A tunable ultra-broadband and ultra-high sensitivity far-infrared metamaterial absorber based on VO2 and graphene.

We proposed a far-infrared tunable metamaterial absorber using vanadium dioxide (VO2) and graphene as controlling materials. The properties of the absorber are investigated theoretically using the finite-difference time-domain (FDTD) technique. It was found that when the Fermi energy level of graphene is fixed at zero, VO2 is in the insulated state, and the metasurface exhibits far-infrared broadband absorption performance, with absorptance exceeding 90% in the wavelength range of 12.6 µm to 23.2 µm. In addition, by elevating the Fermi energy level of graphene, the absorption bandwidth of the device is expanded continuously. When the VO2 is in the metallic state, the device can flexibly transform into a far-infrared narrowband absorber. The device also has the advantage of being insensitive to changes in polarization and incident angle. The origin of the absorption and the tuning principle of the device were analyzed and verified successfully by using an equivalent circuit model (ECM). Besides, we also studied the refraction index sensing characteristics of the absorber. Surprisingly, the absorber exhibits excellent sensing characteristics, and its sensitivity (S) reaches 14.108 µm per RIU and the figure of merit (FOM) is 6.13 per RIU.

Difunctional terahertz metasurface with switchable polarization conversion and absorption by VO2 and photosensitive silicon.

In this study, a terahertz (THz) switchable multifunctional metasurface is proposed to realize polarization conversion and absorption. When vanadium dioxide (VO2) is in the dielectric state, the structure demonstrates polarization conversion with double broadband transmission. The transmittance of the double broadband is greater than 80% in the frequency ranges of 2.05-2.38 THz and 3.38-3.68 THz, and the polarization conversion rate (PCR) is greater than 90%. Excellent asymmetric transmission (AT) properties are exhibited by polarization conversion. The transmittance of the double broadband can be modulated dynamically by the pump light by controlling the conductivity of the photosensitive silicon (PS). When VO2 is in the metallic state, the metasurface is switched to be a bidirectional THz absorber for TE and TM wave incidence, and the maximum absorptance of the absorber can reach more than 95%. Furthermore, the absorption is insensitive to the angle of incidence, and the absorption frequency and intensity can be dynamically tuned by changing the polarization angle. By changing the conductivity of the PS, the intensity and frequency of the absorption can also be adjusted. Using the metasurface, we achieved a dynamic multiplexing imaging function for linearly polarized waves. The metasurface showed a new vision for multifunctional THz devices and exhibited a wide application prospect in the field of THz imaging.

A switchable and tunable multifunctional terahertz polarization converter based on a graphene metasurface.

We propose a switchable and tunable terahertz metasurface polarization converter based on graphene. The metasurface is composed of a bottom gold film, a lower SiO2 layer, an intermediate N-type graphene layer, an upper SiO2 layer, and a top layer of square graphene resonant rings. By using the CST Microwave Studio, we studied their polarization conversion properties. The results show that the metasurface enables a versatile range of functions, including x-to-y linear polarization conversion within the frequency range of 4.23-7 THz, the transformation of linearly polarized waves into right-hand circularly polarized and left-hand circularly polarized waves. Moreover, at 3.873 THz, the metasurface can exhibit circular dichroism (CD) with a high CD value of 0.7. Based on the effect, a strategy to detect three representative avian influenza viruses was proposed and tested, which implies that the metasurface can be applied in biosensing.

Ultrafast and low-power multichannel all-optical switcher based on multilayer graphene.

A metal-insulator-metal waveguide structure composed of a hexagonal resonator cavity and a ring with a slit is proposed. By using the finite difference time domain method, the transmission properties of the structure were studied. It was found that three distinct plasmon-induced transparency peaks appear in the visible and near-infrared bands, and the transmissivity of the three peaks is more than 80%. By tuning the structure size, the positions of the peaks can be adjusted. Then we introduced graphene, covering the surface of the cavity. By adjusting the refraction index of the graphene using light, the position of the three transmission peaks can be changed correspondingly. Based on the effect, we designed an all-optical switcher with ultrafast optical response time (about 2 ps) and low light absorption (about 2.3%). The proposed waveguide structure provides a way for the development of visible and near-infrared filters and all-optical switchers.

Size effect on nonlinear optical properties and ultrafast dynamics of silver nanoparticles.

Z-scan technology was used to study the nonlinear absorption (NLA) and nonlinear refraction (NLR) of silver nanoparticles (Ag NPs) with various sizes under different laser intensities. The results demonstrate that the NLA and NLR of Ag NPs were size-dependent. Specifically, the 10 nm Ag NPs exhibit saturation absorption (SA) and insignificant NLR. The 20 and 40 nm Ag NPs show the coexistence of SA and reverse saturation absorption (RSA). SA is believed to result from ground-state plasma bleaching, whereas RSA originates from excited state absorption (ESA). The 20 nm and 40 nm Ag NPs shows increasing self-defocusing with the increase of laser intensity. It was observed that the energy relaxation of Ag NPs mainly includes two processes of electron-phonon and phonon-phonon couplings on the order of picoseconds.

Double transformation of the nonlinear absorption in silver nanoparticles.

The nonlinear absorption of 40 nm Ag nanoparticles (Ag NPs) was investigated using open aperture (OA) Z-scan technique at 532 nm. Experiments show that the nonlinear absorption behavior of Ag NPs is intensity dependent. Specifically, under low laser energy the Ag NPs shows saturable absorption (SA). At medium laser energy, the transformation of nonlinear absorption from SA to reverse saturable absorption (RSA) happens. While under stronger laser energy, double transformation (SA→RSA→SA) of nonlinear absorption occurs. The experimental results were analyzed theoretically using a model based on single-photon absorption and two-photon absorption saturation.

Ultrafast nonlinear absorption with multiple transformations and transient dynamics of gold nanobipyramids.

The process and condition of saturable absorption (SA) and reverse saturable absorption (RSA) of ultrafast nonlinear optics in metal nanoparticles are essential for applications including light generation, amplification, modulation, and switching. Here, we first discover and explore the multiple transformations (SA-RSA-SA) of ultrafast nonlinear absorption behavior of metal nanoparticles in femtosecond pulses. Correspondingly, the energy level model and fitting formula of multiple transformations are established to illustrate the process of optical response. The femtosecond transient absorption spectra provide information about their ultrafast dynamics process and vibrational mode, which further reveals the multiple transformation mechanisms of nonlinear absorption in gold nanobipyramids (Au-NBPs). Furthermore, Au-NBPs exhibit a significantly higher SA modulation depth up to 42% in the femtosecond, which is much higher than the reported values of other nanomaterials. Our results indicate that Au-NBPs can be used as broadband ultrafast Q-switching and mode-locking, and the conversion offers new opportunities for metal nanostructures in applications of optical switching.

Tunable dual-band mid-infrared absorber based on the coupling of a graphene surface plasmon polariton and Tamm phonon-polariton.

We propose and demonstrate a tunable dual-band mid-infrared absorber structure based on the coupling effect of a surface plasmon polariton (SPP) and Tamm phonon-polariton (TPhP). The structure is composed of the distributed Bragg reflector (DBR), air layer, SiC and graphene ribbons. In the air layer, the graphene ribbons are embedded to realize the localized SPP (LSPP), which makes the structure support both the graphene LSPP (GLSPP) and TPhP. The absorption properties of the structure are investigated theoretically and numerically. It is found that strong coupling of the GLSPP and TPhP can be realized by choosing reasonable parameters, which causes a dual-frequency perfect absorption and makes the maximum Rabi splitting of the coupled mode reach 5.76 meV. Furthermore, the mode coupling and absorption intensity can be tuned by adjusting the thickness of the air layer and the Fermi level of the graphene ribbons. This work might provide new possibilities for the development of mid-infrared band sensors, filters and emitters based on the coupling of multiple modes.

Broadband terahertz absorber with tunable frequency and bandwidth by using Dirac semimetal and strontium titanate.

A bifunctional broadband absorber in the terahertz band based on patterned bulk Dirac semimetal (BDS) and strontium titanate (STO) is proposed. The properties of the absorber are investigated using the finite-difference time-domain (FDTD) method. The results show that the width of absorption can be modulated from 0.59 THz to 0.7 THz when the Fermi energy of the BDS is independently shifted from 40 meV to 50 meV. By tuning the temperature from 250 K to 400K, the center frequency of the broadband absorption spectrum can be changed from 1.311 THz to 1.505 THz, and the absorption bandwidth broadens from 0.66 THz to 0.81 THz. In addition, the simulation results show that the absorber is insensitive to electromagnetic wave polarization, and can still maintain a stable broadband absorption effect when the oblique incidence is within 40° for TE and TM modes. Based on the impedance matching theory, the physical mechanism of the broadband absorption is analyzed theoretically. This work can provide an alternative way to design high-performance multifunctional tunable terahertz devices.

Detection of GDF11 by using a Ti3C2-MXene-based fiber SPR biosensor.

In the research of resistant aging, the concentration of Growth differentiation factor-11(GDF11) is an indispensable parameter. So the accurate detection of GDF11 is very important in life science and medical cosmetology. Hereby, we proposed and demonstrated a simple method to detect low concentration GDF11 by using fiber surface plasmon resonance (SPR) sensor decorated with two-dimension (2D) material Ti3C2-MXene and gold nanosphere. The sensitivity of the fiber SPR sensor was increased to be 4804.64nm/RIU. After functionalized with GDF11 antibody, the fiber SPR sensor could specifically recognize GDF11, and the limit of detection (LOD) can reach 0.577pg/L which is 100 times lower than that of single-molecule ELISA method.

A study of the microwave actuation of a liquid crystalline elastomer.

We present a method for actuating LCE materials by microwave radiation. The microwave actuation performance of a polysiloxane-based nematic liquid crystalline elastomer (LCE) was investigated. The microwave-material interaction caused a dipolar loss, which created a heating effect to trigger the nematic-isotropic transition of the LCE matrix, thus leading to the deformation actuation of the LCE material. This energy conversion from radiant energy to thermal energy provided a contactless pathway to actuate the LCE material without the aid of other components acting as energy converters. The LCE demonstrated rapid maximum contraction upon microwave irradiation, and this microwave-stimulated response was fully reversible when the microwave irradiation was switched off. More importantly, the microwave actuation exhibited superiority relative to photo-actuation, which is the usual method of contactless actuation. The microwaves can penetrate the opaque thick barriers to effectively actuate the LCE due to their strong penetrability; they can also penetrate multiple LCE samples and actuate them almost simultaneously. By taking advantage of the salient features of microwave actuation, a microwave detector system, implementing the LCE as an actuator material, was fabricated. This demonstrated the performance of monitoring microwave irradiation intensities with good sensitivity and convenient manipulation.

Fiber SPR refractive index sensor with the variable core refractive index.

In this paper, a refractive index sensor based on the control of the fiber core refractive index is proposed. By employing ultraviolet curable adhesive with the different refractive index and hollow capillary fiber, the special fiber with a variable core refractive index is fabricated. Using the special fiber, a novel, to the best of our knowledge, refractive index surface plasmon resonance (SPR) sensor with a controllable detecting range of refractive index is realized. Functional testing of the sensing probes with the core refractive indices of 1.590, 1.516, and 1.454 is performed respectively, indicating that their sensitivities are 1580 nm/RIU, 2220 nm/RIU, and 3467 nm/RIU, respectively, and their detecting ranges of refractive index are 1.385-1.435 RIU, 1.365-1.415 RIU, and 1.335-1.385 RIU, respectively. Furthermore, in order to explore the detection effect of the sensing probe with the higher-core refractive index, we conducted theoretical calculation using the Kretchmann model. The experimental and simulating results indicates that, with the increase of the core refractive index, the magnitude of refractive index that can be detected increases. This study provides a new method for the detection of high refractive index solutions and a new idea for the fabrication of wavelength-division multiplexing distributed SPR sensors.

Wavelength-dependent nonlinear absorption and ultrafast dynamics process of Au triangular nanoprisms.

The nonlinear absorption and ultrafast dynamics process of Au triangular nanoprisms were investigated by using broadband (ranging from 550 to 700 nm) nanosecond Z-scan measurements and femtosecond time-resolved transient absorption spectrum, respectively. We found that Au triangular nanoprisms exhibit saturation absorption (SA) at low excitation intensities. With the increase of incident intensity, a switch from SA to reverse saturation absorption (RSA) occurs. Photo-dynamics process was found to be a double-exponential energy relaxation with a fast and a slow decay component. Interestingly, when probe wavelength is away from the plasma resonance peak, the decay of relaxation also shows the modulation due to the vibration mode of the coherent excitation.

Photo actuation of liquid crystalline elastomer nanocomposites incorporated with gold nanoparticles based on surface plasmon resonance.

In this work, according to the characteristic of surface plasmon resonance (SPR) of metallic nanoparticles, we investigated the photo actuation performance of a liquid crystalline elastomer (LCE) nanocomposite with incorporated gold nanoparticles (nano-gold/LCE nanocomposite). The nano-gold/LCE nanocomposites were fabricated by incorporating gold nanoparticles into a polysiloxane-based LCE matrix via a novel experimental protocol, and characterized by a well-developed SPR absorption band in the visible spectrum range. The nano-gold/LCE nanocomposites demonstrated strong actuation upon irradiation with a quasi-daylight source; the reason lay in that the SPR response of gold nanoparticles performed efficient energy conversion from light energy to thermal energy, and thus offered an activation pathway for the nematic-isotropic transition of the LCE matrix. The nano-gold/LCE nanocomposites underwent rapid maximum axial contraction up to about one third of the original length under light irradiation, and this photo-stimulated muscle-like actuation was fully reversible via the on-off switching of the light source. The photo actuation properties of nano-gold/LCE nanocomposites with varying irradiation intensities and gold nanoparticle content were also investigated. In addition, the nano-gold/LCE nanocomposites demonstrated superior optical nonlinear properties, and revealed potential for the application area of mode-locking for laser technology.

Ultrafast optical properties of type-II CdZnS/ZnSe core-shell quantum dots.

In this study, the ultrafast optical properties of type-II CdZnS/ZnSe core-shell quantum dots were investigated using the Z-scan and transient absorption technique with femtosecond pulses. With 800-nm wavelength excitation, the CdZnS/ZnSe quantum dots exhibited two-photon absorption, and the two-photon absorption cross section was obtained as about 3.37 × 106 GM. In addition, the transfer time of electrons and the recombination lifetime of a single exciton were obtained. For the photoluminescence of the CdZnS/ZnSe quantum dots at temperatures from 80 to 280 K, the peak position redshifted by 60 meV, width broadened by 3 meV, and intensity decreased by a factor of four.

Ultrafast spectroscopic studies of composition-dependent near-infrared-emitting alloyed CdSeTe quantum dots.

In this study, optical and structural characterizations of near-infrared-emitting alloyed CdSeTe quantum dots (QDs) are measured after the dissolution in toluene. Luminescence spectra are obtained from alloyed CdSeTe QDs under 800 nm femtosecond laser excitation. With increasing pump fluence, the line width or full width at half maximum (FWHM) of photoluminescence (PL) spectrum becomes larger than 10 nm due to increasing temperature. Ultrafast spectroscopic properties of CdSeTe QDs are investigated by means of time-resolved PL, transient absorption (TA) and Z scan techniques. Moreover, open-aperture (OA) Z scan measurement is used to clarify the composition and pump fluence dependence of optical nonlinearity under femtosecond laser excitation. With increasing pump fluence, evolution from saturable absorption to reverse saturable absorption in CdSeTe QDs is observed. The transition process is analyzed via a phenomenological model based on nonlinear absorption coefficient and saturation intensity, which indicates that CdSeTe QDs have potential for applications in all-optical switching devices.

Fiber birefringence measurement by an external applied strain method and a polarimetric fiber laser sensor.

In this work, a high-sensitivity and low-cost sensing scheme for measuring intrinsic and induced fiber birefringence change is reported based on a polarimetric fiber laser sensor interrogated by the beat frequency technique. The fiber birefringence measurement is achieved by an external applied strain method. A detailed theoretical analysis of the principle for fiber birefringence measurement is carried out. Two alternative equations are given for determining the change of fiber birefringence, which make it very convenient for users to choose different order beat signals. To verify the performance of the sensing system, the external applied strain-induced fiber birefringence change is measured experimentally. The experiment result shows that the fiber birefringence experiences a linear increase with the increase of applied strain. A strain response coefficient of 4.646×10-11/µÏµ is obtained. Furthermore, the repeatability and stability performances of the polarimetric fiber laser sensor are also investigated.

Ethylenediamine-modified graphene oxide covalently functionalized with a tetracarboxylic Zn(ii) phthalocyanine hybrid for enhanced nonlinear optical properties.

Tetracarboxylic Zn(ii) phthalocyanine-amino functionalized graphene oxide (ZnPcC4-NGO) hybrid materials have been prepared by a covalent functionalization method. The characterizations indicate that the amino-functionalization of GO has an important influence on the structure and photophysical properties of the ZnPcC4-NGO hybrid. The ZnPcC4-NGO hybrid exhibits enhanced photo-induced electron transfer or energy transfer (PET/ET), compared to the ZnPcC4 covalent functionalized GO (ZnPcC4-GO), owing to the presence of the extended sp(2) carbon configurations, along with the partial reduction of the NGO nanosheets and the introduction of electron-donating ethylenediamine. The nonlinear optical (NLO) properties of the hybrids were investigated using the Z-scan technique at 532 nm with 4 ns laser pulses. The results show that the efficient covalent functionalization and partial reduction of NGO cause the ZnPcC4-NGO hybrid to possess evidently larger NLO properties than the individual NGO, ZnPcC4 and the ZnPcC4-GO hybrid. The enhanced NLO performance can be attributed to the increased excited state absorption from the extended sp(2) carbon configurations of the NGO moiety, reverse saturable absorption arising from ZnPcC4 moiety, and the contribution of the efficient PET/ET process between the ZnPcC4 and NGO moieties in the hybrid.

The effects of central metals on the photophysical and nonlinear optical properties of reduced graphene oxide-metal(II) phthalocyanine hybrids.

Reduced graphene oxide-metal(II) phthalocyanine (RGO-MPc, M = Cu, Zn and Pb) hybrid materials have been prepared by the covalent functionalization method. The resultant RGO-MPc hybrids are characterized by scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared, ultraviolet-visible absorption and fluorescence spectroscopy. The RGO-MPc hybrids exhibit strong fluorescence quenching by means of the photo-induced electron transfer or the energy transfer (PET/ET) process between the RGO and MPc moieties. The PET/ET process particularly depends on the fluorescence quantum yield of MPc molecules with different central metals. The nonlinear optical (NLO) properties of the RGO-MPc hybrids are investigated by using the Z-scan technique at 532 nm with 4 ns laser pulses. The results show that the NLO properties of MPc molecules increase in the order of Zn < Pb < Cu, but the RGO-MPc hybrids exhibit NLO performance in the inverse sequence of Zn > Pb > Cu, implying that the NLO response arising from the efficient PET/ET process between RGO and MPc may play a more important role in the NLO properties of RGO-MPc hybrids than that originating from the MPc moiety.

A side-illuminated plasmonic planar lens.

Planar lens based on nanoscale slits has been demonstrated theoretically and experimentally. In this paper, we propose a 2D model of a similar planar lens but with side-illumination. The lens consists of a main bus waveguide to transport plasmonic wave and grooves functioning as antennas. The shapes and filling materials of the waveguide and grooves are assumed to be invariant in the third direction. The phase retardation needed for wavefront shaping comes from the transverse propagation of the plasmonic wave in the waveguide and the well-designed groove positions. The concept is applied to the design of planar lenses and axicons. The simulation results demonstrate that such structures can work as good diffractive elements. The side-illumination property of such structure enables the potential integration of lens on chip.