Ultrafast Photonics Research Articles

Gain-assisted magnetoplasmonic switching in metal–dielectric–metal plasmonic waveguides

Optical switches in metal–dielectric–metal (MIM) structures attracted much interest for chip-scale plasmonic modulator merging nanoelectronics and ultrafast photonics. The absorption switches employed gain modulations; thus, their switching speed is limited to the sub-gigahertz range because of the spontaneous lifetime of the transition. In this paper, we theoretically predicted that the function of the absorption switches is achieved employing magnetic field reversal and constant gain instead of gain modulations. We investigated analytically and numerically transmission in the presence of both gain and gyration and revealed that the transmission shift by the external magnetic field is maximized at a resonance value of gain. The switching speed can reach the THz range thanks to the large optical bandwidth in the MIM stub structures and the advanced magnetization switching technology. The MIM structure with combined gain and magnetoplasmonic properties enhances interaction between light and magnetic field on nanoscale and enables the device footprint down to the deep subwavelength scale λ2/50.

Wavelength-tunable broadband lasers based on nanomaterials.

Nanomaterials are widely used in the fields of sensors, optoelectronics, biophotonics and ultrafast photonics due to their excellent mechanical, thermal, optical, electrical and magnetic properties. Particularly, owing to their nonlinear optical properties, fast response time and broadband operation, nanomaterials are ideal saturable absorption materials in ultrafast photonics, which contribute to the improvement of laser performance. Therefore, nanomaterials are of great importance to applications in wavelength-tunable broadband pulsed lasers. Herein, we review the integration and applications of nanomaterials in wavelength-tunable broadband ultrafast photonics. Firstly, the two integration methods, which are direct coupling and evanescent field coupling, and their characteristics are introduced. Secondly, the applications of nanomaterials in wavelength-tunable broadband lasers are summarized. Finally, the development of nanomaterials and broadband tunable lasers is reviewed and discussed.

Transparent nanopaper for ultrashort pulse generation in the near-infrared region.

Transparent nanopaper (T-paper) can be applied in the field of electromagnetic shielding materials, antistatic materials, composite conductive materials, electric pool materials, super capacitors, and thermal management systems. However, this kind of T-paper has not been employed in ultrafast photonics yet. For the first time, to our knowledge, transparent electrical nanopaper is used in fiber lasers, different from the conventional pulsed fiber laser, which operates in the Q-switched regime under low pump power and then in the mode-locked regime under high pump power. Mode-locking is achieved first with a pulse duration of 550fs under low pump power (166mW). When further increasing the pump power up to 198mW, the proposed fiber laser can be converted from a mode-locked to Q-switched state, which is a result of the two-photon absorption effect. The proposed fiber laser based on T-paper can be potentially applied in optical tomography, metrology, spectroscopy, micro-machining technology, and biomedical diagnostics.

Determination of microwave instability threshold of storage ring based on longitudinal bunch asymmetry characterization

In this study, a novel method is proposed to measure the microwave instability (MWI) threshold of the storage ring by means of the ultrafast photon detection streak camera system. It provides an additional and complementary diagnostic tool for determination of the MWI threshold of the current storage ring. Compared to other conventional measurement methods, this scheme can realize the more accurate measurement of the MWI threshold current with the remarkable advantages of simple optical architecture, good robustness, and high accuracy. In the experiment, the MWI threshold current of the Hefei Light Source-II (HLS-II) is obtained as 12.77 ± 0.54 mA corresponding to 123.6 kV of the nominal radio-frequency (RF) voltage by measuring the current dependence of the bunch asymmetry factor, under single-bunch operation mode. With a further adjust RF voltage to 100.2, 110.2, 130.3, and 140.2 kV, the turbulent threshold currents are acquired to be 11.11 ± 0.90, 11.74 ± 0.47, 13.21 ± 0.68, and 13.78 ± 0.82 mA, respectively. Additionally, the applicability, feasibility, and reliability of this method are further verified by experimentally observing the beam characteristic behaviors of the energy spread broadening and horizontal beam size widening. From experimental verification and comparative analysis, it is proved that this reported approach is valid for measuring the MWI threshold of the storage ring. This method is importance for the study of the beam instability mechanisms and related longitudinal beam dynamics in storage ring, which is also useful for machine debugging and beam status analysis.

Emerging 2D Materials with Nonparabolic Bands for Ultrafast Photonics

In recent years, 2D materials have been widely used in optical and photonic applications such as mode lockers, optical switches, polarizers, and optical modulators. As is known, many 2D optical materials are parabolic semiconductors. The electronic states determines the material's optical characteristics. The energy band structure with a bandgap dominates the electron dynamics process associated with intrinsic light absorption. Particularly, the Dirac cone and flat band (FB) are special nonparabolic energy band structures in materials. Dirac materials are characterized by a zero bandgap, and FB materials have a completely flat dispersionless band in the Brillouin zone. These materials are used as emergent and promising 2D materials for saturable absorbers (SAs) due to their excellent nonlinear optical response, and they become potential candidates for ultrafast photonics applications. Herein, the focus is on the characteristics of the energy band structure; the carrier dynamics are reviewed, and application in ultrafast photonics are summarized.

Sodium-doped InP/ZnSeS/ZnS quantum dots as a saturable absorber for passive Q-switched fiber lasers

In this work, sodium-doped InP/ZnSeS/ZnS (InP–Na) QDs was fabricated by thermal injection method for the first time. The nonlinear optical properties were first demonstrated by realizing a modulation depth of 7.85% and saturable intensity of 2.2 MW cm−2 by two-detector method. The Q-switching operations were carried out by integrating InP–Na QDs as saturable absorber (SA) in a fiber laser ring cavity system. Q-switched laser pulses with a pulse width of 139 ns were generated. The ability of effective pulse width compression can be attributed to the short exciton recovery time of SA, as confirmed by transient absorption and time-resolved photoluminescence. This is the first demonstration of Na-doped InP/ZnSeS/ZnS QDs and using it as SA for generating ultrafast laser pulses. Therefore, this study extends the research scope of InP-based QDs in optical communications, nonlinear photonics and ultrafast photonics.

Interface Charge Transfer in MXenes/Graphene Heterostructures for Ultrafast Photonics

AbstractTwo‐dimensional (2D) transition‐metal carbides, nitrides, and carbocyanides, known as MXenes, have attracted tremendous attention in photonics and electronics because of their promising optical, electrical, and mechanical properties. However, the potential of MXenes‐based heterostructures for ultrafast photonics has not yet been explored. In this study, a Ti3C2Tx/graphene vertical heterostructure (TG‐VHS), an excellent saturable absorber (SA), is successfully used to generate femtosecond laser pulses via passive mode‐locking. The modulation depth of the TG‐VHS is measured to be 28%, which is ≈12.7 and 1.5 times higher than that of Ti3C2Tx MXenes and graphene, respectively. Density functional theory calculations show that various heterostructure systems are formed owing to the presence of various functional groups in Ti3C2Tx, and multiple charge transfers significantly enhance the nonlinear optical characteristics of TG‐VHS. Moreover, utilizing TG‐VHS as the SA, a stable mode‐locked operation with a pulse duration as short as 356 fs at a wavelength of 1559.16 nm is achieved; harmonic mode‐locking with the highest repetition rate of 603.1 MHz is also obtained. This study provides an important strategy for developing VHS SA for ultrafast pulse fiber lasers and opens new perspectives for designing advanced MXenes‐based photonic devices.

Ag/MXene Composite as a Broadband Nonlinear Modulator for Ultrafast Photonics

Ag/MXene Composite as a Broadband Nonlinear Modulator for Ultrafast Photonics

Beneficial cooperation

AbstractMotivated by the first laboratory demonstration of a self‐referenced femtosecond frequency comb using a fiber laser [1], Toptica Photonics and the Ultrafast Phenomena and Photonics research group of Alfred Leitenstorfer at the University of Konstanz have established a direct link of collaboration since 2004. This long‐standing knowledge transfer has now been recognized with the Technology Transfer Prize of the German Physical Society (DPG).

Interfacial modification of GO-FePt hybrid film leads to giant ultrafast optical nonlinearity

Excellent optical nonlinearity is highly promising for the development of multifunctional ultrafast photonics techniques and integrated optoelectronic devices. However, existing numerous optical materials commonly suffer from several challenges, such as bad functional integration, slow response speed, and weak optical nonlinearity thus severely hindering their extensive applications. Here, we first engineer new scalable graphene oxide (GO)-FePt films by chemistry induced self-assembly and then demonstrate the enhanced ultrafast optical nonlinearity by resorting to their interfacial modifications. It is found that the FePt magnetic nanoparticles (NPs) are easily assembled on the surface of the GO by exchanging the surface ligands of the former with evaporated solvent, leading to good water solubility and super-paramagnetism. The femtosecond Z-scan test results show that both the nonlinear saturation absorption and optical Kerr refraction of the pure GO are subjected to reversion once the FePt NPs are introduced. More importantly, we can greatly enhance the ultrafast nonlinear optics response of as-prepared GO-FePt hybrid film by promoting the concentration of magnetic NPs. In addition, the associated optical nonlinearity can be further boosted with femtosecond vectorial beams excitation instead of the linearly polarized one. In principle, the enhanced optical nonlinearity may arise from direct metal-to-semiconductor interfacial charge transfer transition (DICTT) related to the difference in work function and Fermi level together with complicated interaction between the vectorial light fields and the hybrid films. The GO-based magnetic hybrid films with giant ultrafast optical nonlinearity have potential wide-ranging applications in integrated optoelectronics, ultrafast nanophotonics, opto-magnetic storage, and beyond.

Nonlinear optical properties of 3d transition metal oxides and applications in fiber lasers: Review and prospect

Transition metal oxides (TMOs) are distinguished by their thermal and chemical stability as well as excellent optical, electrical, and magnetic properties. Moreover, the 3[Formula: see text]-orbital electrons of the transition metal ions in TMOs have a strong [Formula: see text] Coulomb interaction, enabling them to exhibit a variety of unusual physical phenomena, dramatically different from conventional metals and semiconductors. Additionally, TMOs possess excellent nonlinear optical properties, such as third-order nonlinear response and saturation absorption, which make them broadly applicable for ultrafast photonics devices. Herein, a review is presented of the nonlinear optical properties of typical TMOs, as well as their employment as saturable absorbers in near-infrared pulsed fiber lasers in recent years. Finally, some challenges and promising prospects for nonlinear ultrafast photonic devices based on TMOs are discussed.

Harmonic Mode‐Locked Er‐Doped Fiber Laser Based on a Microfiber‐Based PbS Nanoparticle Saturable Absorber

AbstractLead sulfide (PbS) is a nanomaterial with excellent optical and chemical properties, such as a narrow bandgap (0.37 eV), high thermal damage threshold, and high stability. Obviously, it is appropriate as a saturable absorber (SA) device for ultrafast photonics. However, PbS nanoparticles (NPs) as the SA of ultrashort harmonic mode‐locked pulse still haven't been demonstrated at present. In this paper, the PbS NPs are made into an SA‐device‐based microfiber by optical deposition method and connected in an integrated Erbium‐doped fiber laser. And both characteristics and nonlinear optical properties of PbS NPs have been systemically investigated. A fundamental frequency mode‐locked pulsed laser is proposed, whose central wavelength is 1560 nm, and the pulse width is 1 ps. In addition, high repetition rate operations are achieved, with a maximum repetition rate of 833 MHz. This is the first time that PbS NPs are used to generate 96th‐order harmonic mode‐locking, and the corresponding pulse duration is 987 fs. It is demonstrated that PbS NPs are a kind of SA photonic material with excellent performance. It can improve the communication capacity by applying fiber communication, and it has potential application value even in material processing and optical comb.

Direct Observation of Photon Induced Giant Band Renormalization in 2D PdSe2 Dichalcogenide by Transient Absorption Spectroscopy.

Insight into fundamental light-matter interaction as well as underlying photo-physical processes is crucial for the development of novel optoelectronic devices. Palladium diselenide (PdSe2 ), an important representative of emerging 2D noble metal dichalcogenides, has gain considerable attention owing to its unique optical, physical, and chemical properties. In this study, 2D PdSe2 nanosheets (NSs) are prepared using the liquid-phase exfoliation method. A broadband carrier relaxation dynamics from visible to near-infrared bands are revealed using a time-resolved transient absorption spectrometer, giving results that indicate band filling and bandgap renormalization (BGR) effects in the 2D PdSe2 NSs. The observed blue-shift of the transient absorption spectra at the primary stage and the subsequent red-shift can be ascribed to this BGR effect. These findings reveal the many-body character of the 2D TMDs material and may hold keys for applications in the field of optoelectronics and ultrafast photonics.

Nitrogen-doped graphene quantum dots for femtosecond ultrafast pulsed fiber lasers

Nitrogen-doped graphene quantum dots (N-GQDs) are a new class of quantum dot with unique electronic properties, whose electrons transport and biochemical properties have been extensively studied. However, the optical properties of N-GQDs, especially their nonlinear optical response and applications in ultrafast photonics and photoelectronic, need to be further explored. The nonlinear optical response and ultrafast carrier dynamics of N-GQDs in an aqueous solution are systematically investigated in this work. N-GQDs have an ultrafast carrier cooling delay of about 143 fs in the visible range. In the study of the nonlinear optical response of N-GQDs, the saturable absorption is transformed into reverse saturable absorption as the incident pulse intensity increases, which makes N-GQDs an effective saturable absorber for ultrashort pulse generation in mode-locked Er-doped fiber lasers. Our results may lead to further investigations into the optical properties of N-GQDs and demonstrate potential opportunities for ultrafast photonic applications.

Deep learning enhanced ultra-fast SPECT/CT bone scan in patients with suspected malignancy: quantitative assessment and clinical performance

Objectives. To evaluate the clinical performance of deep learning-enhanced ultrafast single photon emission computed tomography/computed tomography (SPECT/CT) bone scans in patients with suspected malignancy. Approach. In this prospective study, 102 patients with potential malignancy were enrolled and underwent a 20 min SPECT/CT and a 3 min SPECT scan. A deep learning model was applied to generate algorithm-enhanced images (3 min DL SPECT). The reference modality was the 20 min SPECT/CT scan. Two reviewers independently evaluated general image quality, Tc-99m MDP distribution, artifacts, and diagnostic confidence of 20 min SPECT/CT, 3 min SPECT/CT, and 3 min DL SPECT/CT images. The sensitivity, specificity, accuracy, and interobserver agreement were calculated. The lesion maximum standard uptake value (SUVmax) of the 3 min DL and 20 min SPECT/CT images was analyzed. The peak signal-to-noise ratio (PSNR) and structure similarity index measure (SSIM) were evaluated. Main results. The 3 min DL SPECT/CT images showed significantly superior general image quality, Tc-99m MDP distribution, artifacts, and diagnostic confidence than the 20 min SPECT/CT images (P < 0.0001). The diagnostic performance of the 20 min and 3 min DL SPECT/CT images was similar for reviewer 1 (paired X 2 = 0.333, P = 0.564) and reviewer 2 (paired X 2 = 0.05, P = 0.823). The diagnosis results for the 20 min (kappa = 0.822) and 3 min DL (kappa = 0.732) SPECT/CT images showed high interobserver agreement. The 3 min DL SPECT/CT images had significantly higher PSNR and SSIM than the 3 min SPECT/CT images (51.44 versus 38.44, P < 0.0001; 0.863 versus 0.752, P < 0.0001). The SUVmax of the 3 min DL and 20 min SPECT/CT images showed a strong linear relationship (r = 0.991; P < 0.0001). Significance. Ultrafast SPECT/CT with a 1/7 acquisition time can be enhanced by a deep learning method to achieve comparable image quality and diagnostic value to those of standard acquisition.

Open-ended exploration of ultrashort pulse lasers: an innovative design strategy for devices based on 2D materials

Ultrashort pulse lasers have vital significance in the field of ultrafast photonics. A saturable absorber (SA) as the core device to generate ultrashort pulses has innovative design strategies; the most interesting of which is the integration strategy based on 2D materials. This review presents recent advances in the optoelectronic properties of 2D materials and in the way the materials are prepared, characterized, and integrated into devices. We have done a comprehensive review of the optical properties of materials and material-based devices and their current development in the field of fiber lasers and solid-state lasers. Finally, we offer a look at future applications for 2D materials in ultrafast lasers and their prospects.

Bismuth oxysulfide nanosheets as a novel saturable absorber for Er-doped and Tm-doped ultrafast fiber lasers

Novel layered bismuth oxysulfide (Bi2O2S) has shown great potential in photoelectric detection, photocatalysis, and photochemical cells due to its excellent properties such as excellent environmental stability, high carrier mobility, and favorite band gap (about 1∼1.5 eV). These excellent physical properties also make it advantageous for applications in the field of ultrafast photonics. However, to date, there has been no reported research on Bi2O2S in ultrafast fiber lasers. In this work, we report a facile method to synthesize Bi2O2S nanosheets by a combination of the hydrothermal and liquid-phase exfoliation (LPE) methods with good crystallinity and thickness less than 10 nm. Further, stable mode-locked pulses are achieved by applying the Bi2O2S SA in Er-doped and Tm-doped fiber lasers, respectively. The Er-doped mode-locked fiber laser has a pulse width of 453.6 fs and the optical spectrum is stable, with a maximum output power of 5.1 mW in the mode-locked state. The obtained Tm-doped mode-locked fiber laser based on Bi2O2S has an estimated pulse width of 2.69 ps and a maximum output power of 7.13 mW. Our results provide new opportunities for the development of Bi2O2S together with its application in ultrafast photonics.

Tunable optical nonlinearities and surface plasmon resonance in Fe3O4 nanoparticles-decorated Ti3C2 MXene composite for ultrafast fiber lasers

In this work, the Ti3C2 MXene was modified with Fe3O4 nanoparticles to obtain a composite with in-plane optical anisotropy and surface plasmon properties. Based on a side-polished D-shaped fiber (DSF), this Fe3O4/Ti3C2 MXene (F/M) composite exhibits a tunable nonlinear optical (NLO) absorption response under 1.5 μm laser excitation with different polarization states. The modulation depths can be obtained from 0.9% to 12.1%. As an application, the ultrafast erbium-doped fiber laser (EDFL) with multiple pulsed laser operations was demonstrated using the F/M-DSF device as a saturable absorber (SA). Both stable Q-switching and mode-locking operations were demonstrated in the same all-fiber laser cavity, which generated pulsed laser with microsecond and sub-picosecond durations respectively. In addition, the bound state (BS) of solitons in harmonic mode-locked lasers with the pulse time interval of 3 ps and the highest order of 30th was also realized. This versatile and simple-structured all-fiber laser has great potential in various applications, such as optical communication. With the numerical simulations, the dynamic of F/M composite-based soliton mode-locking was also explored, illustrating the mechanism of soliton formation and propagation. This work not only expands the advanced applications of F/M composite in ultrafast photonics but also provides a new perspective for designing and optimizing its ultrafast modulating performance.

Enhanced electro-optical properties of nematic liquid crystal doped with Nitrogen doped graphene quantum dots

The first step of this study was to prepare mixtures that contained different concentrations of N doped graphene quantum dots (NGQDs) into nematic liquid crystals (NLCs). Then, through the Z-scan technology, the optical measurements in the nonlinear regime were conducted with a continuous wave (CW) at 632 nm wavelength and a low power laser beam. The nonlinear absorption (NLA) coefficient, nonlinear refractive (NLR) index, and third-order nonlinear susceptibility of NLCs doped with different concentrations of the NGQDs at distinctive laser powers and diverse applied voltages of external electric field was experimentally measured. The Z-scan results demonstrate that prepared cells have two photon absorption (2PA) and self-focusing effects. As the result, the concentration of the NGQDs in E7 and power of laser were optimum at 1% wt. and 11 mW, respectively. The effect of a DC electric field on optical nonlinearity was also investigated in the mixtures of NLCs and NGQDs at the concentration of 1% wt. Based on these findings, this sample can be used in both nonlinear optics and ultrafast broadband photonics.

Promoting spintronic terahertz radiation via Tamm plasmon coupling

Spectral fingerprint and terahertz (THz) field-induced carrier dynamics demands the exploration of broadband and intense THz signal sources. Spintronic THz emitters (STEs), with high stability, a low cost, and an ultrabroad bandwidth, have been a hot topic in the field of THz sources. One of the main barriers to their practical application is lack of an STE with strong radiation intensity. Here, through the combination of optical physics and ultrafast photonics, the Tamm plasmon coupling (TPC) facilitating THz radiation is realized between spin THz thin films and photonic crystal structures. Simulation results show that the spectral absorptance can be increased from 36.8% to 94.3% for spin THz thin films with TPC. This coupling with narrowband resonance not only improves the optical-to-spin conversion efficiency, but also guarantees THz transmission with a negligible loss (∼4%) for the photonic crystal structure. According to the simulation, we prepared this structure successfully and experimentally realized a 264% THz radiation enhancement. Furthermore, the spin THz thin films with TPC exhibited invariant absorptivity under different polarization modes of the pump beam and weakening confinement on an obliquely incident pump laser. This approach is easy to implement and offers possibilities to overcome compatibility issues between the optical structure design and low energy consumption for ultrafast THz opto-spintronics and other similar devices.