Optical Absorption Resonance Research Articles

Gold Nanoparticles for Retinal Molecular Optical Imaging.

The incorporation of gold nanoparticles (GNPs) into retinal imaging signifies a notable advancement in ophthalmology, offering improved accuracy in diagnosis and patient outcomes. This review explores the synthesis and unique properties of GNPs, highlighting their adjustable surface plasmon resonance, biocompatibility, and excellent optical absorption and scattering abilities. These features make GNPs advantageous contrast agents, enhancing the precision and quality of various imaging modalities, including photoacoustic imaging, optical coherence tomography, and fluorescence imaging. This paper analyzes the unique properties and corresponding mechanisms based on the morphological features of GNPs, highlighting the potential of GNPs in retinal disease diagnosis and management. Given the limitations currently encountered in clinical applications of GNPs, the approaches and strategies to overcome these limitations are also discussed. These findings suggest that the properties and efficacy of GNPs have innovative applications in retinal disease imaging.

Single-molecule detection of SARS-CoV-2 N protein on multilayered plasmonic nanotraps with surface-enhanced Raman spectroscopy

The spread of the SARS-CoV-2 virus has had an unprecedented impact, both by posing a serious risk to human health and by amplifying the burden on the global economy. The rapid identification of the SARS-CoV-2 virus has been crucial to preventing and controlling the spread of SARS-CoV-2 infections. In this study, we propose a multilayered plasmonic nanotrap (MPNT) device for the rapid identification of single particles of SARS-CoV-2 virus in ultra-high sensitivity by surface-enhanced Raman scattering (SERS). The MPNT device is composed of arrays of concentric cylindrical cavities with Ag/SiO2/Ag multilayers deposited on the top and at the bottom. By varying the diameter of the cylinders and the thickness of the multilayers, the resonant optical absorption and local electric field were optimized. The SERS enhancement factors of the proposed device are of the order of 108, which enable the rapid identification of SARS-CoV-2 N protein in concentrations as low as 1.25 × 10−15－12.5 × 10−15 g mL−1 within 1 min. The developed MPNT SERS device provides a label-free and rapid detection platform for SARS-CoV-2 virus. The general nature of the device makes it equally suitable to detect other infectious viruses.

Hydrogen on Colloidal Gold Nanoparticles.

Colloidal gold nanoparticles (AuNPs) have myriad scientific and technological applications, but their fundamental redox chemistry is underexplored. Reported here are titration studies of oxidation and reduction reactions of aqueous AuNP colloids, which show that the AuNPs bind substantial hydrogen (electrons + protons) under mild conditions. The 5 nm AuNPs are reduced to a similar extent with reductants from borohydrides to H2 and are reoxidized back essentially to their original state by oxidants, including O2. The reactions were monitored via surface plasmon resonance (SPR) optical absorption, which was shown to be much more sensitive to surface H than to changes in solution conditions. Reductions with H2 occurred without pH changes, demonstrating that hydrogenation forms surface H rather than releasing H+. Computational studies suggested that an SPR blueshift was expected for H atom addition, while just electron addition likely would have caused a redshift. Titrations consistently showed a maximum redox change of the 5 nm NPs, independent of the reagent, corresponding to 9% of the total gold or ∼30% hydrogen surface coverage (∼370 H per AuNP). Larger AuNPs showed smaller maximum fractional surface coverages. We conclude that H binds to the edge, corner, and defect sites of the AuNPs, which explains the stoichiometric limitation and the size effect. The finding of substantial and stable hydrogen on the AuNP surface under mild reducing conditions has potential implications for various applications of AuNPs in reducing environments, from catalysis to biomedicine. This finding contrasts with the behavior of bulk gold and with the typical electron-focused perspective in this field.

Photodriven electron-transfer dynamics in a series of heteroleptic Cu(I)-anthraquinone dyads.

Solar fuels catalysis is a promising route to efficiently harvesting, storing, and utilizing abundant solar energy. To achieve this promise, however, molecular systems must be designed with sustainable components that can balance numerous photophysical and chemical processes. To that end, we report on the structural and photophysical characterization of a series of Cu(I)-anthraquinone-based electron donor-acceptor dyads. The dyads utilized a heteroleptic Cu(I) bis-diimine architecture with a copper(I) bis-phenanthroline chromophore donor and anthraquinone electron acceptor. We characterized the structures of the complexes using x-ray crystallography and density functional theory calculations and the photophysical properties via resonance Raman and optical transient absorption spectroscopy. The calculations and resonance Raman spectroscopy revealed that excitation of the Cu(I) metal-to-ligand charge-transfer (MLCT) transition transfers the electron to a delocalized ligand orbital. The optical transient absorption spectroscopy demonstrated that each dyad formed the oxidized copper-reduced anthraquinone charge-separated state. Unlike most Cu(I) bis-phenanthroline complexes where increasingly bulky substituents on the phenanthroline ligands lead to longer MLCT excited-state lifetimes, here, we observe a decrease in the long-lived charge-separated state lifetime with increasing steric bulk. The charge-separated state lifetimes were best explained in the context of electron-transfer theory rather than with the energy gap law, which is typical for MLCT excited states, despite the complete conjugation between the phenanthroline and anthraquinone moieties.

Visualizing ultrafast photothermal dynamics with decoupled optical force nanoscopy

The photothermal effect in nanomaterials, resulting from resonant optical absorption, finds wide applications in biomedicine, cancer therapy, and microscopy. Despite its prevalence, the photothermal effect in light-absorbing nanoparticles has typically been assessed using bulk measurements, neglecting near-field effects. Beyond standard imaging and therapeutic uses, nanosecond-transient photothermal effects have been harnessed for bacterial inactivation, neural stimulation, drug delivery, and chemical synthesis. While scanning probe microscopy and electron microscopy offer single-particle imaging of photothermal fields, their slow speed limits observations to milliseconds or seconds, preventing nanoscale dynamic investigations. Here, we introduce decoupled optical force nanoscopy (Dofn), enabling nanometer-scale mapping of photothermal forces by exploiting unique phase responses to temporal modulation. We employ the photothermal effect’s back-action to distinguish various time frames within a modulation period. This allows us to capture the dynamic photothermal process of a single gold nanorod in the nanosecond range, providing insights into non-stationary thermal diffusion at the nanoscale.

Optical absorption of angulon in metal halide perovskites

We theoretically study the optical absorption of an angulon in the metal halide perovskites (MHP) based on the improved Devreese–Huybrechts–Lemmens model, where the formation of quasiparticle angulon states originates from the organic cation rotating in the inorganic octahedral cage of MHP. We find that the resonance optical absorption peaks appear when the energy of the incident photon matches the quantum levels of angulon. Moreover, the intensity of absorption depends on the quantum states of phonon angular momentum. These theoretical results provide significant insight to study the redistribution of angular momenta for the rotating molecules immersed in the many-body environment.

Temperature-sensitive hybridization of propagating and localized surface phonon polaritons in polar 4H-SiC nano-resonators

Polar 4H-SiC nano-resonators can host localized surface phonon polaritons (SPhPs) with low optical loss and fully exploit light–matter interactions for promising nanophotonic applications such as surface-enhanced spectroscopies and thermal imaging. To expand the mid-infrared and infrared application spaces, a sound knowledge of temperature effect on infrared dielectric functions and SPhPs of 4H-SiC is required, yet it remains largely unexplored. Herein, we focus on exploiting the temperature influence on dielectric functions, hybridization of propagating and localized SPhPs, and tailed spectral radiation properties of 4H-SiC nanopillar arrays through spectroscopic ellipsometry (SE) measurements as well as multiscale simulations. The 4H-SiC crystal is grown using the physical vapor transport method, and SE experiments measure infrared dielectric functions at temperatures between 300 and 800 K. Finite-element electromagnetic simulations confirm the emerged Monopole and transverse dipoles (TD1 and TD2) resonance modes in 4H-SiC nano-resonators, which agrees with the literature experiment. At high temperatures with strong lattice vibration, the amplitudes of resonant optical absorption peaks gradually decrease and the linewidths broaden, accompanied by the weakened electric resonances. First-principles calculations show that the anharmonic phonon scattering strengthens and less optical phonons are coupled to incident photons as temperature increases. Moreover, the propagating and localized SPhPs’ hybridization and spectral radiation properties of 4H-SiC nanopillar arrays can be largely tailed by modifying the morphology and incident angle of light. This work provides physical insights into the temperature-induced spectral tuning of 4H-SiC nano-resonators and helps exploit their applications in the high-temperature working conditions.

Explanation of the Opposing Shifts in the Absorption Edge and the Optical Resonance in CuFeS2 Nanoparticles

Size-dependent change of the electronic band structure is one of the key features of nanoparticles in the quantum confinement region. CuFeS2 nanoparticles have a strong absorption feature in the visible region that has, controversially, been described as neither an excitonic transition nor a free carrier plasmon oscillation. Instead, the absorption feature in CuFeS2 nanoparticles has been attributed to quasi-static optical resonances from inter-band transitions between the valence band (VB), intermediate band (IB), and conduction band (CB). As such, we hypothesized that the feature should be subject to quantum confinement effects through modification of the electronic bands. In this paper, we show experimentally that the optical resonance absorption peak red-shifts and the optical band gap blue-shifts as the particle size decreases. Through density functional theory (DFT) and the tight binding (TB) modeling, we elucidate the size dependence of the band structure, especially focusing on the change in the IB. Using a Lorentzian oscillator optical model to simulate the absorption spectrum with inputs from the DFT-calculated band structure and band shifts from the TB model, we find that the size-dependent shifts of the optical resonance peak position in CuFeS2 are due to a tri-band quantum confinement effect that results in both the VB to IB and IB to CB gap expansion that accompanies a decrease in particle size. We also find that the transitions between the IB and CB play only a minor role in the optical spectrum. Moreover, the linear optical Lorentzian model predicts that the optical resonance peak is tunable across the visible range by partially filling the IB, lowering the CB, or expanding the IB.

Transformation Pathways in Colloidal CdTeSe Magic-Size Clusters.

A rarely studied transformation in colloidal ternary magic-size clusters (MSCs) is addressed. We report the first observation of the transformation from ternary CdTeSe MSC-399 to MSC-422, which occurs at room temperature. These two MSC types display sharp optical absorption resonances at 399 and 422 nm, respectively, and are related in that they are quasi isomers, together with their counterpart precursor compounds (PCs). Binary CdTe and CdSe samples were prepared in the prenucleation stage also called the induction period (IP). After they were mixed and placed in a mixture of toluene and octylamine, the transformation was found to take place and to be assisted by the addition of the CdSe IP sample. A binary IP sample contains corresponding binary PCs and monomers (Mo) and fragments (Fr). We argue that the transformation pathway is enabled by the corresponding ternary PCs, involving the substitution reaction, namely CdTeSe PC-399 + CdSe (Mo/Fr)-1 ⇒ CdTeSe PC-422 + CdSe (Mo/Fr)-2. The present study provides an in-depth understanding of the formation characteristics of the MSCs.

Transformation Pathways in Colloidal CdTeSe Magic‐Size Clusters

AbstractA rarely studied transformation in colloidal ternary magic‐size clusters (MSCs) is addressed. We report the first observation of the transformation from ternary CdTeSe MSC‐399 to MSC‐422, which occurs at room temperature. These two MSC types display sharp optical absorption resonances at 399 and 422 nm, respectively, and are related in that they are quasi isomers, together with their counterpart precursor compounds (PCs). Binary CdTe and CdSe samples were prepared in the prenucleation stage also called the induction period (IP). After they were mixed and placed in a mixture of toluene and octylamine, the transformation was found to take place and to be assisted by the addition of the CdSe IP sample. A binary IP sample contains corresponding binary PCs and monomers (Mo) and fragments (Fr). We argue that the transformation pathway is enabled by the corresponding ternary PCs, involving the substitution reaction, namely CdTeSe PC‐399 + CdSe (Mo/Fr)‐1 ⇒ CdTeSe PC‐422 + CdSe (Mo/Fr)‐2. The present study provides an in‐depth understanding of the formation characteristics of the MSCs.

Intraconduction band-related optical absorption in coupled (In,Ga)N/GaN double parabolic quantum wells under temperature, coupling and composition effects

Intraconduction subband related optical absorption coefficient in non-polar m-plane symmetric linked (In,Ga)N/GaN double parabolic quantum wells is theoretically investigated in this study taking into consideration the presence of a single dopant hydrogen-like donor impurity moving down the growth axis. Calculations for a finite parabolic potential are performed using the impurity position, structural dimension, indium fraction, and temperature as adjusting variables. Using the finite difference approach, the Schrodinger equation is numerically solved to obtain the eigenvalues and their corresponding eigenvectors in the effective-mass approximation. The results reveal that increasing the coupling barrier and/or well concentration causes a considerable blueshift in the position of the absorption peak. Temperature, size change, and/or an impurity moving from the left band edge to the middle of the structure all cause a significant redshift in the absorption spectra. It has also been established that resonant optical absorption characteristics can be controlled by a careful choice of temperature and structure size, opening up new possibilities for high-performance infrared optical devices, optical modulators, solar cells, and laser applications.

Biosynthesis and characterization of Gynocardia odorata R. Br. mediated silver nanoparticles and evaluation of its antimicrobial activity

The present study was designed to synthesize Silver Nanoparticles (Ag-NPs) in aqueous medium using leaf extract of Gynocardia odorata R. Br. (Achariaceae). The synthesized Ag-NPs were characterized using different technique such as UV-Visible Spectroscopy, X-Ray Diffraction (XRD) Spectroscopy, Fourier Transform Infrared Spectroscopy (FTIR) and Transmission Electron Microscopy (TEM). The reduction of Ag ions to initiate nucleation and subsequent Ostwald Ripening to form nanoparticles was made possible by the presence of various antioxidants in the leaves of Gynocardia odorata. These antioxidants served both as reducing and capping agents. The synthesized Ag-NPs were found to be polydispersed in nature and spherical in shape. With the Surface Plasmon Resonance (SPR) optical absorption band peak at ~440 nm was observed using UV-Vis spectrophotometer. FTIR confirmed the presence of methoxy and allyl groups in the synthesized Ag-NPs and nearly 15-45 nm diameter spherical shaped NPs was validated using TEM. The synthesized Ag-NPs were stable for a long period (more than six months) and showed good antibacterial activity against both gram positive and gram negative bacterial strains and the effect was higher as compared to the normal aqueous extract.

Ground and first five low-lying excited states related optical absorption in In.1Ga.9N/GaN double quantum wells: Temperature and coupling impacts

In the present article, we investigated theoretically the total absorption coefficient related to inter conduction subband transitions in symmetric In.1Ga.9N/GaN double rectangular quantum wells within the effective-mass and parabolic band approximations making an allowance for temperature and structure's size influences. The finite difference method is adopted to solve numerically one-dimensional Schrödinger equation and to obtain the six first low-lying allowed eigen-values and their corresponding eigen-functions in view of finite potential barrier height, effective-mass and dielectric constant mismatches. The optical absorption coefficients are computed based on the compact density matrix approach within an iterative procedure. The numerical results are reported for two different values of temperature. Obvious temperature impact is obtained mainly for narrow coupling barrier compared to large one. Apart from 3S–3P transition and regardless the coupling width, momentous red-shift is found with increasing temperature. Moreover, 2S–2P, 2S–3P and 3S–3P associated TOAC spectra are blue-shifted while all others show a substantial red-shift with increasing the coupling width. Also, it is found that resonant optical absorption characteristics can be tailored by an appropriate selection of temperature and structure's size leading to new prospective for good performance infrared optical devices, optical modulators, solar cells and laser applications.

Nonlinear Absorption and Ultrafast Dynamics of Ag Nanoparticle

Resonant nonlinear optical absorption of silver nanoparticles was studied experimentally via open aperture Z-scan using 130 fs, 400 nm laser pulses. Experimental results show that, at low laser intensity, silver nanoparticles can exhibit saturated absorption. While at high laser intensity, it shows reverse saturated absorption. The saturable absorption is explained in terms of ground state plasmon bleaching, while the reverse saturable absorption is believed to be from two-photon absorption. Saturable optical intensity and two-photon absorption coefficient were obtained to be 1.3×1010 W/m2 and 3.3×10−10 m/W, respectively. The energy relaxation process of Ag nanoparticles after laser excitation was studied via pump-probe technique at 400 nm. Experimental results demonstrated that energy relaxation included electron-phonon coupling process with time constant τ1=(713±50) fs, and phonon-phonon coupling process with time constant τ2=(25.2±3) ps, respectively.

Electrically enhanced graphene-metal plasmonic antenna for infrared sensing

An optical graphene-metal hybrid antenna geometry, permitting the efficient tuning of plasmon resonance with unity absorption is reported. The total optical absorption and optical field enhancement are accomplished by designing a graphene-metal antenna. The antenna is comprised of gold hexagon radiator with bilayer graphene on the top, functioning as a double plasmonic resonant structure. The tunability of absorption and optical field enhancement is realized by electrical gating. The strong coupling takes place between the gold plasmons and graphene plasmons, resulting in the strong enhancement of optical fields. The proposed design is modeled in CST Microwave Studio and is simulated through FDTD solver. Moreover, the dynamic tuning of the resonance frequency and optical absorption is achieved by increasing the chemical potential of graphene layers through gate voltage. The tuning range of the designed antenna is optimized in a bandwidth starting from 30 THz to 34 THz. Although at 33 THz the antenna meets matching conditions by having high input impedance, low admittance, and almost unity absorption. The functioning bandwidth of the antenna is preferable for plasmonic applications i.e. infrared sensing and imaging, where high absorption and enhanced field characteristics are required.

Parts-per-billion detection of carbon monoxide: A comparison between quartz-enhanced photoacoustic and photothermal spectroscopy

We report on a comparison between two optical detection techniques, one based on a Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) detection module, where a quartz tuning fork is acoustically coupled with a pair of millimeter-sized resonator tubes; and the other one based on a Photothermal Spectroscopy (PTS) module where a Fabry-Perot interferometer acts as transducer to probe refractive index variations. When resonant optical absorption of modulated light occurs in a gas sample, QEPAS directly detects acoustic waves while PTS probes refractive index variations caused by local heating. Compact QEPAS and PTS detection modules were realized and integrated in a gas sensor system for detection of carbon monoxide (CO), targeting the fundamental band at 4.6 μm by using a distributed-feedback quantum cascade laser. Performance was compared and ultimate detection limits up to ∼ 6 part-per-billion (ppb) and ∼15 ppb were reached for QEPAS and the PTS module, respectively, using 100 s integration time and 40 mW of laser power.

Quantum–classical mechanics: On the problem of a two-photon resonance band shape in polymethine dyes

In the framework of a new physical theory — quantum–classical mechanics, on the basis of a dynamic electron-nuclear-reorganization resonance and the law of conservation of energy, an explanation is given for the strong detuning of the resonance and the associated significant parasitic transformation of the shape of the resonant optical absorption band as a result of the transition from linear to nonlinear, two-photon absorption in polymethine dyes in solutions. Based on this explanation and model band shapes of the theoretical fit to the experimental optical bands (Hales et al. (2010)), the conditions for reconstructing the resonance shape of the band for two-photon absorption and its redshift are predicted.

InAsSb pillars for multispectral long-wavelength infrared absorption

InAsSb pillars were investigated for multispectral photodetection in the long wavelength infrared (LWIR) region. An InAs0.19Sb0.81 thin film was successfully grown on Si (100) substrate, utilizing an AlSb buffer layer to alleviate the large lattice mismatch. X-ray diffraction studies showed a majority [100] orientation of the as-grown films, with minor orientations arising as a result of twinning. Arrays of InAsSb pillars with diameters ranging from 1700 nm to 4000 nm were fabricated by a top-down reactive ion etching process. The arrays showed resonant optical absorption peaks in the LWIR region from 8 to 16 μm wavelength, dependent on the pillar diameter. The peak absorptance wavelength increased by 0.46 μm for each 100 nm increase in pillar diameter, demonstrating the multispectral tunability of such arrays.

Near Resonant Optical Absorption by a System Coupled with Two Laser Beams

The structure of a solution of the generalized Maxwell–Bloch system of equations describing the strongly pumped interacting two-level atoms is discussed. This structure is represented by means of the corresponding differential equations for each contributing process. The interaction between the processes is introduced through the interaction integral and is illustrated by the specific system of graphs. The method allows one to describe the quantum-field-induced long-range interaction prevailing over short-range collisions and causing the broadening, narrowing, and shifts of an absorption line shape. The description is given in terms of the interaction integrals which couple the collective atomic polarization and population inversion. The contributions from different effects are analyzed with the use of the additivity of the corresponding absorption/reemission rates.

Intersubband Transition Engineering in the Conduction Band of Asymmetric Coupled Ge/SiGe Quantum Wells

n-type Ge/SiGe asymmetric coupled quantum wells represent the building block of a variety of nanoscale quantum devices, including recently proposed designs for a silicon-based THz quantum cascade laser. In this paper, we combine structural and spectroscopic experiments on 20-module superstructures, each featuring two Ge wells coupled through a Ge-rich SiGe tunnel barrier, as a function of the geometry parameters of the design and the P dopant concentration. Through a comparison of THz spectroscopic data with numerical calculations of intersubband optical absorption resonances, we demonstrated that it is possible to tune, by design, the energy and the spatial overlap of quantum confined subbands in the conduction band of the heterostructures. The high structural/interface quality of the samples and the control achieved on subband hybridization are promising starting points towards a working electrically pumped light-emitting device.