A parametrization of density matrices of ddimensions in terms of the raising J+and lowering J−angular momentum operators is established together with an implicit connection with the generalized Bloch-GellMann paramete...A parametrization of density matrices of ddimensions in terms of the raising J+and lowering J−angular momentum operators is established together with an implicit connection with the generalized Bloch-GellMann parameters. A general expression for the density matrix of the composite system of angular momenta j1and j2is obtained. In this matrix representation violations of the Bell-Clauser-Horne-Shimony-Holt inequalities are established for the X-states of a qubit-qubit, pure and mixed, composite system, as well as for a qubit-qutrit density matrix. In both cases maximal violation of the Bell inequalities can be reached, i.e., the Cirel’son limit. A correlation between the entanglement measure and a strong violation of the Bell factor is also given. For the qubit-qutrit composite system a time-dependent convex combination of the density matrix of the eigenstates of a two-particle Hamiltonian system is used to determine periodic maximal violations of the Bell’s inequality.展开更多
Magnetic films with low Gilbert damping are crucial for magnonic devices,which provide a promising platform forrealizing ultralow-energy devices.In this study,low Gilbert damping and coercive field were observed in Bi...Magnetic films with low Gilbert damping are crucial for magnonic devices,which provide a promising platform forrealizing ultralow-energy devices.In this study,low Gilbert damping and coercive field were observed in Bi/In-dopedyttrium iron garnet(BiIn:YIG)thin films.The BiIn:YIG(444)films were deposited onto different substrates using pulsedlaser deposition.Low coercivity(<1 Oe)with saturation magnetization of 125.09 emu/cc was achieved along the in-planedirection of BiIn:YIG film.The values of Gilbert damping and inhomogeneous broadening of ferromagnetic resonance inBiIn:YIG films were obtained to be as low as 4.05×10^(-4)and 5.62 Oe,respectively.In addition to low damping,the giantFaraday rotation angles(up to 2.9×10^(4)deg/cm)were also observed in the BiIn:YIG film.By modifying the magneticstructure and coupling effect between Bi^(3+)and Fe^(3+)of Bi:YIG,doped In^(3+)plays a key role on variation of the magneticproperties.The low damping and giant Faraday effect made the BiIn:YIG film an appealing candidate for magnonic andmagneto-optical devices.展开更多
Laser diodes are widely used and play a crucial role in myriad modern applications including nonlinear optics and photonics.Here,we explore the four-wave mixing effect in a laser diode gain medium induced by the feedb...Laser diodes are widely used and play a crucial role in myriad modern applications including nonlinear optics and photonics.Here,we explore the four-wave mixing effect in a laser diode gain medium induced by the feedback from the high-Q microring resonator.This phenomenon can be observed at a laser frequency scan close to the microresonator eigenfrequency,prior to the transition of the laser diode from a free-running to a self-injection locking regime.The effect opens up the possibility for generation of remarkably low-noise,stable,and adjustable microwave signals.We provide a detailed numerical study of this phenomenon proven with experimental results and demonstrate the generation of the signals in the GHz range.The obtained results reveal the stability of such regime and disclose the parameter ranges enabling to achieve it.Cumulatively,our findings uncover,to our knowledge,a novel laser diode operation regime and pave the way for the creation of new types of chip-scale,low-noise microwave sources,which are highly demanded for diverse applications,including telecommunication,metrology,and sensing.展开更多
Quantum communication provides an enormous advantage over its classical counterpart： security of communications based on the very principles of quantum mechanics. Researchers have proposed several approaches for user...Quantum communication provides an enormous advantage over its classical counterpart： security of communications based on the very principles of quantum mechanics. Researchers have proposed several approaches for user identity authentication via entanglement. Unfortunately, these protocols fail because an attacker can capture some of the particles in a transmitted sequence and send what is left to the receiver through a quantum channel. Subsequently, the attacker can restore some of the confidential messages, giving rise to the possibility of information leakage. Here we present a new robust General N user authentication protocol based on N-particle Greenberger-Horne-Zeilinger （GHZ） states, which makes eavesdropping detection more effective and secure, as compared to some current authentication protocols. The security analysis of our protocol for various kinds of attacks verifies that it is unconditionally secure, and that an attacker will not obtain any information about the transmitted key. Moreover, as the number of transferred key bits N becomes larger, while the number of users for transmitting the information is increased, the probability of effectively obtaining the transmitted authentication keys is reduced to zero.展开更多
Semiconductor devices are strong competitors in the race for the development of quantum computational systems.In this work,we interface two semiconductor building blocks of different dimensionalities with complementar...Semiconductor devices are strong competitors in the race for the development of quantum computational systems.In this work,we interface two semiconductor building blocks of different dimensionalities with complementary properties:(1)a quantum dot hosting a single exciton and acting as a nearly ideal single-photon emitter and(2)a quantum well in a 2D microcavity sustaining polaritons,which are known for their strong interactions and unique hydrodynamic properties,including ultrafast real-time monitoring of their propagation and phase mapping.In the present experiment,we can thus observe how the injected single particles propagate and evolve inside the microcavity,giving rise to hydrodynamic features typical of macroscopic systems despite their genuine intrinsic quantum nature.In the presence of a structural defect,we observe the celebrated quantum interference of a single particle that produces fringes reminiscent of wave propagation.While this behavior could be theoretically expected,our imaging of such an interference pattern,together with a measurement of antibunching,constitutes the first demonstration of spatial mapping of the self-interference of a single quantum particle impinging on an obstacle.展开更多
We introduce a programmable eight-port interferometer with the recently proposed error-tolerant architecture capable of performing a broad class of transformations.The interferometer has been fabricated with femtoseco...We introduce a programmable eight-port interferometer with the recently proposed error-tolerant architecture capable of performing a broad class of transformations.The interferometer has been fabricated with femtosecond laser writing,and it is the largest programmable interferometer of this kind to date.We have demonstrated its advantageous error tolerance by showing an operation in a broad wavelength range from 920 to 980 nm,which is particularly relevant for quantum photonics due to efficient photon sources existing in this wavelength range.Our work highlights the importance of developing novel architectures of programmable photonics for information processing.展开更多
We introduce a programmable eight-port interferometer with the recently proposed error-tolerant architecture capable of performing a broad class of transformations.The interferometer has been fabricated with femtoseco...We introduce a programmable eight-port interferometer with the recently proposed error-tolerant architecture capable of performing a broad class of transformations.The interferometer has been fabricated with femtosecond laser writing,and it is the largest programmable interferometer of this kind to date.We have demonstrated its advantageous error tolerance by showing an operation in a broad wavelength range from 920 to 980 nm,which is particularly relevant for quantum photonics due to efficient photon sources existing in this wavelength range.Our work highlights the importance of developing novel architectures of programmable photonics for information processing.展开更多
We propose a practical scheme for end-to-end optical backpropagation in neural networks. Using saturable absorption for the nonlinear units, we find that the backward-propagating gradients required to train the networ...We propose a practical scheme for end-to-end optical backpropagation in neural networks. Using saturable absorption for the nonlinear units, we find that the backward-propagating gradients required to train the network can be approximated in a surprisingly simple pump-probe scheme that requires only simple passive optical elements. Simulations show that, with readily obtainable optical depths, our approach can achieve equivalent performance to state-of-the-art computational networks on image classification benchmarks, even in deep networks with multiple sequential gradient approximation. With backpropagation through nonlinear units being an outstanding challenge to the field, this work provides a feasible path toward truly all-optical neural networks.展开更多
The stabilization and manipulation of laser frequency by means of an external cavity are nearly ubiquitously used in fundamental research and laser applications. While most of the laser light transmits through the cav...The stabilization and manipulation of laser frequency by means of an external cavity are nearly ubiquitously used in fundamental research and laser applications. While most of the laser light transmits through the cavity, in the presence of some back-scattered light from the cavity to the laser, the self-injection locking effect can take place, which locks the laser emission frequency to the cavity mode of similar frequency. The self-injection locking leads to dramatic reduction of laser linewidth and noise. Using this approach, a common semiconductor laser locked to an ultrahigh-Q microresonator can obtain sub-Hertz linewidth, on par with state-of-the-art fiber lasers. Therefore it paves the way to manufacture high-performance semiconductor lasers with reduced footprint and cost. Moreover, with high laser power, the optical nonlinearity of the microresonator drastically changes the laser dynamics, offering routes for simultaneous pulse and frequency comb generation in the same microresonator. Particularly, integrated photonics technology, enabling components fabricated via semiconductor CMOS process, has brought increasing and extending interest to laser manufacturing using this method. In this article, we present a comprehensive tutorial on analytical and numerical methods of laser self-injection locking, as well a review of most recent theoretical and experimental achievements.展开更多
The mid-infrared spectrum can be recorded from almost any material,making mid-infrared spectroscopy an extremely important and widely used sample characterization and analysis technique.However,sensitive photoconducti...The mid-infrared spectrum can be recorded from almost any material,making mid-infrared spectroscopy an extremely important and widely used sample characterization and analysis technique.However,sensitive photoconductive detectors operate primarily in the near-infrared(NIR),but not in the mid-infrared,making the NIR more favorable for accurate spectral analysis.Although the absorption cross section of vibrational modes in the NIR is orders of magnitude smaller compared to the fundamental vibrations in the mid-infrared,different concepts have been proposed to increase the detectability of weak molecular transitions overtones.Yet,the contribution of magnetophotonic structures in the NIR absorption effect has never been explored so far.Here we propose high-Q magnetophotonic structures for a supersensitive detection of weak absorption resonances in the NIR.We analyze the contributions of both magnetic and nonmagnetic photonic crystal configurations to the detection of weak molecular transitions overtones.Our results constitute an important step towards the development of highly sensitive spectroscopic tools based on high-Q magnetophotonic sensors.展开更多
We propose theoretically and demonstrate experimentally the generation of light pulses whose polarization varies temporally to cover selected areas of the Poincare´sphere with both tunable swirling speed and tota...We propose theoretically and demonstrate experimentally the generation of light pulses whose polarization varies temporally to cover selected areas of the Poincare´sphere with both tunable swirling speed and total duration(1 ps and 10 ps,respectively,in our implementation).The effect relies on the Rabi oscillations of two polariton polarized fields excited by two counter-polarized and delayed pulses.The superposition of the oscillating fields result in the precession of the Stokes vector of the emitted light while polariton lifetime imbalance results in its drift from a circle of controllable radius on the Poincare´sphere to a single point at long times.The positioning of the initial circle and final point allows to engineer the type of polarization spanning,including a full sweeping of the Poincare´sphere.The universality and simplicity of the scheme should allow for the deployment of time-varying full-Poincare´polarization fields in a variety of platforms,timescales,and regimes.展开更多
The results of an optoelectronic system—frequency-shifted feedback(FSF)laser experimental examination are presented.The considered FSF laser is seeded only with optical amplifer spontaneous emission(ASE)and operates ...The results of an optoelectronic system—frequency-shifted feedback(FSF)laser experimental examination are presented.The considered FSF laser is seeded only with optical amplifer spontaneous emission(ASE)and operates in the mode-locked regime,whereby the output radiation is sequence of short pulses with a repetition rate determined by the delay time in its optical feedback circuit.In the frequency domain,the spectrum of such a pulse sequence is an optical frequency comb(OFC).These OFCs we call initial.We consider the possibility of tunable acousto-optic(AO)dual and quad-comb frequency spacing downconversion in the FSF laser seeded with ASE and operating in the mode-locked regime.The examined system applies a single frequency shifting loop with single AO tunable flter as the frequency shifter that is fed with several radio frequency signals simultaneously.The initial OFCs with frequency spacing of about 6.5 MHz may be obtained in the wide spectral range and their width,envelope shape and position in the optical spectrum may be tuned.The dual-combs are obtained with a pair of initial OFCs aroused by two various ultrasound waves in the acousto-optic tunable flter(AOTF).The dual-combs frequency spacing is determined by the frequency diference of the signals applied to the AOTF piezoelectric transducer and can be tuned simply.The quad-combs are obtained with three initial OFCs,forming a pair of dual-combs,appearing when three ultrasound frequencies feed the AOTF transducer.The quad-combs frequency spacing is defned by the diference between the frequency spacing of dual-combs.Quad-combs with more than 5000 spectral lines and tunable frequency spacing are observed.The successive frequency downconversion gives the possibility to reduce the OFC frequency spacing form several MHz for initial OFC to tens of kHz for quad-combs.展开更多
The inverse Faraday effect induced in magnetic films by ultrashort laser pulses allows excitation and control of spins at gigahertz and sub-terahertz frequencies. The frequency of the optically excited magnetization p...The inverse Faraday effect induced in magnetic films by ultrashort laser pulses allows excitation and control of spins at gigahertz and sub-terahertz frequencies. The frequency of the optically excited magnetization precession is easily tunable by the external magnetic field. On the other hand, the initial phase of the precession marginally depends on the magnetic field. Here we demonstrate an approach for the control of the precession phase by variation of the pump beam direction. In particular, we consider the case when the magnetization precession is excited by obliquely incident pump pulses in a magnetic dielectric film placed in the in-plane magnetic field.Theoretical consideration predicts that the initial phase should appear for a non-zero in-plane component of the pump wavevector orthogonal to the external magnetic field. Experimental studies confirm this conclusion and reveal that the phase grows with increase of the in-plane wavevector component. Variation of phase by 15 deg is demonstrated. Potentially, the phase could be changed even more pronouncedly by more than 90 deg. This work provides a simple way for additional manipulation with optically excited magnetization dynamics, which is of importance for different spintronic applications.展开更多
文摘A parametrization of density matrices of ddimensions in terms of the raising J+and lowering J−angular momentum operators is established together with an implicit connection with the generalized Bloch-GellMann parameters. A general expression for the density matrix of the composite system of angular momenta j1and j2is obtained. In this matrix representation violations of the Bell-Clauser-Horne-Shimony-Holt inequalities are established for the X-states of a qubit-qubit, pure and mixed, composite system, as well as for a qubit-qutrit density matrix. In both cases maximal violation of the Bell inequalities can be reached, i.e., the Cirel’son limit. A correlation between the entanglement measure and a strong violation of the Bell factor is also given. For the qubit-qutrit composite system a time-dependent convex combination of the density matrix of the eigenstates of a two-particle Hamiltonian system is used to determine periodic maximal violations of the Bell’s inequality.
基金supported by the National Key Research and Development Program of China(Grant No.2023YFE0201000)the National Science Fund for Distinguished Young Scholars(Grant No.52225201)+2 种基金the National Natural Science Foundation of China(Grant Nos.52372004 and 52072085)the Fundamental Research Funds for the Central Universities(Grant Nos.2023FRFK06001 and HIT.BRET.2022001)Heilongjiang Touyan Innovation Team Program.
文摘Magnetic films with low Gilbert damping are crucial for magnonic devices,which provide a promising platform forrealizing ultralow-energy devices.In this study,low Gilbert damping and coercive field were observed in Bi/In-dopedyttrium iron garnet(BiIn:YIG)thin films.The BiIn:YIG(444)films were deposited onto different substrates using pulsedlaser deposition.Low coercivity(<1 Oe)with saturation magnetization of 125.09 emu/cc was achieved along the in-planedirection of BiIn:YIG film.The values of Gilbert damping and inhomogeneous broadening of ferromagnetic resonance inBiIn:YIG films were obtained to be as low as 4.05×10^(-4)and 5.62 Oe,respectively.In addition to low damping,the giantFaraday rotation angles(up to 2.9×10^(4)deg/cm)were also observed in the BiIn:YIG film.By modifying the magneticstructure and coupling effect between Bi^(3+)and Fe^(3+)of Bi:YIG,doped In^(3+)plays a key role on variation of the magneticproperties.The low damping and giant Faraday effect made the BiIn:YIG film an appealing candidate for magnonic andmagneto-optical devices.
文摘Laser diodes are widely used and play a crucial role in myriad modern applications including nonlinear optics and photonics.Here,we explore the four-wave mixing effect in a laser diode gain medium induced by the feedback from the high-Q microring resonator.This phenomenon can be observed at a laser frequency scan close to the microresonator eigenfrequency,prior to the transition of the laser diode from a free-running to a self-injection locking regime.The effect opens up the possibility for generation of remarkably low-noise,stable,and adjustable microwave signals.We provide a detailed numerical study of this phenomenon proven with experimental results and demonstrate the generation of the signals in the GHz range.The obtained results reveal the stability of such regime and disclose the parameter ranges enabling to achieve it.Cumulatively,our findings uncover,to our knowledge,a novel laser diode operation regime and pave the way for the creation of new types of chip-scale,low-noise microwave sources,which are highly demanded for diverse applications,including telecommunication,metrology,and sensing.
文摘Quantum communication provides an enormous advantage over its classical counterpart： security of communications based on the very principles of quantum mechanics. Researchers have proposed several approaches for user identity authentication via entanglement. Unfortunately, these protocols fail because an attacker can capture some of the particles in a transmitted sequence and send what is left to the receiver through a quantum channel. Subsequently, the attacker can restore some of the confidential messages, giving rise to the possibility of information leakage. Here we present a new robust General N user authentication protocol based on N-particle Greenberger-Horne-Zeilinger （GHZ） states, which makes eavesdropping detection more effective and secure, as compared to some current authentication protocols. The security analysis of our protocol for various kinds of attacks verifies that it is unconditionally secure, and that an attacker will not obtain any information about the transmitted key. Moreover, as the number of transferred key bits N becomes larger, while the number of users for transmitting the information is increased, the probability of effectively obtaining the transmitted authentication keys is reduced to zero.
基金the ERC project Elecopter grant number 780757 for financial supportCorrect Systems Lab funded by the state of Upper Austria and the Austrian Science Fund(FWF):P29603 for financial support+2 种基金the project FISR—C.N.R.“Tecnopolo di nanotecnologia e fotonica per la medicina di precisione”-CUP B83B17000010001“Progetto Tecnopolo per la Medicina di precisione,Deliberazione della Giunta Regionale n.2117 del 21/11/2018”the PRIN project Inphopol.
文摘Semiconductor devices are strong competitors in the race for the development of quantum computational systems.In this work,we interface two semiconductor building blocks of different dimensionalities with complementary properties:(1)a quantum dot hosting a single exciton and acting as a nearly ideal single-photon emitter and(2)a quantum well in a 2D microcavity sustaining polaritons,which are known for their strong interactions and unique hydrodynamic properties,including ultrafast real-time monitoring of their propagation and phase mapping.In the present experiment,we can thus observe how the injected single particles propagate and evolve inside the microcavity,giving rise to hydrodynamic features typical of macroscopic systems despite their genuine intrinsic quantum nature.In the presence of a structural defect,we observe the celebrated quantum interference of a single particle that produces fringes reminiscent of wave propagation.While this behavior could be theoretically expected,our imaging of such an interference pattern,together with a measurement of antibunching,constitutes the first demonstration of spatial mapping of the self-interference of a single quantum particle impinging on an obstacle.
基金State Atomic Energy Corporation ROSATOM(868-1.3-15/15-2021,Roadmap for Quantum computing)Russian Science Foundation(22-12-00353)。
文摘We introduce a programmable eight-port interferometer with the recently proposed error-tolerant architecture capable of performing a broad class of transformations.The interferometer has been fabricated with femtosecond laser writing,and it is the largest programmable interferometer of this kind to date.We have demonstrated its advantageous error tolerance by showing an operation in a broad wavelength range from 920 to 980 nm,which is particularly relevant for quantum photonics due to efficient photon sources existing in this wavelength range.Our work highlights the importance of developing novel architectures of programmable photonics for information processing.
基金State Atomic Energy Corporation ROSATOM(868-1.3-15/15-2021,Roadmap for Quantum computing)Russian Science Foundation(22-12-00353).
文摘We introduce a programmable eight-port interferometer with the recently proposed error-tolerant architecture capable of performing a broad class of transformations.The interferometer has been fabricated with femtosecond laser writing,and it is the largest programmable interferometer of this kind to date.We have demonstrated its advantageous error tolerance by showing an operation in a broad wavelength range from 920 to 980 nm,which is particularly relevant for quantum photonics due to efficient photon sources existing in this wavelength range.Our work highlights the importance of developing novel architectures of programmable photonics for information processing.
文摘We propose a practical scheme for end-to-end optical backpropagation in neural networks. Using saturable absorption for the nonlinear units, we find that the backward-propagating gradients required to train the network can be approximated in a surprisingly simple pump-probe scheme that requires only simple passive optical elements. Simulations show that, with readily obtainable optical depths, our approach can achieve equivalent performance to state-of-the-art computational networks on image classification benchmarks, even in deep networks with multiple sequential gradient approximation. With backpropagation through nonlinear units being an outstanding challenge to the field, this work provides a feasible path toward truly all-optical neural networks.
基金The results presented in Sections 2.5 and 3.2 were obtained with the support of the Russian Science Foundation(project 22-22-00872)The results presented in Sections 2.3,3.4 and 4 were obtained with the support of the Russian Science Foundation(Project 20-12-00344)+5 种基金Y.-H.L.acknowledges support from the China Postdoctoral Science Foundation(Grant No.2022M721482)W.L.acknowledges support from the National Natural Science Foundation of China(Grant No.62075233)the CAS Project for Young Scientists in Basic Research(Grant No.YSBR-69)J.L.acknowledges support from the National Natural Science Foundation of China(Grant No.12261131503)Shenzhen−Hong Kong Cooperation Zone for Technology and Innovation(HZQB-KCZYB2020050)from the Guangdong Provincial Key Laboratory(2019B121203002).
文摘The stabilization and manipulation of laser frequency by means of an external cavity are nearly ubiquitously used in fundamental research and laser applications. While most of the laser light transmits through the cavity, in the presence of some back-scattered light from the cavity to the laser, the self-injection locking effect can take place, which locks the laser emission frequency to the cavity mode of similar frequency. The self-injection locking leads to dramatic reduction of laser linewidth and noise. Using this approach, a common semiconductor laser locked to an ultrahigh-Q microresonator can obtain sub-Hertz linewidth, on par with state-of-the-art fiber lasers. Therefore it paves the way to manufacture high-performance semiconductor lasers with reduced footprint and cost. Moreover, with high laser power, the optical nonlinearity of the microresonator drastically changes the laser dynamics, offering routes for simultaneous pulse and frequency comb generation in the same microresonator. Particularly, integrated photonics technology, enabling components fabricated via semiconductor CMOS process, has brought increasing and extending interest to laser manufacturing using this method. In this article, we present a comprehensive tutorial on analytical and numerical methods of laser self-injection locking, as well a review of most recent theoretical and experimental achievements.
基金Israeli Innovation Authority-Kamin Program,(62045 Year 2)Russian Foundation for Basic Research(19-02-00856a)Russian Science Foundation(18-72-00233)。
文摘The mid-infrared spectrum can be recorded from almost any material,making mid-infrared spectroscopy an extremely important and widely used sample characterization and analysis technique.However,sensitive photoconductive detectors operate primarily in the near-infrared(NIR),but not in the mid-infrared,making the NIR more favorable for accurate spectral analysis.Although the absorption cross section of vibrational modes in the NIR is orders of magnitude smaller compared to the fundamental vibrations in the mid-infrared,different concepts have been proposed to increase the detectability of weak molecular transitions overtones.Yet,the contribution of magnetophotonic structures in the NIR absorption effect has never been explored so far.Here we propose high-Q magnetophotonic structures for a supersensitive detection of weak absorption resonances in the NIR.We analyze the contributions of both magnetic and nonmagnetic photonic crystal configurations to the detection of weak molecular transitions overtones.Our results constitute an important step towards the development of highly sensitive spectroscopic tools based on high-Q magnetophotonic sensors.
基金We acknowledge funding from the MIUR project Beyond Nano,the ERC Grant POLAFLOW(308136)the IEF project SQUIRREL(623708)and the support from IRSES project POLAPHEN.
文摘We propose theoretically and demonstrate experimentally the generation of light pulses whose polarization varies temporally to cover selected areas of the Poincare´sphere with both tunable swirling speed and total duration(1 ps and 10 ps,respectively,in our implementation).The effect relies on the Rabi oscillations of two polariton polarized fields excited by two counter-polarized and delayed pulses.The superposition of the oscillating fields result in the precession of the Stokes vector of the emitted light while polariton lifetime imbalance results in its drift from a circle of controllable radius on the Poincare´sphere to a single point at long times.The positioning of the initial circle and final point allows to engineer the type of polarization spanning,including a full sweeping of the Poincare´sphere.The universality and simplicity of the scheme should allow for the deployment of time-varying full-Poincare´polarization fields in a variety of platforms,timescales,and regimes.
基金Russian Science Foundation.Sections 1,2,and 3.3 were supported by Grant 23-12-00057,Sects.3.1 and 3.2 were supported by Grant 20-12-00344.
文摘The results of an optoelectronic system—frequency-shifted feedback(FSF)laser experimental examination are presented.The considered FSF laser is seeded only with optical amplifer spontaneous emission(ASE)and operates in the mode-locked regime,whereby the output radiation is sequence of short pulses with a repetition rate determined by the delay time in its optical feedback circuit.In the frequency domain,the spectrum of such a pulse sequence is an optical frequency comb(OFC).These OFCs we call initial.We consider the possibility of tunable acousto-optic(AO)dual and quad-comb frequency spacing downconversion in the FSF laser seeded with ASE and operating in the mode-locked regime.The examined system applies a single frequency shifting loop with single AO tunable flter as the frequency shifter that is fed with several radio frequency signals simultaneously.The initial OFCs with frequency spacing of about 6.5 MHz may be obtained in the wide spectral range and their width,envelope shape and position in the optical spectrum may be tuned.The dual-combs are obtained with a pair of initial OFCs aroused by two various ultrasound waves in the acousto-optic tunable flter(AOTF).The dual-combs frequency spacing is determined by the frequency diference of the signals applied to the AOTF piezoelectric transducer and can be tuned simply.The quad-combs are obtained with three initial OFCs,forming a pair of dual-combs,appearing when three ultrasound frequencies feed the AOTF transducer.The quad-combs frequency spacing is defned by the diference between the frequency spacing of dual-combs.Quad-combs with more than 5000 spectral lines and tunable frequency spacing are observed.The successive frequency downconversion gives the possibility to reduce the OFC frequency spacing form several MHz for initial OFC to tens of kHz for quad-combs.
文摘The inverse Faraday effect induced in magnetic films by ultrashort laser pulses allows excitation and control of spins at gigahertz and sub-terahertz frequencies. The frequency of the optically excited magnetization precession is easily tunable by the external magnetic field. On the other hand, the initial phase of the precession marginally depends on the magnetic field. Here we demonstrate an approach for the control of the precession phase by variation of the pump beam direction. In particular, we consider the case when the magnetization precession is excited by obliquely incident pump pulses in a magnetic dielectric film placed in the in-plane magnetic field.Theoretical consideration predicts that the initial phase should appear for a non-zero in-plane component of the pump wavevector orthogonal to the external magnetic field. Experimental studies confirm this conclusion and reveal that the phase grows with increase of the in-plane wavevector component. Variation of phase by 15 deg is demonstrated. Potentially, the phase could be changed even more pronouncedly by more than 90 deg. This work provides a simple way for additional manipulation with optically excited magnetization dynamics, which is of importance for different spintronic applications.
