Niranjan Shivaram

A low-cost spatial light modulator for use in undergraduate and graduate optics labs

Spatial light modulators (SLMs) are a versatile tool for teaching optics, but the cost associated with an SLM setup prevents its adoption in many undergraduate and graduate optics labs. We describe a simple method for creating a low-cost SLM by extracting components from a commercial LCD projector. We demonstrate the pedagogical applications of this SLM design by providing examples of its use in teaching diffraction and interference phenomena. We also discuss an SLM’s potential as a research tool in graduate labs. In particular, we demonstrate its use in holography and in the generation of optical vortices.

Ultrafast dynamics of neutral superexcited oxygen: a direct measurement of the competition between autoionization and predissociation.

Using ultrafast extreme ultraviolet pulses, we performed a direct measurement of the relaxation dynamics of neutral superexcited states corresponding to the nlσ(g)(c(4)Σ(u)(-)) Rydberg series of O(2). An extreme ultraviolet attosecond pulse train was used to create a temporally localized Rydberg wave packet and the ensuing electronic and nuclear dynamics were probed using a time delayed femtosecond near-infrared pulse. We investigated the competing predissociation and autoionization mechanisms in superexcited oxygen molecules and found that autoionization is dominant for the low n Rydberg states. We measured an autoionization lifetime of 92±6 fs and 180±10 fs for the (5s,4d)σ(g) and (6s,5d)σ(g) Rydberg state groups, respectively. We also determine that the disputed neutral dissociation lifetime for the ν=0 vibrational level of the Rydberg series is 1100±100 fs.

In situ spatial mapping of Gouy phase slip for high-detail attosecond pump–probe measurements

Attosecond pump-probe experiments routinely utilize extreme ultraviolet (XUV) and IR fields, with relative phase being the variable parameter. However, the Gouy phase slip between the focused IR and XUV pulses inevitably leads to a certain amount of phase averaging and loss of accuracy. By using ion imaging, we establish a one-to-one mapping between the local phase slip and the spatial coordinates of the focal volume, thus performing in situ characterization of the Gouy phase of a complex beam and its role in ionization of He and Xe. We demonstrate that spatially discriminated ion imaging enhances the contrast of a phase-dependent XUV+IR ionization signal. We utilize our technique to unmask a weak ionization asymmetry, thus opening pathways for further high-precision attosecond studies.

Optimization of few-cycle pulse generation: spatial size, mode quality and focal volume effects in filamentation based pulse compression.

We demonstrate the key role played by the spatial characteristics and focusing conditions of a femtosecond multi-cycle laser pulse in optimization of filament output for the purpose of obtaining compressed light pulses in the few-cycle regime. We find that for a given beam profile and focal parameters, driving the filament with energy above a certain limiting value can negatively impact pulse compression. However, for a given energy, a smaller and cleaner input beam mode obtained by using a hard aperture can substantially improve the pulse compression ability. In addition, we show that a larger focal volume can assist in creation of a shorter output pulse.

Attosecond Quantum-Beat Spectroscopy in Helium

The evolution of electron wavepackets determines the course of many physical and chemical phenomena and attosecond spectroscopy aims to measure and control such dynamics in real-time. Here, we investigate radial electron wavepacket motion in Helium by using an XUV attosecond pulse train to prepare a coherent superposition of excited states and a delayed femtosecond IR pulse to ionize them. Quantum beat signals observed in the high resolution photoelectron spectrogram allow us to follow the field-free evolution of the bound electron wavepacket and determine the time-dependent ionization dynamics of the low-lying 2p state.

Attosecond Quantum Beat Spectroscopy

We investigate electron wavepacket dynamics in Helium using an attosecond pulse train to prepare a superposition of states. The wavepacket is probed by a femtosecond infrared pulse on a 900 femtosecond timescale with attosecond resolution.

Observing the Real-Time Evolution of Helium Atoms in a Strong Laser Field

The interaction of a strong laser field with an atom significantly modifies its atomic structure. Such an atom can be modeled using the Floquet theory in which the atomic states are described by Floquet states composed of several Fourier components. We use high-order harmonics present in extreme-ultraviolet (XUV) attosecond pulse trains (APTs) to create excited states in infra-red(IR) laser dressed He atoms which are ionized by the dressing laser field itself. The quantum interference between different components of the Floquet states leads to oscillation in the ion yield as a function of XUV-IR time delay. We measure the phase of this quantum interference process through the phase of the ion yield signal which allows us to follow the evolution of the dressed atom, in real-time, as the intensity of the IR field is varied. We observe a transition from a 5p Floquet state dominated ionization to a 2p Floquet state dominated ionization with increasing IR intensity.