Dichroism is often associated with chiral systems and circularly polarized light that is used as a chiral reagent. The two mirror images of a chiral systems are discerned by studying the propagation of left- and right-circularly polarized light. Here we demonstrate (a) efficient chiral sensitivity with linearly polarized helical light beams carrying an orbital angular momentum, and (b) that dichroism is intrinsic to achiral systems including amorphous materials. We model helical dichroism by considering induced multi-pole moments in light-matter interactions. Our research challenges the conventional wisdom that dichroism does not exist in achiral systems, enables to control light-matter interaction and opens new opportunities in chiroptical spectroscopy.
An electron that multiphoton ionizes is immediately subject to the light’s electric field that will control its short-term future. This control enables a gas of atoms to produce intense VUV or soft X-ray beams. Since we can precisely control the infrared beam, we can synthesize attosecond soft X-ray pulses – pulses that are the shortest controlled events ever systematically produced. For complex atom (such as xenon), the recollision electron shares its energy in any multi-electron interaction. Measuring the energy share encodes multielectron dynamics such as the Fano resonance structure in helium and the Giant Plasmon resonance in Xenon.
Attosecond pulses are generated by electrons that are extracted from a quantum system by an intense light pulse and travel through the continuum under the influence of the electric field of the light. Portions of each electron wave packet are forced to re-collide with its parent ion after the field reverses direction. Upon re-collision, the electron and ion can recombine, emitting soft x-ray radiation that can be in the form of attosecond pulses. This highly nonlinear process occurs in atoms, molecules and solids and offers unique measurement opportunities – for measuring the attosecond pulse itself; the orbital(s) from which it emerged; and the band structure of material in which the wave packets moved.
We investigate damage formation on the surface of fused silica by two femtosecond laser pulses, a tightly focused 266 nm (UV) pulse followed by a loosely focused 800 nm (IR) pulse. We show that the damage size is determined by the UV pulse, and only a small fraction of the normal UV damage threshold energy is needed to cause damage when combined with the properly delayed IR pulse. Our results, analyzed with a rate equation model, suggest that the UV pulse generates seed electrons through multiphoton absorption and the IR pulse utilizes these electrons to cause damage by avalanche ionization. By tuning such parameters like pulse energy, time delay, IR pulse duration and polarization, we further demonstrate that damage profile can be controlled.
We report on the experimental results of micro- and nanostructures fabricated on the surface of fused silica by a train of two femtosecond laser pulses, a tightly focused 266 nm (ultraviolet, UV) pulse followed by a loosely focused 800 nm (infrared, IR) pulse. By controlling the fluence of each pulse below the damage threshold, micro- and nanostructures are fabricated using the combined beams. The resulting damage size is defined by the UV pulse, and a reduction of UV damage threshold is observed when the two pulses are within ~ 1 ps delay. The effects of IR pulse duration on the UV damage threshold and shapes are investigated. These results suggest that the UV pulse generates seed electrons through multiphoton absorption and the IR pulse utilizes these electrons to cause damage by avalanche process. A single rate equation model based on electron density can be used to explain these results. It is further demonstrated that structures with dimensions of 124 nm can be fabricated on the surface of fused silica using 0.5 NA objective. This provides a possible route to XUV (or even shorter wavelength) laser nano-machining with reduced damage threshold.
Spatial characterization of high harmonics (HH) and XUV coherent radiation is of paramount importance, along
with its temporal characterization. For many applications it will be necessary to accurately measure the beam
properties, just as it is important to know the beam characteristics for many laser experiments. For example,
high harmonics and attosecond pulses are being proposed as a front-end for the next generation X-ray free
electron lasers. This oscillator-amplifier-like arrangement will require well characterized high harmonic sources.
On the other hand, the electromagnetic radiation carries the combined signature of underlying quantum physical
processes at the molecular level and of the cooperative phase matching. For example, accurate reconstruction of
the high harmonic spatial wavefront, along with its temporal profile, gives us a complete range of tools to apply
to the fundamental quantum properties and dynamics associated with high harmonic generation. We present
a new concept of frequency resolved wavefront characterization that is particularly suitable for characterizing
XUV radiation. In keeping with tradition in the area we give it an acronym - SWORD (Spectral Wavefront
Optical Reconstruction by Diffraction). Our approach is based on an analysis of the diffraction pattern of a slit
situated in front of a flat-field spectrometer. As the slit is scanned, the spectrally resolved diffraction pattern
is recorded. Analyzing the measured diffractogram, we can reconstruct the wavefront. The technique can be
easily extended beyond the XUV spectral region. When combined with temporal characterization techniques all
information about the beam can be measured.
We report on the first experiments of high-order harmonic generation done with the 100 Hz high-energy optical
parametric amplifier (OPA) of the Advanced Laser Light Source. Using krypton and argon as targets, we show that the
OPA's signal beam − with a wavelength range from 1200 nm to 1600 nm, 1.3 mJ to 0.8 mJ of pulse energy and 100 fs
pulse duration − can generate fully tunable XUV radiation down to a wavelength of 15 nm. We have also started to
investigate the use of the OPA pulses for molecular imaging. Inducing molecular alignment with 800 nm, 70 fs pulses,
we have measured the high harmonics spectra generated with 1300 nm pulses from nitrogen molecules oriented at
various angles with respect to the ionizing field, in order to study for the first time the technique of molecular orbital
tomography with a laser wavelength different than 800 nm.
With the proper choice of laser parameters focused femtosecond laser light creates long-range self-assembled planar nanocracks inside and on the surface of fused silica glass. The orientation of the crack planes is normal to the laser polarization direction and can be precisely controlled. The arrays of cracks when properly oriented and combined with chemical etching produce high aspect ratio micro- and nanofluidic channels. Direct femtosecond laser writing without any chemical etching can be used to fabricate embedded nanoporous capillaries in bulk fused silica for biofiltering and electrophoresis applications. The morphology of the porous structures critically depends on the laser polarization and pulse energy and can be used to control the transmission rates of fluids through the capillaries. Finally high aspect ratio, polarization-dependent, self-ordered periodic nanoslots can be fabricated from nanocracks produced on the surface of fused silica wafers. Control of the surface slot width from 10 to 60 nm is achieved through selective chemical etching. This technique, which may be useful for Surface Enhanced Raman Scattering (SERS) applications, has sub-diffraction limited resolution and features high throughput writing over centimeters.
High harmonics produced in aligned molecules contain the structural information of bound-state electronic states. We
have produced high harmonics from N2 molecules aligned to arbitrary directions with 5-degrees steps. From the set of
high harnionic spectra, we have successfully reconstructed tomographic images of the highest occupied molecular
orbital (HOMO) of N2.
Self-organized nanostructures have been recently observed when femtosecond laser pulses were focused inside fused silica glass. We have shown that these nanostructures extend throughout the focal volume and their order is preserved over macroscopic distances when the focus is scanned. We discuss the present understanding of the formation of the nanostructures including a model based on transient nanoplasmonics. The model predicts the periodicity of nanoplanes to scale as λ/2 in the medium. This is experimentally verified at 800 nm and 400 nm light with which we obtain nanoplane spacing of 250 ± 20 nm and 140 ± 20 nm respectively, which scale as predicted. Another requirement of the model is that ionization occurs preferentially at regions that have previously been ionized. This allows an initially inhomogeneous plasma to develop into an ordered nanoplasma array. Using transmission measurements we show that the required "memory" exists in the case of fused silica.
High harmonics produced in aligned molecules contain the structural information of the outermost electron orbital that preferentially ionizes in intense laser fields. We show a method to reconstruct a 3-dimensional (3D) structure of the molecular orbital. The method is based on the technologies to align molecules and to produce attosecond XUV pulses, both of which utilize intense ultrashort laser pulses. We measured a set of high harmonic spectra produced in differently aligned N2 molecules, and successfully reconstructed the image of the highest occupied molecular orbital (HOMO) with sub-angstrom resolution.
We have investigated the full three dimensional momentum correlation between the electrons emitted from strong field double ionization of neon when the re-collision energy of the first electron is on the order of the ionization potential of the singly charged neon ion. We find that the momentum correlation in the direction perpendicular to the laser field depends on the time difference of the two electrons leaving the ion. Our results are consistent with double ionization proceeding through transient double excited states that field ionize.
When an atom is ionized by an x-ray pulse in the presence of a laser field,the drift velocity of photoelectrons shows the phase dependence on the dressing field.We show how to use this effect to characterize single attosecond x-ray pulses.(i)Attosecond streak camera - the distortion of the photoelectron spectra induced by the laser field is used to map the temporal shape of the x-ray pulse to the photoelectron spectra.(ii) Attosecond SPIDER (spectral phase interferometry for direct electric-field reconstruction) - the spectral shearing interferometry of photoelectrons is used to directly retrieve the spectral phase of the x-ray pulse from the photoelectron spectra.
This paper contrasts the photosensitivity responses and processing windows between two extreme approaches in laser structuring of photonic devices: ultrafast and deep-ultraviolet F2 lasers. Low-loss single mode waveguides were formed by scanning in fused silica the focused light from a 50-fs Ti:Sapphire laser and a 157-nm 15-ns F2 laser. The latter source represents the first known demonstration of writing buried waveguide structures in bulk glass without driving ultrafast-laser interaction physics. For the ultrafast laser, a refractive index change of 1.0 x 10-3 was noted after an accumulated fluence of ~10 kJ/cm2, a high scanning speed of ~100 micrometers /s, and 100-kHz repetition rate. Longitudinal and side-writing techniques were employed and waveguides were characterized at 0.633-micrometers and 1.5-micrometers wavelengths. For the F2 laser, photosensitivity responses were similar in germanosilicate planar waveguides, and ~10-fold smaller in fused silica. Waveguide writing speeds were ~100-fold slower than for the ultrafast laser because of the smaller 100-Hz repetition rate. Overall, ultrafast lasers and ultraviolet lasers offer strong photosensitivity responses in silica-based glasses that address niche applications in fabricating complex three-dimensional photonic structures and trimming optical circuits for telecommunication applications.
The powerful transition from electronic to photonic systems in today's Internet-driven communication industry is driving the development of processes to miniaturize and integrate optical components. New processing and packaging technologies are now required that can precisely shape and assemble transparent optical components to sub-wavelength accuracy. Laser microfabrication technology is beginning to play a role here. Our groups are exploring two extremes in laser technology- ultrafast lasers and very short wavelength F2 lasers- to microstructure optical surfaces and to profile refractive-index structures inside transparent glasses. In this paper, we compare photosensitivity responses, spatial resolution, and processing windows for the deep-ultraviolet and ultrafast laser approaches, and discuss prospects for laser printing and trimming of optical waveguide components and circuits.
As a result of rapid engineering progress, femtosecond lasers will soon be available for industrial applications such as micro machining. I will review the physics behind laser machining of metals, semiconductors and transparent materials. Then I will briefly discuss the current state-of- the-art in femtosecond laser technology of interest to micro machining.
The physical principles of a streak camera capable of sub-femtosecond time resolution are discussed. In place of the traditional photocathode, a distributed medium is used. In place of rapidly changing voltage applied to the deflection plates, we use an electromagnetic field propagating in free space. The basic technology for the sub-femtosecond streak camera is the same as that of above-threshold ionization experiments that have become very common in studies of strong laser field interactions with atoms.
A short pulse will develop temporal sub-structures as it self- focuses. We show how these substructures lead to both the catastrophic expansion of the spectrum and the remarkable beam stability against self-focusing.
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