Yachen Gao

Optical temperature sensing in β-NaLuF4:Yb3+/Er3+/Tm3+ based on thermal, quasi-thermal and non-thermal coupling levels

Three methods for optical temperature sensing are investigated in the NaLuF4:Yb3+/Er3+/Tm3+ phosphor, which is prepared by a hydrothermal method. The temperature-dependent luminescence is investigated under 980 nm excitation. Utilizing fluorescence intensity ratio (FIR) technique, the temperature sensing behaviors are studied in the range of 300–600 K based on thermal coupling levels (2H11/2 and 4S3/2 of Er3+), “quasi-thermal coupling levels” (4F7/2 of Er3+ and 3F2 of Tm3+) and non-thermal coupling levels (1D2 and 1G4 of Tm3+), respectively. The maximum sensitivity is 604 × 10−4 K−1 at 300 K, which is based on non-thermal coupling levels of Tm3+. Meanwhile, the temperature dependent emission colors are discussed, which are changed from bluish green to light blue with the temperature rising. These significant results, high sensitivity and the temperature dependent multicolor emissions, indicate that the NaLuF4:Yb3+/Er3+/Tm3+ phosphor is robust for optical temperature sensing.

Temperature and composition dependent excitonic luminescence and exciton-phonon coupling in CdSeS nanocrystals

The yellow- and red-emitting CdSeS nanocrystals (NCs) synthesized through one-step organometallic synthesis method are uniformly assembled in polymethyl methacrylate (PMMA). A higher-energy emission band originates from band-edge excitonic state appeared at low temperature. With the Se dopant concentration increasing, the luminescent spectra of CdSeS NCs have a red-shifted emission peak and a shorter luminescent lifetime, which is attributed to the existence of trapping state caused by surface defect and Se dopant. CdSeS NC shows a shorter luminescence lifetime and higher energy emission peak in PMMA matrix than that in toluene, indicating that the former is more favorable to transfer energy through exciton-phonon coupling. The upconversion luminescence (UCL) is observed using 800 nm femtosecond laser excitation. The pump power dependence demonstrated UCL spectra of yellow-emitting CdSeS NCs has a slope of 2.2, while that of red-emitting CdSeS NCs has a slope of 1.4. The results demonstrate that the two-photon absorption plays a dominating role when Se concentration of CdSeS NCs is lower, while phonon-assisted UCL by one-photon excitation gradually takes place with the amount of Se dopants increasing.

Stark sublevels of Er3+–Yb3+ codoped Gd2(WO4)3 phosphor for enhancing the sensitivity of a luminescent thermometer

We report a strategy for enhancing the sensitivity of an optical thermometer by utilizing the Stark sublevels. Under 980 nm excitation, the upconversion emission originating from the Stark levels of Er3+ (4S3/2(2)/4S3/2(1) and 4F9/2/4F9/2(1)) is observed in a monoclinic phase Gd2(WO4)3:Er3+/Yb3+ phosphor, which is synthesized through the co-precipitation method. The temperature sensing behavior is studied over the range of 296–620 K based on thermal coupling levels 2H11/2/4S3/2(2), 2H11/2/4S3/2, 2H11/2/4S3/2(1), 4S3/2(2)/4S3/2(1) and 4F9/2(2)/4F9/2(1) using fluorescence intensity ratio (FIR) technology. The sensitivity based on Stark sublevels 2H11/2/4S3/2(2) or 2H11/2/4S3/2(1) is almost twice more than that based on the traditional 2H11/2/4S3/2 levels. The obtained maximum sensitivity of the optical thermometer is 16.5 × 10−3 K−1 at 395 K (2H11/2/4S3/2(2)). These results suggest that the use of Stark levels is a promising approach for enhancing the sensitivity of optical thermometers.

Nonlinear Absorptions of CdSeTe Quantum Dots under Ultrafast Laser Radiation

The oil-soluble alloyed CdSeTe quantum dots QDs are prepared by the electrostatic method. The basic properties of synthesized CdSeTe QDs are characterized by UV-Vis absorption spectroscopy, photoluminescence spectroscopy, inductively coupled plasma mass spectrometry, and transmission electron microscope. The off-resonant nonlinear optical properties of CdSeTe QDs are studied by femtosecond Z-scan at 1 kHz low-repetition rate and 84 MHz high-repetition rate. Nonlinear absorption coefficients are calculated under different femtosecond laser excitations. Due to the long luminescent lifetime of CdSeTe QDs, under the conditions of high-repetition rate, for open-aperture curve, heat accumulation and bleaching of ground state are responsible for the decrease of two-photon absorption TPA coefficient.

The role of sp2/sp3 hybrid carbon regulation in the nonlinear optical properties of graphene oxide materials

Rational regulation of localized sp2/sp3 hybrid carbon structure in graphene oxide systems plays a very important role in developing advanced carbon-based hybrid materials. Here, we report a simple ethanol solvothermal method toward precise control of the growth of the sp2 hybrid carbon configurations/clusters in the sp3 carbon matrix so as to regulate the structure of electronic energy bands in the graphene oxide system. The results of morphology observation, XPS, solid-state 13C MAS NMR, FT-IR and Raman spectroscopy proved that controllable generation of the sp2 hybrid carbon configurations/clusters can be achieved based on an executive oxidation/reduction strategy. Upon excitation by a 532 nm laser with 4 ns pulses, the obtained reduced graphene oxide (160-rGO-6) with a large number of sp2 hybrid carbon configurations displays greater nonlinear reverse saturable absorption response and a higher nonlinear absorption coefficient β of 560 cm GW−1 than graphene oxide with different oxidation degree (GO-X, X = 6, 8, 10 and 12), and reduced graphene oxide with relatively few sp2 hybrid carbon configuration ratios (Y-rGO-6, Y = 80, 100, 120, 140 and 180). The significantly enhanced nonlinear reverse saturable absorption of 160-rGO-6 is attributed to the two photon absorption and excited state absorption originating from the sp2 hybrid carbon configuration system.

Carrier dynamics and optical nonlinearity of alloyed CdSeTe quantum dots in glass matrix

Size and pump-fluence dependent ultrafast carrier dynamics of CdSeTe QDs are investigated using femtosecond pump-probe techniques operating at two different repetition rates: 1 kHz (low-repetition rate), and 76 MHz (high-repetition rate). With a low-repetition rate laser and 3.1 eV excitation photon energy, multiple exciton generation (MEG) is observed and the optical responses of alloyed QDs clearly show three components: a fast decay ascribed to carrier recombination, an intermediate component associated with MEG decay, and a slow decay associated with radiative Auger recombination. With a high-repetition rate laser and excitation photon energy resonant with band-edge energy, obvious coherent phonon oscillations are observed in 4 nm CdSeTe QDs due to impulsive stimulated Raman scattering. Open-aperture Z-scan measurement is used to clarify the size and pump-fluence dependence of optical nonlinearity under femtosecond laser excitation. With increasing laser power, an evolution from saturable absorption to reverse saturable absorption in CdSeTe QDs is observed. The transition process is analyzed using a phenomenological model based on nonlinear absorption coefficient and saturation intensity. These results indicate that CdSeTe QDs in a glass matrix are a class of materials for potential application in all-optical switching devices.

Reducing the actuation threshold by incorporating a nonliquid crystal chain into a liquid crystal elastomer

Liquid crystal elastomers (LCEs) are important smart materials that can undergo reversible deformation in response to liquid crystal (LC) phase transitions. A low threshold temperature for LC phase transition is advantageous because the LCE material can be more conveniently actuated by the applied stimulus. In this work, we investigated the effect of a nonliquid crystal chain on the reduction of threshold temperature of the LC phase transition by linking a nonliquid crystal side chain, 4-methoxyphenyl-1-hexenyloxy (MOCH3), to the network backbone of a classical polysiloxane-based side-chain nematic LCE. The nematic–isotropic transition temperature (Tni) of the MOCH3 incorporated nematic LCE was lower than that of the normal nematic LCE without the incorporation of a nonliquid crystal chain by about 27 °C. Compared to the normal nematic LCE or its nanocomposite, the MOCH3 incorporated nematic LCE or its nanocomposite demonstrated more rapid thermo-actuated deformation or photo-actuated deformation, and can be actuated to attain full axial contraction at an obviously lowered temperature or by light with obviously lowered intensity, while the maximum contraction ratio basically did not vary. These research results indicate that some nonliquid crystal chains show potential for improving the characteristics and enhancing the application significance of LCE materials.

Femtosecond laser induced ripples and nanohole arrays on silicon

We investigate the formation of ripple and nanohole induced by femtosecond laser pulses on the surface of silicon. Periodic ripples aligned perpendicular to the direction of laser polarization has been observed. The period of the periodic ripples decreases with the increasing pulse number. Particularly aperiodic ripples with orientation parallel to the laser polarization are formed depend on the number of laser pulses and energy. The nanohole arrays are formed on the overlapped areas of periodic and aperiodic ripples. The interference between the surface scattered or excited wave and the laser itself is proposed to explain the decrease of ripple period.

Multi-beam laser heterodyne measurement with ultra-precision for Young modulus based on oscillating mirror modulation

This paper proposes a novel method of multi-beam laser heterodyne measurement for Young modulus. Based on Doppler effect and heterodyne technology, loaded the information of length variation to the frequency difference of the multi-beam laser heterodyne signal by the frequency modulation of the oscillating mirror, this method can obtain many values of length variation caused by mass variation after the multi-beam laser heterodyne signal demodulation simultaneously. Processing these values by weighted-average, it can obtain length variation accurately, and eventually obtain value of Young modulus of the sample by the calculation. This novel method is used to simulate measurement for Young modulus of wire under different mass by MATLAB, the obtained result shows that the relative measurement error of this method is just 0.3%.

Optical limiting of platinum nanoparticle stabilized by C 60 derivative

The optical limiting performance of platinum nanoparticle protected by C60 derivative was investigated by using 8-ns laser pulses at a wavelength of 532nm. The investigation indicated the sample could provide enhanced optical limiting even better than benchmark optical limiting material, fullerene C60 dissolved in toluene. The origins of the strong optical limiting were discussed based on reverse saturable absorption and nonlinear scattering.