Excitation Laser Intensity Research Articles

A Raman spectroscopy study of MBE-grown Hg1−xCdxSe alloys grown on GaSb (2 1 1) by molecular beam epitaxy

This work presents a Raman spectroscopy study of Hg1−xCdxSe alloys grown on GaSb (2 1 1) substrates by molecular beam epitaxy. For the Raman spectra of Hg1−xCdxSe (0 ≤ x ≤ 0.6) measured with low laser excitation intensity, no sample damage is observed and the Raman spectra are dominated by the HgSe-like phonon modes. However, with a higher laser excitation intensity, material damage in the form of void formation is observed due to Hg loss and the resultant Raman spectra are dominated by CdSe-like phonon modes. For a constant laser excitation intensity the material damage was found to increase with increasing Cd content due to weakening of the Hg-Se bond in the presence of Cd. A separate Raman study undertaken on the cross-section of a compositionally graded HgCdSe sample clearly indicates that the integrated Raman intensity exhibits an essentially linear dependence on Cd content for the composition range, 0.30 ≤ x ≤ 0.45. The results of this study provide a useful guide for the development of any thermal procedure to be used in any detector fabrication process, as well as indicating that Raman spectroscopy has the potential to be developed into a general purpose tool capable of assessing HgCdSe compositional uniformity.

Investigation of the factors affecting the photothermal therapy potential of small iron oxide nanoparticles over the 730-840 nm spectral region.

The use of superparamagnetic iron oxide nanoparticles (SPIONs) as a sensitizer in photothermal therapy (PTT) is relatively new and the origin of such a phenomenon is not known. Usually, large crystals and aggregated particles are preferred in the literature, suggesting that these increase the absorbance of particles at the irradiation wavelength, and hence, provide a larger temperature increase. This study has two major goals: identification of the key factors that affect the photo-induced temperature increase in well-controlled experiments and the influence of laser irradiation on nanoparticle properties. Small, biocompatible poly(acrylic acid) coated SPIONs (PAA/SPIONs) were used since they are more practical for future medical use than large aggregates. We studied the impact of three major laser-dependent variables, namely the wavelength (between 728 and 838 nm), intensity (1.85-9.76 W cm-2) and power (105-800 mW) as well as attenuation at the irradiation wavelength, on photothermal heating achieved with PAA/SPIONs. Within the studied range of these variables, only the laser power plays a critical role on the magnitude of photothermal heating in solutions. There is no strong correlation between the attenuation at the excitation wavelength and the temperature increase. In addition, extensive characterization of SPIONs before and after irradiation revealed no significant difference, which supports the re-usability of SPIONs. Lastly, the PTT potential of these small PAA/SPIONs was demonstrated in vitro on HeLa cells. At these low laser powers no temperature increase in SPION-free water or cell death in SPION-free cells was detected. Hence, this study provides a new insight into the photothermal effect of SPIONs, provides a clear and repeatable experimental procedure and demonstrates great potential for small SPIONs to be exploited in PTT.

Charge and thermal modeling of a semiconductor-based optical refrigerator

Despite multiple attempts to achieve optical refrigeration in very high (99.5%) external quantum efficiency (EQE) GaAs, no cooling has been observed to date. In this study, we investigate optical refrigeration in GaAs by numerically solving the transient drift-diffusion equation coupled to Poisson's equation. The charge carrier distributions we obtain, together with the heat diffusion equation, allow us to observe the spatial and temporal evolution of cooling/heating within GaAs. Our results indicate that maximum cooling occurs at a laser intensity different from that which maximizes EQE. An 11-fold difference in intensity exists with a corresponding 6-fold difference in cooling power. We ultimately find that samples suspended in vacuum using a 250 μm SiO2 fiber cool to 88 K, starting from room temperature. These results emphasize the critical importance of choosing an appropriate laser excitation intensity to achieve optical refrigeration along with minimizing the conductive heat load on the refrigerator. Furthermore, results of this study are applicable towards analyzing the optical response of other optoelectronic systems where accurate charge and/or heat diffusion modeling is critical.

Millimeter-Scale Nonlocal Photo-Sensing Based on Single-Crystal Perovskite Photodetector

SummaryOrganometal trihalide perovskites (OTPs) are promising optoelectronic materials for high-performance photodetectors. However, up to now, traditional polycrystal OTP-based photodetectors have demonstrated limited effective photo-sensing range. Recently, bulk perovskite single crystals have been seen to have the potential for position-sensitive photodetection. Herein, for the first time, we demonstrate a position-dependent photodetector based on perovskite single crystals by scanning a focused laser beam over the device perpendicular to the channel. The photodetector shows the best-ever effective photo-sensing distance up to the millimeter range. The photoresponsivity and photocurrent decrease by nearly an order of magnitude when the beam position varies from 0 to 950 μm and the tunability of carrier diffusion length in CH3NH2PbBr3 with the variation of the exciting laser intensity is demonstrated. Furthermore, a numerical model based on transport of photoexcited carriers is proposed to explain the position dependence. This photodetector shows excellent potential for application in future nanoelectronics and optoelectronics systems.

Synthesis of 9,10-distyrylanthracene derivative and its one- and two-photon induced emission in solid state

Solid fluorescence plays an important role in optoelectronic devices. In this paper, 9,10-bis(6-dimethylamino benzoxazole styryl)-anthracene (1) was synthesized. 1 displayed a strong emission in solid state (microcrystalline state, amorphous solid state and polymeric thin film) resulted from aggregation-induced emission mechanism, and a large absolute fluorescence quantum yield (ϕf = 0.65) was obtained. 1 also exhibited up-conversion emission in solid state and a strong yellow emission was observed upon excitation with 800–1064 nm, the quadratic dependence of the fluorescence on the excitation laser intensity confirmed that the up-conversion emission resulted from two-photon process.

The Effect of Light Intensity, Temperature, and Oxygen Pressure on the Photo-Oxidation Rate of Bare PbS Quantum Dots.

The oxidation speed of PbS quantum dots has been a subject of controversy for some time. In this study, we reveal the precise functional form of the oxidation rate constant for bare quantum dots through analysis of their photoluminescence as a function of temperature, oxygen pressure, and excitation-laser intensity. The combined effect of these factors results in a reduced energy barrier that allows the oxidation to proceed at a high rate. Each absorbed photon is found to have a 10−8 probability of oxidizing a PbS atomic pair. This highlights the importance of photo-excitation on the speed of the oxidation process, even at low illumination conditions. The procedure used here may set up a quantitative standard useful for characterizing the stability of quantum dots coated with ligands/linkers, and to compare different protection schemes in a fair quantitative way.

Strong Transient Absorption of Trapped Holes in Anatase and Rutile TiO2 at High Laser Intensities

During transient absorption spectroscopic investigations we found that the intensity of the transient absorption signal of the trapped holes monitored in the microsecond time domain drastically increases at high excitation laser intensity. This increase has been related to the presence of long-lived Ti3+ centers formed upon high laser exposure via a surface reorganization. The Coulomb interaction of the trapped holes with long-lived Ti3+ centers leads to an increased absorption coefficient of the former resulting in much higher transient absorption signals below 450 nm rather than in the wavelength region above where the trapped electrons absorb. The surface reorganization induced via the excitation source can be avoided in the case of anatase if the measurements are conducted at low laser intensities, while in the case of rutile already at low excitation conditions the transient absorption enhancement of the trapped holes occurs.

Measurements of the energy distribution of a high brightness rubidium ion beam

The energy distribution of a high brightness rubidium ion beam, which is intended to be used as the source for a focused ion beam instrument, is measured with a retarding field analyzer. The ions are created from a laser-cooled and compressed atomic beam by two-step photoionization in which the ionization laser power is enhanced in a build-up cavity. Particle tracing simulations are performed to ensure the analyzer is able to resolve the distribution. The lowest achieved full width 50% energy spread is (0.205 ± 0.006) eV, which is measured at a beam current of 9 pA. The energy spread originates from the variation in the ionization position of the ions which are created inside an extraction electric field. This extraction field is essential to limit disorder-induced heating which can decrease the ion beam brightness. The ionization position distribution is limited by a tightly focused excitation laser beam. Energy distributions are measured for various ionization and excitation laser intensities and compared with calculations based on numerical solutions of the optical Bloch equations including ionization. A good agreement is found between measurements and calculations.

DLS Setup for in Situ Measurements of Photoinduced Size Changes of Microgel-Based Hybrid Particles.

Photoinduced size changes in microgel particles loaded with gold nanoparticles (AuNPs) were investigated with an extended multiangle dynamic light scattering (DLS) setup. The DLS setup was equipped with a conventional laser (λ = 633 nm) to determine the microgel particle size. Additionally, a laser (λ = 532 nm) is installed to study the photoresponsive behavior of the AuNP-microgel hybrids. The wavelength of 532 nm is close to the absorption maximum of the plasmon resonance of the AuNPs used in the present study (i.e. spherical AuNPs with a diameter of 14 nm). The extended DLS setup enables us to follow in situ the change in microgel size during irradiation. The light stimulus is directly correlated with the size changes of the hybrid particles and the photothermal effect depends on the intensity of the excitation laser. The increase in excitation laser intensity results in a size reduction of hybrid particles because of the ability of AuNPs to partially transform the absorbed photon energy into heat which is emitted into the surrounding microgel network.

Laser crystallization of germanium nanowires fabricated by electrochemical deposition

AbstractGermanium (Ge) nanowires were fabricated by electrochemical deposition on titanium‐coated silicon substrate. Arrays of nanowires with different mean diameters were obtained using indium nanoparticles of various sizes as centers of nucleation. Raman spectroscopy was used to establish the structural properties of nanowires under different laser excitation intensities. Cycles of measurements with different intensities demonstrated irreversible changes in the structure. Initial study with minimal excitation intensity of 3 W/cm2 displayed that the spectra are descriptive of amorphous Ge. Further study at increased intensities led to the transformation of Raman spectra to the shapes that can be attributed to crystalline Ge. Detailed analysis of spectra shapes for samples with different mean diameters of Ge nanowires after exposure to high intensity laser radiation allows to estimate the fraction of the crystalline phase in Ge nanowires. The spectra of a sample irradiated by He–Ne laser were compared with thermally annealed sample in a vacuum at 150 °C. A similar shape of the obtained spectra indicates on the thermal nature of the effect, which leads to a change in the observed structural properties. This change under even slight heating can be explained by thermal isolation of nanowires from the substrate.

Variation of the band structure in DKDP crystal excited by intense sub-picosecond laser pulses

The nonlinear absorption (NLA) properties of potassium dideuterium phosphate crystals at 515 nm under different excitation laser intensities are investigated with the Z-scan technique. Two critical intensities are highlighted: the critical intensity for exciting the NLA and the critical intensity of the multiphoton absorption mechanism transition. Experimental results indicate the existence of defect states located in the band gap, which can be manipulated by varying laser intensity. A model based on the change of multiphoton absorption mechanism induced by the transformation of defect species is proposed to interpret the experiments. Modeling results are in good agreement with the experiment data.

Two-photon absorption and upconversion luminescence of colloidal CsPbX3 quantum dots

The nonlinear optical and the upconversion luminescence (UCL) properties of CsPbX3 (X = Br or its binary mixtures with Cl, I) quantum dots (QDs) are investigated by femtosecond open-aperture (OA) Z-scan and time-resolved luminescence techniques in nonresonant spectral region. The OA Z-scan results show that CsPbX3 QDs have strong reverse saturable absorption (RSA), which is ascribed to two-photon absorption. Partially changing halide composition from Cl to Br, to I, two-photon absorption cross sections become larger at the same laser excitation intensity. The composition-tunable nonlinear absorption should be attributed to the gradual decrease of the lowest direct band gaps with the halide substitute. Moreover, the strong UCL can be observed under near infrared femtosecond laser excitation. Halide composition-tunable UCL dynamics of CsPbX3 QDs is analyzed by use of two-exponential fitting with deconvolution. When CsPbX3 QDs have similar sizes (10–13 nm), with partially changing halide composition from Cl to Br, to I, the average UCL lifetime becomes longer due to the variation of Kane energy. Our findings suggest all-inorganic perovskite QDs can be used as excellent gain medium for high-performance frequency-upconversion lasers and provide reference to engineer such QDs toward practical optoelectronic applications.

Superlinear Photoluminescence Dynamics in Plasmon–Quantum-Dot Coupling Systems

Light–matter interaction exploiting plasmons is attracting great interest in terms of a new twist, hot electrons. We designed a basic configuration to couple plasmonic metasurfaces with a layer of quantum dots (QDs) embedded in semiconductors and experimentally investigated the photoluminescence (PL) dynamics in the coupled systems of III–V semiconductor QDs with plasmonic metasurfaces. The QDs of InAs, which emit luminescence at telecom wavelengths near 1300 nm, were grown on GaAs substrates with a molecular-beam-epitaxy technique. The plasmonic metasurfaces were fabricated on top of the GaInAs substrates, using a numerical design for single-layer Au metasurfaces. Here we show that the PL responses through the plasmonic metasurface becomes more active in the coupled systems than those in the QDs without the plasmonic metasurfaces, being superlinear with respect to the excitation laser intensity, even under weak continuous-wave excitation. The superlinear responses are successfully described in a general ...

Experimental investigation of mesoscopic heterogeneous motion of laser-activated self-propelling Janus particles in suspension.

The mesoscopic collective motion of self-propelling active particle suspension is experimentally investigated. The active particles are silica micro spheres with Au hemisphere coating, and their propelling strength is activated by laser irradiation. The suspension is driven from equilibrium to near equilibrium and far from equilibrium by tuning the excitation laser intensity. By use of the long-term particle tracking technique, the time evolution of a large amount of active particles is resolvable. For low laser intensity, the suspension is driven to near equilibrium state with homogeneous superdiffusion motion. The strength of enhanced superdiffusion is monotonically related to the laser intensity. For high laser intensity, the motility-induced phase separation with the coexistence of dense cluster and very dilute individual particle are observed. It leads to highly heterogeneous dynamic with less mobile jammed cluster and fast-moving particles and subsequently suppresses the enhanced superdiffusion. Such heterogeneous dynamics is similar to many far from equilibrium systems. Finally, the degree away from equilibrium (Gaussian dynamics) triggered by propelling strength is quantified by non-Gaussian parameters.

Carrier dynamics of InAs quantum dots with GaAs1−xSbx barrier layers

Photoluminescence properties of InAs/GaAs1−xSbx quantum dots (QDs) are investigated with respect to the Sb-composition of x = 0, 0.15, and 0.25. The QDs demonstrate a type-II band alignment for x = 0.15 and 0.25. In contrast, with well-defined InAs/GaAs QDs, the InAs/GaAs1−xSbx QDs exhibit overlapping spectral features with increasing laser excitation intensity, which are determined to be the result of different carrier recombination routes, including both type-I and type-II pathways. Our investigation indicates that the optical behavior and the carrier dynamics in these InAs/GaAs1−xSbx QDs are much more complicated than in the InAs/GaAs QD counterparts. It provides very useful information for understanding the band structure and carrier dynamics of type-II InAs/GaAs1−xSbx QDs.

Two/multi-photon induced up-conversion emission of dicyanoisophorone derivative

Two/multi-photon induced up-conversion emission based on an organic dye is described. Bis(dicyanoisophorone)-1,4-divinylbenzene (1) was synthesized by treatment of 2-(3,5,5-trimethylcyclohex-2-en-1-ylidene)malononitrile and terephthalaldehyde. 1 exhibited strong aggregation-induced emission in solid state, and a large fluorescent quantum yield (ϕf=63%) was obtained. 1 also exhibited nonlinear optical property, a strong up-conversion emission was observed upon excitation with 800–1500nm, and the quadratic dependence of the fluorescence on the excitation laser intensity confirmed the two/multi-photon process.

(Invited) Transient Absorption Study of Singlet Excitons in Crystalline Peropyrene Derivatives: Examination of Singlet Fission Process

Transient absorption study of singlet excitons in crystalline peropyrene derivatives: examination of singlet fission process Akihiro Furube*1, Daiki Yamanaka1, Shinichiro Yanagiya1, Hiroyuki Matsuzaki2, Kazuyuki Uchida3, Takashi Kubo3 1Tokushima Univ,. 2-1, Minamijosanjima-cho, Tokushima, 770-8506, Japan 2AIST, 1-1-1 Umezono, Tsukuba, Ibaraki, 305-8568, Japan 3Osaka Univ., 1-1, Machikaneyama, Toyonaka, Osaka 560-0043, Japan Furube.akihiro@tokushima-u.ac.jp Singlet fission process in organic crystal is attracting considerable interest due to its potential application of organic photovoltaic (OPV) with the internal quantum efficiency of 200%, since one absorbed photon can produce two triplet excitons that can generate two electron-hole pairs at the interface between the organic crystal and a suitable electron donor material such as fullerene derivatives and carbon nanotubes. Peropyrene, which is one of polycyclic aromatic hydrocarbon compounds, is expected to exhibit singlet fission in the crystal form. The SF is a reaction in which the excited singlet state (S1) spilt into two triplet excited states (T1 + T1). Energy matching (S1 ~ T1 + T1) reported [1] for peropyrene molecule interests researchers for development of derivatives of this molecule to control the energy levels and crystal structure for efficient singlet fission. In the present study, two kinds of 2,9-disubstituted peropyrene derivatives, one with two phenyl groups and the other with two n-butyl groups, were newly synthesized and tested. Time-resolved transient absorption spectroscopy in the femtosecond and nanosecond timescales, time-resolved fluorescence spectroscopy, and steady-state absorption/fluorescence measurements were carried out in order to reveal the relaxation processes of the singlet excitons of these peropyrene derivatives in the crystalline and solution phases. Transient absorption spectra and kinetics of phenyl peropyrene in the crystalline state are shown in Figure 1a. Here, diffuse reflectance spectroscopy was applied [2, 3]. At 10 ps after excitation, a broad absorption band appeared around 700 nm, which showed gradual decay up to 500 ps. It should be noted that the excitation laser intensity (at 400 nm wavelength) was adjusted to be weak enough so as not to undergo any non-linear decay process such as exciton–exciton annihilation. Since the absorption peak wavelength (700 nm) is close to the one observed in solution (775 nm), the absorption band can be ascribed to the singlet exciton in crystalline state. The decay component in the picosecond timescale showed a time constant of 150 ps. The involvement of singlet fission in the decay of a singlet exciton would result in the appearance of the T1absorption band being similar to the band observed in solution around 525 nm. However, we did not observe such an absorption band in the picosecond time scale. This experimental finding implies that the energy level of the S1 state is lower than twice the energy of the T1 state in the crystalline state. In this case, singlet fission is expected to occur from highly excited singlet states (Sn). Under high-density excitation conditions, two singlet excitons collide each other to produce Sn and S0 through singlet-singlet exciton annihilation (S1 + S1 → Sn + S0). Thereafter the resulting Sn state can undergo singlet fission. Figure 1b shows the transient absorption. Indeed, a broad absorption band centered at 570 nm was observed in the transition absorption spectrum measured at 1 ns, after the disappearance of the absorption band of the S1 state around 700 nm. This spectral feature is similar to that of the T1 state observed in solution, suggesting that the T1 state is generated from the highly excited singlet state Snthrough singlet fission. Interestingly, n-butyl peropyrene crystal did not show singlet fission either in weak or strong excitation condition. Based on these experiments, it was revealed that S1exciton in phenyl peropyrene is energetically low for undergoing singlet fission, therefore requiring additional energy. Further detailed discussion is given with other experimental results. Figure.1Transient absorption spectra of phenyl peropyrene crystal power samples excited by 400 nm laser with weak (a) and strong (b) intensity, respectively. Acknowledgement: A part of this work is supported by KAKENHI (16K13669) Japan.

Femtosecond, two-photon, planar laser-induced fluorescence of carbon monoxide in flames.

Two-photon, planar laser-induced fluorescence (TP-PLIF) of carbon monoxide was performed in steady and driven flames using femtosecond (fs) laser pulses at 1kHz. Excitation radiation at 230.1nm (full-width at half-maximum bandwidth of 270 cm-1) was used to pump many rovibrational two-photon transitions in the B1∑+←X1∑+ system. Visible fluorescence in the range 362-516nm was captured using an image intensifier and high-speed camera. The signal dependence on excitation energy and wavelength is presented. Photolytic interferences from the ultraviolet laser were explored in a sooting diffusion flame. Using an excitation laser intensity of 1010 W/cm2, negligible photolytic interferences were observed, and PLIF imaging of dynamic flame events was performed at 1 kHz.

Light-Enhancing Plasmonic-Nanopore Biosensor for Superior Single-Molecule Detection.

A stacked plasmonic nanowell-nanopore biosensor strongly suppresses the background fluorescence from the bulk and yields net more than tenfold enhancement of the fluorescence intensity. The device offers extremely high signal-to-background (S/B) ratio for single-molecule detection at ultralow excitation laser intensities, while maintaining extremely high temporal bandwidth for single-DNA sensing.

Nanomembrane-Based, Thermal-Transport Biosensor for Living Cells.

Knowledge of materials' thermal-transport properties, conductivity and diffusivity, is crucial for several applications within areas of biology, material science and engineering. Specifically, a microsized, flexible, biologically integrated thermal transport sensor is beneficial to a plethora of applications, ranging across plants physiological ecology and thermal imaging and treatment of cancerous cells, to thermal dissipation in flexible semiconductors and thermoelectrics. Living cells pose extra challenges, due to their small volumes and irregular curvilinear shapes. Here a novel approach of simultaneously measuring thermal conductivity and diffusivity of different materials and its applicability to single cells is demonstrated. This technique is based on increasing phonon-boundary-scattering rate in nanomembranes, having extremely low flexural rigidities, to induce a considerable spectral dependence of the bandgap-emission over excitation-laser intensity. It is demonstrated that once in contact with organic or inorganic materials, the nanomembranes' emission spectrally shift based on the material's thermal diffusivity and conductivity. This NM-based technique is further applied to differentiate between different types and subtypes of cancer cells, based on their thermal-transport properties. It is anticipated that this novel technique to enable an efficient single-cell thermal targeting, allow better modeling of cellular thermal distribution and enable novel diagnostic techniques based on variations of single-cell thermal-transport properties.