated for the first Stokes wave generation at 1,174 nm. A 1.0 at.% Nd3+ and 1.0 at.% La3+-doped CaNb2O6 crystal in dimensions 3 × 3 × 14.3 mm3 was used as the self-Raman laser crystal. Doping 1 at.% La3+ ions into this crystal could subdue the fluorescence quenching caused by cross-relaxation between Nd3+ ions and finally improve the laser output performance. Under the incident pump power of 9.9 W, the first Stokes wave at 1,174 nm with a maximum output power up to 928 mW was obtained, with the diode to Stokes conversion efficiency of about 9.4%. The results show that the Nd,La:CaNb2O6 is also a promising self-Raman crystal for efficient fundamental and Raman laser operation.

Watt-level gigahertz femtosecond fiber laser system at 920 nm.

We demonstrate a watt-level femtosecond fiber laser system at 0.9 µm with a repetition rate of >1 GHz, which is the highest value reported so far for a fundamental mode-locked fiber laser. The fiber laser system is seeded by a fundamental mode-locked fiber laser constructed with a home-made highly Nd3+-doped fiber. After external amplification and pulse compression, an output power of 1.75 W and a pulse duration of 309 fs are obtained. This compact fiber laser system is expected to be a promising laser source for biological applications, particularly two-photon excitation microscopy.

Self‐Frequency‐Mixing Raman Laser Based on RbTiOPO4

AbstractIn this work, the generation of yellow laser emission from a self‐frequency‐mixing Raman laser utilizing RbTiOPO4 (RTP) is reported. The RTP crystal is positioned within the cavity of a Nd:YAG/Cr4+:YAG composite, passively Q‐switched laser. By utilizing this intracavity configuration, Raman conversion of the fundamental laser wavelength to Stokes wavelengths and frequency mixing of these near‐infrared wavelengths to the visible wavelength range is achieved using a single RTP crystal. Multiple visible wavelengths are generated, with a primary line at 574.3 nm and additional weaker lines at 552.3 and 563.9 nm. An average output power of 180 mW is measured from the output coupler end of the laser setup (note that laser emission in the visible wavelength range is also generated toward the input end of the laser cavity) for an incident pump power of 11.4 W. The results demonstrate the potential of RTP as a self‐frequency‐mixing, Raman crystal, which may be used for the development of efficient and compact yellow lasers.

Controlling LIPSS formation on Ni surface using near-infrared laser beam and its low-order harmonics

Ultrafast laser ablation of metal surfaces is a common physical process used to generate periodic surface structures with tuneable multifunctionality for various applications in science and technology. In this work, the formation of femtosecond laser-induced periodic surface structures (LIPSSs) is investigated with 50 kHz repetition rate laser of 40 fs pulse duration and a central wavelength () along with its second () and third ) harmonic components. The variation in the LIPSS periodicity is demonstrated by changing the wavelength of the ablating laser pulses. LIPSS with periodicity ∼0.85 m at the fundamental ablating pulses, ∼0.42 m periodicity at the second harmonic, and ∼0.35 m periodicity at the third harmonic were observed. The third harmonic radiation was generated in an air-filament with minimal temporal delay with respect to the fundamental driving pulses to structure Nickel surfaces at ambient conditions. The approach demonstrates the potential for generating very fine LIPSS on materials’ surfaces using the efficient high-order harmonics of an infrared fundamental laser beam.

Compact passively Q-switched KTA self-frequency doubled Raman laser with 671 cm−1 shift

Compact yellow laser source based on KTA self-frequency-doubled Raman conversion was demonstrated. Cr4+:YAG bonded on Nd: YAG crystal was selected for passively Q-switched fundamental laser generation and making the system more compact. Both stimulated Raman scattering and frequency doubling were successfully realized in the same x-cut KTA crystal. Single wavelength laser at 573 nm was generated by frequency doubling of the first-Stokes at 1146 nm with the Raman shift of 671 cm−1. Under an incident pump power of 10.5 W, the average output power of 240 mW and conversion efficiency of 2.3% are obtained from the output coupler end of the laser setup. Frequency doubling generated towards the pump input end of the laser cavity was not collected. The pulse repetition frequency and pulse width are 11.6 kHz and 1.2 ns were detected at the incident pump power of 10.5 W. The narrow pulse width of this positively Q-switched yellow laser resulted in much higher peak power up to 17.2 kW.

Simultaneous Second Harmonic Generation and multiphoton excited photoluminescence in anatase TiO2 nano powders

Second Harmonic Generation (SHG) and Multiphoton Excited Photoluminescence (MEPL) are observed in retro-reflection mode from a 22 nm diameter anatase TiO2 nanoparticle powder. Depth intensity profiles are obtained by scanning the fundamental beam focal point through the TiO2 powder surface into its volume for the energy of the incident photons tuned around the anatase TiO2 band-gap energy of 3.2 eV. Evidence for the appearance of both SHG and MEPL is presented, their relative contribution depending on the conditions of photo-excitation, in particular the fundamental wavelength and the focal point depth location. MEPL dominates at fundamental photon energies matching the band-gap energy whereas below and above the band-gap SHG is clearly seen. Depth intensity profiles exhibiting a rising edge determined by the fundamental laser beam focusing conditions and a decreasing edge determined by extinction and multiphoton scattering strongly differ between SHG and MEPL, SHG profiles being by far narrower than that of MEPL. Re-absorption of the emitted SHG and MEPL light occurs due to the spectral proximity with the TiO2 band gap energy but excitation with fundamental light below band-gap further suggests that the intrinsic difference between the two SHG and MEPL processes is responsible for the contrasted depth profiles. This combined study where both SHG and MEPL occur presents perspectives in the study of the photonic properties of powder materials.

Synergy of electromagnetic effects and thermophysical properties of metals in the formation of laser-induced periodic surface structures

Femtosecond (fs) pulsed lasers have been widely used over the past few decades for precise materials structuring at the micro- and nano-scales. However, in order to realize efficient material processing and account for the formation of laser-induced periodic surface structures (LIPSS), it is very important to understand the fundamental laser–matter interaction processes. A significant contribution to the LIPSS profile appears to originate from the electromagnetic fingerprint of the laser source. In this work, we follow a systematic approach to predict the pulse-by-pulse formation of LIPSS on metals due to the development of a spatially periodic energy deposition that results from the interference of electromagnetic far fields on a non-flat surface profile. On the other hand, we demonstrate that the induced electromagnetic effects alone are not sufficient to allow the formation of LIPSS, therefore we emphasize the crucial role of electron diffusion and electron–phonon coupling on the formation of stable periodic structures. Gold (Au) and stainless steel (SS) are considered as two materials to test the theoretical model while simulation results appear to confirm the experimental results that, unlike with Au, fabrication of pronounced LIPSS on SS is feasible.

Second-Harmonic-Generation Effect and Giant Optical Birefringence in the Weyl Material CaAgAs.

Nonlinear-optical (NLO) materials suitable for the "8-14 μm" atmospheric transparent window are in urgent need. Arsenide is one of the most promising material systems for such an application. However, the second-harmonic-generation (SHG) effect of arsenide is difficult to characterize using the conventional powder SHG technique with a 2 μm fundamental laser. To overcome this problem, we focused on a novel arsenide Weyl material, CaAgAs, with a zero band gap and proposed a modified powder SHG measurement method for narrow-band-gap materials. We successfully observed the SHG signal of CaAgAs, which was approximately 0.7 times that of CdGeAs2. Moreover, on the basis of first-principles calculations, the largest SHG coefficient for CaAgAs is equal to 236 pm/V, and the ionic bonds in the [Ca3As13] motif play an indispensable role for the SHG effect because of the distorted Kagome lattice pattern. CaAgAs is predicted to have strong optical anisotropy with a giant optical birefringence of 0.52.

Temporal characterization of a two-color laser field using tunneling ionization.

The superposition of a fundamental laser pulse and its second harmonic can form an asymmetric laser field that is useful in many applications. The temporal characterization of the two-color laser field becomes necessary. However, the temporal characterization of the two-color laser pulse is a challenging task due to its broad bandwidth and a spectral gap between the two frequency components. Here we demonstrate the temporal characterization of the two-color laser field using multiple ionization yield measurements near the laser focus. This new approach enables the complete temporal characterization of the two-color laser field, including the relative phase between the two frequency components.

10dB Quantum-Enhanced Michelson Interferometer with Balanced Homodyne Detection.

Future generations of gravitational-wave detectors (GWD) are targeting an effective quantum noise reduction of 10dB via the application of squeezed states of light. In the last joint observation run O3, the advanced large-scale GWDs LIGO and Virgo already used the squeezing technology, albeit with a moderate efficiency. Here, we report on the first successful 10dB sensitivity enhancement of a shot-noise limited tabletop Michelson interferometer via squeezed light in the fundamental Gaussian laser mode, where we also implement the balanced homodyne detection scheme that is planned for the third GWD generation. In addition, we achieved a similarly strong quantum noise reduction when the interferometer was operated in higher-order Hermite-Gaussian modes, which are discussed for the GWD thermal noise mitigation. Our results are an important step toward the targeted quantum noise level in future GWDs and, moreover, represent significant progress in the application of nonclassical states in higher-order modes for interferometry, increased spatial resolution, and multichannel sensing.

Generation of 12 W 257 nm laser pulses by sum-frequency generation based on LBO crystals

We have generated high-power deep ultraviolet laser pulses from a Yb-doped rod-type fiber laser through sum-frequency generation (SFG) based on lithium triborate (LBO) crystals. The 1030 nm nanosecond laser pulses from the fiber laser are frequency-tripled to 343 nm pulses with two LBO crystals, and the residual 1030 nm fundamental laser pulses are mixed with the 343 nm pulses in an LBO crystal for the generation of 257 nm pulses. The maximum 257 nm radiation power is 12 W with a repetition rate of 500 kHz, and the conversion efficiency from 1030 nm to 257 nm is 12%. It is observed that the power of 257 nm radiation is limited by the temperature increase in the LBO crystal used for the SFG.

Rotational Doppler effect on reflection upon an ideal rotating propeller

The rotational Doppler shift is the counterpart of the usual linear Doppler effect for rotating bodies. We study by an experimental approach coupled with theoretical considerations the rotational shift of a fundamental laser light reflected on an ideal rotating propeller. We decompose the reflected light on a Laguerre–Gaussian basis and show that only modes having the same rotational symmetry as the propeller are involved in the decomposition. The latter experience a frequency shift proportional to the rotation frequency of the propeller and the topological charge of the beam. Extensions of this work in the microwave domain are then considered.

Metal surface wettability modification by nanosecond laser surface texturing: A review

Laser surface texturing (LST) is a non-contact manufacturing process for fabricating functional surfaces in a manner that improves the corresponding wettability, and is widely used in biomedicine and industry. Laser surface texturing is a facile approach that is compatible with various materials, can result in a hierarchical texture, and enables a high degree of surface wetting (i.e., extreme wetting). In addition to surface structures, surface chemical modification is a primary factor in producing extreme wetting surfaces. This review discusses the effects of various surface textures and surface chemistries on wettability. Optimal laser parameters for the desired surface texture are based on the fundamental wettability and laser mechanism. In particular, bumps in the morphology are conducive to obtaining extreme wetting. Diverse surface chemical strategies result in extreme wetting by different mechanisms. This paper makes a rigorous evaluation of the laser parameters and optimal surface chemical modifications by elucidating the relationships between the surface structure, surface chemical modification, and wettability, and in so doing, determines the final wettability. The unresolved problems of LST are presented in the conclusion. This review provides guidance, development directions, and an integrated framework for LST, which will be useful for fabricating extreme wetting surfaces on various metals.

Self-frequency-conversion nanowire lasers

Semiconductor nanowires (NWs) could simultaneously provide gain medium and optical cavity for performing nanoscale lasers with easy integration, ultracompact footprint, and low energy consumption. Here, we report III–V semiconductor NW lasers can also be used for self-frequency conversion to extend their output wavelengths, as a result of their non-centrosymmetric crystal structure and strongly localized optical field in the NWs. From a GaAs/In0.16Ga0.84As core/shell NW lasing at 1016 nm, an extra visible laser output at 508 nm is obtained via the process of second-harmonic generation, as confirmed by the far-field polarization dependence measurements and numerical modeling. From another NW laser with a larger diameter which supports multiple fundamental lasing wavelengths, multiple self-frequency-conversion lasing modes are observed due to second-harmonic generation and sum-frequency generation. The demonstrated self-frequency conversion of NW lasers opens an avenue for extending the working wavelengths of nanoscale lasers, even to the deep ultraviolet and THz range.

High Repetition Rate, TEM00 Mode, Compact Sub-Nanosecond 532 nm Laser

As a critical transmitter, compact 532 nm lasers operating on high repetition and short pulse widths have been used widely for airborne or space-borne laser active remote sensing. We developed a free space pumped TEM00 mode sub-nanosecond 532 nm laser that occupied a volume of less than 125 mm × 50 mm × 40 mm (0.25 L). The fundamental 1064 nm laser consists of a passively Q-switched composite crystal microchip laser and an off-axis, two-pass power amplifier. The pump sources were two single-emitter semiconductor laser diodes (LD) with 808 nm wavelengths and a maximum continuous wave (CW) power of 10 W each. The average power of the fundamental 1064 nm laser was 1.26 W, with the laser operating at 16 kHz repetition rates and 857 ps pulse widths. Since the beam distortion would be severe in microchip lasers due to the increase in heat load, in order to obtain a high beam quality of 532 nm, the beam distortion in our experiment amplifying the fundamental laser was compensated by adjusting the distribution of the pumping beam. Furthermore, in the critical phase matching (CPM) regime for the second harmonic generation (SHG), a Type I LiB3O5 (LBO) crystal obtained 770 ps, a beam quality of M2 < 1.2, and a 16 kHz pulse output at 532 nm, which was better than 0.6 W average power.

Water-Based Coherent Detection of Broadband Terahertz Pulses.

Both solids and gases have been demonstrated as the materials for terahertz (THz) coherent detection. The gas-based coherent detection methods require a high-energy probe laser beam and the detection bandwidth is limited in the solid-based methods. Whether liquids can be used for THz detection and relax these problems has not yet been reported, which becomes a timely and interesting topic due to the recent observation of efficient THz wave generation in liquids. Here, we propose a THz coherent detection scheme based on liquid water. When a THz pulse and a fundamental laser beam are mixed on a free-flowing water film, a second harmonic (SH) beam is generated as the plasma is formed. Combining this THz-induced SH beam with a control SH beam, we successfully achieve the time-resolved waveform of the THz field with the frequency range of 0.1-18THz. The required probe laser energy is as low as a few microjoules. The sensitivity of our scheme is 1 order of magnitude higher than that of the air-based method under comparable detection conditions. The scheme is sensitive to the THz polarization and the phase difference between the fundamental and control SH beams, which brings direct routes for optimization and polarization sensitive detection. Energy scaling and polarization properties of the THz-induced beam indicate that its generation can be attributed to a four-wave mixing process. This generation mechanism makes simple relationships among the probe laser, THz-induced SH, and THz field, favorable for robustness and flexibility of the detection device.