Randy A. Bartels

Shaped-pulse optimization of coherent emission of high-harmonic soft X-rays

When an intense laser pulse is focused into a gas, the light–atom interaction that occurs as atoms are ionized results in an extremely nonlinear optical process—the generation of high harmonics of the driving laser frequency. Harmonics that extend up to orders of about 300 have been reported, some corresponding to photon energies in excess of 500 eV. Because this technique is simple to implement and generates coherent, laser-like, soft X-ray beams, it is currently being developed for applications in science and technology; these include probing the dynamics in chemical and materials systems and imaging. Here we report that by carefully tailoring the shape of intense light pulses, we can control the interaction of light with an atom during ionization, improving the efficiency of X-ray generation by an order of magnitude. We demonstrate that it is possible to tune the spectral characteristics of the emitted radiation, and to steer the interaction between different orders of nonlinear processes.

Quasi-phase-matched generation of coherent extreme-ultraviolet light

High-harmonic generation is a well-known method of producing coherent extreme-ultraviolet (EUV) light, with photon energies up to about 0.5 keV (refs 1, 2). This is achieved by focusing a femtosecond laser into a gas, and high harmonics of the fundamental laser frequency are radiated in the forward direction. However, although this process can generate high-energy photons, efficient high-harmonic generation has been demonstrated only for photon energies of the order 50–100 eV (ref. 5). Ionization of the gas prevents the laser and the EUV light from propagating at the same speed, which severely limits the conversion efficiency. Here we report a technique to overcome this problem, and demonstrate quasi-phase-matched frequency conversion of laser light into EUV. Using a modulated hollow-core waveguide to periodically vary the intensity of the laser light driving the conversion, we efficiently generate EUV light even in the presence of substantial ionization. The use of a modulated fibre shifts the energy spectrum of the high-harmonic light to significantly higher photon energies than would otherwise be possible. We expect that this technique could form the basis of coherent EUV sources for advanced lithography and high-resolution imaging applications. In future work, it might also be possible to generate isolated attosecond pulses.

Adaptive pulse compression for transform-limited 15-fs high-energy pulse generation

We demonstrate the use of a deformable-mirror pulse shaper, combined with an evolutionary optimization algorithm, to correct high-order residual phase aberrations in a 1-mJ, 1-kHz, 15-fs laser amplifier. Frequency-resolved optical gating measurements reveal that the output pulse duration of 15.2 fs is within our measurement error of the theoretical transform limit. This technique significantly reduces the pulse duration and the temporal prepulse energy of the pulse while increasing the peak intensity by 26%. It is demonstrated, for what is believed to be the first time, that the problem of pedestals in laser amplifiers can be addressed by spectral-domain correction.

Coherent learning control of vibrational motion in room temperature molecular gases

An evolutionary learning algorithm in conjunction with an ultrafast optical pulse shaper was used to control vibrational motion in molecular gases at room temperature and high pressures. We demonstrate mode suppression and enhancement in sulfur hexafluoride and mode selective excitation in carbon dioxide. Analysis of optimized pulses discovered by the algorithm has allowed for an understanding of the control mechanism.

Coherent artifact in modern pulse measurements

All optical pulse cmeasurement techniques necessarily fail in multi-shot measurements of unstable pulse trains because the measurement can only provide a single result, despite of the presence of many different pulse shapes. At the very least, however, the technique should provide a reasonable estimate of a typical pulse in the train and indicate the trains stability. While frequency-resolved optical gating (FROG, [1]) and spectral phase interferometry for direct electric-field reconstruction (SPIDER, [2]) naturally operate single-shot, multi-shot variants are very common, so it is important to understand the effects of instability on multi-shot measurements.

Simplified setup for high-resolution spectroscopy that uses ultrashort pulses

Using a broadband femtosecond laser and a simple optical setup, we demonstrate narrow-bandwidth-tunable excitation of vibrational modes in CCl4 and CHBr3 liquids. The resolution obtained is 80 times higher than the laser bandwidth. A pair of time-shifted, linearly chirped pulses is created by use of a high-order dispersion-compensated prism-interferometer setup. We use this pulse pair to selectively excite Raman-active transitions. Our setup represents a significant simplification with improved resolution, of previous approaches to the use of ultrashort pulses for chemically selective spectroscopy.

Low temperature epitaxial silicon film growth using high vacuum electron‐cyclotron‐resonance plasma deposition

We report on the growth technique and electrical properties of epitaxial Si films grown at low temperatures using an electron‐cyclotron‐resonance plasma deposition technique. We have used standard high vacuum apparatus to grow high quality films at 450–525 °C. A critical step in achieving high quality films is an in situ hydrogen plasma cleaning of the wafer before growth. We have systematically studied the influence of ion bombardment during growth by biasing the substrate, and find that the films are crystalline for substrate bias voltages less negative than about −15 V, but become polycrystalline as the magnitude of the negative bias is increased. The crystallinity of the film was measured using Raman spectroscopy. The undoped films are n type with carrier concentrations in the 1016–1017 cm−3 range. The Hall mobilities measured for the films are comparable to values obtained in bulk Si crystals. We can achieve abrupt profiles in carrier concentrations between the heavy doped substrate and the epilayer,...

Label-free second harmonic generation holographic microscopy of biological specimens.

Second-order nonlinear holographic microscopy for high-speed, three-dimensional imaging is demonstrated. The use of harmonic generation allows image formation of endogenous features in biological samples such as muscle tissue. We have acquired holograms with acquisition times as short as 10 ms, limited by the switching speed of our shutter; frame rates of 100s of Hz are expected to be possible. The samples are imaged with a Yb:KGW femtosecond laser oscillator, whose 1027 nm wavelength is well suited to minimize absorption and scattering. The low average power of the oscillator prevents damage to the sample.

Attosecond Time-Scale Intra-atomic Phase Matching of High Harmonic Generation

Using a model of high-harmonic generation that couples a fully quantum calculation with a semiclassical electron trajectory picture, we show that a new type of phase matching is possible when an atom is driven by an optimal optical waveform. For an optimized laser pulse shape, strong constructive interference is obtained in the frequency domain between emissions from different electron trajectories, thereby selectively enhancing a particular harmonic order. This work demonstrates that coherent control in the strong-field regime is possible by adjusting the peaks of a laser field on an attosecond time scale.

Attosecond time-scale feedback control of coherent X-ray generation

Abstract High-harmonic generation is an extreme, high-order, nonlinear process that converts intense, ultrafast, visible and infrared laser light pulses coherently into the soft X-ray region of the spectrum. We demonstrate that by optimizing the shape of an ultrafast laser pulse, we can selectively enhance this process by promoting strong constructive interference between X-ray bursts emitted from adjacent optical cycles. This work demonstrates that coherent control of highly nonlinear processes in the strong-field regime is possible by adjusting the relative timing of the crests of an electromagnetic wave on a sub-optical cycle, attosecond time scale.