Photoinduced Field Research Articles

Carrier-induced nonlinearities in InGaN/GaN quantum wells with V-pits

Abstract

Multifunctional behaviors of an indium tin oxide/PbLa(ZrTi)O3/indium tin oxide wafer illuminated by ultraviolet light

This article presents multifunctional behaviors of a Pb0.97La0.03(Zr0.52Ti0.48)O3 (3/52/48) wafer subjected to ultraviolet light illumination with a focus on its photoresistive effects. When this PLZT wafer that is spurted with indium tin oxide electrodes and then polarized through thickness is illuminated by ultraviolet light, its resistance increases rather than decreases as observed in conventional photoresistors made of semiconductors. Giant negative voltage and resistance are detected when light is switched off. The bending deformation caused by the photovoltaic and converse piezoelectric effects is examined and the photoinduced electrical field strength is further investigated. The electrical fields in light on–off states are studied. Hysteresis and memristive features of the indium tin oxide/PbLa(ZrTi)O3/indium tin oxide (ITO/PLZT/ITO) sample under repetitive light on–off operations are investigated.

Electron spectrometer in adjustable triode configuration for photo-induced field emission measurements

We have constructed a new ultrahigh vacuum apparatus with a triode configuration for the systematic investigation of photo-induced field emission (PFE) from metallic or semiconducting cathodes. These are exposed to electric fields up to 400 MV∕m and laser irradiation by means of hole or mesh gates. Cathodes and gates are in situ exchangeable and adjustable with high precision to ensure a homogeneous extraction of electrons which are partially transmitted to the fixed electron spectrometer. Its hemispherical sector analyzer provides an energy resolution limit of 8 meV. The commissioning of the measurement system has been performed with a tungsten needle. Its temperature showed up in the high-energy tail of the electron spectrum, while its work function was derived from the spectral low-energy part combined with the integral current-voltage curve. First PFE measurements on B-doped Si-tip arrays yielded a small field emission current increase under green laser illumination. A shift and splitting of the energy spectra was observed which revealed different emission regimes as well as the photosensitivity of the cathode due to carrier excitation into the conduction band. For the full exploitation of the PFE system, a tunable laser over a wide eV-range is required.

Photoinduced electron-transfer reactions and magnetic field effects on the decay rates of a photogenerated biradical from zinc porphyrin–viologen linked compounds in an ionic liquid

Fluorescence and transient absorption spectra of zinc-porphyrin (ZnP)–viologen linked compounds with various methylene groups indicate that intramolecular electron-transfer from the singlet or triplet excited state of ZnP to viologen occurred and a biradical was generated in the ionic liquid (1-butyl-3-methylimidazolium tetrafluoroborate) at various temperatures (283–343K). The decay rate constants of the biradical decreased in 0–0.2∼0.5T and became constant in 0.2∼0.5–1T. Unique effects of temperature and methylene chain length on electron-transfer and magnetic field effects were observed and are probably due to the properties of the ionic liquid as compared with benzonitrile.

Electrical Tuning of Single Nitrogen-Vacancy Center Optical Transitions Enhanced by Photoinduced Fields

We demonstrate precise control over the zero-phonon optical transition energies of individual nitrogen-vacancy (NV) centers in diamond by applying multiaxis electric fields, via the dc Stark effect. The Stark shifts display surprising asymmetries that we attribute to an enhancement and rectification of the local electric field by photoionized charge traps in the diamond. Using this effect, we tune the excited-state orbitals of strained NV centers to degeneracy and vary the resulting degenerate optical transition frequency by >10 GHz, a scale comparable to the inhomogeneous frequency distribution. This technique will facilitate the integration of NV-center spins within photonic networks.

Laminated plate formulation for photostrictive actuators and sensors

This paper presents a model for a laminated plate with surface mounted photostrictive materials. The photo-induced electrical, thermal, and mechanical fields in lanthanum zirconate titanate (PLZT) ceramics with 0-3 polarization are considered as the coupled multi-physics fields in PLZT ceramics. The formulation is presented for general multi-physics fields in PLZT ceramics. The actuating and sensing equations are formulated for a 0-3 polarized PLZT laminated plate and they are also applicable to other photostrictive materials such as polymers and semiconductors. The actuated behaviors of a 0-3 polarized PLZT laminated plate subjected to light illumination are investigated and the numerical results calculated using the present formulations are compared with those available in the literatures.

Reverse phenomena of magnetic field effects and time-resolved EPR spectra in the photogenerated biradical from intramolecular electron-transfer in a phenothiazine–C60 linked compound with a semi-rigid spacer

Photoinduced electron-transfer reactions and magnetic field effects (MFEs) on the decay rates of the photogenerated biradical in a phenothiazine (Ph)–C60 linked compound with a biphenyl group (Ph(BP)C60) were examined in benzonitrile and benzene. Fluorescence and transient absorption spectra indicate that the intramolecular electron-transfer for Ph(BP)C60 from the Ph to the singlet or triplet excited state of C60 was suppressed by the biphenyl group. The decay rates of the photogenerated biradical decreased in the 0–0.2 T magnetic field range and increased in the 0.2–1 T magnetic field range. The reverse phenomena of the MFEs in Ph(BP)C60 were strongly enhanced with increasing temperature and similar to those in Ph(n)C60 (n = 6−12). The MFEs in Ph(BP)C60 can be governed by spin-lattice relaxation and/or spin-spin relaxation mechanisms as observed in Ph(n)C60 (n = 6−12). Time-resolved EPR spectra of Ph(BP)C60 showed absorption, emission, absorption and emission patterns, and are quite different from those in Ph(n)C60 (n = 4−12). The result indicates that the magnitude and distribution of the exchange interaction |2J| in Ph(BP)C60 are smaller than those in Ph(n)C60 (n = 4−12) and charge recombination occurs in the inverted region because the sign of the J is positive.

Anti-phase domain structuring in oxide ferroelectrics: Physical properties and application

The results from studies of domain borders by the optical and acoustic topography methods in LiNbO3:Fe samples are presented. The formation of Fe ion concentration gradients, a change in the refractive index, and a gradient of the photoinduced field and parameters of acoustic waves near the domain borders were observed. It is shown that main reason for the distortion of crystalline structure parameters is the formation of photoinductive field gradients of spatially separated Fe2+ and Fe3+ ions.

New organic FET-like photoactive device, experiments and DFT modeling

We present the possible construction of an organic FET-like photoactive device in which source-drain current through a phthalocyanine ( H(2)Pc film is affected by a photo-induced dipolar field in a photoactive "gate" electrode. The influence of the dipolar electric field on charge transfer between H(2)Pc molecules is modeled by DFT quantum-chemical calculations on H(2)Pc dimers and tetramers.

Photoinduced electron-transfer and magnetic field effects on the dynamics of the radical pair in a C60 cluster–phenothiazine system

Le transfert photo-induit des electrons et les effets de champ magnetique dans le systeme clusters de C 6 0 derives (C 6 0 N + )-methylphenothiazine (MePH) ont ete etudies. Les clusters de C 6 0 N + ontete prepares par dissolution dans le solvant mixte THF-H 2 O. Le diametre des clusters a ete estime, par mesure de diffusion dynamique de la lumiere et AFM, comme etant de l'ordre de 100 nm. Les spectres d'absorption transitoire du systeme clusters de C 6 0 N + -MePH obtenus par excitation laser a 355 nm indiquent que le transfert intermoleculaire photo-induit des electrons s'effectue depuis l'etat triplet excite de la phenothiazine (PH) vers le cluster C 6 0 . Le rendement de formation du radical de la phenothiazine augmente avec le champ magnetique. Les effets de champ magnetique suggerent la generation d'une paire radicalaire triplet.

Kinetics of photoinduced changes in Ag nanoparticles deposited on an indium tin oxide surface

An enhancement of the UV-visible optical absorption spectra of Ag nanoparticles (NPs) deposited on indium tin oxide (ITO) surfaces of different resistance by a seed-mediated growth technique is presented. A bicolour coherent beam, obtained from a pulsed Nd:YAG laser providing a 1060-nm fundamental wavelength and a double-frequency one at 530 nm, was used. A significant change in the optical density is observed for Ag NPs on low-resistance ITO surfaces (≈4 Ω/square), while almost nothing occurs with high-resistance (≈50 Ω/square surfaces). The spectral position of the maximal absorption at 440 nm is almost preserved in both cases. This result is explained in terms of a local photoinduced static field and surface-plasmon polaritons interacting with photoexcited phonons. A grating effect, arising from the coherent bicolour light forming a superstructure as for quantum crystals, could be another contribution.

Ultrafast differential transmission spectroscopy of excitonic transitions in InGaN/GaN multiple quantum wells

Room-temperature carrier dynamics in InGaN/GaN multiple quantum wells are studied by employing ultrafast pump–probe spectroscopy. Specifically, the observed differential spectral signatures are characteristic of changes in the absorption coefficient through both a reduction of the quantum-confined Stark shift due to the photoinduced in-well field screening (low carrier densities) and excitonic absorption quenching (high carrier densities). The comparison of the differential absorption spectra at different injected carrier densities allows us to separate field screening from excitonic bleaching. The estimated in-well field at the transition point between field screening and excitonic bleaching is consistent with the theoretical value of the piezoelectric field in the strained InGaN well.

Non-contact evaluation of semiconductors using a laser SQUID microscope

The distribution of a photo-induced magnetic field from a p–n junction is measured by a laser SQUID microscope that consists of a high temperature superconductor SQUID magnetometer and a laser diode. The maximum magnetic field of several pico-tesla is detected at the depletion layer of the p–n junction. At the point where the laser spot and p–n junction have symmetry, the magnetic field reaches the minimum. The minority carrier diffusion length is also obtained by using two wavelengths of excitation light. The photon flux density ratio is measured in each wavelength so that the photo-induced magnetic field can be equal. The minority carrier diffusion length is related to the photon flux density ratio. The results are in good agreement with the value obtained by spreading resistance profiling method. Since the laser SQUID microscope can evaluate the minority carrier diffusion length without any contact, this method is useful for contamination free in-process monitoring of the p–n junction.

Femtosecond pump and probe spectroscopy of optical nonlinearities in an InGaN/GaN heterostructure

ABSTRACTThe magnitudes and evolutions of two photoinduced absorption nonlinearities, absorption bleaching and field screening, were compared and investigated by employing electroabsorption and femtosecond pump-probe spectroscopy in a biased InGaN/GaN p-i-n double heterostructure. Steady state electroabsorption measurements indicate the field induced absorption coefficient changes in this structure are caused by the Franz-Keldysh effect. The temporal resolution of the absorption bleaching spectra suggests that the photoinduced carriers rapidly relaxed to the InGaN band edge within several picoseconds. As the applied reverse bias field was increased, the transition of the differential absorption spectral signature from the signature for absorption bleaching to the signature for field screening was observed. The magnitude of the change in absorption due to photoinduced carrier screening of the applied field is quantified and compared to absorption bleaching.

Photo-induced magnetic field imaging of p–n junction using a laser SQUID microscope

We have constructed a laser superconducting quantum interference device (SQUID) microscope for imaging photo-induced magnetic fields by focused laser beam. This system can improve the spatial resolution for semiconductor samples at room temperature. Our system consists of the following essential components, a laser diode, a high temperature superconductor SQUID gradiometer, a piezo driving non-magnetic ceramics XY stage, a lock-in amplifier and a video system. Under the practical condition without using a magnetic shield, the p–n junction of single crystal silicon was measured. The magnetic field gradient (d B z /d x) of 600 fT/cm was obtained as images with spatial resolution of 20 μm. The laser SQUID microscope has the possibility of becoming a new tool for evaluation, because it offers noninvasive and non-contact access to semiconductors.

Dynamics of photoinduced field screening; THz-pulse and second harmonic generation from semiconductor surface

The equation to describe the evolution of a space charge electric field in a semiconductor after an ultra-short laser pulse excitation is extended to be valid for both drift-diffusion and collisionless transport regimes. Numerical calculations of this equation performed for a semiconductor surface depletion region show that the character of the surface electric field screening varies from monotonic to oscillatory with increasing photoexcitation level. It is found that THz-pulse emission from the excited surface depletion layer is determined by the second time derivative of the surface potential, and its waveform shows bipolar or oscillatory behaviour depending on whether drift-diffusion or collisionless transport regime is realized. The depletion field induced second harmonic (SH) contribution is analysed.

A pump and probe study of photoinduced internal field screening dynamics in an AlGaN/GaN single-quantum-well structure

Photogenerated carrier dynamics in an AlGaN/GaN single quantum well has been studied using a conventional degenerate pump and probe technique at room temperature. Photoinduced absorption at the exciton resonance has been observed. It is explained by the absorption coefficient change, through the quantum-confined Stark effect and the quantum-confined Franz–Keldish effect, caused by the photoinduced internal electric-field screening. In comparison with biased GaAs multiple quantum wells, a slower time evolution of differential transmission signals has been also found. Its origin is attributed to the longer carrier sweep-out time due to the potential profile of the sample in conjunction with the longer carrier recombination time.

Photoinduced domains in lithium niobate

We have observed the optically induced occurrence of domain structure with polarization opposite to the spontaneous polarization of a single-domain sample of lithium niobate. The appearance of the domains can be explained by a redistribution of the density of the Fe2+ and Fe3+ impurity ions under the action of the photoinduced field and subsequent additional inverse polarization of the environment surrounding the Fe2+ ions.

Temporal and spectral dynamics in multiquantum-well semiconductor saturable absorbers

Performing femtosecond pump-probe measurements on multiquantum-well (MQW) semiconductor saturable absorbers we find a strong spectral dependence of the absorption dynamics on a picosecond time scale. While for long wavelengths a pump pulse induced reduction of the absorption is observed, at short wavelengths the absorption increases due to the pump. We explain this behavior on the basis of a model including photo-induced field screening.

Influence of the internal photoinduced electric field on the formation of self-pumped phase conjugation in doped (K_05Na_05)_02(Sr_075Ba_025)_09Nb_2O_6 crystals

The obvious influence of the internal photoinduced electric field on the formation of self-pumped phase conjugation in doped (K(0.5) Na(0.5))(0.2) (Sr(0.75) Ba(0.25))(0.9) Nb(2)O(6) crystals has been investigated with intermittent and continuous laser beams. An analysis is given that involves the migration of charge carriers driven by the photovoltaic (PV) and the pyroelectric (PY) electric field. Deviation from the optimal state of energy transfer owing to the PV effect and the self-adjusting of holograms owing to the PY field are the two important dominant processes in the self-pumped phase conjugator. The influence of the photoinduced electric field on the conjugation fidelity and the stability of the phase-conjugate output is also observed and analyzed. The bump feature of the two-wave mixing and the oscillating situation of the self-pumped phase conjugator are observed and are explained in view of the photoinduced internal field.