Laser-induced Thermoelectric Voltage Research Articles

Transverse laser-induced thermoelectric voltages in tilted La2−xSrxCuO4 thin films

The transverse laser-induced thermoelectric voltages in the tilted La2−xSrxCuO4 (LSCO) thin films are observed for the first time. The detected signals are demonstrated to originate from the anisotropy of thermoelectric power and depend on the Sr-doping level drastically. The largest voltage is observed in the LSCO films at x=0.15 under the irradiation of different lasers with the wavelength in the spectrum range from infrared to ultraviolet. The dependence of the signals on Sr-doping content is associated with a two dimensional charge transport behavior in LSCO films near the optimum doping content.

Improving the performance of thermoelectric devices by doping Ag in LaPbMnO 3 thin films

A series of Ag-doped La 0.6Pb 0.4MnO 3 thin films were grown on vicinal cut substrates by pulsed laser deposition (PLD). Laser-induced thermoelectric voltages (LITV) had been observed in these films, and these LITV signals had been demonstrated to originate from the anisotropic Seebeck effect. By doping Ag to an optimum value, it was found that the peak values ( U P) of the LITV signals were maximized, and the full-width at half-maximum ( τ) of the response curves of LITV were minimized at the same time. The figure of merit ( F m) of the device used as photodetector is greatly improved by doping Ag in La 0.6Pb 0.4MnO 3 thin films. The possible reason for these improvements had been well discussed.

Influence of Load Resistance on Time Constants of Laser-Induced Thermoelectric Voltage Signal in ${\rm La}_{1\hbox{-}x}{\rm Ca}_{x} {\rm MnO} _{3}$ and ${\rm YBa} _{2}{\rm Cu}_{3} {\rm O} _{7\hbox{-}y}$ Thin Films

It is found that in laser-induced thermoelectric voltage (LITV) signal in La1-xCaxMnO3 and YBa2Cu3O7-y.H thin films, both the rise-time and decay time are increasing linearly with the total parallel-connected resistance of the load and the film. To obtain fast laser-induced thermoelectric voltage signal, smaller load should be connected in parallel with he film.

A novel time-integral type laser energy meter based on anisotropic Seebeck effect

We have posed the design of a time-integral type laser energy meter based on anisotropic Seebeck effect for the first time. Anisotropic Seebeck effect is responsible for the laser-induced thermoelectric voltage effect in high temperature superconductor (HTSC) cuprates and colossal magnetoresistance (CMR) manganites thin films grown on tilted single crystal substrates. In this study, for an example, an epitaxial La 2/3Ca 1/3MnO 3 thin film prepared on a tilted LaAlO 3 substrate by standard pulsed-laser deposition (PLD) method is tested with a 1064-nm Q-switched Nd:YAG laser and its 2nd (532 nm), 3rd (355 nm), and 4th (266 nm) harmonics from room temperature to 16 K. The integral of the voltage signal with time shows a good linear relation with the laser energy per pulse in the measured wavelength and temperature range, which confirms the theoretical analysis given in this letter and can be used to design a time-integral type laser energy meter. The sensitivity increases as the film thickness increases or as the thermal diffusion constant decreases, which makes the time-integral type laser energy meter low cost as compared with the peak-voltage type. It operates with fast (nanosecond range) and broad-spectrum (from infrared to ultraviolet) response in wide temperature range (from room temperature to 10s K), and can be useful replacements for pyroelectric power/energy meters.

Influence of annealing oxygen pressure on the laser-induced thermoelectric voltage effect in YBa2Cu3O7-x thin films

The laser-induced thermoelectric voltage (LITV) signals were measured in the YBa2Cu3O7-x (YBCO) films annealed under different oxygen pressure. It was found that the detected LITV signals were enhanced 2—4 times with the increasing of annealing oxygen pressure. The effect can be explained by the dependence of thermoelectric anisotropy on the oxygen content in YBCO. When the films were illuminated by continuous lasers in the wavelength range of 473—808nm， the measured signals in the films annealed under 5000Pa oxygen pressure were larger than those in other films. Meanwhile， pulse ultraviolet laser illumination produced the largest signal in the films annealed under oxygen pressure of 105Pa. The response time of LITV signals was observed to become shorter with the increasing of annealing oxygen pressure， which was shown to have resulted from the increasing of thermal conductivity theoretically. The films annealed under 5000Pa oxygen pressure had the fastest response to the pulsed ultraviolet laser. The response time was 70ns and the rise time of 29ns was comparable to the duration of the applied laser pulse (about 28ns).

Oxygen pressure effect on lattice parameters of SrTiO3 and SrNb0.2Ti0.8O3 thin films and laser-induced thermoelectric voltages effect

SrTiO3 and SrNb0.2Ti0.8O3 thin films were prepared by pulsed laser deposition technique on LaAlO3 (100) single crystal substrates. X-ray diffraction shows that all the thin films have［001］ orientation. With the oxygen pressure increasing, the lattice parameter c of SrTiO3 thin films decreases, while the lattice parameter c of SrNb0.2Ti0.8O3 thin films decreases and then increases. Meanwhile, the optimal oxygen pressure to prepare (SrTiO3/SrNb0.2Ti0.8O3)L multilayer of two-dimensional electron gas was found to be 1.0×10-2Pa. In addition, laser-induced thermoelectric voltage effect was observed on SrNb0.2Ti0.8O3 thin film prepared on vicinal-cut LaAlO3 (100) substrate for the first time.

Enhanced Phonon Scattering in Oxide Superlattices to Improve Laser Induced Thermoelectric Voltages

AbstractOptimizing the figure of merit for thermoelectric applications, ZT = S2σ2T/κ is currently at the core of materials oriented research in thermoelectricity. Here, one promising approach is to reduce ther thermal conductivity without sacrificing the electrical conductivity. Constructing superlattices of structurally compatible materials is one way to accomplish this goal. We report an enhanced laser induced thermoelectric voltage (LITV) effect observed in (YBa2Cu3O7/La1-xPbxMnO3)n multilayer thin films for the first time. Two groups of multilayer thin films grown on vicinal cut LaAlO3 substrates were prepared by pulsed laser deposition technique. The first group were grown on different substrates vicinal cut at different angles, and were used for checking the mechanism of the induced voltages. The second group samples were made at different period number n and for studying the number dependence of the peak values of LITV. The substrate angle dependence proved that this is a thermoelectric effect [1]. It was found that the LITV signals were enhanced significantly for these multilayer thin films comparing with the single layer ones. It is natural that the conductivity is going to be anisotropic due to the layered structure, and the same holds for the Seebeck coefficients. The enlarged Seebeck anisotropy will lead to higher induced voltages. Another possible reason is the reduced thermal conductivity in the layered structure. The maximum enhancement of LITV signals takes place at period number of 7, which seems in agreement with the prediction of minimum thermal conductivity in superlattices by Simkin and Mahan.

Laser-induced thermoelectric voltage in normal state MgB 2 thin films

Laser-induced voltage has been observed in c-axis oriented MgB 2 thin film at room temperature. The amplitude of the signal is approximately proportional to the film thickness. For the film with the thickness of 150 nm, a very fast response has been detected when the film was irradiated by a 308 nm pulsed laser of 20 ns duration. The rise time and full width at half-maximum of the signal are about 3 and 25 ns, respectively. The physical origin of the laser-induced voltage can be attributed to a transverse thermoelectricity due to the anisotropic thermopower in MgB 2.

Laser induced thermoelectric voltage effect in La0.67Pb0.33MnO3 thin films doped with Ag

The La0.67Pb0.33MnO3 (LPMO) thin films doped with Ag on vicinal-cut LaAlO3 substrates were prepared using standard pulsed laser deposition technique, in which laser induced thermoelectric voltage (LITV) effect was found. With Ag doping level x (x is defined as,x=massAg/massLPMO) increasing from 0.00 to 0.10, the response time of LITV signals in LPMO thin films firstly decreases (reaching minimum at 0.06), and then increases (still less than that with zero Ag doping). Further, an optimal film thickness was found, at which the peak voltage and response time of LITV signals to reaches maximum and minimum, respectively. The response time of LITV in the LPMO thin films are substantially smaller than that of La0.67Ca0.33MnO3 thin films at the same Ag doping level.

Figure of Merit for Detectors Based on Laser-InducedThermoelectric Voltages in La1−xCaxMnO3 andYBa2Cu3O7−σ Thin Films

A figure of merit (FOM) Z = Up/τr, where Up is the peakvoltage, and τr is the rise time of the laser-inducedthermoelectric voltage (LITV) signal, is defined for photodetector basedon the LITV and the influence of the parameters on FOM is analysed basedon the time dependence of LITVs in La1−xCaxMnO3 (LCMO)and YBa2Cu3O7−σ (YBCO) thin films grown on vicinal-cutsubstrates. We find that the FOM increases as the photon penetrationdepth decreases, and linearly increases with the thermal diffusionconstant D. To achieve the highest FOM, the film thickness d has to becontrolled to an optimum value. We also find that the FOM is directlyproportional to the laser absorption coefficient α0, thelaser energy density per pulse E, the illuminated length of film l,sin(2α) [αis the vicinal-cut angle], the Seebeckcoefficient anisotropy (Sab−Sc), and is inverselyproportional to the mass density ρ and the specific heat c0.

Optimum thickness for laser induced thermoelectric voltage effect in high Tc superconductor and CMR thin film

Based on the formula of the time dependence of laser-induced voltage in high Tc superconductor(HTSC) and collosal magneto-resistance (CMR) thin films, we have successfully deduced the function of the optimum thickness for producing largest laser-induced thermoelectric voltage. It is indicated that a direct proportional relationship between laser-induced thermoelectric voltage and the reciprocal of thickness does not correctly describe the situation. In fact, an optimum thickness must exist. This function provides the main factor of affecting the design for fast-time-response device. We have discussed the parameter of thin film influencing the optimum thickness, and compared it with experimental result.

Time dependence of laser-induced thermoelectric voltages in La1−xCaxMnO3 and YBa2Cu3O7−δ thin films

The time dependence of laser-induced thermoelectric voltage (LITV) in La1−xCaxMnO3 (LCMO) or YBa2Cu3O7−δ (YBCO) thin films grown on a vicinal SrTiO3 substrate is derived. From the fitted curve of the measured time evolution of the LITV, several parameters of a LCMO thin film are deduced.

Laser-induced off-diagonal thermoelectric voltage in La 1− xCa xMnO 3 thin films

Laser induced off-diagonal thermoelectric voltages were observed in La 0.67Ca 0.33MnO 3 thin films deposited on vicinal cut SrTiO 3 substrates, as well as in films of other doping levels. The dependence of this voltage on photon fluence, illuminated length of the film, film thickness and orientation are in remarkable analogy to the results reported for YBa 2Cu 3O 7− x thin films. That suggests an identical physical mechanism in both materials which based on the existence of off-diagonal elements in the Seebeck tensors. It demonstrates that an intrinsically anisotropic charge transport exists in these series of materials, in spite of the quasi-cubic crystal structure of the base material LaMnO 3. From the results obtained on films with a systematic variation of the doping level and materials, the anisotropy of the Seebeck tensor is suggested to be a contribution of the long-range cooperative Jahn–Teller distortions.

Laser-induced anisotropic thermoelectric voltages in thin films

Transverse thermoelectric voltages have been observed to occur when thin Mo and W films are excited by pulsed laser light at normal incidence to the film. Wavelengths in the range 0.46–1.06 μm and pulse widths of ∼ 3–300 nsec were used. A maximum thermoelectric voltage of ∼50 mV occurring across a load resistance of 50 Ω for an ∼ 1-kW incident laser pulse has been observed. A correlation between intrinsic film stress and output voltage suggests that the stress gives rise to a nonscalar absolute thermoelectric power (Seebeck coefficients) even though the transport properties of these bulk materials are isotropic.