Multi-functional Gallium Arsenide Nanoparticles and Nanostructures Fabricated Using Picosecond Laser Ablation

We report the fabrication and performance evaluation of hybrid surface-enhanced Raman scattering (SERS) substrates involving laser ablation and chemical routes for the trace-level detection of various analyte molecules. Initially, picosecond laser ablation experiments under ambient conditions were performed on pure silver (Ag) and gold (Au) substrates to achieve distinct nanosized features on the surface. The properties of the generated surface features on laser-processed portions of Ag/Au targets were systematically analyzed using UV–visible reflection and field emission scanning electron microscopy studies. Later, hybrid-SERS substrates were achieved by grafting the chemically synthesized Au nanostars on the plain and laser-processed plasmonic targets. Subsequently, we employed these as SERS platforms for the detection of a pesticide (thiram), a molecule used in explosive compositions [ammonium nitrate (AN)], and a dye molecule [Nile blue (NB)]. A comparative SERS study between the Au nanostar-decorated bare glass, silicon, Ag, Au, and laser-processed Ag and Au targets has been established. Our studies and the obtained data have unambiguously determined that laser-processed Ag structures have demonstrated reasonably good enhancements in the Raman signal intensities for distinct analytes among other substrates. Importantly, the fabricated hybrid SERS substrate of “Au nanostar-decorated laser-processed Ag” exhibited up to eight times enhancement in the SERS intensity compared to laser-processed Ag (without nanostars), as well as up to three times enhancement than the Au nanostar-loaded plain Ag substrates. Additionally, the achieved detection limits from the Au nanostar-decorated laser-processed Ag SERS substrate were ∼50 pM, ∼5 nM, and ∼5 μM for NB, thiram, and AN, respectively. The estimated enhancement factors accomplished from the Au nanostar-decorated laser-processed Ag substrate were ∼106, ∼106, and ∼104 for NB, thiram, and AN, respectively.

Hybrid Surface-Enhanced Raman Scattering Substrates for the Trace Detection of Ammonium Nitrate, Thiram, and Nile Blue

Enhancing infrared window transmittance of zinc sulfide (ZnS) is an effective way to improve the infrared detection imaging quality. In this study, the antireflective subwavelength structure (ASS) is designed and fabricated on ZnS substrate, aiming to suppress surface Fresnel reflection and achieve high transmittance within the far-infrared band. The structure parameters of ASS such as height, period, and fill factor are initially set based on the effective medium approximation theory, and are optimized via the rigorous coupled-wave analysis (RCWA) method. Moreover, the antireflective ASS mechanism is explored by using the local light field selective enhancements in structures for capturing incident photons. In addition, a femtosecond laser Bessel beam is used to experimentally fabricate the designed ASS on the ZnS substrate for obtaining large array height, resulting in transmittance enhancement in the far-infrared region. Ultimately, the broadband and wide-angle ASS with the anti-laser damage properties and hydrophobicity is achieved on the optical window. Hence, the clarity of the infrared image captured by the infrared detection system is improved. 

Broadband antireflective subwavelength structures for infrared detection

We report the proof-of-concept of an operational laser active medium with a depressed cladding waveguide manufactured in the volume of a Cr 2+ :ZnS single-crystalline sample and antireflection microstructures fabricated on its facets exclusively by femtosecond laser processing techniques. This allowed us to achieve transmittance in a broad range from 2 to 8 µm, approaching a maximum of over 90% near 2.5 µm, and lasing at 2275 nm at the average output power of 20 mW for the absorbed pump power of 500 mW with the slope efficiency of 5.5 %. This demonstration opens up a route towards the industrial fabrication of compact integrable laser sources and sensors based on II-VI materials. 

All-laser-microprocessed waveguide Cr:ZnS laser

Infrared antireflection of a zinc sulfide (ZnS) surface is important to improve performance of infrared detector systems. In this paper, double-pulse femtosecond laser micro-machining is proposed to fabricate a subwavelength quasi-periodic array (SQA) on ZnS substrate for infrared antireflection. The SQA consisting of approximately 30 million holes within a 2 × 2 cm2 area is uniformly formed in a short time. The double-pulse beam can effectively suppress the surface plasma shielding effect, resulting in obtaining a larger array depth. Further, the SQA depth is tunable by changing pulse energy and pulse delay, and can be used to readily regulate the infrared transmittance spectra as well as hydrophobicity. Additionally, the optical field intensity distributions of the SQA simulated by the rigorous coupled-wave analysis method indicate the modulation effect by the array depth. Finally, the infrared imaging quality captured through an infrared window embedded SQA is evaluated by a self-built infrared detection system.

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Subwavelength Quasi-Periodic Array for Infrared Antireflection

As an important mid-infrared to far-infrared optical window, ZnS is extremely important to improve spectral transmission performance, especially in the military field. However, on account of the Fresnel reflection at the interface between the air and the high-strength substrate, surface optical loss occurs in the ZnS optical window. In this study, the concave antireflective sub-wavelength structures (ASS) on ZnS have been experimentally investigated to obtain high transmittance in the far-infrared spectral range from 6 μm to 10 μm. We proposed a simple method to fabricate microhole array ASS by femtosecond Bessel beam, which further increased the depth of the microholes and suppressed the thermal effects effectively, including the crack and recast layer of the microhole. The influence of different Gaussian and Bessel beam parameters on the microhole morphology were explored, and three ASS structures with different periods were prepared by the optimized Bessel parameters. Ultimately, the average transmittance of the sample with the ASS microhole array period of 2.6 μm increased by 4.1% in the 6 μm to 10 μm waveband, and the transmittance was increased by 5.7% at wavelength of 7.2 μm.

Fabrication of an Anti-Reflective Microstructure on ZnS by Femtosecond Laser Bessel Beams.

An effective and simple method is proposed for fabricating the micro/nano hybrid structures on metal surfaces by adjusting femtosecond laser fluence, scanning interval, and polarization. The evolution of surface morphology with the micro/nano structures is discussed in detail. Also, the mechanism of light absorption by the micro/nano hybrid structures is revealed. Compared with the typical periodic light-absorbing structures, this type of micro/nano hybrid structures has an ultralow average reflectivity of 2% in the 250–2300 nm spectral band and the minimum 1.5% reflectivity in UV band. By employing this method, large areas of the micro/nano hybrid structures with high consistency could be achieved.

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Effective strategy to achieve a metal surface with ultralow reflectivity by femtosecond laser fabrication

The invention relates to a double light path debugging beam combining method and system. By means of the method and system, the problem than two paths of light beams fail to be combined precisely in an existing laser beam combining mode is solved. The method comprises the following steps that 1) a first light path is adjusted, specifically, 1.1) a light spot of the first light path is adjusted tobe in the middle, 1.2) the levelness and perpendicularity of the first light path are adjusted roughly, and 1.3) the levelness and perpendicularity of the first light path are adjusted accurately; 2)a second light path is adjusted, specifically, 2.1) a light spot of the second light path is adjusted to be in the middle, 2.2) the levelness and perpendicularity of the second light path are adjustedroughly, and 2.3) the levelness and perpendicularity of the second light path are adjusted accurately; and 3) beam combining and detecting are performed, specifically, 3.1) beams of the first light path and the second light path are combined, and 3.2) beam combining detecting is performed.

Double light path debugging beam combining method and system

We propose an effective way to achieve an enhanced optical absorption surface of titanium alloy 7 (Ti7) fabricated by a femtosecond (fs) laser assisted with airflow pressure. The effect of laser scanning speed and laser power on the surfaces’ morphology and average reflectivity was studied. In order to further reduce the surface’s reflectivity, different airflow pressure was introduced during the fabrication of Ti7 by a fs laser. Furthermore, the average reflectivity of samples fabricated under different laser parameters assisted with airflow was presented. In addition, the high and low temperature tests of all samples were performed to test the stability performance of the hybrid micro/nanostructures in extreme environments. It is demonstrated that the airflow pressure has an important influence on the micro/nanostructures for light trapping, the average reflectivity of which could be as low as 2.31% over a broad band of 250–2300 nm before high and low temperature tests, and the reflection for specific wavelengths can go below 1.5%.

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Enhanced optical absorption surface of titanium fabricated by a femtosecond laser assisted with airflow pressure

The invention relates to an anti-reflection micro-nano structure array based on space shaping, and the array comprises a plurality of anti-reflection micro-nano structures which are arranged in an array, and the surface shape of each anti-reflection micro-nano structure is an equilateral triangle or an isosceles trapezoid; the horizontal spacing and the vertical spacing of the adjacent equilateraltriangles in the array are both P; or, the horizontal spacing and the vertical spacing of the adjacent isosceles trapezoids in the array are both P; P is specifically determined according to the wavelength of antireflection. According to the invention, the technical problem of low spectral bandwidth precision caused by the fact that a micro group hole shadow part of an existing micro-nano structure array is not distributed according to a theoretical P value is solved; the two types of anti-reflection micro-nano structure arrays are distributed from each direction according to a theoretical Pvalue, so that the selection precision of the spectral bandwidth is improved, and the requirements of the anti-reflection transmittance, the spectral bandwidth, the incident angle and other characteristics can be better met.

Li Xun

Papers

Fabrication of an Anti-Reflective Microstructure on ZnS by Femtosecond Laser Bessel Beams.

Effective strategy to achieve a metal surface with ultralow reflectivity by femtosecond laser fabrication

Double light path debugging beam combining method and system

Enhanced optical absorption surface of titanium fabricated by a femtosecond laser assisted with airflow pressure

Anti-reflection micro-nano structure array based on space shaping