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1.
J Phys Chem A ; 123(30): 6491-6495, 2019 Aug 01.
Artigo em Inglês | MEDLINE | ID: mdl-31329435

RESUMO

The investigation of the photoelectron circular dichroism (PECD) in the strong field regime (800 nm, 6.9 × 1013 W/cm2) on methyloxirane (MOX) reveals a flip of the sign of PECD between different fragmentation channels. This finding is of great importance for future experiments and applications in chemistry or pharmacy using PECD in the strong field regime as analysis method. We suggest that the observed sign change of PECD is not caused by ionization from different orbitals but by effectively selecting differently oriented nonisotropic subsamples of molecules via the fragmentation channel.

2.
Opt Express ; 27(7): 10009-10021, 2019 Apr 01.
Artigo em Inglês | MEDLINE | ID: mdl-31045148

RESUMO

Precise knowledge of modal behavior is of essential importance for understanding light guidance, particularly in hollow-core fibers. Here we present a semi-analytical model that allows determination of bands formed in revolver-type anti-resonant hollow-core fibers. The approach is independent of the actual arrangement of the anti-resonant elements, does not enforce artificial lattice arrangements and allows determination of the effective indices of modes of preselected order. The simulations show two classes of modes: (i) low-order modes exhibiting effective indices with moderate slopes and (ii) a high number of high-order modes with very strong effective index dispersion, forming a quasi-continuum of modes. It is shown that the mode density scales with the square of the normalized frequency, being to some extent similar to the behavior of multimode fibers.

3.
Appl Opt ; 57(29): 8529-8535, 2018 Oct 10.
Artigo em Inglês | MEDLINE | ID: mdl-30461919

RESUMO

Understanding polarization in waveguides is of fundamental importance for any photonic device and is particularly relevant within the scope of fiber optics. Here, we investigate the dependence of the geometry-induced polarization behavior of single-ring antiresonant hollow-core fibers on various parameters from the experimental perspective, showing that structural deviations from an ideal polygonal shape impose birefringence and polarization-dependent loss, confirmed by a toy model. The minimal output ellipticity was found at the wavelength of lowest loss near the center of the transmission band, whereas birefringence substantially increases toward the resonances. The analysis that qualitatively also applies to other kinds of hollow-core fibers showed that maximizing the amount of linearly polarized light at the fiber output demands both operating at the wavelength of lowest loss, as well as carefully choosing the relative orientation of input polarization. This should correspond to the situation in which the difference of the core extent along the two corresponding orthogonal polarization directions is minimal. Due to their practical relevance, we expect our findings to be very important in fields such as nonlinear photonics or metrology.

4.
Sci Adv ; 4(4): eaaq1526, 2018 04.
Artigo em Inglês | MEDLINE | ID: mdl-29713685

RESUMO

Spectroscopy in the wavelength range from 2 to 11 µm (900 to 5000 cm-1) implies a multitude of applications in fundamental physics, chemistry, as well as environmental and life sciences. The related vibrational transitions, which all infrared-active small molecules, the most common functional groups, as well as biomolecules like proteins, lipids, nucleic acids, and carbohydrates exhibit, reveal information about molecular structure and composition. However, light sources and detectors in the mid-infrared have been inferior to those in the visible or near-infrared, in terms of power, bandwidth, and sensitivity, severely limiting the performance of infrared experimental techniques. This article demonstrates the generation of femtosecond radiation with up to 5 W at 4.1 µm and 1.3 W at 8.5 µm, corresponding to an order-of-magnitude average power increase for ultrafast light sources operating at wavelengths longer than 5 µm. The presented concept is based on power-scalable near-infrared lasers emitting at a wavelength near 1 µm, which pump optical parametric amplifiers. In addition, both wavelength tunability and supercontinuum generation are reported, resulting in spectral coverage from 1.6 to 10.2 µm with power densities exceeding state-of-the-art synchrotron sources over the entire range. The flexible frequency conversion scheme is highly attractive for both up-conversion and frequency comb spectroscopy, as well as for a variety of time-domain applications.

5.
J Phys Chem Lett ; 8(13): 2780-2786, 2017 Jul 06.
Artigo em Inglês | MEDLINE | ID: mdl-28582620

RESUMO

Most large molecules are chiral in their structure: they exist as two enantiomers, which are mirror images of each other. Whereas the rovibronic sublevels of two enantiomers are almost identical (neglecting a minuscular effect of the weak interaction), it turns out that the photoelectric effect is sensitive to the absolute configuration of the ionized enantiomer. Indeed, photoionization of randomly oriented enantiomers by left or right circularly polarized light results in a slightly different electron flux parallel or antiparallel with respect to the photon propagation direction-an effect termed photoelectron circular dichroism (PECD). Our comprehensive study demonstrates that the origin of PECD can be found in the molecular frame electron emission pattern connecting PECD to other fundamental photophysical effects such as the circular dichroism in angular distributions (CDAD). Accordingly, distinct spatial orientations of a chiral molecule enhance the PECD by a factor of about 10.

6.
Light Sci Appl ; 6(12): e17124, 2017 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-30167225

RESUMO

Ultrafast supercontinuum generation in gas-filled waveguides is an enabling technology for many intriguing applications ranging from attosecond metrology towards biophotonics, with the amount of spectral broadening crucially depending on the pulse dispersion of the propagating mode. In this study, we show that structural resonances in a gas-filled antiresonant hollow core optical fiber provide an additional degree of freedom in dispersion engineering, which enables the generation of more than three octaves of broadband light that ranges from deep UV wavelengths to near infrared. Our observation relies on the introduction of a geometric-induced resonance in the spectral vicinity of the ultrafast pump laser, outperforming gas dispersion and yielding a unique dispersion profile independent of core size, which is highly relevant for scaling input powers. Using a krypton-filled fiber, we observe spectral broadening from 200 nm to 1.7 µm at an output energy of ∼ 23 µJ within a single optical mode across the entire spectral bandwidth. Simulations show that the frequency generation results from an accelerated fission process of soliton-like waveforms in a non-adiabatic dispersion regime associated with the emission of multiple phase-matched Cherenkov radiations on both sides of the resonance. This effect, along with the dispersion tuning and scaling capabilities of the fiber geometry, enables coherent ultra-broadband and high-energy sources, which range from the UV to the mid-infrared spectral range.

7.
Proc Natl Acad Sci U S A ; 113(51): 14651-14655, 2016 12 20.
Artigo em Inglês | MEDLINE | ID: mdl-27930299

RESUMO

Quantum tunneling is a ubiquitous phenomenon in nature and crucial for many technological applications. It allows quantum particles to reach regions in space which are energetically not accessible according to classical mechanics. In this "tunneling region," the particle density is known to decay exponentially. This behavior is universal across all energy scales from nuclear physics to chemistry and solid state systems. Although typically only a small fraction of a particle wavefunction extends into the tunneling region, we present here an extreme quantum system: a gigantic molecule consisting of two helium atoms, with an 80% probability that its two nuclei will be found in this classical forbidden region. This circumstance allows us to directly image the exponentially decaying density of a tunneling particle, which we achieved for over two orders of magnitude. Imaging a tunneling particle shows one of the few features of our world that is truly universal: the probability to find one of the constituents of bound matter far away is never zero but decreases exponentially. The results were obtained by Coulomb explosion imaging using a free electron laser and furthermore yielded He2's binding energy of [Formula: see text] neV, which is in agreement with most recent calculations.

8.
Opt Lett ; 40(14): 3432-5, 2015 Jul 15.
Artigo em Inglês | MEDLINE | ID: mdl-26176487

RESUMO

We present an approach how to combine large-mode field diameters with effective single-mode guidance in a hollow-core antiresonant optical fiber. We demonstrate experimentally and in simulations that single-mode guidance is achieved in a simplified hollow-core fiber design with a core diameter of 30 µm by shifting the effective indices of the first cladding modes close to those of higher order core modes. Our fiber shows low loss propagation and effective single-mode operation from the near infrared to deep ultraviolet wavelengths down to 270 nm on a loss level of approximately 3 dB/m.

9.
Opt Express ; 23(3): 2557-65, 2015 Feb 09.
Artigo em Inglês | MEDLINE | ID: mdl-25836120

RESUMO

Recently, a novel antiresonant hollow core fiber was introduced having promising UV guiding properties. Accompanying simulations predicted ten times lower loss than observed experimentally. Increasing loss is observed in many antiresonant fibers with the origin being unknown. Here, two possible reasons for the enhanced loss are discussed: strand thickness variation and surface roughness scattering. Our analysis shows that the attenuation is sensitive to thickness variations of the strands surrounding the hollow-core which strongly increase loss at short wavelengths. The contribution of surface roughness stays below the dB/km level and can be neglected. Thus, preventing structural irregularities by improved fabrication approaches is essential for decreasing loss.

10.
Opt Express ; 22(16): 19131-40, 2014 Aug 11.
Artigo em Inglês | MEDLINE | ID: mdl-25320999

RESUMO

Guiding light inside the hollow cores of microstructured optical fibers is a major research field within fiber optics. However, most of current fibers reveal limited spectral operation ranges between the mid-visible and the infrared and rely on complicated microstructures. Here we report on a new type of hollow-core fiber, showing for the first time distinct transmission windows between the deep ultraviolet and the near infrared. The fiber, guiding in a single mode, operates by the central core mode being anti-resonant to adjacent modes, leading to a novel modified tunneling leaky mode. The fiber design is straightforward to implement and reveals beneficial features such as preselecting the lowest loss mode (Gaussian-like or donut-shaped mode). Fibers with such a unique combination of attributes allow accessing the extremely important deep-UV range with Gaussian-like mode quality and may pave the way for new discoveries in biophotonics, multispectral spectroscopy, photo-initiated chemistry or ultrashort pulse delivery.

11.
Opt Express ; 21(3): 3170-81, 2013 Feb 11.
Artigo em Inglês | MEDLINE | ID: mdl-23481776

RESUMO

We present a novel approach to directly measure the bend loss of individual modes in few-mode fibers based on the correlation filter technique. This technique benefits from a computer-generated hologram performing a modal decomposition, yielding the optical power of all propagating modes in the bent fiber. Results are compared with rigorous loss simulations and with common loss formulas for step-index fibers revealing high measurement fidelity. To the best of our knowledge, we demonstrate for the first time an experimental loss discrimination between index-degenerated modes.


Assuntos
Modelos Teóricos , Fibras Ópticas , Refratometria/instrumentação , Refratometria/métodos , Simulação por Computador , Módulo de Elasticidade , Transferência de Energia , Desenho de Equipamento , Análise de Falha de Equipamento
12.
Opt Express ; 20(13): 13777-88, 2012 Jun 18.
Artigo em Inglês | MEDLINE | ID: mdl-22714443

RESUMO

We report on the possibilities of nanoscale optical fibers with all-normal dispersion behavior for pulse-preserving and coherent supercontinuum generation at deep ultraviolet wavelengths. We discuss the influence of important parameters such as pump wavelength and fiber diameter, for both optical nanofibers and nanoscale suspended-core optical fibers. Simulations reveal that by appropriate combination of fiber geometry and input pulse parameters, intensive spectral components well below 300 nm are generated. In addition, the impact of preceding taper transitions used for input coupling purposes is discussed in detail.


Assuntos
Tecnologia de Fibra Óptica/instrumentação , Iluminação/instrumentação , Nanoestruturas/química , Nanotecnologia/instrumentação , Desenho de Equipamento , Análise de Falha de Equipamento , Estudos de Viabilidade , Nanoestruturas/ultraestrutura , Raios Ultravioleta
13.
Opt Express ; 19(21): 20151-8, 2011 Oct 10.
Artigo em Inglês | MEDLINE | ID: mdl-21997026

RESUMO

Nonlinear pulse compression based on the generation of ultra-broadband supercontinuum (SC) in an all-normal dispersion photonic crystal fiber (ANDi PCF) is demonstrated. The highly coherent and smooth octave-spanning SC spectra are generated using 6 fs, 3 nJ pulses from a Ti:Sapphire oscillator for pumping a 13 mm piece of ANDi PCF. Applying active phase control has enabled the generation of 4.5 fs pulses. Additional spectral amplitude shaping has increased the bandwidth of the SC spectra further leading to nearly transform-limited pulses with a duration of 3.64 fs, which corresponds to only 1.3 optical cycles at a central wavelength of 810 nm. This is the shortest pulse duration achieved via compression of SC spectra generated in PCF to date. Due to the high stability and the smooth spectral intensity and phase distribution of the generated SC, an excellent temporal pulse quality exhibiting a pulse contrast of 14 dB with respect to the pre- and post-pulses is achieved.

14.
Opt Express ; 19(15): 13873-9, 2011 Jul 18.
Artigo em Inglês | MEDLINE | ID: mdl-21934748

RESUMO

We demonstrate nonlinear pulse compression based on recently introduced highly coherent broadband supercontinuum (SC) generation in all-normal dispersion photonic crystal fiber (ANDi PCF). The special temporal properties of the octave-spanning SC spectra generated with 15 fs, 1.7 nJ pulses from a Ti:Sapphire oscillator in a 1.7 mm fiber piece allow the compression to 5.0 fs high quality pulses by linear chirp compensation with a compact chirped mirror compressor. This is the shortest pulse duration achieved to date from the external recompression of SC pulses generated in PCF. Numerical simulations in excellent agreement with the experimental results are used to discuss the scalability of the concept to the single-cycle regime employing active phase shaping. We show that previously reported limits to few-cycle pulse generation from compression of SC spectra generated in conventional PCF possessing one or more zero dispersion wavelengths do not apply for ANDi PCF.

15.
Opt Express ; 19(13): 12275-83, 2011 Jun 20.
Artigo em Inglês | MEDLINE | ID: mdl-21716464

RESUMO

Recently, coherent pulse-preserving and octave-spanning supercontinuum (SC) generation was theoretically predicted and experimentally shown in photonic crystal fibers (PCFs) with all-normal dispersion behavior. Since this behavior is due only to the all-normal dispersion profile and not to the photonic crystal cladding, other all-normal optical waveguides exhibit these properties as well. We extend this concept to suspended-core fibers and optical nanofibers and show experimental demonstrations of this way of SC generation. We show that optical suspended-core fibers and optical nanofibers of appropriate dimensions exhibit all-normal dispersion and address octave-spanning single pulse SC generation in the visible (VIS) and ultra violet (UV) wavelength range. In addition, we discuss the feasibility of fiber taper transitions for suitable input coupling schemes in sub-micron diameter fibers and show the importance of short adiabatic transition profiles for utilizing high-energy pulses to obtain maximum spectral broadening. They are essential for coherent broadband UV SC generation in optical nanofibers.


Assuntos
Tecnologia de Fibra Óptica/instrumentação , Tecnologia de Fibra Óptica/métodos , Luz , Nanofibras , Raios Ultravioleta , Lasers de Gás , Microscopia Eletrônica de Varredura , Nanofibras/ultraestrutura , Dinâmica não Linear
16.
Opt Express ; 19(8): 7742-9, 2011 Apr 11.
Artigo em Inglês | MEDLINE | ID: mdl-21503084

RESUMO

Recently, the generation of coherent, octave-spanning, and recompressible supercontinuum (SC) light has been demonstrated in optical fibers with all-normal group velocity dispersion (GVD) behavior by femtosecond pumping. In the normal dispersion regime, soliton dynamics are suppressed and the SC generation process is mainly due to self-phase modulation and optical wave breaking. This makes such white light sources suitable for time-resolved applications. The broadest spectra can be obtained when the pump wavelength equals the wavelength of maximum all-normal GVD. Therefore each available pump wavelength requires a specifically designed optical fiber with suitable GVD to unfold its full power. We investigate the possibilities to shift the all-normal maximum dispersion wavelength in microstructured optical fibers from the near infra red (NIR) to the ultra violet (UV). In general, a submicron guiding fiber core surrounded by a holey region is required to overcome the material dispersion of silica. Photonic crystal fibers (PCFs) with a hexagonal array of holes as well as suspended core fibers are simulated for this purpose over a wide field of parameters. The PCFs are varied concerning their air hole diameter and pitch and the suspended core fibers are varied concerning the number of supporting walls and the wall width. We show that these two fiber types complement each other well in their possible wavelength regions for all-normal GVD. While the PCFs are suitable for obtaining a maximum all-normal GVD in the NIR, suspended core fibers are well applicable in the visible wavelength range.

17.
Opt Express ; 19(4): 3775-87, 2011 Feb 14.
Artigo em Inglês | MEDLINE | ID: mdl-21369202

RESUMO

We present the first detailed demonstrations of octave-spanning SC generation in all-normal dispersion photonic crystal fibers (ANDi PCF) in the visible and near-infrared spectral regions. The resulting spectral profiles are extremely flat without significant fine structure and with excellent stability and coherence properties. The key benefit of SC generation in ANDi PCF is the conservation of a single ultrashort pulse in the time domain with smooth and recompressible phase distribution. For the first time we confirm the exceptional temporal properties of the generated SC pulses experimentally and demonstrate their applicability in ultrafast transient absorption spectroscopy. The experimental results are in excellent agreement with numerical simulations, which are used to illustrate the SC generation dynamics by self-phase modulation and optical wave breaking. To our knowledge, we present the broadest spectra generated in the normal dispersion regime of an optical fiber.

18.
Opt Express ; 18(4): 3754-61, 2010 Feb 15.
Artigo em Inglês | MEDLINE | ID: mdl-20389385

RESUMO

Reducing the waist of an optical fiber taper to diameters below 1 microm can be interpreted as creating an optical nanofiber with propagation properties different from conventional optical fibers. Although there is theoretically no cutoff of the fundamental mode expected, a steep decline in transmission can be observed when the fiber diameter is reduced below a specific threshold diameter. A simple estimation of this threshold diameter applicable to arbitrary taper profiles and based on the diameter variation allowing adiabatic transmission behavior is introduced and experimentally verified. In addition, this threshold behavior is supported by investigating the variation of the power distribution of the nanofiber fundamental mode as a function of the fiber diameter.


Assuntos
Modelos Teóricos , Nanoestruturas/ultraestrutura , Nanotecnologia/instrumentação , Fibras Ópticas , Refratometria/métodos , Simulação por Computador , Desenho Assistido por Computador , Desenho de Equipamento , Análise de Falha de Equipamento , Luz , Espalhamento de Radiação
19.
Invest Radiol ; 43(4): 229-35, 2008 Apr.
Artigo em Inglês | MEDLINE | ID: mdl-18340246

RESUMO

OBJECTIVES: To analyze the effects of the sliding-thin-slab averaging algorithm on low-contrast performance in MDCT imaging and to find reasonable parameters for clinical routine work. MATERIALS AND METHODS: A low-contrast phantom simulating hypodense lesions (20 HU object contrast) was scanned with a 16-slice spiral CT scanner using different mAs-settings of 25, 50, 100, and 195 mAs. Other scan parameters were as follows: tube voltage = 120 kVp, slice collimation = 0.625 mm, pitch = 1.375 (high speed), reconstruction interval = 0.5 mm. Images were reconstructed with soft, standard, and bone algorithms, resulting in a total of 12 datasets. A sliding-thin-slab averaging algorithm was applied to these primary datasets, systematically varying the slab thickness between 0.5 and 5.0 mm. The low-contrast performance of the resulting datasets was semi-automatically analyzed using a statistical reader-independent approach: A size-dependent analysis of the image noise within the phantom was used to empirically generate a contrast discrimination function (CDF). The ratio between the actual contrast and the minimum contrast necessary for the detection (as given by the CDF) was calculated for all lesions in each dataset and used to evaluate the low-contrast detectability of the different lesions at increasing slab thickness. The results were compared with the original datasets to calculate the improvement in low-contrast detectability. RESULTS: Using the sliding-thin-slab algorithm, low-contrast performance was increased by a factor between 1.1 and 1.7 when compared with the primary dataset. The improvement of the visibility index at optimal slab thickness when compared with the original slice thickness (0.625 mm) was statistically significant (P < 0.05, Student t test) for the following datasets: 8 mm: all datasets; 6 mm: 25 mAs/soft, 195 mAs/bone, 25 mAs/bone; 5 mm: 25 mAs/soft, 25 mAs/bone. The ideal slab thickness over all datasets was 43% (+/-3%) of the diameter of the lesion to be detected. CONCLUSIONS: The use of an interactive sliding-thin-slab averaging algorithm can be readily applied to optimize low-contrast detectability in thin-collimated CT datasets. As a general rule for daily routine, a slice thickness of approximately 2.5 to 3.0 mm can be regarded as a reasonable preset, resulting in an optimized detectability of lesions with a diameter of 5 mm and above.


Assuntos
Algoritmos , Intensificação de Imagem Radiográfica/métodos , Tomografia Computadorizada por Raios X/métodos , Imageamento Tridimensional , Imagens de Fantasmas , Interpretação de Imagem Radiográfica Assistida por Computador , Estatísticas não Paramétricas
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