Measurement of dispersion and index of refraction of 1-decanol with spectrally resolved white light interferometry.

The high density, high nonlinearity, and stability of liquids make them an attractive medium for spectral broadening and supercontinuum generation in ultrafast experiments. To understand ultrashort pulse propagation in these media, their indices of refraction and dispersions must be characterized. We employ a Mach-Zehnder interferometer to generate a series of interferograms, which we refer to as a spectrogram, to develop a new method of using spectrally resolved white light interferometry to determine the refractive indices of materials. We determine the indices of refraction of BK7, sapphire, ethanol, and 1-decanol at 24°C across the visible and near infrared. To our knowledge, this is the first reported dispersion and index of refraction measurement of 1-decanol.

Amplification of femtosecond pulses based on χ(3) nonlinear susceptibility in MgO.

We experimentally demonstrate large, widely tunable gain using Kerr instability amplification in MgO. By pumping the crystal near optical damage at 1.4×1013W/cm2 by a femtosecond Ti:sapphire laser, we amplify visible and near-infrared pulses by factors >5000 or a gain g≈17/mm. We temporally characterize the pulses to show that they are 42 fs in duration, much shorter than the pump pulse. In the non-collinear setup, the angle between the pump and seed selects the amplified wavelength, where we find certain angles amplify both the visible and near-infrared simultaneously. We find that near the maximum pumping intensities, higher-order nonlinearities may play a role in the amplification process.

Linking high harmonics from gases and solids.

When intense light interacts with an atomic gas, recollision between an ionizing electron and its parent ion creates high-order harmonics of the fundamental laser frequency. This sub-cycle effect generates coherent soft X-rays and attosecond pulses, and provides a means to image molecular orbitals. Recently, high harmonics have been generated from bulk crystals, but what mechanism dominates the emission remains uncertain. To resolve this issue, we adapt measurement methods from gas-phase research to solid zinc oxide driven by mid-infrared laser fields of 0.25 volts per ångström. We find that when we alter the generation process with a second-harmonic beam, the modified harmonic spectrum bears the signature of a generalized recollision between an electron and its associated hole. In addition, we find that solid-state high harmonics are perturbed by fields so weak that they are present in conventional electronic circuits, thus opening a route to integrate electronics with attosecond and high-harmonic technology. Future experiments will permit the band structure of a solid to be tomographically reconstructed.

High-harmonic generation in solids driven by counter-propagating pulses.

We used two 800 nm laser pulses propagating in the opposite directions, to drive the emission of high-order vacuum ultra-violet harmonics off of the surface of an MgO (100) single crystal. We demonstrated the advantages that our approach provides compared to a single beam geometry, in both forward and backward emission.

Harmonic generation in solids with direct fiber laser pumping.

High harmonic generation in solids presents the possibility for bringing attosecond techniques to semiconductors and a simple source for frequency comb spectroscopy in the vacuum ultraviolet. We generate up to the seventh harmonic of a Tm fiber laser by focusing in silicon or zinc oxide. The harmonics are strong and stable, with no indication of material damage. Calculations show the potential for generating nineteenth harmonic photons at 12 eV photons of energy.

Controlling attosecond angular streaking with second harmonic radiation.

High harmonic generation, which produces a coherent burst of radiation every half cycle of the driving field, has been combined with ultrafast wavefront rotation to create a series of spatially separated attosecond pulses, called the attosecond lighthouse. By adding a coherent second harmonic beam with polarization parallel to the fundamental, we decrease the generating frequency from twice per optical cycle to once. The increased temporal separation increases the pulse contrast. By scanning the carrier envelope phase, we see that the signal is 2π periodic.

All-Optical Reconstruction of Crystal Band Structure.

The band structure of matter determines its properties. In solids, it is typically mapped with angle-resolved photoemission spectroscopy, in which the momentum and the energy of incoherent electrons are independently measured. Sometimes, however, photoelectrons are difficult or impossible to detect. Here we demonstrate an all-optical technique to reconstruct momentum-dependent band gaps by exploiting the coherent motion of electron-hole pairs driven by intense midinfrared femtosecond laser pulses. Applying the method to experimental data for a semiconductor ZnO crystal, we identify the split-off valence band as making the greatest contribution to tunneling to the conduction band. Our new band structure measurement technique is intrinsically bulk sensitive, does not require a vacuum, and has high temporal resolution, making it suitable to study reactions at ambient conditions, matter under extreme pressures, and ultrafast transient modifications to band structures.

Creating high-harmonic beams with controlled orbital angular momentum.

A beam with an angular-dependant phase Φ = ℓϕ about the beam axis carries an orbital angular momentum of ℓℏ per photon. Such beams are exploited to provide superresolution in microscopy. Creating extreme ultraviolet or soft-x-ray beams with controllable orbital angular momentum is a critical step towards extending superresolution to much higher spatial resolution. We show that orbital angular momentum is conserved during high-harmonic generation. Experimentally, we use a fundamental beam with |ℓ| = 1 and interferometrically determine that the harmonics each have orbital angular momentum equal to their harmonic number. Theoretically, we show how any small value of orbital angular momentum can be coupled to any harmonic in a controlled manner. Our results open a route to microscopy on the molecular, or even submolecular, scale.

Near-threshold harmonics from a femtosecond enhancement cavity-based EUV source: effects of multiple quantum pathways on spatial profile and yield.

We investigate the photon flux and far-field spatial profiles for near-threshold harmonics produced with a 66 MHz femtosecond enhancement cavity-based EUV source operating in the tight-focus regime. The effects of multiple quantum pathways in the far-field spatial profile and harmonic yield show a strong dependence on gas jet dynamics, particularly nozzle diameter and position. This simple system, consisting of only a 700 mW Ti:Sapphire oscillator and an enhancement cavity produces harmonics up to 20 eV with an estimated 30-100 µW of power (intracavity) and > 1µW (measured) of power spectrally-resolved and out-coupled from the cavity. While this power is already suitable for applications, a quantum mechanical model of the system indicates substantial improvements should be possible with technical upgrades.

Simple method to determine dispersion of high-finesse optical cavities.

We present a simple and quick, yet accurate method to measure the dispersion of high finesse optical cavities. By exciting the cavity with a femtosecond frequency comb and measuring the resonance condition as a function of optical frequency, the cavity's dispersion can be determined with minimal uncertainty. Measurement results are presented from an evacuated reference cavity with low group delay dispersion as well as several differential, intra-cavity measurements of well known optical materials demonstrating the dynamic range and accuracy of this technique.

Vectorizing the spatial structure of high-harmonic radiation from gas.

Strong field laser physics has primarily been concerned with controlling beams in time while keeping their spatial profiles invariant. In the case of high harmonic generation, the harmonic beam is the result of the coherent superposition of atomic dipole emissions. Therefore, fundamental beams can be tailored in space, and their spatial characteristics will be imparted onto the harmonics. Here we produce high harmonics using a space-varying polarized fundamental laser beam, which we refer to as a vector beam. By exploiting the natural evolution of a vector beam as it propagates, we convert the fundamental beam into high harmonic radiation at its focus where the polarization is primarily linear. This evolution results in circularly polarized high harmonics in the far field. Such beams will be important for ultrafast probing of magnetic materials.

Cavity-enhanced high harmonic generation for extreme ultraviolet time- and angle-resolved photoemission spectroscopy.

With its direct correspondence to electronic structure, angle-resolved photoemission spectroscopy (ARPES) is a ubiquitous tool for the study of solids. When extended to the temporal domain, time-resolved (TR)-ARPES offers the potential to move beyond equilibrium properties, exploring both the unoccupied electronic structure as well as its dynamical response under ultrafast perturbation. Historically, ultrafast extreme ultraviolet sources employing high-order harmonic generation (HHG) have required compromises that make it challenging to achieve a high energy resolution-which is highly desirable for many TR-ARPES studies-while producing high photon energies and a high photon flux. We address this challenge by performing HHG inside a femtosecond enhancement cavity, realizing a practical source for TR-ARPES that achieves a flux of over 1011 photons/s delivered to the sample, operates over a range of 8-40 eV with a repetition rate of 60 MHz. This source enables TR-ARPES studies with a temporal and energy resolution of 190 fs and 22 meV, respectively. To characterize the system, we perform ARPES measurements of polycrystalline Au and MoTe2, as well as TR-ARPES studies on graphite.

Light amplification by seeded Kerr instability.

Amplification of femtosecond laser pulses typically requires a lasing medium or a nonlinear crystal. In either case, the chemical properties of the lasing medium or the momentum conservation in the nonlinear crystal constrain the frequency and the bandwidth of the amplified pulses. We demonstrate high gain amplification (greater than 1000) of widely tunable (0.5 to 2.2 micrometers) and short (less than 60 femtosecond) laser pulses, up to intensities of 1 terawatt per square centimeter, by seeding the modulation instability in an Y3Al5O12 crystal pumped by femtosecond near-infrared pulses. Our method avoids constraints related to doping and phase matching and therefore can occur in a wider pool of glasses and crystals even at far-infrared frequencies and for single-cycle pulses. Such amplified pulses are ideal to study strong-field processes in solids and highly excited states in gases.

Controlling the orbital angular momentum of high harmonic vortices.

Optical vortices, which carry orbital angular momentum (OAM), can be flexibly produced and measured with infrared and visible light. Their application is an important research topic for super-resolution imaging, optical communications and quantum optics. However, only a few methods can produce OAM beams in the extreme ultraviolet (XUV) or X-ray, and controlling the OAM on these beams remains challenging. Here we apply wave mixing to a tabletop high-harmonic source, as proposed in our previous work, and control the topological charge (OAM value) of XUV beams. Our technique enables us to produce first-order OAM beams with the smallest possible central intensity null at XUV wavelengths. This work opens a route for carrier-injected laser machining and lithography, which may reach nanometre or even angstrom resolution. Such a light source is also ideal for space communications, both in the classical and quantum regimes.

Direct current cataphoretic effects on Hg 2537-A radiation from Hg + Ar discharges (dc fluorescent lamps).

The Hg 2537-A intensity from Hg + Ar discharges was measured as an independent function of mercury vapor pressure (as influenced by a cold spot temperature) at various constant currents and tube radii with ~4-Torr argon. As a function of mercury pressure, the intensity rises to a peak (which defines an optimum mercury cold spot temperature) and then decreases with further increase in mercury pressure due to the combination of self-absorption and electron deexcitation. The behavior of the optimum mercury cold spot temperature is dependent upon ac or dc conditions. For ac, the optimum mercury pressure is ~7 mTorr (corresponding to a Hg cold spot temperature of ~40 degrees C) and comparatively insensitive to current. By contrast, the optimum mercury cold spot temperature for the dc case is dependent upon whether the anode end or cathode end is cooled. The dc lamp with anode end cooled yields an optimum mercury cold spot temperature less than 40 degrees C and decreases with increasing current, while the optimum with cathode cooled is greater than 40 degrees C and increases with increasing current. We believe that the peak intensity always occurs at the same real mercury density (because that determines the self-absorption), but the Hg cold spot temperature required to achieve this density is affected by dc cataphoretic pumping phenomena.

Effect of Electron Deexcitation and Self-Absorption on the Intensity of the Hg 2537-A Radiation from Hg + Ar Discharges (ac Fluorescent Lamps).

The intensity of the Hg 2573-A radiation from Hg + Ar discharges was measured as an independent function of mercury pressure (0.2-50 mTorr), ac current (50-2100 mA) and tube radius (0.79 cm and 1.27 cm) at a constant Ar pressure of ~4 Torr. For various constant mercury pressures, the Hg 2537-A intensity initially rises linearly with increasing current, but then tends to bend over and approach an asymptotic limit. The nonlinear, asymptotic behavior is due to electron deexcitation of the Hg 6(3)P(1) state at the higher currents in the presence of Hg 2537-A self-absorption. The Hg 2537-A intensity was also measured as a function of mercury pressure at various constant currents. The intensity rises to a peak (which defines an optimum Hg pressure) and then decreases with further increase in mercury pressure due to the combination of self-absorption and electron deexcitation. For high ac currents, the optimum Hg pressure is independent of current but varies inversely with the tube diameter. All this behavior is relevant to the problem of obtaining high efficiency from fluorescent lamps at high powers.

Initial afterglow of the self-absorbed hg 2537-a radiation from hg + ar discharges.

A limitation on the high frequency modulation of gas discharge lamps is the duration of the radiative afterglow which is often dilated by self-absorption. Introducing a foreign gas into the discharge alters the absorption line shape and width by collisions, thus reducing self-absorption and the afterglow decay time. This is a general technique for extending the high frequency modulability. For the experiments, the effect of variable argon pressures on the self-absorbed Hg 2537-A initial radiative decay time (tau) was measured from abruptly terminated discharges as independent functions of the mercury pressure (0.8-70 mTorr) and argon pressure (5-200 Torr). tau increases with the mercury density but is substantially reduced by the argon pressure in quantitative agreement with the theory of Holstein and Walsh and the concept that the initial decay is primarily limited by self-absorption for our range of variables. A detailed theoretical analysis indicates that there are several ways that additional argon reduces the Hg 2537-A self-absorption: (1) the Hg 237-A line gets broader simply because the additional argon atoms increase the Hgndash;Ar collision frequency; (2) adding argon causes the gas temperature to rise, and this drives the Hgndash;Ar collision frequency still higher; (3) the rise in gas temperature also causes an increase in the Hg 2537-A Doppler width. Thus, a general technique for substantially increasing the modulability of a gas discharge lamp emitting self-absorbed radiation has been theoretically and experimentally demonstrated. These results are consistent with our previous analysis performed on measurements of the Hg 2537-A intensity in these discharges relevant to fluorescent lamps. These phenomena may also be relevant in some gas lasers.

Effects of argon atoms on the self-absorption and the intensity of hg 2537-a radiation in hg + ar discharges.

A study was made of the feasibility of increasing the efficiency of fluorescent lamps at high powers by increasing the Hg 2537-A resonance radiation through a reduction of self-absorption. Specifically, we attempted to reduce the Hg 2537-A self-absorption by introducing a higher pressure of a foreign gas (argon) to alter the Hg 2537-A absorption line shape and width by collision broadening. The intensity of the Hg 2537-A line in Hg + Ar discharges was measured as an independent function of mercury pressure (0.7 mTorr to 27 mTorr), argon pressure (5 Torr to 400 Torr), and dc input power (5.5 W to 97 W). A detailed theoretical analysis indicates that there are four ways that additional argon reduces the Hg 2537-A self-absorption: (1) The Hg 2537-A line gets broader simply because the additional argon atoms increase the Hg-Ar collision frequency; (2) adding argon causes the gas temperature to rise and this drives the Hg-Ar collision frequency still higher; (3) the rise in gas temperature also causes an increase in the Hg 2537-A doppler width; (4) the additional argon changes the Hg 2537-A line shape from doppler dominated to a collision dominated profile. The experiments demonstrate, however, that no gain is achieved in the Hg 2537-A intensity with the addition of extra argon in spite of the beneficial effect on the self-absorption escape rate. This advantage is apparently offset by the argon's reduction of the electron energy which leads to fewer mercury atoms excited to the Hg 6(3)P(1) state.

Theoretical model of visible radiation from rare gas flashlamps.

A simple theoretical model of visible light emission from xenon flashlamps is presented. The continuum light emission is calculated from the rate of electron-ion recombination in the xenon plasma, which is treated as a greybody in thermal equilibrium. The effect of radiation reabsorption is calculated in terms of the temperature-dependent greybody emissivity. The model predictions of radiated power and energy are compared to measured data. Reasonable agreement is obtained over a wide range of parameters of practical interest. Thus the model provides a useful analytical tool for first-order engineering design of xenon flash-lamp illumination systems.