Volume 8,Issue 1,2012 Table of Contents
2012, 8(1):1-3.
Abstract:
ZnS:Mn2+ nanocrystals (NCs) with particle size from 1.9 nm to 3.2 nm are synthesized via chemical precipitation method with different [S2−]/[Zn2+] ratios. The size dependent decay for Mn emission exhibits a double exponential behavior. And two lifetime values, in millisecond time domain, can both be shortened with size increasing, which is attributed to enhanced interaction between host and Mn2+ impurity. A molecular structure model is proposed to interpret the tendency of two lifetime components, which is correlated to the number of S vacancy (Vs) defects around Mn2+.
ZnS:Mn2+ nanocrystals (NCs) with particle size from 1.9 nm to 3.2 nm are synthesized via chemical precipitation method with different [S2−]/[Zn2+] ratios. The size dependent decay for Mn emission exhibits a double exponential behavior. And two lifetime values, in millisecond time domain, can both be shortened with size increasing, which is attributed to enhanced interaction between host and Mn2+ impurity. A molecular structure model is proposed to interpret the tendency of two lifetime components, which is correlated to the number of S vacancy (Vs) defects around Mn2+.
2012, 8(1):4-8.
Abstract:
ZnO nanorods prepared by a solution-phase method are annealed at different temperatures in oxygen ambient. The luminescence properties of the samples are investigated. In the same excitation condition, the photoluminescence (PL) spectra of all samples show an ultraviolet (UV) emission and a broad strong visible emission band. The asymmetric visible emission band of annealed samples has a red-shift as the annealing temperature increasing from 200 °C to 600 °C and it can be deconvoluted into two subband emissions centered at 535 nm (green emission) and 611 nm (orange-red emission) by Gaussian-fitting analysis. Analyses of PL excitation (PLE) spectra and PL spectra at different excitation wavelengths reveal that the green emission and the orange-red emission have a uniform initial state, which can be attributed to the electron transition from Zn interstitial (Zni) to oxygen vacancy (Vo) and oxygen interstitial (Oi), respectively.
ZnO nanorods prepared by a solution-phase method are annealed at different temperatures in oxygen ambient. The luminescence properties of the samples are investigated. In the same excitation condition, the photoluminescence (PL) spectra of all samples show an ultraviolet (UV) emission and a broad strong visible emission band. The asymmetric visible emission band of annealed samples has a red-shift as the annealing temperature increasing from 200 °C to 600 °C and it can be deconvoluted into two subband emissions centered at 535 nm (green emission) and 611 nm (orange-red emission) by Gaussian-fitting analysis. Analyses of PL excitation (PLE) spectra and PL spectra at different excitation wavelengths reveal that the green emission and the orange-red emission have a uniform initial state, which can be attributed to the electron transition from Zn interstitial (Zni) to oxygen vacancy (Vo) and oxygen interstitial (Oi), respectively.
2012, 8(1):9-12.
Abstract:
The output characteristics of nonlinear optical fiber-loop mirror are analyzed in detail when the pump pulses with the same wavelength are input in the both directions for recovering the clock component of the signal spectrum. It is found that the double output pulses are produced in the transmission port of the nonlinear optical fiber-loop mirror. The output pulse peaks are located in time domain at the rising and falling edges of the pump pulses. It is demonstrated that the rising and falling edges of the pump pulse can be directly extracted by this method. Through numerical simulation, the effects of the relative delay of pump pulses and the dispersion of fiber on the characteristics of output pulses are studied. By spectrum analysis, it is found that the spectrum of output pulse sequence includes the clock components of the pump pulse sequence, and a new idea is provided for all-optical clock extraction.
The output characteristics of nonlinear optical fiber-loop mirror are analyzed in detail when the pump pulses with the same wavelength are input in the both directions for recovering the clock component of the signal spectrum. It is found that the double output pulses are produced in the transmission port of the nonlinear optical fiber-loop mirror. The output pulse peaks are located in time domain at the rising and falling edges of the pump pulses. It is demonstrated that the rising and falling edges of the pump pulse can be directly extracted by this method. Through numerical simulation, the effects of the relative delay of pump pulses and the dispersion of fiber on the characteristics of output pulses are studied. By spectrum analysis, it is found that the spectrum of output pulse sequence includes the clock components of the pump pulse sequence, and a new idea is provided for all-optical clock extraction.
Zheng Yi-bo
,
YAO Jian-qua
,
ZHANG Lei
,
WANG Yuan
,
WEN Wu-qi
,
JING Lei
,
DI Zhi-gang
,
KANG Jian-yi
2012, 8(1):13-16.
Abstract:
The thermal properties of photonic crystal fiber (PCF) laser with 18 circularly distributed cores are investigated by using full-vector finite element method (FEM). The results show that the 18-core PCF has a more effective thermal dispersion construction compared with the single core PCF and 19-core PCF. In addition, the temperature distribution of 18-core PCF laser with different thermal loads is simulated. The results show that the core temperature approaches the fiber drawing value of 1800 K approximately when the thermal load is above 80 W/m which corresponds to the pumping power of 600 W approximately, while the coating temperature approaches the damage value of about 550 K when the thermal load is above 15 W/m which corresponds to the pumping power of 110 W approximately. Therefore the fiber cooling is necessary to achieve power scaling. Compared with other different cooling systems, the copper cooling scheme is found to be an effective method to reduce the thermal effects.
The thermal properties of photonic crystal fiber (PCF) laser with 18 circularly distributed cores are investigated by using full-vector finite element method (FEM). The results show that the 18-core PCF has a more effective thermal dispersion construction compared with the single core PCF and 19-core PCF. In addition, the temperature distribution of 18-core PCF laser with different thermal loads is simulated. The results show that the core temperature approaches the fiber drawing value of 1800 K approximately when the thermal load is above 80 W/m which corresponds to the pumping power of 600 W approximately, while the coating temperature approaches the damage value of about 550 K when the thermal load is above 15 W/m which corresponds to the pumping power of 110 W approximately. Therefore the fiber cooling is necessary to achieve power scaling. Compared with other different cooling systems, the copper cooling scheme is found to be an effective method to reduce the thermal effects.
2012, 8(1):17-20.
Abstract:
A wavelength splitter with ultra-compact and simple structure is proposed and analyzed by using both plane wave expansion (PWE) method and finite difference time domain (FDTD) method. The device is based on directional coupling between two parallel lithium niobate (LiNbO3, LN) nanowire optical waveguides. The wavelength splitter with a coupling region length of 5 m can separate 1.31μm and 1.55μm wavelengths for corresponding outputs with transmittance higher than 97%.
A wavelength splitter with ultra-compact and simple structure is proposed and analyzed by using both plane wave expansion (PWE) method and finite difference time domain (FDTD) method. The device is based on directional coupling between two parallel lithium niobate (LiNbO3, LN) nanowire optical waveguides. The wavelength splitter with a coupling region length of 5 m can separate 1.31μm and 1.55μm wavelengths for corresponding outputs with transmittance higher than 97%.
2012, 8(1):21-24.
Abstract:
The tunable micro-cavity based on one-dimensional (1D) photonic crystal doped by KTP is designed. The optical transmission properties in the doped one-dimensional defect photonic crystals are analyzed using transfer matrix method (TMM). According to the electro-optic effect, the refractive index ellipsoid equation is established with the applied alternating current at both coordinate axes, and the characteristics of temperature-optics and modulation are studied. Numerical calculations and experimental results show that the tuning range is ∼40 nm.
The tunable micro-cavity based on one-dimensional (1D) photonic crystal doped by KTP is designed. The optical transmission properties in the doped one-dimensional defect photonic crystals are analyzed using transfer matrix method (TMM). According to the electro-optic effect, the refractive index ellipsoid equation is established with the applied alternating current at both coordinate axes, and the characteristics of temperature-optics and modulation are studied. Numerical calculations and experimental results show that the tuning range is ∼40 nm.
2012, 8(1):25-28.
Abstract:
We present a novel mechanism,which is formed by periodically changing the radii of dielectric rods in the middle row of a photonic crystal, to control and stop light. Using the Bloch theory and coupled-mode theory, the dispersion characteristic of such a photonic crystal coupled cavity optical waveguide is obtained. We also theoretically demonstrate that the group velocity of a light pulse in this system can be modulated by dynamically changing the refractive index or radii of the selected dielectric rods, and the light stopping can be achieved.
We present a novel mechanism,which is formed by periodically changing the radii of dielectric rods in the middle row of a photonic crystal, to control and stop light. Using the Bloch theory and coupled-mode theory, the dispersion characteristic of such a photonic crystal coupled cavity optical waveguide is obtained. We also theoretically demonstrate that the group velocity of a light pulse in this system can be modulated by dynamically changing the refractive index or radii of the selected dielectric rods, and the light stopping can be achieved.
2012, 8(1):29-32.
Abstract:
The characteristics of noncollinear phase matching and quasi-phase-matching in the THz-wave parametric oscillator (TPO) are investigated. The expression of the effective parametric gain length under the condition of noncollinear phase matching configuration is deduced. The relationship between the poling period of periodically poled LiNbO3 crystal and the generated THz frequency under the condition of quasi-phase-matching configuration is analyzed. Based on the analyses above we propose a new TPO configuration which ensures the three mixing waves interact collinearly. The effects of operation temperature on phase matching are analyzed.
The characteristics of noncollinear phase matching and quasi-phase-matching in the THz-wave parametric oscillator (TPO) are investigated. The expression of the effective parametric gain length under the condition of noncollinear phase matching configuration is deduced. The relationship between the poling period of periodically poled LiNbO3 crystal and the generated THz frequency under the condition of quasi-phase-matching configuration is analyzed. Based on the analyses above we propose a new TPO configuration which ensures the three mixing waves interact collinearly. The effects of operation temperature on phase matching are analyzed.
2012, 8(1):33-36.
Abstract:
A novel fiber laser spectral beam combining scheme based on a concave grating is presented. The principle of the presented system is analyzed, and a concave grating with blazed structure for spectral beam combining is designed. The combining potential of the system is analyzed, and the results show that 39 Yb-doped fiber laser can be spectrally beam combined via the designed system. By using scalar diffraction theory, the combining effect of the system is analyzed. The results show that the diffraction efficiency of the designed concave grating is higher than 72% over the whole gain bandwidth, and the combining efficiency is 73.4%. With output power of 1 kW for individual fiber laser, combined power of 28.6 kW can be achieved.
A novel fiber laser spectral beam combining scheme based on a concave grating is presented. The principle of the presented system is analyzed, and a concave grating with blazed structure for spectral beam combining is designed. The combining potential of the system is analyzed, and the results show that 39 Yb-doped fiber laser can be spectrally beam combined via the designed system. By using scalar diffraction theory, the combining effect of the system is analyzed. The results show that the diffraction efficiency of the designed concave grating is higher than 72% over the whole gain bandwidth, and the combining efficiency is 73.4%. With output power of 1 kW for individual fiber laser, combined power of 28.6 kW can be achieved.
2012, 8(1):37-39.
Abstract:
A new kind of glass-ceramic phosphor, which contains crystalline phases with green emissions, is explored. The glassceramic is prepared through semi-melt-quenching procedure with a nominal composition of (Ca0.99Eu0.01)3Si2O7. The greenemitting crystals are precipitated and identified to be β-Ca2SiO4: Eu2+ which is responsible for 510 nm-peaked much broader band emissions holding at 1550 °C for half an hour. In terms of the available light scattering theory, the appearance of opaque is discussed by closely associating with size and morphology of luminous β-Ca2SiO4 crystalline phase in glass.
A new kind of glass-ceramic phosphor, which contains crystalline phases with green emissions, is explored. The glassceramic is prepared through semi-melt-quenching procedure with a nominal composition of (Ca0.99Eu0.01)3Si2O7. The greenemitting crystals are precipitated and identified to be β-Ca2SiO4: Eu2+ which is responsible for 510 nm-peaked much broader band emissions holding at 1550 °C for half an hour. In terms of the available light scattering theory, the appearance of opaque is discussed by closely associating with size and morphology of luminous β-Ca2SiO4 crystalline phase in glass.
2012, 8(1):40-42.
Abstract:
An all-optical transient waveform equivalent time sampling system based on cross-gain modulation (XGM) in semiconductor optical amplifier (SOA) is presented. A noisy SOA dynamic model and PIN equivalent circuit function are employed for system evaluating. The results show this SOA-XGM sampler with subtracting postprocessing can achieve picosecond sampling window. The shape of sampling window can be adjusted by SOA bias current and amplitude of control pulse. Compared with amplified spontaneous emission (ASE) noise of SOA and thermal noise in PIN, the jitter of sampling control pulse causes most sampling error. Simulations show that this SOA-XGM sampler can be a capable candidate for transient waveform sampling.
An all-optical transient waveform equivalent time sampling system based on cross-gain modulation (XGM) in semiconductor optical amplifier (SOA) is presented. A noisy SOA dynamic model and PIN equivalent circuit function are employed for system evaluating. The results show this SOA-XGM sampler with subtracting postprocessing can achieve picosecond sampling window. The shape of sampling window can be adjusted by SOA bias current and amplitude of control pulse. Compared with amplified spontaneous emission (ASE) noise of SOA and thermal noise in PIN, the jitter of sampling control pulse causes most sampling error. Simulations show that this SOA-XGM sampler can be a capable candidate for transient waveform sampling.
2012, 8(1):43-47.
Abstract:
One of the key problems to hinder the realization of optical burst switching (OBS) technology in the core networks is the losses due to the contention among the bursts at the core nodes. Burst segmentation is an effective contention resolution technique used to reduce the number of packets lost due to the burst losses. In our work, a burst segmentation-deflection routing contention resolution mechanism in OBS networks is proposed. When the contention occurs, the bursts are segmented according to the lowest packet loss probability of networks firstly, and then the segmented burst is deflected on the optimum routing. An analytical model is proposed to evaluate the contention resolution mechanism. Simulation results show that high-priority bursts have significantly lower packet loss probability and transmission delay than the low-priority. And the performance of the burst lengths, in which the number of segments per burst distributes geometrically, is more effective than that of the deterministically distributed burst lengths.
One of the key problems to hinder the realization of optical burst switching (OBS) technology in the core networks is the losses due to the contention among the bursts at the core nodes. Burst segmentation is an effective contention resolution technique used to reduce the number of packets lost due to the burst losses. In our work, a burst segmentation-deflection routing contention resolution mechanism in OBS networks is proposed. When the contention occurs, the bursts are segmented according to the lowest packet loss probability of networks firstly, and then the segmented burst is deflected on the optimum routing. An analytical model is proposed to evaluate the contention resolution mechanism. Simulation results show that high-priority bursts have significantly lower packet loss probability and transmission delay than the low-priority. And the performance of the burst lengths, in which the number of segments per burst distributes geometrically, is more effective than that of the deterministically distributed burst lengths.
2012, 8(1):48-51.
Abstract:
Tianjin University of Technology and Springer-Verlag Berlin Heidelberg 2012 C We propose the sub-picosecond chirped soliton pulse propagation in concave-dispersion-flattened fibers (CDFF). The effects of pulse characteristics and the fiber dispersion parameters on propagation characteristics of the chirped soliton pulse are numerically investigated in the CDFF by the split-step Fourier method (SSFM). The unchirped soliton pulse can stably propagate with unchanged pulse width in the CDFF. The temporal full width at half maximum (FWHM) of the chirped soliton performs a damped oscillation with the increase of propagation distance. The period and amplitude of the oscillation increase with the increase of the chirp parameter |C|. The effect of high-order dispersion (β 3−β 6) on soliton propagation characteristics can be neglected. The soliton pulse slightly broadens with the increase of propagation distance and still maintains soliton characteristics when the fiber loss (ATT) is further considered. The variation of root-meansquare (RMS) spectral width with propagation distance is opposite to that of the temporal width. The output spectrum of soliton has a single peak for the unchirped case, while has multi-peak for chirped case. The temporal width of the soliton obviously increases with the increase of the initial width, decreases with the increase of dispersion peak D 0 of the fiber, and slightly increases with the decrease of dispersion coefficients k 1 and k 2 of the fiber.
Tianjin University of Technology and Springer-Verlag Berlin Heidelberg 2012 C We propose the sub-picosecond chirped soliton pulse propagation in concave-dispersion-flattened fibers (CDFF). The effects of pulse characteristics and the fiber dispersion parameters on propagation characteristics of the chirped soliton pulse are numerically investigated in the CDFF by the split-step Fourier method (SSFM). The unchirped soliton pulse can stably propagate with unchanged pulse width in the CDFF. The temporal full width at half maximum (FWHM) of the chirped soliton performs a damped oscillation with the increase of propagation distance. The period and amplitude of the oscillation increase with the increase of the chirp parameter |C|. The effect of high-order dispersion (β 3−β 6) on soliton propagation characteristics can be neglected. The soliton pulse slightly broadens with the increase of propagation distance and still maintains soliton characteristics when the fiber loss (ATT) is further considered. The variation of root-meansquare (RMS) spectral width with propagation distance is opposite to that of the temporal width. The output spectrum of soliton has a single peak for the unchirped case, while has multi-peak for chirped case. The temporal width of the soliton obviously increases with the increase of the initial width, decreases with the increase of dispersion peak D 0 of the fiber, and slightly increases with the decrease of dispersion coefficients k 1 and k 2 of the fiber.
2012, 8(1):52-55.
Abstract:
Based on the principle of virtual equivalent gain, the evolution of self-similar pulses in a normal dispersion-decreasing fiber is investigated. The occurrence of wave breaking during the process under highly nonlinear conditions and the effect of its revealed four-wave mixing on the process are also analyzed. The results indicate that the pulse spectrum broadening in the initial stage is dominated by self-phase modulation while by four-wave mixing in the later stage. The more intense nonlinearity is, the faster the pulse evolution converges to the self-similarity synchronously in both the time domain and the spectrum domain.
Based on the principle of virtual equivalent gain, the evolution of self-similar pulses in a normal dispersion-decreasing fiber is investigated. The occurrence of wave breaking during the process under highly nonlinear conditions and the effect of its revealed four-wave mixing on the process are also analyzed. The results indicate that the pulse spectrum broadening in the initial stage is dominated by self-phase modulation while by four-wave mixing in the later stage. The more intense nonlinearity is, the faster the pulse evolution converges to the self-similarity synchronously in both the time domain and the spectrum domain.
2012, 8(1):56-59.
Abstract:
An all-optical format conversion from non-return-to-zero (NRZ) to return-to-zero (RZ) is presented based on cross-phase modulation (XPM) in a silicon waveguide with a detuned optical bandpass filter (OBPF). The simulation results show that the tunable bandwidth of the OBPF leads to RZ signals with tunable pulse width. The conversion efficiency (CE) and the pattern effect of the RZ signal are attributed to the parameters of the pump pulse and the OBPF. The converted RZ signal exhibits lower timing jitter than the NRZ signal.
An all-optical format conversion from non-return-to-zero (NRZ) to return-to-zero (RZ) is presented based on cross-phase modulation (XPM) in a silicon waveguide with a detuned optical bandpass filter (OBPF). The simulation results show that the tunable bandwidth of the OBPF leads to RZ signals with tunable pulse width. The conversion efficiency (CE) and the pattern effect of the RZ signal are attributed to the parameters of the pump pulse and the OBPF. The converted RZ signal exhibits lower timing jitter than the NRZ signal.
2012, 8(1):60-62.
Abstract:
Design and realization of random measurement scheme for compressed sensing (CS) are presented in this paper, and lower limits of the measurement number are achieved when the precise reconstruction is realized. Four kinds of random measurement matrices are designed according to the constraint conditions of random measurement. The performance is tested employing the algorithm of stagewise orthogonal matching pursuit (StOMP). Results of the experiment show that lower limits of the measurement number are much better than the results described in Refs.[13–15]. When the ratios of measurement to sparsity are 3.8 and 4.0, the mean relative errors of the reconstructed signals are 8.57 × 10−13 and 2.43 × 10−14, respectively, which confirms that the random measurement scheme of this paper is very effective.
Design and realization of random measurement scheme for compressed sensing (CS) are presented in this paper, and lower limits of the measurement number are achieved when the precise reconstruction is realized. Four kinds of random measurement matrices are designed according to the constraint conditions of random measurement. The performance is tested employing the algorithm of stagewise orthogonal matching pursuit (StOMP). Results of the experiment show that lower limits of the measurement number are much better than the results described in Refs.[13–15]. When the ratios of measurement to sparsity are 3.8 and 4.0, the mean relative errors of the reconstructed signals are 8.57 × 10−13 and 2.43 × 10−14, respectively, which confirms that the random measurement scheme of this paper is very effective.
2012, 8(1):63-66.
Abstract:
A scheme is proposed for involving programmable quantum logic gates via teleportation, which is a unique technique in quantum mechanics. In our scheme, considering the inevitable decoherence caused by noisy environment, the quantum states are not maximally entangled. We show the implementation of single qubit quantum gates and controlled-NOT (CNOT) gate, which are universal quantum gates. Hence, any quantum gate can be implemented by using teleportation with non-maximally entangled states. Furthermore, two schemes in differet connections of universal gates are proposed and compared, and our results show the parallel connection outperforms the cascade connection.
A scheme is proposed for involving programmable quantum logic gates via teleportation, which is a unique technique in quantum mechanics. In our scheme, considering the inevitable decoherence caused by noisy environment, the quantum states are not maximally entangled. We show the implementation of single qubit quantum gates and controlled-NOT (CNOT) gate, which are universal quantum gates. Hence, any quantum gate can be implemented by using teleportation with non-maximally entangled states. Furthermore, two schemes in differet connections of universal gates are proposed and compared, and our results show the parallel connection outperforms the cascade connection.
2012, 8(1):67-71.
Abstract:
Based on the definition of fractional Fourier transform (FrFT) in the cylindrical coordinate system, the propagation properties of a controllable dark-hollow beam (CDHB) are investigated in detail. An analytical formula is derived for the FrFT of a CDHB. By using the derived formula, the properties of a CDHB in the FrFT plane are illustrated numerically. The results show that the properties of the intensity of the beam in the FrFT are closely related to not only the fractional order but also initial beam parameter, beam order and the lens focal length of the optical system for performing FrFT. The derived formula provides an effective and convenient way for analyzing and calculating the FrFT of a CDHB.
Based on the definition of fractional Fourier transform (FrFT) in the cylindrical coordinate system, the propagation properties of a controllable dark-hollow beam (CDHB) are investigated in detail. An analytical formula is derived for the FrFT of a CDHB. By using the derived formula, the properties of a CDHB in the FrFT plane are illustrated numerically. The results show that the properties of the intensity of the beam in the FrFT are closely related to not only the fractional order but also initial beam parameter, beam order and the lens focal length of the optical system for performing FrFT. The derived formula provides an effective and convenient way for analyzing and calculating the FrFT of a CDHB.
2012, 8(1):72-75.
Abstract:
In this paper we study the influence of the relative phase between the probe and driving fields on propagation effect in an open Doppler broadening V-type three-level atomic system with spontaneously generated coherence (SGC) by using the calculation result of the density matrix motion equations and the propagation equations of the driving and probe fields. It is shown that the relative phase (Φ) has remarkable periodical influence on the propagation effect, and the period is 2π. By selecting appropriate value of Φ, we can get larger lasing without inversion (LWI) gain and longer propagation distance in which gain exists, and hence obtain higher probe field (i.e. LWI) intensity. When Φ=π/2, the largest LWI gain and probe field intensity can be got. In addition, the atomic exit rate (γ 0) and ratio (S) of the atomic injection rates also have a considerable modulation role on the phase-dependent propagation effect. In certain value range of γ 0 (S), LWI gain and probe field intensity increase with γ 0 (S) increasing. In the open system, LWI gain and probe field intensity much larger than those in the corresponding closed system can be obtained.
In this paper we study the influence of the relative phase between the probe and driving fields on propagation effect in an open Doppler broadening V-type three-level atomic system with spontaneously generated coherence (SGC) by using the calculation result of the density matrix motion equations and the propagation equations of the driving and probe fields. It is shown that the relative phase (Φ) has remarkable periodical influence on the propagation effect, and the period is 2π. By selecting appropriate value of Φ, we can get larger lasing without inversion (LWI) gain and longer propagation distance in which gain exists, and hence obtain higher probe field (i.e. LWI) intensity. When Φ=π/2, the largest LWI gain and probe field intensity can be got. In addition, the atomic exit rate (γ 0) and ratio (S) of the atomic injection rates also have a considerable modulation role on the phase-dependent propagation effect. In certain value range of γ 0 (S), LWI gain and probe field intensity increase with γ 0 (S) increasing. In the open system, LWI gain and probe field intensity much larger than those in the corresponding closed system can be obtained.
2012, 8(1):76-80.
Abstract:
Radiation forces and trapping stability of metallic (i.e. gold) Rayleigh particle by combined beam are analyzed, and the combined beam is formed by superimposing two partially coherent off-axis flat-topped beams. The dependences of radiation forces on off-axis distance parameter, correlation length and particle radius are illustrated by numerical examples. The results show that there exist critical values d 0,c and σ 0,c for the combined beam. For 0<d⩽d 0,c or 0<σ 0⩽σ 0,c the Gaussianlike intensity profile takes place at the geometrical focal plane, so that the transverse gradient force can act as restoring force. As the off-axis distance parameter increases or the correlation length decreases, the maximal intensity, the radiation force and trapping stiffness become smaller, while the transverse and longitudinal trapping ranges become larger. In comparison with a single beam, the combined beam is more favourable for trapping metallic Rayleigh particle owing to the stronger trapping stiffness and the larger trapping range.
Radiation forces and trapping stability of metallic (i.e. gold) Rayleigh particle by combined beam are analyzed, and the combined beam is formed by superimposing two partially coherent off-axis flat-topped beams. The dependences of radiation forces on off-axis distance parameter, correlation length and particle radius are illustrated by numerical examples. The results show that there exist critical values d 0,c and σ 0,c for the combined beam. For 0<d⩽d 0,c or 0<σ 0⩽σ 0,c the Gaussianlike intensity profile takes place at the geometrical focal plane, so that the transverse gradient force can act as restoring force. As the off-axis distance parameter increases or the correlation length decreases, the maximal intensity, the radiation force and trapping stiffness become smaller, while the transverse and longitudinal trapping ranges become larger. In comparison with a single beam, the combined beam is more favourable for trapping metallic Rayleigh particle owing to the stronger trapping stiffness and the larger trapping range.
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