Low Optical Intensities Research Articles

Optically Induced Symmetry Breaking Due to Nonequilibrium Steady State Formation in Charge Density Wave Material 1T-TiSe2.

The strongly correlated charge density wave (CDW) phase of 1T-TiSe2 is of interest to verify the claims of a chiral order parameter. Characterization of the symmetries of 1T-TiSe2 is critical to understand the origin of its intriguing properties. Here we use very low-power, continuous wave laser excitation to probe the symmetries of 1T-TiSe2 by using the circular photogalvanic effect. We observe that the ground state of the CDW phase (D3d) is achiral. However, laser excitation above a threshold intensity transforms 1T-TiSe2 into a nonequilibrium chiral phase (C3), which changes the electronic correlations in the material. The inherent sensitivity of the photogalvanic technique to structural symmetries provides evidence of the different optically driven phase of 1T-TiSe2, which allows us to assign symmetry groups to these states. Our work demonstrates that optically induced phase change can occur at extremely low optical intensities in strongly correlated materials, providing a pathway to engineer new phases using light.

Photoconductivity, pyroconductivity, and pyroelectricity effects in photodetection of layer-structured CuInP2S6/MoS2 heterojunction

Layer-structured ferroelectric material-modulated heterojunctions are promising for use as photodetectors. However, the mechanism behind their photodetection performance, including photoconductivity, pyroconductivity, and pyroelectricity effects, has not yet been fully studied. Herein, we present a CuInP2S6/MoS2 heterojunction with a layered structure, in which the narrow bandgap of MoS2 extends the detection wavelength range with a specific detectivity (D*) of 108–1010 Jones. The pyroelectric effect, dominated by polarization, introduces significant current peaks and valleys when the light is turned on and off. A competitive mechanism is discovered among the photoconductivity, pyroconductivity, and pyroelectricity effects. At low optical power intensities, the pyroconductivity effect is dominant under 405 nm light, while the pyroelectricity effect dominates under 808 nm light. However, at high optical power intensities, both the photoconductivity and pyroconductivity effects become dominant, regardless of the light wavelength. The responsivity (R), external quantum efficiency (EQE), and D* of the CuInP2S6/MoS2 heterojunction are two orders of magnitude for 405 nm light compared to 808 nm light. These results not only demonstrate the coupling effect among the photoconductivity, pyroconductivity, and pyroelectricity effects in the photodetection of layer-structured CuInP2S6/MoS2 heterojunctions but also highlight the potential applications of these heterojunctions in multifunctional devices, provided that these effects can be clearly distinguished.

Self-powered, ultraviolet-to-near infrared broadband photodetector based on Ag-doped CsPbI3/PEDOT:PSS heterojunction.

Metal halide perovskites are studied for photodetection applications because of their outstanding optical and electrical properties. A self-powered ultraviolet-to-near infrared broadband photodetector based on a Ag-doped CsPbI3/PEDOT:PSS heterojunction was investigated. The photodetector using a CsPbI3:Ag/PEDOT:PSS heterostructure with a planar photoconductive structure operated over a broad 355-1560 nm wavelength range in self-powered mode. A terahertz signal was modulated with the CsPbI3:Ag/PEDOT:PSS structure at low optical excitation intensity to investigate its photodetection mechanism. The experimentally designed detector can present images of the letters "C", "N" and "U" in the visible and near-infrared wavelengths, indicating a potential broadband imaging application.

All-optical graphene-on-silicon slot waveguide modulator based on graphene’s Kerr effect

All-optical graphene-based optical modulators have recently attracted much attention because of their ultrafast and broadband response characteristics (bandwidth larger than 100 GHz) in comparison with the previous graphene-based optical modulators, which are electrically tuned via the graphene Fermi level. Silicon photonics has some benefits such as low cost and high compatibility with CMOS design and manufacturing technology. On the other hand, graphene has a unique large nonlinear Kerr coefficient, which we calculate using graphene's tight-binding model based on the semiconductor Bloch equations. Its real and imaginary parts are negative at the wavelength of 1.55 µm and EF=0.1eV. To simultaneously use the benefits mentioned above, we present an all-optical, CMOS-compatible, and graphene-on-silicon slot (GOSS) waveguide extinction and phase modulator that consists of two different geometries. The first one consists of a one-stage GOSS waveguide with a single layer of graphene. To increase the light-graphene interaction and consequently enhance the modulation efficiency (ME), another stage of the GOSS waveguide is placed over the first one. This two-stage configuration is called a graphene-on-silicon double-slot (GOSDS) waveguide. The ME, insertion loss (IL), and modulation depth (MD) for a 12.5 µm GOSDS waveguide modulator with a double layer of graphene can reach 0.241 dB/µm, 1.31 dB, and 77%, respectively, at optical pump intensities about 9MWcm-2. Our design has a smaller waveguide length (17.6 times) than the previous all-optical graphene-on-silicon ribbon waveguide extinction modulator and high MD (about 2 times) in comparison with a graphene-clad microfiber all-optical extinction modulator. Compared with an all-optical Mach-Zehnder interferometer phase modulator, our design has short graphene coated waveguide length (≈0.1 times) and low local optical intensities (≈0.043 times) needed for π phase shift. This study may promote the design and realization of high-performance, wideband, compact, and all-optical control on a single chip with a reasonable contrast level.

A Visible‐Blind Photodetector and Artificial Optoelectronic Synapse Using Liquid‐Metal Exfoliated ZnO Nanosheets

AbstractAtomically thin 2D materials are highly sought for high‐performance electronic and optoelectronic devices. Despite being a widely recognized functional material for a plethora of applications, ultra‐thin nanosheets of zinc oxide (ZnO) at a millimeter‐scale for developing high‐performance electronic/optoelectronic devices have not been reported. This has prevented the exploration of electronic and optical properties of ZnO when it is only a few atoms thick. Here, a liquid metal exfoliation technique is used that takes advantage of the van der Waals forces between the interfacial oxide and the chosen substrate to obtain ZnO nanosheets with lateral dimensions in the millimeter scale and thickness down to 5 nm. Their suitability for applications is shown by demonstrating a visible‐blind photodetector with high figures of merit as compared to other ZnO morphologies. At extremely low operating bias of 50 mV and low optical intensity of 0.5 mW cm−2, the ZnO photodetector demonstrates an external quantum efficiency (EQE), responsivity (R), and detectivity (D*) of 4.3 × 103%, 12.64 A W−1, and 5.81 × 1015 Jones at a wavelength of 365 nm. The trap‐mediated photoresponse in the ZnO nanosheets is further utilized to demonstrate optoelectronic synapses. Versatile synaptic functions of the nervous systems are optically emulated with the ultra‐thin ZnO nanosheets.

Granular Polymeric Carbon Nitride with Carbon Vacancies for Enhanced Photocatalytic Hydrogen Evolution

Considering the high charge recombination rate, low optical adsorption intensity and limited active sites greatly constrict the solar‐to‐chemical conversion efficiency of polymer carbon nitride. Herein, a facile approach is reported to produce defected polymeric carbon nitride (PCN) with abundant granular bulks with fractured boundaries by thermal reduction treatment in CO atmosphere. The photocatalytic hydrogen evolution over defected PCN exhibits a rate of 3281.2 μmol g−1 h−1, 3.5 times higher than the pristine, which is most possibly ascribed to the following factors. The unique defected and porous structure not only provides higher specific surface area, more exposed active edges, abundant charge separation sites, and active centers for hydrogen generation but also is beneficial to rapid mass and charge transfer, interior diffusing of incident light, and shortening carrier transport length, thus enhancing the optical adsorption and accelerating the photocatalytic reaction kinetics. Furthermore, the electron delocalization at carbon vacancies sites on one hand facilitates the separation rate of electron–hole pairs and prolonged the carrier lifetime; on the other hand, the electronic polarization caused by C atoms loss helps increasing the affinity between catalyst and substrate reactant. Moreover, the higher electron donor density and lower conduction band minimum (CBM) potential further enhance the reduction capacity for H2 evolution.

Efficient terahertz generation and detection in cadmium telluride using ultrafast ytterbium laser

In this work, we study THz generation and detection using cadmium telluride (CdTe) crystals pumped by amplified (1.025 μm wavelength) and oscillator (1.045 μm wavelength) ytterbium (Yb) lasers. For each laser, we compare the performances of the CdTe THz emitter and detector to those of GaP crystals. Under optimum conditions, we demonstrate that the former shows 3 and 5 times better performances compared with the latter for detection and generation, respectively. When pumped by an amplified Yb laser, we find that the CdTe crystal is more efficient than the GaP crystal for emission at optical fluences lower than 250 μJ/cm2. Although CdTe has some limitations in comparison with GaP, such as high THz absorption above 1 THz and the appearance of two-photon absorption at relatively low optical intensity, our findings demonstrate the potential of this crystal to be used as the emitter and detector in combination with the Yb laser.

Thermo-elasto-plasto-dynamics of ultrafast optical ablation in polycrystalline metals. Part I: Theoretical formulation

Ultrafast laser-induced ablation is a complex function of many coupled parameters including laser intensity, pulse duration, optical wavelength, material thermophysical properties, and grain size. The predominant belief is that ultrafast laser ablation involves either heterogeneous or homogeneous nucleation that results in rapid phase transition or the material been superheated. However, surface morphology and fragmentation ejection by ultrafast heating with low laser fluence along with the fact that the corresponding lattice temperature is lower than the melting temperature suggest that a different removal mechanism other than phase transition and explosion is at work. Hot carrier emissions and their impact on the dissipation of the electron subsystem are also commonly ignored when describing electron and electric transports. A non-thermal ablation mechanism underlying femtosecond laser–polycrystalline material interaction and the induced surface morphology are formulated for low optical intensity using a comprehensive ablation model. The formulation follows the time scales characteristic of each of the underlying dynamics and obeys the principle of energy conservation when establishing the energy transports of the electron and lattice subsystems. The implications for properly describing low-intensity ultrafast ablation include providing the knowledge base essential for mitigating fracture and fragmentation ejection.

Atomistic modeling and rational design of optothermal tweezers for targeted applications.

Optical manipulation of micro/nanoscale objects is of importance in life sciences, colloidal science, and nanotechnology. Optothermal tweezers exhibit superior manipulation capability at low optical intensity. However, our implicit understanding of the working mechanism has limited the further applications and innovations of optothermal tweezers. Herein, we present an atomistic view of opto-thermo-electro-mechanic coupling in optothermal tweezers, which enables us to rationally design the tweezers for optimum performance in targeted applications. Specifically, we have revealed that the non-uniform temperature distribution induces water polarization and charge separation, which creates the thermoelectric field dominating the optothermal trapping. We further design experiments to systematically verify our atomistic simulations. Guided by our new model, we develop new types of optothermal tweezers of high performance using low-concentrated electrolytes. Moreover, we demonstrate the use of new tweezers in opto-thermophoretic separation of colloidal particles of the same size based on the difference in their surface charge, which has been challenging for conventional optical tweezers. With the atomistic understanding that enables the performance optimization and function expansion, optothermal tweezers will further their impacts.

Next-Generation Optical Nanotweezers for Dynamic Manipulation: From Surface to Bulk.

Optical traps based on strongly confined electromagnetic fields at metal-dielectric interfaces are far more efficient than conventional optical tweezers. Specifically, these near-field nanotweezers allow the trapping of smaller particles at lower optical intensities, which can impact diverse research fields ranging from soft condensed matter physics to materials science and biology. A major thrust in the past decade has been focused on extending the capabilities of plasmonically enhanced nanotweezers beyond diffusion-limited trapping on surfaces such as to achieve dynamic control in the bulk of fluidic environments. Here, we review the recent efforts in optical nanotweezers, especially those involving hybrid forcing schemes, covering both surface and bulk-based techniques. We summarize the important capabilities demonstrated with this promising approach, with niche applications in reconfigurable nanopatterning and on-chip assembly as well as in sorting and separating colloidal nanoparticles.

Opto-thermoelectric pulling of light-absorbing particles

Optomechanics arises from the photon momentum and its exchange with low-dimensional objects. It is well known that optical radiation exerts pressure on objects, pushing them along the light path. However, optical pulling of an object against the light path is still a counter-intuitive phenomenon. Herein, we present a general concept of optical pulling—opto-thermoelectric pulling (OTEP)—where the optical heating of a light-absorbing particle using a simple plane wave can pull the particle itself against the light path. This irradiation orientation-directed pulling force imparts self-restoring behaviour to the particles, and three-dimensional (3D) trapping of single particles is achieved at an extremely low optical intensity of 10−2 mW μm−2. Moreover, the OTEP force can overcome the short trapping range of conventional optical tweezers and optically drive the particle flow up to a macroscopic distance. The concept of self-induced opto-thermomechanical coupling is paving the way towards freeform optofluidic technology and lab-on-a-chip devices.

Nonreciprocal Transmission in Nonlinear PT‐Symmetric Metamaterials Using Epsilon‐Near‐Zero Media Doped with Defects

AbstractNonreciprocal transmission forms the basic operation mechanism of optical diodes and isolators and requires the tantalizing task of breaking the Lorentz reciprocity law. In this work, strong nonreciprocal transmission is demonstrated by using a compact nonlinear parity‐time (PT) symmetric system based on epsilon‐near‐zero (ENZ) materials photonically doped with gain and loss defects and separated by an ultrathin air gap. The nonlinear response of this scalable configuration is triggered at relatively low optical intensities due to the strong electric field confinement in the defects. The extreme asymmetric field distribution achieved upon excitation from opposite incident directions, combined with the enhanced nonlinear properties of the proposed system, results in a pronounced self‐induced nonreciprocal transmission. Cascade configurations with optimized geometrical dimensions are used to achieve self‐induced nonreciprocal transmission with a maximum contrast, ideal for the design of new all‐optical diodes. The presented robust nonreciprocal response occurs by operating at a frequency slightly shifted off the exceptional point but without breaking the PT‐symmetric phase, different compared to previous works. The findings of this work can have a plethora of applications, such as nonreciprocal ultrathin coatings for the protection of sources or other sensitive equipment from external pulsed signals, circulators, and isolators.

Gravimetry through non-linear optomechanics

Precision gravimetry is key to a number of scientific and industrial applications, including climate change research, space exploration, geological surveys and fundamental investigations into the nature of gravity. A variety of quantum systems, such as atom interferometry and on-chip-Bose–Einstein condensates have thus far been investigated to this aim. Here, we propose a new method which involves using a quantum optomechanical system for measurements of gravitational acceleration. As a proof-of-concept, we investigate the fundamental sensitivity for gravitational accelerometry of a cavity optomechanical system with a trilinear radiation pressure light-matter interaction. The phase of the optical output encodes the gravitational acceleration g and is the only component which needs to be measured. We prove analytically that homodyne detection is the optimal readout method and we predict an ideal fundamental sensitivity of Δg = 10−15 ms−2 for state-of-the-art parameters of optomechanical systems, showing that they could, in principle, surpass the best atomic interferometers even for low optical intensities. Further, we show that the scheme is strikingly robust to the initial thermal state of the oscillator.

Direct Particle Tracking Observation and Brownian Dynamics Simulations of a Single Nanoparticle Optically Trapped by a Plasmonic Nanoaperture

Optical trapping using plasmonic nanoapertures has proven to be an effective means for the contactless manipulation of nanometer-sized particles under low optical intensities. These particles have included polystyrene and silica nanospheres, proteins, coated quantum dots and magnetic nanoparticles. Here we employ fluorescence microscopy to directly observe the optical trapping process, tracking the position of a polystyrene nanosphere (20 nm diameter) trapped in water by a double nanohole (DNH) aperture in a gold film. We show that position distribution in the plane of the film has an elliptical shape. Comprehensive simulations are performed to gain insight into the trapping process, including of the distributions of the electric field, temperature, fluid velocity, optical force, and potential energy. These simulations are combined with stochastic Brownian diffusion to directly model the dynamics of the trapping process, that is, particle trajectories. We anticipate that the combination of direct particle...

Noise performance of PbS colloidal quantum dot photodetectors

We report on the noise characterization of photoconductors based on PbS colloidal quantum dots. The devices operate in the near infrared region with peak responsivity exceeding 70 A/W at 1.3 μm at low optical intensity and low voltage bias. The large responsivity, combined with the low dark current of high resistance devices, provides a specific detectivity D* as large as 1011 cm Hz1/2 W−1. The noise characteristics are investigated using noise current spectra measured at different biases both in dark and under optical excitation. The analysis revealed that the noise is clearly dominated by the flicker component up to 100 kHz. The noise performance is investigated at different optical intensities and for different device dimensions and voltage biases.

Sub-picosecond temporal resolution of anomalous Hall currents in GaAs

The anomalous Hall (AH) and spin Hall effects are important tools for the generation, control, and detection of spin and spin-polarized currents in solids and, thus, hold promises for future spintronic applications. Despite tremendous work on these effects, their ultrafast dynamic response is still not well explored. Here, we induce ultrafast AH currents in a magnetically-biased semiconductor by optical femtosecond excitation at room temperature. The currents’ dynamics are studied by detecting the simultaneously emitted THz radiation. We show that the temporal shape of the AH currents can be extracted by comparing its THz radiation to the THz radiation emitted from optically induced currents whose temporal shape is well known. We observe a complex temporal shape of the AH currents suggesting that different microscopic origins contribute to the current dynamics. This is further confirmed by photon energy dependent measurements revealing a current inversion at low optical excitation intensities. Our work is a first step towards full time resolution of AH and spin Hall currents and helps to better understand the underlying microscopic origins, being a prerequisite for ultrafast spintronic applications using such currents.

Hybrid intracerebral probe with integrated bare LED chips for optogenetic studies

This article reports on the development, i.e., the design, fabrication, and validation of an implantable optical neural probes designed for in vivo experiments relying on optogenetics. The probes comprise an array of ten bare light-emitting diode (LED) chips emitting at a wavelength of 460 nm and integrated along a flexible polyimide-based substrate stiffened using a micromachined ladder-like silicon structure. The resulting mechanical stiffness of the slender, 250-μm-wide, 65-μm-thick, and 5- and 8-mm-long probe shank facilitates its implantation into neural tissue. The LEDs are encapsulated by a fluropolymer coating protecting the implant against the physiological conditions in the brain. The electrical interface to the external control unit is provided by 10-μm-thick, highly flexible polyimide cables making the probes suitable for both acute and chronic in vivo experiments. Optical and electrical properties of the probes are reported, as well as their in vivo validation in acute optogenetic studies in transgenic mice. The depth-dependent optical stimulation of both excitatory and inhibitory neurons is demonstrated by altering the brain activity in the cortex and the thalamus. Local network responses elicited by 20-ms-long light pulses of different optical power (20 μW and 1 mW), as well as local modulation of single unit neuronal activity to 1-s-long light pulses with low optical intensity (17 μW) are presented. The ability to modulate neural activity makes these devices suitable for a broad variety of optogenetic experiments.

Laser Processing of Low Optical Reflection Micro/Nano-patterned Si Substrates for SERS

ABSTRACTLight collection efficiency and specific molecular detection are crucial factors for the performance of bio-chemical molecule sensors. In this paper, the low optical-reflection silicon (Si) substrates which combine reduced optical reflectivity by light trapping effect and high Raman enhancement ability of gold nanoparticles (Au-NPs) coated textured Si substrates are investigated for surface-enhanced Raman scattering (SERS). A fast, single-step and highly controllable nanosecond (ns) laser processing technique is employed to fabricate textured Si substrates under ambient conditions. Parallel arrays of micro-pyramids are fabricated on Si surface by direct laser writing two-dimensional structures. SEM micrographs clearly show well-ordered surface features in the form of micro-pyramid shape with well-defined sharp tips on the laser processed Si substrates. The aggregation of Si micro/nanoparticles on Si surface forms nanocavities and nanogaps and further enhances the surface roughness in order to minimize the optical reflection. The low optical reflection Si substrates exhibit optical reflection below 15% over a broad wavelength range from 300 nm to 1200 nm. The textured Si substrates with high signal reproducibility are successfully applied as SERS substrates to detect a very small concentration of Rhodamine B molecules with an average enhancement factor of the order of ∼107. The low optical reflection and SERS signal amplification are also altered by the variation of laser pulse energy resulting into low optical reflection and high SERS signal intensity over the entire laser-patterned area. The effect of surface roughness on water contact angle was studied after the modification with Polydimethylsiloxane (PDMS), the surfaces show perfect superhydrophobicity with almost no water adhesion. This approach provides a novel high-speed and cost-effective method for fabricating SERS substrate with micro/nano-scale surfaces roughness and low optical reflection for high Raman signal enhancement.

Nonlinear optical conductivity of a generic two-band system with application to doped and gapped graphene

We present a general formulation to calculate the dynamic optical conductivity, beyond the linear response regime, of any electronic system whose quasiparticle dispersion is described by a two band model. Our phenomenological model is based on the optical Bloch equations. In the steady state regime it yields an analytic solution for the population inversion and the interband coherence, which are nonlinear in the optical field intensity, including finite doping and temperature effects. We explicitly show that the optical nonlinearities are controlled by a single dimensionless parameter which is directly proportional to the incident field strength and inversely proportional to the optical frequency. This identification leads to a unified way to study the dynamical conductivity and the differential transmission spectrum across a wide range of optical frequencies, and optical field strength. We use our formalism to analytically calculate the nonlinear optical conductivity of doped and gapped graphene, deriving the well known universal ac conductivity of $\sigma_0={e^2}/4\hbar$ in the linear response regime of low optical intensities (or equivalently high frequencies) and non-linear deviations from it which appear at high laser intensities (or low frequencies) including the impact of finite doping and band-gap opening.

Optical and fundamental band gaps disparity in transparent conducting oxides: new findings for the and systems

The optical band gap, extracted from absorption measurements, defines the figure of merit for transparent conducting oxides (TCOs). In many oxides, such as or , inversion symmetry introduces a selection rule that blocks transitions from the valence-band maximum to the conduction-band minimum. This raises the absorption threshold and enlarges the optical gap relative to the fundamental band gap. Here, we present density-functional computations identifying two optical gaps, either of which can be detected, depending on the optical light intensity. Under strong illumination, weak transitions from -points near the valence-band maximum contribute significantly to the absorption spectrum and define an optical gap matching the fundamental gap. Low optical intensities by contrast give prominence to the large optical gap determined by the selection rule. While experimental conditions have favored observation of the former optical gap in , in contrast, absorption measurements in have focused on the latter. Our findings explain the disparity between the optical and fundamental gaps in bixbyite and predict that, measured under low illumination, the optical gap for rutile will increase, from 3.60 eV to 4.34 eV.