Input Beam Power Research Articles

Observation of nonlinear fractal higher order topological insulator

Higher-order topological insulators (HOTIs) are unique materials hosting topologically protected states, whose dimensionality is at least by 2 lower than that of the bulk. Topological states in such insulators may be strongly confined in their corners which leads to considerable enhancement of nonlinear processes involving such states. However, all nonlinear HOTIs demonstrated so far were built on periodic bulk lattice materials. Here, we demonstrate the first nonlinear photonic HOTI with the fractal origin. Despite their fractional effective dimensionality, the HOTIs constructed here on two different types of the Sierpiński gasket waveguide arrays, may support topological corner states for unexpectedly wide range of coupling strengths, even in parameter regions where conventional HOTIs become trivial. We demonstrate thresholdless spatial solitons bifurcating from corner states in nonlinear fractal HOTIs and show that their localization can be efficiently controlled by the input beam power. We observe sharp differences in nonlinear light localization on outer and multiple inner corners and edges representative for these fractal materials. Our findings not only represent a new paradigm for nonlinear topological insulators, but also open new avenues for potential applications of fractal materials to control the light flow.

Extreme transport of light in spheroids of tumor cells

Extreme waves are intense and unexpected wavepackets ubiquitous in complex systems. In optics, these rogue waves are promising as robust and noise-resistant beams for probing and manipulating the underlying material. Localizing large optical power is crucial especially in biomedical systems, where, however, extremely intense beams have not yet been observed. We here discover that tumor-cell spheroids manifest optical rogue waves when illuminated by randomly modulated laser beams. The intensity of light transmitted through bio-printed three-dimensional tumor models follows a signature Weibull statistical distribution, where extreme events correspond to spatially-localized optical modes propagating within the cell network. Experiments varying the input beam power and size indicate that the rogue waves have a nonlinear origin. We show that these nonlinear optical filaments form high-transmission channels with enhanced transmission. They deliver large optical power through the tumor spheroid, and can be exploited to achieve a local temperature increase controlled by the input wave shape. Our findings shed light on optical propagation in biological aggregates and demonstrate how nonlinear extreme event formation allows light concentration in deep tissues, paving the way to using rogue waves in biomedical applications, such as light-activated therapies.

Quantitative analysis of the synthetic food colorants with optical nonlinearity

The self-focusing transmissions of light beams in five food dye suspensions including Red 40, Red 2, Red 4, Yellow 6, Blue 1 are studied. We qualitatively analyze the key influencing factors of the self-focusing phenomenon with five widely-used food dyes, including the concentration of suspensions, the input-beam power and the kinds of the food dyes. It is found that, the self-focusing phenomenon is mainly attributed to the shift of focus caused by thermal effects. Particularly, Red 40 presents a best nonlinear performance with a 5 cm self-focusing propagation at the wavelength of 532 nm. The significant difference of the self-focusing effects of the five food dyes are further explained by investigating their optical absorption and optical nonlinearity including nonlinear refractive index and absorption coefficient. It is found that a stronger absorption and nonlinearity, originated from the kinds of the dyes or the concentration of the dyes, leads to a stronger nonlinear self-focusing effect. Our work provides a systematic study on the quantitative analysis of the synthetic food colorants with optical nonlinearity, demonstrating a strong nonlinear self-focusing propagation and high nonlinear refractive index in red 40, red 2, red 4. This may have potential applications in nonlinear optics with soft matters.

Nematic Dispersive Shock Waves from Nonlocal to Local

The structure of optical dispersive shock waves in nematic liquid crystals is investigated as the power of the optical beam is varied, with six regimes identified, which complements previous work pertinent to low power beams only. It is found that the dispersive shock wave structure depends critically on the input beam power. In addition, it is known that nematic dispersive shock waves are resonant and the structure of this resonance is also critically dependent on the beam power. Whitham modulation theory is used to find solutions for the six regimes with the existence intervals for each identified. These dispersive shock wave solutions are compared with full numerical solutions of the nematic equations, and excellent agreement is found.

Stable vortex soliton in nonlocal media with orientational nonlinearity.

We report on the first experimental observation of stable vortex solitons in nematic liquid crystals with nonlocal nonlinear reorientational response. We show how these nonlinear vortex beams can be formed and confined in extraordinary optical waves by employing the cell with no lateral boundary conditions and the application of an external magnetic field that effectively controls the molecular direction and propagation of the self-trapped beams. We also find that these vortex solitons can be generated in certain ranges of the input beam power.

Demonstration of Bessel-like beam with variable parameters generated using cross-phase modulation.

We propose a new method to generate Bessel-like beam using cross-phase modulation. The hot rubidium atomic sample is shown to have the ability to vary a Gaussian beam (probe beam) into a Bessel-like beam when the sample is illuminated with a counter-propagating Gaussian beam (pump beam) tuned close to the atomic resonances. It is demonstrated that the Bessel-like beam exhibits self-healing after encountering an obstruction on the beam path. The parameters of the Bessel-like beam are found to be easily adjusted by the pump beam power and sample temperature. Moreover, this method is even applicable to the probe beam of low power, having more practical value than the method using self-phase modulation which needs high input beam power. The merits of variable parameter, no requirement for input beam power, simple setup, and low cost would make this method of significance in a variety of applications, especially in those areas where Bessel beam power is needed to be low and parameter to be adjusted easily without changing the setup.

Atomic vapor as a source of tunable, non-Gaussian self-reconstructing optical modes

In this manuscript, we demonstrate the ability of nonlinear light-atom interactions to produce tunably non-Gaussian, partially self-healing optical modes. Gaussian spatial-mode light tuned near to the atomic resonances in hot rubidium vapor is shown to result in non-Gaussian output mode structures that may be controlled by varying either the input beam power or the temperature of the atomic vapor. We show that the output modes exhibit a degree of self-reconstruction after encountering an obstruction in the beam path. The resultant modes are similar to truncated Bessel-Gauss modes that exhibit the ability to self-reconstruct earlier upon propagation than Gaussian modes. The ability to generate tunable, self-reconstructing beams has potential applications to a variety of imaging and communication scenarios.

Inhibition of multi-filamentation of high-power laser beams.

We experimentally demonstrate the control and complete elimination of multi-filamentation in condensed matter by varying the focusing geometry. In particular, increasing the input beam power enables the extension of the filament length without generating multi-filaments up to 1400 times the critical power in fused silica at an 800nm wavelength. Furthermore, the generated single filament exhibits spatial solitary wave behavior.

Microlens beam homogenizer for excimer laser processing

This paper reports the results of developed microlens beam homogenizer for the 248 nm excimer laser. Excimer laser has a complex irradiance profile, and it is difficult to design a homogenizer for this kind of light source. We present a method to design microlens beam homogenizer according to the propagation properties of the irradiance profile. Since the intensity distribution image of excimer laser raw beam can be photographed by the laser beam analyzer, we use matlab to analyze the image and extract relevant information to calculate. With the calculated data, we simulate a virtual excimer laser source which has the same irradiance profile as the initial beam. According to the energy spatial distribution characteristics of the virtual excimer laser source, a microlens beam homogenizer is designed. The designed microlens homogenizer has been fabricated and tested. Measurements of the irradiance profile is presented and compared to the ideal design. After shaping and homogenization, a highly uniform beam spot is obtained finally. The profile of this beam spot is a square flat-top with dimensions of approximately 17 × 17 mm2. This optical system can transform approximately 80% of the total input beam power into the work plane of output beam, and beam uniformity of better than ±3% is realized. The successful development of this optical system can provide a technical support for the excimer laser processing with high quality and efficiency.

Diode amplifier of modulated optical beam power

Analytical relations are obtained between characteristics of modulated light at the output and input of an optical diode power amplifier operating in the highly saturated gain regime. It is shown that a diode amplifier may act as an amplitude-to-phase modulation converter with a rather large bandwidth . The low sensitivity of the output power of the amplifier to the input beam power and its high energy efficiency allow it to be used as a building block of a high-power multielement laser system with coherent summation of a large number of optical beams.

Self-Focusing of Ultraintense Femtosecond Optical Vortices in Air

Our experiments show that the critical power for self-focusing collapse of femtosecond vortex beams in air is significantly higher than that of a flattop beam and grows approximately linearly with the vortex order. With less than 10% of initial transverse intensity modulation of the beam profiles, the dominant mode of self-focusing collapse is the azimuthal breakup of the vortex rings into individual filaments, the number of which grows with the input beam power. The generated bottlelike distributions of plasma filaments rotate on propagation in the direction determined by the sense of vorticity.

Realization of all-optical multi-logic functions and a digital adder with input beam power management for multi-input injection locking in a single-mode Fabry-Pérot laser diode

We propose a novel idea for the suppression of the dominant mode of the single-mode Fabry-Pérot laser diode (SMFP-LD) to realize all-optical multi-logic functions and a digital adder. The basic principle of the proposed scheme is the power management of input beams to suppress the dominant mode of the SMFP-LD for multi-input injection locking. The proposed principle is explained and implemented to realize all-optical multi-logic functions and a digital adder at an input data rate of 10 Gbps. A clear eye opening with an extinction ratio of about 12 dB and a rising-falling time of less than 40 ps are observed at the outputs. The bit error rate (BER) performance is measured for all logic gates and half adder operation. We found there is no BER floor up to BER of 10(-12) and the maximum power penalty of about 1.2 dB at a BER of 10(-9).

Self-focusing of Gaussian Laser Beam Through Collisionless Plasmas and Its Effect on Second Harmonic Generation

This paper presents an investigation of self-focusing of gaussian laser beam in a collisionless plasma and its effect on second harmonic generation. In the presence of gaussian beam, the carriers get redistributed from the high field region to low field region by ponderomotive force and transverse density gradient is established in plasma which in turn generates the plasma wave at pump frequency. This plasma wave interacts with incident laser beam and a second harmonic is generated. Furthermore, if the initial power of input beam is more than the critical power for self-focusing, the beam gets self-focused and hence the generated plasma wave and second harmonic which depend upon the background electron concentration and power of the main beam also get modified accordingly.

Nonlinearity-mediated soliton ejection from trapping potentials in nonlocal media

We address the properties of optical solitons in thermal nonlinear media with a local refractive index defect that is capable to trap solitons launched even close to the sample boundary despite the boundary-mediated forces that tend to deflect all beams toward the center of the sample. We show that while such forces become more pronounced with increasing the input beam power the defect can trap only light below a critical power above which solitons are ejected. The dynamics of soliton ejection and the subsequent propagation may be controlled.

Three-dimensional mapping of multiple filament arrays

We study experimentally and numerically the spatiotemporal dynamics of the multiple filament arrays excited by self-focusing of intense elliptical laser beams in fused silica. Our results demonstrate that although multiple filament arrays emerge as apparently regular patterns in the space domain, the spatiotemporal dynamics of the individual filaments is governed by the input-beam power and the input-beam ellipticity. In the case of moderate input-beam ellipticity, the individual filaments propagate in curved trajectories arising from skewed (spatiotemporal) coherence. The spatiotemporal propagation dynamics is regularized by increasing the input-beam ellipticity, and in part due to permanent modifications of fused silica that occur under intense irradiation. In this case, strong pulse reshaping and shock-front generation are observed, which yields a regular array of very short ($<5$ fs) superluminally propagating localized peaks in the leading front, followed by the subpulses centered on the input-pulse top, and trailed by subluminally propagating pulses with rather complex transverse intensity distribution.

Sub-critical regime of femtosecond inscription

We apply well known nonlinear diffraction theory governing focusing of a powerful light beam of arbitrary shape in medium with Kerr nonlinearity to the analysis of femtosecond (fs) laser processing of dielectric in sub-critical (input power less than the critical power of self-focusing) regime. Simple analytical expressions are derived for the input beam power and spatial focusing parameter (numerical aperture) that are required for achieving an inscription threshold. Application of non-Gaussian laser beams for better controlled fs inscription at higher powers is also discussed.

Near-field imaging of nonlinear pulse propagation in planar silica waveguides

A simplified near-field scanning optical microscope is employed to image the propagation of short laser pulses in planar silica waveguides, in the anomalous dispersion regime, under varying conditions of input beam power and width. Our results show a complex evolution of the transverse intensity profiles of the beam when there is a pronounced difference between the input diffraction and dispersion lengths. Numerical simulations confirm that these complex spatial dynamics are intimately related to the temporal and spectral evolution of the pulse.

Split-Cavity Monotrons Achieving 40 Percent Electronic Efficiency

The simplest of the microwaves tubes, the monotron consists of a circular cylindrical cavity driven by a linear electron beam. Without needing an externally applied magnetic field, and operating in circular TM/sub 0n0/ modes, the device yields the maximal electronic efficiency of 20.0%. However, an electronic efficiency of 40% can be achieved by two monotrons in tandem as this paper demonstrates. This is accomplished by using, as in RF linacs and split-cavity oscillators, a cylindrical cavity bisected by a conducting grid with RF fields oscillating p radians out of phase in the twin partitions. Specifically discussed are the design and operating characteristics of split-cavity monotrons driven by 10 keV, 50-70-A current beams. Dictated by the injection beam energy, the optimum cavity aspect ratio of length/spl I.bar/radius=0.37 is central to achieving single p-mode operation while suppressing competing TM/sub 0n0/ modes. Rooted in one-dimensional analysis, a 9.2-GHz split-cavity monotron is synthesized with the cavity capacitively coupled to an output TM/sub 01/ waveguide for external utilization of the RF power internally generated at 40.0% conversion efficiency. Numerical particle-in-cell simulation with a 0.4-cm thick, 70-A current beam crossing a transparent grid gives the overall efficiency of 35.7% relative to 700-kW input beam power.

Beam shaping with a plano-aspheric lens pair

A pair of plano-aspheric lenses can be used to transform a collimated, radially symmetric, Gaussian beam to a radially symmetric flat-top beam. Diffraction of the output beam due to the choice of irradiance profile, as well as the finite aperture of the optics, must be considered if a propagating beam is required. Choosing both lenses to be positive, one can show that the aspheric surfaces are strictly convex, which facilitates fabrication by magnetorheological figuring. A fused silica lens pair is demonstrated, which can be used at any wavelength from 250 to 1550 nm to transform a Gaussian to a flat-top beam. Measurements of both the irradiance profile and phase of the output beam are presented and compared to the ideal design. These optics transform 78% of the total input beam power into the flat-top region of the output beam, which is uniform to better than 5% rms. For applications requiring uniform illumination, this represents a fourfold improvement in power utilization over the Gaussian input. The output wavefront is flat to a quarter wave at 514 nm, resulting in a beam that propagates approximately 0.5 m without significant change in profile.

A New High Power Microwave Source Operated at Low MagneticField

A new type of high power microwave source operated at low magneticfield is proposed and studied by the particle-in-cell (PIC) method. Anoversized uniform backward-wave-oscillator-like structure is connected to a taperedslow-wave structure by a resonant cavity. In this device, the electronbeam current is effectively used to yield microwaves with highefficiency, and the mode is locked in a wide range of diode voltage toreduce the requirement to the voltage wave form. The PIC simulationresults show that a peak microwave power of 2.1 GW (averaged 1.1 GW)with a pure TM01 mode at 10.7 GHz is radiated with 4.1 GW inputbeam power, the frequency remains approximately the same when the diodevoltage changed from 300 kV to 700 kV.