Pump Light Intensity Research Articles

Techniques for measuring transverse relaxation time of xenon atoms in nuclear-magnetic-resonance gyroscopes and pump-light influence mechanism

The transverse relaxation time of noble-gas atoms in nuclear-magnetic-resonance gyroscopes is a very important indicator for measuring the gyroscope’s performance. In this paper, the optimized free-induction-decay method, together with fitting-ratio method and magnetic-resonance-broadening method in the rotating coordinate system, is analyzed for the first time, in order to accurately measure the transverse relaxation time. An experimental device was designed, and the transverse-relaxation-time measurements were carried out by using these three methods under specific conditions. The advantages of these three methods are analyzed by comparison of experimental process and experimental results. The influence of pump-light intensity on the transverse relaxation time of noble-gas atoms is further studied on this basis, and verified through experiments.

Dynamics of non‐equilibrium charge carriers in p‐germanium doped by gallium

The low‐temperature capture processes of non‐equilibrium holes into gallium acceptors in moderately doped p‐germanium (NA ≈ 2 × 1015 cm−3) has been investigated by a single‐color pump–probe experiment using the free electron laser FELBE. The capture time decreases with increasing average photon flux density of the excitation pulse from about 10.9 ns (at ∼1.2 × 1024 cm−2 s−1) to ∼1.2 ns (∼2 × 1026 cm−2 s−1). Relaxation inside the valence band is almost independent on pump light intensity and its characteristic time is about 200 ps. In Addition, the intracenter relaxation times of the lowest excited Ga states were measured. The lifetimes scale with the phonon density of states controlling the bound hole − acoustic phonon interaction. The lifetime of the lowest excited state, , was measured to be ∼275 ps; while the lifetimes of the higher excited states, and , were found to be ∼160 ps.

Photonic band gaps structure properties of two-dimensional function photonic crystals

The tunable two-dimensional photonic crystals band gap, absolute photonic band gap and semi-Dirac point are beneficial to designing the novel optical devices. In this paper, tunable photonic band gaps structure was realized by a new type two-dimensional function photonic crystals, which dielectric constants of medium columns are functions of space coordinates. However for the two-dimensional conventional photonic crystals the dielectric constant does not change with space coordinates. As the parameter adjustment, we found that the photonic band gaps structures are dielectric constant function coefficient, medium columns radius, dielectric constant function form period number and pump light intensity dependent, namely, the photonic band gaps position and width can be tuned. we also obtained absolute photonic band gaps and semi-Dirac point in the photonic band gaps structures of two-dimensional function photonic crystals. These results provide an important theoretical foundation for design novel optical devices.

On the optical super-resolution dynamic readout effects of metal thin films

The physical understanding for retrieving super-resolution pits using metal thin films in optical information storage is given, where the reflection modulations both of bright and dark fields are demonstrated experimentally due to the reflection intensity change of metal thin films. A pump–probe setup is constructed to explore the transient reflection intensity change under different pump light intensities. The results show that the reflectivity increases for Al thin films while the reflectivity decreases for Cu and Zn thin films, corresponding to the reflection modulation of the bright and dark fields, respectively. The frequency response function also verifies that the cutoff frequency is extended through reflection modulation. This work gives an understanding on the optical super-resolution readout effect of metal thin films and is very useful in promoting the development of nano-optical information storage.

Long-range surface-plasmon-enhanced all-optical switching and beam steering through nonlinear Kerr effect

A proposition toward all-optical tuning of long-range surface-plasmon-enhanced beam shifts and all-optical switching is implemented analytically by exploitation of Kerr effect induced refractive index change through varying pump light intensity in a multilayer long-range surface-plasmon configuration at 1550 nm. Through the optimized design comprising polydiacetylene para-toulene sulfonate as a Kerr polymer, a high-contrast all-optical switch with a pump intensity threshold of 0.15 GW/cm2 is proposed. The design also provides giant spatial and angular Goos–Hanchen and Imbert–Fedorov shifts at mm and µrad ranges along with wide range of tunability of beam shifts with the varying pump light intensity and the varying incident angle. Moreover, exact beam position and beam steering considering the conjoint effect of spatial and angular Goos–Hanchen and Imbert–Fedorov shifts for different incident polarizations are also obtained. This idea proffers a new possibility for pulse generation, optical sensors applications, optical modulation, geodetic surveying, etc.

The influence of signals correlation on the long-term stability of a tandem of quantum magnetometers with laser pumping

The results of studies of the long-term frequency stability as a function of the correlation coefficient for a tandem of two quantum magnetometers with laser pumping of 87Rb in wall-coated vapour cell are represented. Measurement scheme includes a low-frequency self-generating magnetometer and a quantum microwave discriminator working at magnetic dipole transitions of radio-optical end state resonance. The difference of synchronously detected signals is processed to determine the Allan variance as a function of averaging time and correlation coefficient of signals. These parameters are essentially dependent both on the pumping light intensity and polarization and the intensity of radio fields that are produced in the working cell.

Impact of band tail distribution on carrier trapping in hydrogenated amorphous silicon for solar cell applications

Carrier trapping in hydrogenated amorphous silicon (a-Si:H) has been investigated by means of an optical pump-probe technique. The trapped carriers (electrons) at the conduction band tail are detected as an increment of the photocurrent, and their density is quantitatively determined under the assumption of carrier generation and recombination kinetics. We find that carrier trapping strongly depends on the band tail distribution in addition to the pump light intensity. Specifically, the trapped electron density increases with the Urbach energy that characterizes the valence band tail broadening. Under the condition of a pump light intensity of 10mW/cm2 operated at 532nm, the trapped electron density is determined to be ≈4×1017cm−3 for an intrinsic a-Si:H film with an Urbach energy of 45mV. The effects of carrier trapping on the device performance are studied in single-junction a-Si:H p-i-n solar cells. The results suggest that carrier trapping causes a reduction in the fill factor of the solar cells.

The design of an optical triode

Under the action of pump light, a conventional photonic crystal can be turned into a functional photonic crystal. In the paper, we have designed an optical triode with a one-dimensional functional photonic crystal, and analyzed the effect of period number, medium thickness, refractive index, incident angle, the irradiation frequency and the intensity of the pump light on the optical triode magnification. We obtain some valuable results, which shall help to optimize the design of optical triodes.

Laser-Induced Thermal Effect for Tunable Filter Employing Ferrofluid and Fiber Taper Coupler

In this letter, an all-optically tunable filter based on laser-induced thermal effect has been proposed by employing a fiber taper coupler immersed in the magnetic fluids. Tunable spectral characteristics of the optically controlled fiber filter have been theoretically analyzed and experimentally investigated. Experimental results indicate that as the pump power varies from 0 to 216.1 mW, and the proposed filter shows a large wavelength shift of 58.2 nm. Moreover, it exhibits a nonlinear spectral shift behavior in response to the applied pump light intensity with good spectral repeatability. With such desirable advantages as compactness, ease of fabrication, and high spectral tunability, our proposed all-optically tunable filter would be a promising candidate for fiber laser and amplification, fiber-optic sensing, and optical fiber communications applications.

Mathematical models of light waves in Brillouin-scattering fiber-optic gyroscope resonator

The Brillouin-scattering fiber-optic gyroscope (BFOG) is a new-generation miniaturized high-precision fiber-optic gyroscope. By focusing on the light characteristics of the BFOG resonator, we derive a recursive relationship for the intensity of light of different orders by using the coupled equations for stimulated Brillouin scattering. We then establish mathematical models of the light characteristics, threshold power analysis, and optimum resonance condition analysis. Based on these models, we derive a functional relationship between the incident pump light intensity, light intensity in the cavity, the intensity of each Stokes wave order, and emitted light intensity. Finally, we simulate and quantify the threshold power and resonance critical condition influenced by several main factors including the insertion loss, coupling coefficient, and transmission loss.

Ultrafast beam steering using gradient Au-Ge₂Sb₂Te₅-Au plasmonic resonators.

Beam steering devices have gained extensive interests in the fields of optical interconnects, communications, displays and data storages. However, the challenge lies in obtaining an ultrafast beam steering structure in the optical regime. Here, we propose phase-array-like plasmonic resonators based on metal/phase-change materials (PCMs)/metal trilayers for all-optical ultrafast beam steering in the mid-infrared (MIR) region. We numerically demonstrate an angle beam steering of 11° for transmitted wave (front lobe) and 22° for reflected wave (back lobe) by switching between the amorphous and crystalline states of the PCM (Ge₂Sb₂Te₅). A photothermal model is used to study the temporal variation of the temperature of the Ge₂Sb₂Te₅ film to show potential for switching the phase of Ge₂Sb₂Te₅ by optical heating. Generation of the beam steering in this structure exhibits a fast beam steering time of 3.6 ns under a low pump light intensity of 2.6 μW/μm₂. Our design possesses a simple geometry which can be fabricated using standard photolithography patterning and is essential for exploiting the ultrafast beam steering in various applications in the MIR regime.

An actively ultrafast tunable giant slow-light effect in ultrathin nonlinear metasurfaces

A slow-light effect based on metamaterial-induced transparency (MIT) possesses great practical applications for integrated photonic devices. However, to date, only very weak slow-light effects have been obtained in metamaterials because of the intrinsic loss of metal. Moreover, no active control of slow-light has been achieved in metamaterials. Here, we report the realization of a giant slow-light effect on an ultrathin metasurface that consists of periodic arrays of gold nanoprism dimers with a thickness of 40 nm sandwiched between a multilayer-graphene micro-sheet/zinc oxide nanoparticle layer and a monolayer graphene/polycrystalline indium tin oxide layer. The strong field confinement of the plasmonic modes associated with the MIT ensures a tremendous reduction in the group velocity around the transparency window. A group index of more than 4 × 103 is achieved, which is one order of magnitude greater than that of previous reports. A large tunable wavelength range of 120 nm is achieved around the center of the transparency window when the pump light intensity is only 1.5 kW cm−2. The response time is as fast as 42.3 ps. These results demonstrate the potential for the realization of various functional integrated photonic devices based on metasurfaces, such as all-optical buffers and all-optical switches. The achievement of a giant slow-light effect in an ultrathin metamaterial could prove useful for constructing ultrafast optical memories. Cuicui Lu and co-workers from Peking University in China made an engineered nonlinear metasurface from a periodic array of gold nanoprism dimers (40-nanometre-thick equilateral triangles with a side length of 400 nanometres). They patterned the gold nanoprisms onto a glass sheet that had been coated with a conductive film of indium tin oxide and a monolayer of graphene. The scientists then coated the prisms with zinc oxide nanoparticles and several layers of graphene. The resulting metasurface sandwich exhibited strong, ultrafast nonlinear properties. In particular, a giant, tunable slow light effect was reported with a group index of more than 4,000 and a tunable wavelength of 120 nanometres.

Ultrafast and low-power all-optical switch based on asymmetry electromagnetically induced transparency in MIM waveguide containing Kerr material

We have proposed the analog of electromagnetically induced transparency (EIT) in a metal–insulator–metal (MIM) plasmonic waveguide side coupled with two stub resonators containing Kerr material. The mechanism of the EIT-like transmission sp ectra in our structure is theoretically analyzed and numerically investigated by using the Finite-Difference Time-Domain (FDTD) method. It is found that the symmetry of the EIT-like spectra can be broken and the asymmetry degree of the EIT-like spectra can be enhanced by increasing the width of the double stub resonators. Taking advantage of the asymmetry EIT phenomenon in the proposed plasmonic system, an ultrafast and low-power all-optical switch with femtosecond-scale feedback time and a required bistable pump light intensity as low as 24.2MW/cm2 was proposed and numerically investigated. The proposed all-optical switch may find potential important applications in highly integrated optical circuits.

Spectral investigation of nonlinear local field effects in Ag nanoparticles

The capability of Ag nanoparticles to modulate their optical resonance condition, by optical nonlinearity, without an external feedback system was experimentally demonstrated. These optical nonlinearities were studied in the vicinity of the localized surface plasmon resonance (LSPR), using femtosecond pump-and-probe spectroscopy with a white-light continuum probe. Transient transmission changes ΔT/T exhibited strong photon energy and particle size dependence and showed a complex and non-monotonic change with increasing pump light intensity. Peak position and change of sign redshift with increasing pump light intensity demonstrate the modulation of the LSPR. These features are discussed in terms of the intrinsic feedback via local field enhancement.

Ultrafast tunable chirped phase-change metamaterial with a low power.

We numerically demonstrate an all-optical tunable dual-band double negative (DNG) index chirped metamaterial (MM) in the mid-infrared (M-IR) region. This MM possesses an ultrafast and significant tunability under low pump light power, realized by combining phase change material (PCM). It has a configuration of elliptical nanohole array (ENA) penetrating through metal/PCM/metal (Au-Ge(2)Sb(2)Te(5)-Au) films. Here, we consider the case when the chirp is introduced by displacing the positions of the ENA along the short axis of the elliptical apertures inside the primitive cell, which can achieve multiple internal surface-plasmon polariton (SPP) modes at the inner metal-dielectric interfaces of the structure and thus providing a dual-band negative index with simultaneous negative permittivity and permeability. The influence of amorphous and crystalline states of Ge(2)Sb(2)Te(5) on the effective optical parameters of the structure is analyzed. Switching between these states provides a large wavelength shift of the structure's effective optical parameters. A photothermal model is used to study the temporal variation of the temperature of the Ge(2)Sb(2)Te(5) layer to show a potential to switch the phase of Ge(2)Sb(2)Te(5) by optical heating. Generation of the tunable dual-band DNG index presents clear advantages as it possesses a fast tuning time of 0.4 ns, a low pump light intensity of 7.3μW/μm(2), and a large tunable wavelength range of 978 nm. We expect that our design may have potential applications in actively tunable multi-band nanodevices.

Fast Tuning of Double Fano Resonance Using A Phase-Change Metamaterial Under Low Power Intensity

In this work, we numerically demonstrate an all-optical tunable Fano resonance in a fishnet metamaterial(MM) based on a metal/phase-change material(PCM)/metal multilayer. We show that the displacement of the elliptical nanoholes from their centers can split the single Fano resonance (FR) into a double FR, exhibiting higher quality factors. The tri-layer fishnet MMs with broken symmetry accomplishes a wide tuning range in the mid-infrared(M-IR) regime by switching between the amorphous and crystalline states of the PCM (Ge2Sb2Te5). A photothermal model is used to study the temporal variation of the temperature of the Ge2Sb2Te5 film to show the potential for switching the phase of Ge2Sb2Te5 by optical heating. Generation of the tunable double FR in this asymmetric structure presents clear advantages as it possesses a fast tuning time of 0.36 ns, a low pump light intensity of 9.6 μW/μm2, and a large tunable wavelength range between 2124 nm and 3028 nm. The optically fast tuning of double FRs using phase change metamaterials(PCMMs) may have potential applications in active multiple-wavelength nanodevices in the M-IR region.

Temporal response and reflective behaviors in all-optical switch based on InAs/GaAs one-dimensional quantum-dot resonant photonic crystal

The temporal responses and the reflected digital signal behaviors of a novel InAs/GaAs one-dimensional quantum dot resonant photonic crystal (QD-RPC) with 400 periods of unit cell, consisting of an InAs quantum dot (QD) layer and other GaAs barrier layer, are theoretically investigated by using a transfer matrix method. We demonstrate that the stopband (resonant photonic band gap) has fast response and the Nyquist rate is up to 1.15THz centered at Bragg reflection wavelength 1242.3nm. The square-wave time series with the duration of 10ps reflected by the stopband is transmitted and the intensity of pump light utilized is only 0.32MW/cm2 when this structure acts as an all-optical switch for a given temperature 45K and relative size fluctuation 0.1%. In addition, the extinction ratios of this optical switch associated with temperatures and signal pulse durations are analyzed and discussed.

Ultrafast all-optical polarization QDs switch room temperature properties

All-optical switching consisting of Bragg-spaced quantum dots layers is investigated in theory. The quantum dots are applied in this switch for their spin-dependent polarization and ultrafast full recovery of the sample after passage of a circle-polarized control light. The circular dichroism and birefringence are induced, and the polarization of the signal pulse is changed, which can be applied to design the all-optical switch. This switching structure shows great advantages of lower requirement of pump light intensity, and is a promising candidate for the ultrafast all-optical switching devices at room temperature comparing to the similar quantum well structure.

All-Optical Plasmonic Switches Based on Coupled Nano-disk Cavity Structures Containing Nonlinear Material

All-optical plasmonic switches based on a novel coupled nano-disk cavity configuration containing nonlinear material are proposed and numerically investigated. The finite difference time domain simulation results reveal that the single-disk plasmonic structure can operate as an “on–off” switch with the presence/absence of pumping light. We also demonstrate that the proposed T-shaped plasmonic structure with two disk cavities can switch signal light from one port to another under an optical pumping light, functioning as a bidirectional switch. The proposed nano-disk cavity plasmonic switches have many advantages such as compact size, requirement of low pumping light intensity, and ultra-fast switching time at a femto-second scale, which are promising for future integrated plasmonic devices for applications such as communications, signal processing, and sensing.

All-optical polarization switching based on Bragg-spaced quantum dots layers

One kind of Bragg-spaced all-optical switching has been proposed in this paper, and the quantum dots ensembles are used in it as active layers. By one-dimensional photonic crystal theory and transmission matrix method, we have studied the reflectivity stop band and switching effect based on the ac Stark effect. The reflectivity stop band of this switch can be suppressed or recovered, and the circular dichroism and birefringence are induced by a circle-polarized control light, which result in a significant polarization switching effect. This switching structure shows great advantages of lower requirement of pump light intensity, larger contrast ratio than that of Bragg-spaced quantum wells with the same period, especially this switching can be used at room temperature theoretically. So we predict that there are prodigious prospects for its using in high speed optical communications as all-optical switching.