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This paper reports the fabrication of parallel integrated sapphire fiber Bragg gratings (SFBGs) probe sensor by the femtosecond laser point by point (PbP) method. The SFBGs probe sensor transmits optical signals through quartz multimode fiber, and takes an 8.5 mm sapphire fiber as the sensing probe, which can work stably for a long time at 1200 °C. This method can not only quickly prepare and obtain SFBGs with small bandwidth, but also obtain gratings with high reflectivity, which is about 15%. The mode field distribution in sapphire fibers with different lengths is studied by the beam quality analyzer. High-order modes are easily excited as the length of sapphire fibers increases. In addition, a spherical lens is prepared at the end of sapphire fiber, which effectively improve the signal-to-noise ratio (SNR) to 22 dB. The good spectral quality can still be maintained after holding at 1200 °C for 24 hours, and the temperature sensitivity can reach 30.19 pm/°C in the high temperature range, which is more than 2 times higher than that of quartz FBGs sensors. This parallel integrated SFBGs probe sensor overcomes the problems of doped quartz fiber element diffusion at high temperature and the instability of microstructure caused by material softening, and can be developed for long-term stable operation at 1200 °C high temperature and harsh environments. [ABSTRACT FROM AUTHOR]

As the main component of silicon (Si)-based Optic Electronics Integrated Circuit (OEIC) chip, Si-based infrared photodetector operating at communication wavebands is very important. In this paper, we report a kind of chromium (Cr)-hyperdoped black Si material fabricated using femtosecond laser pulses irradiation. The concentration of the Cr atoms in the black Si layer exceeds 1020 cm−3 and the Cr-hyperdoped Si has a large sub-bandgap absorptance (~60% for $1.31 \boldsymbol {\mu }\text{m}$). The deep-energy level introduced by Cr impurity is 0.39 eV below the bottom of conduction band and thus the ionized electron concentration is very low (~1015 cm−3). Owing to the excellent sub-bandgap absorption of Cr-hyperdoped Si, face-to-face black Si photodiodes are fabricated. Under illumination of $1.31 \boldsymbol {\mu }\text{m}$ light, the responsivity of the photodiodes based on N+-N junction reaches 0.57 A/W at 4.3 V bias. In addition, the rise and delay time of the device to the infrared light are on the order of milliseconds. [ABSTRACT FROM AUTHOR]

Blazed gratings are widely used in optical detection devices with the unique property of concentration of light energy to a desired diffraction order. However, traditional substrates for fabricating blazed gratings are easily damaged with the wear and tear in long-term usage under harsh conditions. Here, we propose the fabrication of blazed gratings on sapphire with high mechanical stability by femtosecond laser lithography combined with subsequent reactive ion etching (RIE). Various sapphire blazed gratings with periods from 2 μm to 10 μm and heights from 260 nm to 2 μm were realized to concentrate light energy (wavelengths of 405 nm, 532 nm and 633 nm) to different diffraction orders (1st, 2nd, and 3rd). The results provide great insights for the construction of micro-optical devices that have potential applications under harsh conditions. [ABSTRACT FROM AUTHOR]

Femtosecond laser additive manufacturing based on two-photon polymerization performs a useful technique for fabricating high-precision 3D structures. However, poor mechanical strength and low processing efficiency brought by near polymerization threshold processing to achieve high accuracy are still urgent problems. In this work, a definition-reinforcement-solidification (DRS) processing strategy is proposed to solve the current problems, in which DRS processing includes three steps: utilize a near-threshold femtosecond laser to accurately define the surface profile of 3D structures, perform enhanced scanning on the above structure's inner surface with a high-power femtosecond laser to increase the mechanical strength, and rapidly solidify the inner unpolymerized resin through one exposure of ultraviolet light to improve the processing efficiency. Compared with conventional near-threshold point-by-point direct writing, microstructures fabricated by the DRS strategy for a given geometry and polymer have hardness and Young's modulus values increase to 154% and 199%, respectively, and exhibit a 10000% improvement in machining efficiency without the loss of processing accuracy. Arbitrary complex 3D microstructures can be implemented rapidly via this strategy. This work will inject new vitality into two-photon polymerization additive manufacturing. [ABSTRACT FROM AUTHOR]

In recent years, femtosecond laser direct writing technology has made great progress in integrated quantum photonic circuits due to its advantages of applicability to an extensive range of materials and true 3-D ability. However, a waveguide prepared via a femtosecond laser shows a non-circular cross section and high birefringence, which seriously restricts the development of polarization encoded quantum photonic integrated chips towards greater integration and higher functions. Here, we propose an amplitude-phase double shaping method to reduce waveguide birefringence, which involves using a cylindrical lens and slits. A waveguide prepared using this method shows a circularity cross-section that reaches up to 97.6% and a birefringence that is as low as 1.49 × 10−6. Based on such an ultralow birefringence waveguide, a polarization independent Hong-Ou-Mandel quantum interferometer was fabricated. Interference visibilities for identical photon pairs in different polarization states were found to be more than 95% with a standard deviation of 0.6%. The ultra-low birefringence single-mode waveguide and the polarization-independent interferometer realized using this approach will play an important role in large-scale polarization encoding integrated quantum photonic chips. [ABSTRACT FROM AUTHOR]

Microlens array has been demonstrated to be an effective method for laser beam homogenization. However, the homogenization of high-fluence laser beam is still a great challenge due to the serious damage to the microlens-array-based homogenizer. Here, an ultra-smooth sapphire concave microlens-array-based homogenizer, which is fabricated by dry-etching-assisted femtosecond laser machining, is proposed for shaping high-fluence pulsed laser beam to a flat-top profile intensity distribution. By taking advantages of high transmittance (>70%) and low surface roughness (1.1 nm), high-fluence (5 J/cm2) and wide-band (266 nm, 532 nm and 808 nm) laser beams homogenization is successfully realized by the sapphire concave microlens arrays. [ABSTRACT FROM AUTHOR]

Here, we report embedded spiral zone plates (SZPs) processed by tightly focused femtosecond pulses, inducing a refractive index change in a sapphire crystal. The change in the index is investigated both experimentally and theoretically and found to be 0.93% lower than that before the laser irradiation. Benefiting from the investigation of the photorefraction, a high diffractive efficiency of up to 78% is achieved for the SZPs. In addition, the SZPs are embedded under the surface of the sapphire, which ensures that the components possess stable optical properties in the case of external refractive index changes. The high-performance elements having a high diffractive efficiency and stable optical performance will have tremendous applications in optical communication and optical manipulation. [ABSTRACT FROM AUTHOR]

A crystalline silicon (Si) surface was modified using nanosecond laser pulses in an argon atmosphere. The laser-modified Si (M-Si) samples have a higher performance and thermostable absorption in the broadband range (400–2400 nm) than conventional Si. The concentration of carrier electrons in the M-Si layer is at least five orders of magnitude greater than the carrier concentration of the Si substrate. Using the N+–N− junction formed between the M-Si layer and the Si substrate, visible and near-infrared (VIS-NIR) M-Si photodetectors are made. The N+–N− photodiode has good rectification characteristics and a high photoresponse to the subbandgap NIR light at 1310 nm. At the same time, the M-Si photodetector at a low reverse bias shows a large gain to the VIS-NIR light above the bandgap. By comparing the response time of the M-Si photodetector to the light of 900 and 1310 nm, the response speed of the device to the photon above-bandgap energy is faster. [ABSTRACT FROM AUTHOR]

Micro-ripple and micro-bead structures are formed on a silicon (Si) surface after irradiation with femtosecond laser pulses in nitrogen (N2) atmosphere. Simultaneously, supersaturated nitrogen (N) atoms, with a concentration above 1020 cm−3, are doped into the textured black Si layer via laser ablation. The N-doped Si exhibits strong below-bandgap infrared absorption from 1.1 to 2.5~\mu \textm , which remains nearly unchanged after annealing for 30 min at 873 K. The mechanism of this thermally stable infrared absorption is analyzed by first-principles calculations. According to the transmission electron microscopy results, multiple phases (including single crystalline, nanocrystalline, and amorphous phases) are observed in the laser-irradiated layer. Hall Effect measurements prove that N-dopants induce a low background free-carrier concentration ( \sim 1.67\times 10^16\,\,\text cm^-3 ). Finally, a Schottky-based bulk structure photodiode is made. This broadband photodiode exhibits good thermal stability and a photo-responsivity of 5.3 mA/W for 1.31~\mu \textm at a reverse bias of 10 V. [ABSTRACT FROM PUBLISHER]

Optical microcavities have been paid enormous attentions as a basic integrated element in photonic devices including micromodulators, microfilters, and microsensors. In particular, the microcavities that can produce unidirectional and single-mode lasing emissions are greatly demanded to meet the challenge of integrating high-efficiency and high-sensitivity optical functional systems. Herein, a three-dimensional polymer microlaser composed of stacked circular-ring-shaped and spiral-ring-shaped microcavities is fabricated directly on a narrow bandpass filter substrate by the femtosecond laser processing technique. The on-chip polymer microlaser provides a room-temperature, low-threshold, unidirectional, and single-mode laser output, resulting from the coupling between the two stacked microcavities. The fabrication of the on-chip low-threshold polymer microlaser opens up a possibility toward the integration of a variety of polymer microcavities for organic optoelectronic devices. [ABSTRACT FROM PUBLISHER]

Gold (Au)-doped-textured silicon (Si) material with a thermostable absorption below bandgap (>50%) is obtained by femtosecond laser irradiation. Although the concentration of Au impurity (1019 cm−3 ) in textured Si is at least four orders of magnitude greater than the solid solubility of Au in crystalline Si, the sheet carrier density (approximately 1010 cm−2) in Au-doped Si is very low due to a self-compensation effect of Au impurity in Si material. The infrared absorption of Au-doped Si is related to laser-induced-structural defects and sub-band absorption of deep energy levels of Au in Si, which is determined by temperature-dependent Hall Effect measurement. Besides supersaturated doping of Au, a gold silicide phase is formed at textured Si surface. [ABSTRACT FROM PUBLISHER]

Femtosecond (fs) laser annealing has been applied to improve the crystalline quality and absorptance below bandgap of ion implanted silicon (Si) with sulfur (S). The doping concentration of S is up to 10^20 atoms/cm3, which is at least four orders of magnitude higher than the solid solubility of S in crystalline Si. According to the Raman spectra, after fs laser irradiation with appropriate laser energy, the crystal quality is evidently better than primary ion implanted sample. Moreover, the optical absorption coefficient at wavelengths of 1100~2400 nm is up to 1.54 \times 10^4 cm ^\mathrm -1 , which is much larger than the contribution of free carriers. Excessive laser energy will induce serious ablation effect and cause the removing of doping layer. We consider that the high absorption is a combination of sub-bandgap transition of S-related localized states, free carrier absorption, ion implantation induced defect absorption and fs laser irradiation induced defect absorption. In the end, a photo-response of 8.4 mA/W is obtained for the photodiode for 1310 nm detection light. [ABSTRACT FROM PUBLISHER]

We report a novel hybrid refractive–diffractive microlens combined with spiral phase for the generation of optical vortex, which is fabricated via femtosecond laser direct writing technology. The unique optical performance of focusing capability is demonstrated. At the focus position, the hollow focus with different integer topological charges is investigated. Moreover, experimental results are supported by finite-element calculation. The novel microlens generating an optical vortex will fulfill important applications in optical manipulation, multistate information encoding, quantum communication, and computation, particularly in the compaction, integration, and simplification of optical vortex generation system. [ABSTRACT FROM PUBLISHER]

Suspended structures have attracted a great deal of attention in the field of micro-technology and nanotechnology for their important functions. However, the fabrication of the suspended structures is typically cumbersome and time-consuming and requires complicated processes, such as multiple deposition, photolithography, and etching. Here, we report a new method of fabricating flexible suspended structures by femtosecond laser direct writing followed by wet chemical alkaline (KOH) etching. Our proposed method will enable convenient and flexible fabrication of suspended structures for a wide variety of applications in the field of micro-technology and nanotechnology. [ABSTRACT FROM PUBLISHER]

Here, we report a sapphire-based Fresnel zone plate (FZP), which is fabricated by femtosecond laser direct writing assisted with subsequent wet etching. With this method, we solved the problem of high surface roughness caused by ultrafast femtosecond laser processing. We have obtained $\sim 12$ -nm average surface roughness smaller than 1/25 of the optical working wavelength. As-formed sapphire FZP also exhibited a well-defined geometry. More importantly, ultraviolet (UV) light focusing and imaging can be easily achieved. Due to the high material hardness, thermal and chemical stabilities of sapphire, such sapphire FZP, may have great potential in UV imaging and focusing under some harsh environments. [ABSTRACT FROM PUBLISHER]

A metal-coated hemisphere microcavity is realized by a simple self-assembled process. Plasmon-photon hybrid modes are produced by the metallic microcavity, and the lasing wavelength exhibits a blue shift when compared with no-metal cavities. Simulation shows that the blue shift originates from the redistribution of the electric-field intensity in the microcavity. The metal-coated microcavity exhibits excellent lasing performance at room temperature with a quality factor of $\sim 2500$ , owing to high-smooth surface of the hemisphere. Our findings may provide a promising candidate for fundamental investigation of plasmon and photon modes interaction and cavity electrodynamics. [ABSTRACT FROM PUBLISHER]

We report in this letter a concave–convex microlens (CCML) consisting of two different high curvature surfaces. Compared with the conventional plano-convex microlenses, the CCML not only allows for more design freedom, but also offers significantly improved optical performance, particularly minimization of aberration, as is critical when the size of optical components is small. Experimentally, focusing capability at different wavelengths was demonstrated, and the axial chromatic aberration was found significantly reduced to $\sim 4.6$ % of the focal length shift under wavelength 450–660 nm. [ABSTRACT FROM PUBLISHER]

We investigated the strain and surface structural properties of a strain-relaxed Si1–xGex alloy layer with high Ge-content using optical surface second-harmonic generation. Here, the Si1–xGex alloys are heteroepitaxial, and they are deposited onto Si substrates via ultra-high-vacuum-chemical vapor deposition. The in-plane strain and the composition $x$ of the Si1–xGex alloys were determined using Raman spectroscopy. The SH signals generated from three Si1–xGex alloy surfaces versus the rotational angle of the substrate were measured. The SH intensities for the combination of s-input/p-output polarization show fourfold symmetry; however, for s-input/s-output, the SH signals show eightfold symmetry with the rotational angle. Residual strain would induce an enhancement of the isotropic p-output SH component in the Fourier transform coefficient. Finally, the degree of symmetry of the SH signals from these three Si1–xGex alloy surfaces versus the rotational angle was related with the surface pit defects (densities and sizes) and surface roughness. [ABSTRACT FROM AUTHOR]

Micronanostructured silicon surface is irradiated by femtosecond laser pulses at sulfur hexafluoride atmosphere (S-doped black silicon) and photodiodes are successfully fabricated based on this material. From the scanning electronic microscope and atomic force microscope images, the black silicon layer shows micro-nanocomplex structures (microcones covered with nanoparticles). The optoelectrical properties of n+-p junction are formed between S-doped micro-nanostructured silicon-layer and substrate after thermal annealing. For the n+-p photodiode, current–voltage characteristics at different incident light powers have been investigated. The responsivity for 800-nm wavelength is 0.69 A/W at −5 V reverse bias, which is close to that of the usual commercial Si p-i-n photodiode. The present devices are stable and well reproducible. [ABSTRACT FROM PUBLISHER]

Chalcogen-doped microstructural silicon irradiated by femtosecond laser has high near-uniform absorption on a broad spectrum, but the factors leading to infrared absorption are complex and remain an open problem. To clarify the origin of infrared absorption besides hyperdoped Chalcogen atoms, microstructural silicon is fabricated by femtosecond laser under vacuum condition. The relationship between infrared absorption and as-formed new phases (amorphous silicon: $\alpha $ -Si and nanocrystal silicon) is established. It indicates that the infrared absorption is caused by defects related to Urbach states from \alpha $ -Si or nanocrystal Si, and these metastable defects disappeared after a thermal annealing process. From the absorption spectrum of microstructural silicon after etching at different times, it could be figured out that the Urbach states exist in both the surface and subsurface regions of black silicon at a depth of about 2.4 \mu \text{m} . [ABSTRACT FROM PUBLISHER]

We report in this letter the fabrication of edge-alignment-coupled whispering gallery mode photonic-molecule microdisk by femtosecond laser direct writing of dye-doped resins, which results in single-mode lasing output. Under picosecond laser pumping, the excited energy is distributed at the disk periphery and the coupling happens at the overlapping edge of two disks. As a result of mode coupling through Vernier effect, single-mode lasing is achieved in two-microdisk and three-microdisk coupled lasers of various sizes. The effect in the coupled system is confirmed by numerical simulation. This letter may open up a new avenue to the 3-D integrated optoelectronics. [ABSTRACT FROM PUBLISHER]

The fabrication of tilted fiber Bragg gratings (TFBGs) in non-photosensitized single-mode fiber by infrared femtosecond (fs) laser is reported for the first time to the best of our knowledge. The tilted grating structure is obtained by focusing laser pulses through a phase mask into a tilted positioned fiber sample, which is moving along with the interference fringe to form a grating region covering the entire fiber core and partial cladding. A reflective fiber probe based on the TFBG with a gold mirror sputtered on one end has been fabricated and its maximum refractive index (RI) sensitivity is 12.276 nm/RI unit in the RI range of 1.4091–1.4230 for the cladding mode LP020. A wavelength-power demodulated technique based on the sensors has been proposed to measure axial strain and temperature simultaneously, which is found suited to work in harsh environment, especially at high temperature up to 800^\circ\,\ C. [ABSTRACT FROM PUBLISHER]

A novel formation method of long-period fiber gratings (LPFGs) based on periodic microchannels, which are fabricated by femtosecond laser micromachining and selective chemical etching in conventional single-mode fibers, is proposed and experimentally demonstrated. This kind of grating may be as short as only 3 mm and exhibits low-temperature sensitivity of 9.95 pm/^\circC from 30^\circC to 120^\circC. The strain sensitivities of the resonant wavelength and peak loss are -2.4~nm/m\varepsilon and -3.5~dB/m\varepsilon, respectively. High refractive-index (RI) sensitivity with one order of magnitude larger than that of the conventional LPFGs near RI=1.33 is achieved in general single-mode fibers. [ABSTRACT FROM PUBLISHER]

Polaron pair dynamics in regioregular Poly (3-hexylthiophene) (RR-P3HT) film was studied by applying both femtosecond transient absorption and fluorescence up-conversion techniques. The comparison of the dynamics of the pure fluorescence, the ground state bleaching recovery and the polaron pair absorption reveals that the polaron pairs decay back to the ground state of certain morphological places with distinct absorption features, instead of the singlet excited state. [ABSTRACT FROM PUBLISHER]

We report on the band edge alignment in zinc-blended-type CdTe/CdS core-shell quantum dots, using femtosecond spectroscopy. Time-resolved transient absorption spectroscopy directly shows that the system gradually developed from quasi-type-I to type-II as the shell thickness increased. Following photoexcitation of CdS, the injection of a hole from CdS valence band into CdTe valence band is observed. The hole transfer occurs on the 1000 fs time scale in these heteromaterials. Furthermore, a strain ca. 150 meV is obtained by comparing the transient with steady-state spectra. [ABSTRACT FROM PUBLISHER]

Excited state dynamics from molecular fluorescent switches of the donor- or acceptor-derived 5-Methoxy-2-(2-pyridyl)thiazole (2-MPT) fluorescent molecules has been studied by ultrafast spectroscopy. The twisted intramolecular charge transfer (TICT) excited state is found to be created within 1 ps. Then the relaxation of TICT excited state causes further twist of the molecular conformation (especially the moiety 2-MPT) and reduces the transition moment. It takes about 600–700 ps to reach a state with ten nanoseconds of lifetime. This final state is nearly a dark state, which is attributed as the main reason for the drastic decrease of the fluorescence quantum field with respect to the moiety 2-MPT. For MPTEA with a strong electron donor in high polarity solvents, TICT's excited state partially transfers to a planar intramolecular charge transfer (PICT) excited state within 10 ps due to the strong dipole-dipole interaction with solvent molecules. PICT's excited state gives a relatively strong emission with a large red shift. The results demonstrate that the excited state dynamics of these molecular fluorescent switches provides deep insight to their photophysical processes, which is important for exploring novel molecular switches as potential candidates for optical biolabels, sensors and logic gates applications. [ABSTRACT FROM PUBLISHER]

A tip-grounded waveguide microsensor was proposed to overcome the difficulty of quantitative voltage calibration in electrooptical detection for integrated circuit (IC) test. On this basis, we optimized the thickness of the electrooptical material of the sensor to eliminate the influence of the circuit layout on the measured signals. The improved sensor in return made it possible to calibrate the voltage with known reference electric signals quantitatively. This method circumvented the uncertainty of the probe conditions of each measurement point. Finally, a calibration accuracy of better than 6% was obtained, which satisfied broad applications in the IC industry. [ABSTRACT FROM PUBLISHER]

Magnetic field effects on tris-(8-hydroxyquinoline) aluminum-based organic light-emitting devices (OLEDs) by employing \Fe3\O4 as a magnetic dopant in the hole-transport layer (HTL) have been studied. The magnetic doped OLEDs exhibit efficient injection and transport of holes, and its performances are further enhanced after a magnetic field is applied. The enhancement of luminance and current efficiency of 20% and 24% has been obtained from the magnetic doped devices, while they are only 8% and 9%, respectively, for the nondoped devices under an applied magnetic field of 500 mT. Organic magnetoresistance induced by the magnetic doped HTL is the main origin of increased electroluminescence for the magnetic doped OLEDs. [ABSTRACT FROM PUBLISHER]

A strain-scanning second-harmonic generation technique is proposed for high-sensitivity measurement of weak strain in film surfaces or interfaces. The basic idea is the sequential application of tensile and compressive strains to a strained film sample. From the strain-dependent second-harmonic generation (SHG) intensity, the type of the strain can be easily judged from whether the SHG is enhanced or weakened, and its magnitude can be precisely calibrated by an externally applied strain that is known. Thus, the built-in strain of a SiO2/Si interface could be determined as tensile with a magnitude of 3.07\,\times 10^-4. [ABSTRACT FROM PUBLISHER]

This paper reports the two-photon absorption (TPA) and the spectral-narrowed light emission (SNLE) from the crystals of 9, 10-distyrylanthracene (DSA) derivatives. The results obtained by the Z-scan method show that the tested molecules possess high TPA cross sections. Upon near-IR excitation, strong two-photon-excited fluorescence could be observed in the crystals of four types of materials. Furthermore, SNLE with low threshold occurs in three of the four crystals under pumping with the second harmonic generated in the amplifier. Single crystals of the DSA derivatives exhibit unique photonic properties, including strong solid-state fluorescence, large TPA cross section, and stimulated emission. The results demonstrate that these are potential candidates for compact and practical solid-state laser applications. [ABSTRACT FROM PUBLISHER]

The emitting properties of fluorophores can be tailored by surface plasmon polaritons (SPPs), which offers the potential to understand the light-matter interactions at the nanoscale, and lays the groundwork for many future applications. As one of the useful features in SPPs—surface plasmon-coupled emission (SPCE) based on the metal gratings; however, the study on its underlying modulation mechanism remains far from satisfactory. That is, how it is affected by the morphology of the grating surfaces. In this letter, we have addressed this question in the dye-coated corrugated silver gratings that possess different periods and depths. It is found that the properties of SPCE are strongly dependent on the dispersion relations of SPPs modulated by the grating periods. [ABSTRACT FROM PUBLISHER]