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Correction for ‘Extended visible to near-infrared harvesting of earth-abundant FeS2–TiO2 heterostructures for highly active photocatalytic hydrogen evolution’ by Tsung-Rong Kuo et al., Green Chem., 2018, 20, 1640–1647.

Coherent acoustic phonons are generated at terahertz frequencies when semiconductor quantum-well nanostructures are illuminated by femtosecond laser pulses. These phonons-also known as nanoacoustic waves-typically have wavelengths of tens of nanometres, which could prove useful in applications such as non-invasive ultrasonic imaging and sound amplification by the stimulated emission of radiation. However, optical diffraction effects mean that the nanoacoustic waves are produced with spot sizes on the micrometre scale. Near-field optical techniques can produce waves with smaller spot sizes, but they only work near surfaces. Here, we show that a far-field optical technique--which suffers no such restrictions--can be used to spatially manipulate the phonon generation process so that nanoacoustic waves are emitted with lateral dimensions that are much smaller than the laser wavelength. We demonstrate that nanoacoustic waves with wavelengths and spot sizes of the order of 10 nm and 100 nm, respectively, can be generated and detected.

An ideal freestanding graphene in air is centrosymmetric in three dimensions, and its optical second harmonic generation is inhibited. However, we found strong second harmonic generation from suspended grapheme sheets, and attributed this observation to curved sheet in the long range.

In the field of optical microscopy, it is well known that spatial resolution is limited by diffraction of light. There have been variously efforts to achieve resolution beyond diffraction limit. However, most previous methods rely on nonlinearities of fluorescence, and thus require high-intensity lasers or special labeling strategies. In 2011, a novel superresolution technique based on a dielectric microlens and simple bright field microscope was demonstrated. The scheme is unexpectedly simple, since neither labeling nor intense laser is necessary, while the resolution is significantly higher than diffraction limit. Nevertheless, the contrast of bright field microscope is poor. In this work, we combine a dielectric microlens along with confocal laser scanning microscopy to considerably enhance image contrast. By simply inserting a microsphere onto the sample, the resolution is undoubtedly better than diffraction limit of the objective. Meanwhile, the contrast exhibits almost one order of enhancement. Field of view and magnification of the microlens imaging system are also characterized. Comparing with bright field microscope, laser scanning confocal microscopy provides better contrast under this microslens assisted super-resolution scheme. Our finding will contribute to material science and biomedicine research.

Coherent acoustic phonons were generated in the piezoelectric bulk semiconductor through inverse piezoelectric effect by femtosecond laser pulses. While the photocarriers are generated in the depletion region of the piezoelectric semiconductor, the electrons and holes sweep in counter-directions due to the built-in electric field and result in strain pulses through the piezoelectric effect. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The use of a horn sonicator (Misonix, 20 kHz) to recover the flux in the ultrafiltration (UF) of suspended solutions was studied. Two types of solutions, i.e., the Cu2+–polyethylenimine (PEI) solution and the W/O emulsions dispersed in aqueous solution, were selected, which were widely found in binding-UF and liquid surfactant membrane (LSM) processes, respectively, for removal of toxic heavy metals from effluents. The Amicon YM10 membrane (regenerated cellulose, MWCO 10,000) was used. Experiments were performed as a function of tip position, ultrasonic power, and solution properties such as target ion concentration, percentage of emulsification, and volume ratio of W/O emulsions to the external aqueous phase. It was shown that ultrasound associated with a hydraulic pressure of 10 psi was a promising method for the recovery of UF flux, particularly for the solution with less fouling potential. Careful control of the ultrasonic intensity could keep the membranes durable and prevent the organic compounds (PEI in the Cu2+–PEI solution; carrier, surfactant, and solvent in the W/O/W solution) from degradation. Finally, the fouling of membrane during UF of W/O/W solutions was analyzed by the resistance-in-series model.

Liquid surfactant membrane (LSM) process is a novel and effective method for the recovery and separation of heavy metals and amino acids from aqueous solutions. Repeated production and destruction of W/O emulsions plays a crucial role in the LSM process. The aim of this work was to study the efficiency of emulsification with horn ultrasound (20 kHz), in which the organic phase contained the surfactant Span 80 and the acidic carrier di(2-ethylhexyl)phosphoric acid (D2EHPA) in kerosene. The (internal) aqueous phase consisted of divalent Cu(II)-EDTA chelated anions as the tracers. All experiments were performed as a function of emulsification time, surfactant and carrier concentrations, ultrasonic power, and volume ratio of the internal aqueous phase to organic phase. It was shown that in most cases the emulsification was finished with 1 min. With ultrasound, the droplet sizes of W/O emulsions (Sauter diameter, d32) were much smaller than those reported previously by mechanical agitation. The effect of ultrasonic power and surfactant concentration on the droplet size was also investigated. Finally, the possibility of ultrasound demulsification was evaluated and discussed.

We investigate the optical transmission oscillation of multiple quantum wells (MQWs) under the modulation of coherent acoustic phonons. We treat the coherent acoustic phonons as a semiclassical wave obeying continuum elastic equations. Starting from the microscopic electron–phonon interaction Hamiltonian, we obtain expressions for optical absorption modulation of the MQWs due to coherent acoustic phonons. The acoustic phonons introduce a renormalization to single-particle energy and furthermore modify the resonant condition of photon absorption. The optical transmission modulation can be conveniently expressed with the use of sensitivity functions. We derive the analytical expressions for the sensitivity functions by which we calculate the optical transient transmission changes. The calculated results are in good agreement with the experimental observations.

We present a method to obtain complete information of femtosecond pulses. By measuring triple-optical autocorrelation directly with third-harmonic generation, without spectral information, a temporal pulse shape can be obtained by analytical calculation without direction-of-time ambiguity. Combining the resulting optical pulse shape with its corresponding optical spectrum, the exact phase and color variations in time can all be recovered with a Gerchberg-Saxton algorithm through an iterative calculation with an O(n) complexity.

We utilize ultrafast optical spectroscopy to study the quasiparticle relaxation in stoichiometric LiFeAs crystals. According to our temperature-dependent studies, we have observed three electronic phases in LiFeAs. Below the superconducting (SC) temperature ${T}_{c}$ (=15 K), the relaxation time of quasiparticles due to the SC gaps is far longer than 50 ps in our experimental conditions. In addition to SC gaps, we have also found a gaplike feature in the SC state. This gaplike feature is evident up to 40 K, which is above the SC temperature. The quasiparticle relaxation time due to this gaplike feature is in the range of 1--2 ps. We suggest the gaplike feature in LiFeAs is induced by magnetic fluctuations.

Due to the high attenuation in vitreous silica, acoustic attenuations in the THz regime are typically measured by incoherent techniques such as Raman, neutron, and X-ray scattering. Here, we utilized multiple-quantum-well structures to demonstrate acoustic spectroscopy of vitreous silica up to THz regime. The acoustic properties of silica thin films prepared by chemical deposition methods were characterized in the sub-THz regime. This technique may be useful in resolving debated issues relating to Boson peak around 1 THz.

Optical microscope has played an important role in the many fields, such as biology and material study. Because of the wave nature of light, the resolution of optical microscope is limited by diffraction limit, about λ/2 in practice, and that prohibits one from observing more detailed structure of the sample. In the last decade,, some techniques have been developed to overcome the limit, including stimulated emission depletion microscopy (STED), near-field scanning optical microscopy (NSOM), superlens, etc.

We demonstrate stable and tunable light emission in ultraviolet to near infrared regime by using annealed SiOx sample. By adjusting the ratio of Si and O of SiOx, different wavelengths such as ultraviolet, visible and near infrared photoluminescence can be tuned. From the results of transmission electron microscope, various sizes (1~4 nm) of the embedded Si nanoparticles were formed. Nanoparticles with smaller sizes were indeed formed for UV-blue emitting samples and the origin of light emission may be misattributed to the quantum confinement effects. However, we found the efficient and stable light emission in UV-blue regime, with lifetime on the order of nanoseconds, is dominantly from the defects.

We studied acoustic breathing modes of GaN nanorods on silicon substrates by ultrafast acoustic phonon spectroscopy. This technique could be utilized to nondestructively inspect uniformity of nanorods within the area of optical spot.

We present a study on the total internal reflection of nano-acoustic waves (NAWs) at the air∕GaN interface. The coherent NAW was generated and detected by piezoelectric InGaN∕GaN multiple quantum wells using a femtosecond transient transmission technique. With suitably designed sample structures, the fact that strain of the NAW experiences a sign change after total internal reflection at the air∕solid interface is examined directly. The surface roughness of the sample was found to distort the wave front of the NAW and to diminish the measured amplitude of the reflected NAW.

Huge excitonic enhancement of two-photon absorption near the exciton-transition energy was observed in the GaN system. The peak value of the nonlinear absorption coefficient is at least 1500 cm/GW, corresponding to an enhancement factor of >100. The room temperature exciton dephasing time is also obtained based on a spectral analysis to be ∼150 fs in bulk GaN, which is close to the exciton ionization time.

FePt nanoparticles (NPs) have recently been revealed to be significant multifunctional materials for the applications of biomedical imaging, drug delivery and magnetic hyperthermia due to their novel magnetic properties. In this study, a newly discovered photothermal effect activated by the near infrared (NIR) femtosecond laser for FePt NPs was demonstrated. The threshold laser energy to destroy cancer cells was found to be comparable to that of gold nanorods (Au NRs) previously reported. Through the thermal lens technique, it was concluded that the temperature of the FePt NPs can be heated up to a couple of hundreds degree C in picoseconds under laser irradiation due to the excellent photothermal transduction efficiency of FePt NPs. This finding boosts FePt NPs versatility in multifunctional targeted cancer therapy.

CdS, CdSe sensitized solar cells were investigated with excitation-wavelength-dependent, time-resolved photoluminescence technique and charge transport model. This approach is helpful to design semiconductor-sensitized and other types of solar cells.

Hole capture time in p-type GaN was measured by using a femtosecond pump-probe technique. The capture time constant that holes are trapped by Mg-related states was found to be shorter than 10 ps. The hole capture time increases with decreased hole excess energy. By comparing two samples with different doping concentrations, it was found that the hole capture time also decreases with increased doping concentrations.

Triple optical autocorrelation of femtosecond optical pulses was realized simply with third-harmonic-generation technique. This optical technique provides complete knowledge of transient pulse intensity variation directly in time domain. Only analytic calculation is needed to obtain the pulse-shape from data without direction-of-time ambiguity. Combined with a spectral measurement and the Gerchberg–Saxton algorithm, except for pulses with complete temporal and spectral symmetry that will cause a twofold ambiguity, exact phase variation in time can also be retrieved through an iterative calculation with an O(n) complexity.

We present a theoretical model for the photogeneration of coherent acoustic phonons in a piezoelectric p-n junction. In our model, the transport of photoexcited carriers is governed by the drift-diffusion equation, whereas the dynamics of acoustic phonons obeys a loaded string equation. Among various mechanisms, the piezoelectric coupling is found to dominate the acoustic-phonon generation process. The waveform of the photogenerated acoustic pulse is strongly influenced by the various dynamics of the photoexcited carriers, especially the picosecond hole drifting. Our calculation also confirms the crucial role of the built-in electric field in the formation of coherent acoustic phonons under optical excitations.

Spectral characteristics of laser-generated acoustic waves in an InGaN/GaN superlattice structure are studied at room temperature. Acoustic vibrations in the structure are excited with a femtosecond laser pulse and detected via transmission of a delayed probe pulse. Seven acoustic modes of the superlattice are detected, with frequencies spanning a range from 0.36 to 2.5THz. Acoustic waves up to ∼2THz in frequency are not significantly attenuated within the transducer which indicates excellent interface quality of the superlattice. The findings hold promise for broadband THz acoustic spectroscopy.

We demonstrate irregular scattering structure frequency-dependent field control at terahertz frequencies by means of a TM(10) to TM(30) mode converter designed for operation near 300 GHz and fabricated out of lithium niobate. Imaging of the electric fields in the sample, with a Fourier analysis of the time domain signal, yielded the performance as a function of frequency.

We review our current development on nano-ultrasonic by using piezoelectric nano-layers based on the GaN material system. Our study indicated that piezoelectric semiconductor nanostructures can serve as optical piezoelectric transducers to generate and detect nanoacoustic waves through the piezoelectric effect. The nanoacoustic waves, with a wavelength on the order of or shorter than 10 nm, can be used for high accuracy THz electron control, noninvasive subnanometer interface probing, and nano-ultrasonic imaging. Detailed design principles and the applications to the study of lattice/molecular dynamics in condense matters are discussed.

We report a direct determination of the specular scattering probability of acoustic phonons at a crystal boundary by observing the escape of incident coherent phonons from the coherent state during reflection. In the sub-THz frequency range where the phonon wavelength is much longer than the lattice constant, the acoustic phonon-interface interaction is found to agree well with the macroscopic theory on wave scattering from rough surfaces. This examination thus quantitatively verifies the dominant role of atomic-scale corrugations in the Kapitza anomaly observed at 1-10 K and further opens a new path to nondestructively estimate subnanoscale roughness of buried interfaces.

We apply newly developed, phase-sensitive imaging to enable sharply focused visualization of terahertz waves in electro-optic media. This approach allows quantitative characterization of THz waves as they interact with microstructures on or in the sample, yielding new insight into these interactions.

Based on the RGB synthesis technique, a novel colored pulse lasers photography is designed for high speed imaging. A high quality color image can be acquired by assembling the three channels of red, green, and blue images which are acquired from three cameras with three independent laser light sources.

In this work, we review our recent development on nanoultrasonics based on an optical piezoelectric transducer. By embedding strain patterns in piezoelectric nano-layers and by manipulating optical field intensity in temporal and spatial domains, THz nanoacoustic waves with an acoustic wavelength on the order of or shorter than 10 nm can be generated with a nanometer-scaled lateral spot size, much smaller than the excitation optical wavelength. Noninvasive subsurface ultrasonic imaging can thus be realized with a nanometer resolution.

Probing length-dependent thermal conductivity of a given material has been considered as an important experimental method to determine the length of ballistic thermal conduction, or equivalently, the averaged phonon mean free path (l). However, many previous thermal transport measurements have focused on varying the lateral dimensions of samples, rendering the experimental interpretation indirect. Moreover, deducing l is model-dependent in many optical measurement techniques. In addition, finite contact thermal resistances and variations of sample qualities are very likely to obscure the effect in practice, leading to an overestimation of l. We point out that directly investigating one-dimensional length-dependent (normalized) thermal resistance is a better experimental method to determine l. In this regard, we find that no clear experimental data strongly support ballistic thermal conduction of Si or Ge at room temperature. On the other hand, data of both homogeneously-alloyed SiGe nanowires and heterogeneously-interfaced Si-Ge core-shell nanowires provide undisputed evidence for ballistic thermal conduction over several micrometers at room temperature.

We have experimentally obtained the reflection transfer function of a phononic bandgap nano-crystal including phase information by using a single-quantum-well in a phonon cavity. Our time-domain investigations help to resolve the apparent ldquosuperluminalrdquo paradox.

We experimentally demonstrate that acoustic-phonon attenuation in water described by a classic continuum-elastic model fails for sub-THz acoustic frequencies. A dynamic measurement technique for ice-water interface position with a ~10nm resolution will also be presented.

We have demonstrated an optical coherent control technique to generate nano-acoustic waves in InGaN single-quantum-well with tunable acoustic frequency. The acoustic phonon lifetime of the generated nano-acoustic wave was also measured.

Piezoelectric quantum wells can be treated as opto-acoustic transducers to generate and detect acoustic waves with nanometer wavelengths. This opto-acoustic transducer was utilized for 1D ultrasonic scan with 1 nanometer demonstrated resolution.

Piezoelectric semiconductor strained layers can be treated as piezoelectric transducers to generate nanometer-wavelength and THz-frequency acoustic waves. The mechanism of nano-acoustic wave (NAW) generation in strained piezoelectric layers, induced by femtosecond optical pulses, can be modeled by a macroscopic elastic continuum theory. The optical absorption change of the strained layers modulated by NAW through quantum-confined Franz-Keldysh (QCFK) effects allows optical detection of the propagating NAW. Based on these piezoelectric-based optical principles, we have designed an optical piezoelectric transducer (OPT) to generate NAW. The optically generated NAW is then applied to one-dimensional (1-D) ultrasonic scan for thickness measurement, which is the first step toward multidimensional nano-ultrasonic imaging. By launching a NAW pulse and resolving the returned acoustic echo signal with femtosecond optical pulses, the thickness of the studied layer can be measured with

Piezoelectric semiconductor with heterostructure can be treated as a piezoelectric transducer for the generation of acoustic waves with wavelength less than 10 nm (nano-acoustic waves) by optical technique. This optical piezoelectric transducer has also been utilized for the detection of the nano-acoustic waves (NAW). In this paper, we discuss the generation, detection, and propagation of nano-acoustic waves in piezoelectric semiconductors. We demonstrate that the acoustic frequency of the NAW can be tuned by an optical control technique. Besides, we have also developed an acoustic sensor with THz bandwidth which can be used to study NAW propagation control devices such as nano-phononic bandgap crystal. We demonstrated that the roughness of an interface can be evaluated by the NAW with a resolution less than 1 nm through the acoustic phasefront distortion effect. With the optical piezoelectric transducer, nano-ultrasonics, which is analogous to typical ultrasonics but on the nanometer scale, has been successfully developed.

We observed ultrafast electron distribution modulation following the strain oscillation of coherent acoustic nanowaves up to a THz frequency range. Through piezoelectric effects, the acoustic nanowave modifies the electronic subband structure, and further modulates the energy distributions of electrons and holes. With amplitude-fixed acoustic waves, a pronounced enhancement of transmission modulation due to acoustic modulation of electron distribution was observed when the optically probed energy position is close to the Fermi level.

Comparisons between second-harmonic-generation based frequency resolved optical gating (SHG FROG) and triple-optical autocorrelation for direct pulse-shape measurement (TOAD), have been made by computer simulation and experiments. Results show that TOAD has better accuracy than FROG.

Multiple quantum well (MQW) structure piezoelectric semiconductor can be treated as a piezoelectric transducer to generate nanometer wavelength and THz frequency acoustic waves. The generation mechanism of nano acoustic wave (NAW) in quantum wells induced by femtosecond optical pulses can be modeled by a macroscopic elastic continuum theory. The absorption of the MQW's modulated by NAW's through quantum confined Franz-Keldysh (QCFK) effect allows another femtosecond optical probe pulses to monitor the propagating NAW. Many applications in typical ultrasonics can be achieved by NAWs. The simultaneous waveform synthesis is demonstrated by an optical coherent control technique. The phase of the totally reflected NAW is studied. Acoustic coherent control can be achieved by designing the thickness of the cap layer on the MQW. We also demonstrated the feasibility to apply THz NAWs to acoustically control an electronic device with higher operation speed and spatial accuracy. Seismology, which is the first step toward ultrasonic imaging, was also demonstrated. The arrival time of the echo is obtained by processing the transient transmission changes of the probe. Ultrafast technique and nano technology are ready for nano ultrasonics.

By generating coherent nano acoustic waves in InGaN/GaN multiple-quantum-wells and analyzing their echo signals from the sample-air interface, we studied the propagation and reflection properties of the THz acoustic strain waves in GaN

Summary form only given. GaN-based semiconductors have potential applications for optoelectronic devices in the UV-blue wavelength regions and electronic devices capable of operation at high-temperature and high-power conditions. There are great efforts devoted to improve the hole properties in p-type GaN. Presently, p-type GaN doping is achieved mostly with Mg impurities. Defect-related transitions in GaN:Mg will strongly affect its hole behaviors in p-type GaN and are crucial for high-performance device applications. Here we report our investigation on the hole capture time into shallow Mg-related impurities. Ultrashort capture time constants of 10-80 ps depending on Mg doping concentration and excess hole energy are determined.

Summary form only given. Direct third-harmonic-generation (THG) implementation was first realized on a modelocked Cr:forsterite laser with the method of Third Order Autocorrelation for Direct optical pulse-shape measurement (TOAD). In this report, we demonstrate this technique on the commonly used Ti:sapphire modelocked laser for the first time. By using THG process in a single GaN thin film or a thin BBO crystal, the intensity profile of the optical pulses was directly obtained with the background free third-order autocorrelation trace. In addition, we employed the Gerchberg-Saxton algorithm to retrieve the corresponding phase by the obtained intensity and an additional spectral measurement.

We review some experimental and theoretical aspects of coherent acoustic phonon generation in piezoelectric semiconductor multiple quantum wells. In order to model more advanced and complicated nano-acoustic devices, a macroscopic continuum theory for the generation and propagation of coherent acoustic phonons in piezoelectric semiconductor heterostructures is presented. The macroscopic approach is applicable in the coherent regime, and can be easily utilized to study coherent acoustic devices based on piezoelectric semiconductor heterosutructures. For each phonon mode, the corresponding coherent acoustic field obeys a loaded string equation. The driven force has contributions from the piezoelectric and deformation potential couplings. We applied the theory to model the generation of coherent longitudinal acoustic phonons in (0001)-oriented InGaN/GaN multiple quantum wells. The numerical results are in good agreement with the experimental ones. By using the macroscopic theory, we also investigated the crystal-orientation effects on the generation of coherent acoustic phonons in wurtzite multiple quantum wells. It was found that coherent transverse acoustic phonons dominate the generation for certain orientation angles., 10 pages, REVTeX, 8 figures, Paper presented at SPIE Photonics WEST 2003, Ultrafast Phenomena in Semiconductors VII, San Jose, California

Multiple harmonic components have been observed in the Fourier spectra of the coherent acoustic phonon oscillations in InGaN/GaN multiple quantum wells (MQW's). The dependence of the relative magnitudes of the first two Fourier components on the sample geometry was analyzed by conducting time-resolved pump-probe measurements on MQW samples with different well-to-barrier ratios. The results were also compared with the recently proposed loaded-string model [Phys. Rev. B 64, 235316 (2001)], and an overall agreement is obtained which validates the picture of string oscillation with inhomogeneous loadings. We also demonstrated manipulation of the spectral components using the coherent optical control technique. Our demonstration can be regarded as a first step to the realization of wave-form synthesis of nanoacoustic waves.

Huge nonlinear-absorption enhancement was observed in GaN system at the excitonic transition wavelength. This excitonic enhancement is attributed to four-wave-mixing type nonlinear processes. The peak nonlinear coefficient is at least 3000 cm/GW corresponding to an enhancement factor of >200. The dephasing time of the exciton was also observed to be 60-160 fs in GaN thin film at room temperature.

Second harmonic coherent acoustic phonon oscillations were generated in strained InGaN/GaN multiple quantum wells using an optical coherent control technique. This demonstration leads to the nano acoustic waveform synthesis based on optical coherent controls.

We demonstrated an ultrafast electron distribution modulation up to THz frequency by using nano acoustic waves in InGaN/GaN multiple-quantum-wells. With amplitude-fixed acoustic waves, the electron energy distribution modulation was observed by using femtosecond optical technique.

We present ultrafast time-resolved optical spectroscopy on GaN nanorods at room temperature. The studied GaN nanorods, with diameters of ∼50 nm and lengths of ∼400 nm, were grown on the silicon substrate. After femtosecond optical pulses excited carriers in the GaN nanorods, the carriers thermalized within a few picoseconds. Subsequently, the electrons are trapped by the surface states on the order of 20 ps. After the surface electric field was reformed in the GaN nanorods, we found the lifetime of the residue carriers in GaN nanorods is longer than 1.7 ns at room temperature, while the lifetime of carriers in GaN thin film is typically a few hundred picoseconds. Our findings indicate that GaN nanorods have higher electrical quality compared with GaN thin film.

We have experimentally investigated the optical second harmonic generation (SHG) on suspended single-layer and bi-layer graphene sheets. By shining normally incident 800-nm light with polarization along the sample planes, the SHG intensities of single-layer and bi-layer graphene are found to be comparable to the one of polar GaAs with large second order susceptibility, which is unexpected because both have the centrosymmetric property. Our experimental results reveal that the strong SHG is not due to the defects breaking the symmetry. Instead, we suggest that the SHG signals result from the long-range curvature fluctuations on the suspended single-layer and bi-layer graphene sheets.