Frequency domain, IR pump - XUV probe femtosecond spectrometry of small molecules - Time resolved probing of XUV photon interaction with non-equilibrium molecules distorted by the presence of strong IR laser field.
Real-time probing and control of electron and nuclear wavepacket dynamics in atoms/molecules - A velocity map imaging spectrometer is being deployed to analyze the electrons and ion fragments, which will help us to understand the energy redistrubution mechanisms following the interaction of attosecond XUV pulses with atoms/molecules.
Attosecond pulse trains and control of coherent XUV emission - Filmentation, Pulse compression and pulse shaping are used to control the temporal and spectral characteristics of high harmonic generation.
Ultrafast processes in nanoscale and sub-wavelength structures - Femtosecond studies of non-linear effects on surfaces and sub-wavelength structures..
Click the links below to read more on the research topics I have pursued over the last few years -
Attosecond regime XUV pulses and Quasi-Monochromatic XUV generation
Time resolving atomic and molecular dynamics with XUV-IR pump probe
CE Phase stabilization of intense lasers
High energy density plasma physics
Attosecond regime XUV pulses and Quasi-Monochromatic XUV generation
The non-linear interaction of the intense laser pulses with atoms leads to the generation of very high harmonics extending up to 500 eV. The XUV bursts generated in this fashion over each half cycle are of sub-femtosecond duration. It is further possible to confine the XUV emission to a single burst by utilizing < 6fs driving pulse or by employing a polarization gating. In exploring the atomic and molecular processes, the quasi-monochromaticity and selectivity of photon energy is also highly desirable along with time resolution. We have developed a scheme, which could lead to a source that is a compromise between the requirements of time resolution and energy selectivity. This is based on a novel mechanism of non-linear stabilization in a waveguide.
This high harmonic source provides the quasi-monochromaticity with a very easy method for energy tuning via phase matching in a hollow waveguide. In addition, this method could yield a very short temporal profile of the XUV emission, with pulse width of order of less than an optical cycle (~2fs). We have established that non-linear stabilization of XUV generation occurs in a hollow waveguide for <10fs driving pulses, as a result of interplay between transient ionization which determines the phase matching and the instantaneous intensity which determines the high harmonic generation. This mechanism yields to periodic enhancement of XUV flux at certain combinations of transient ionization and instantaneous laser intensity and it repeats cyclically as a function of the gas pressure. Additional advantage of this technique is that it is relatively robust to initial carrier-envelope phase fluctuations. This source is very efficient in the photon energy range between 30 to 50 eV using the gas pressures in the range of 10 to 80 torr. This pressure tuning is easily applicable in a short (4-5 cycle) IR pulse and it yields the very short quasi monochromatic X-ray pulses.
Time resolving atomic and molecular dynamics with XUV-IR pump probe
It is important to time resolve the physical and chemical processes on femtosecond time scales in order gain the understanding and the control of quantum dynamics. The understanding of the dynamics is often achieved by observing the momentum distribution of the end products or fragments in pump-probe studies. However, it is becoming increasingly apparent nowadays that the simple 1D spectroscopy of fragments is no longer sufficient to extract the important information from the complex dynamical processes. A full 3D momentum analysis is very useful in these cases. One such technique is Cold-Target Recoil Ion Mass Spectrometry (COLTRIMS). This technique allows for simultaneous electron-ion measurement in coincidence, while yielding high resolution in all three momentum dimensions. At the same time this technique has close to 4pi collection efficiency. In short this detection system serves as a reaction microscope for atomic and molecular processes.
We are performing the first studies that employ high harmonics in conjunction with this coincidence electron ion 3D momentum imaging technique (COLTRIMS) for the study of ultrafast molecular dynamics. We generate femtosecond EUV pulses at ~ 42 eV photon energy by upconverting intense (> 10^14 Wcm^-2) 25 fs laser pulses in an argon filled waveguide. Using this ultrashort EUV pulse as a pump, we launch simple diatomic molecules D2, N2 and CO into highly excited states near the molecular double-ionization threshold. The dynamics of these highly excited states unfold along different channels, which are identified by observing electron-ion correlation. By employing moderate intensity infrared fields, we show that we can influence and probe these dynamics on femtosecond timescales. We observe that the double ionization yield is significantly enhanced by the presence of an IR field in conjunction with the EUV pump. The kinetic energy release evolves as a function of delay time (internuclear distance), allowing us to map previously unexplored excited state dynamics.
CE Phase stabilization of intense lasers
The ultrafast physics aims to explore and control novel atomic and molecular processes in real time by utilizing time resolution of these pulses that are approaching attosecond regime. The current state-of-art technological requirements for the photon sources pushing the frontiers of the ultrafast physics are - high intensity Carrier-Envelope Phase (CEP) stabilized sub-10 fs laser pulses and the attosecond duration XUV (extreme ultra-violet) bursts to probe the sub-optical-cycle dynamics. We have worked on CEP stabilization of the Ti:Sa amplifier, capable of delivering 1- 2mJ per pulse at 1-5 kHz repetition rate. We have also implemented a hollow fiber compressor to reduce the pulse duration to less than 10fs. On this grating based amplifier, we have been able to demonstrate very good long term CE phase lock stability. The grating based stretcher and compressor scheme, unlike material based scheme is easily scalable to higher pulse energies.
We have measured the first single shot noise characteristics of the CE phase in such a system. The shot-to-shot CE phase noise is 600 mrad over 0.5 sec and mostly limited by the oscillator lock. We observe the intrinsic long term coherence in this system on time scales longer than 30 mins. We can also generate any desired modulation of CEP by mixing the slow electronic waveforms with the front end (oscillator) error signal. This aspect is very important, for example, in experiments where the signal to noise is very weak and lock-in techniques are desirable. The CEP stabilization of the amplifier can allow us an unprecedented control over the interaction of high intensity laser pulses with atoms and molecules. In fact, for exploring the attosecond physics it becomes necessary to control CEP of laser pulses. The absence of such a control leads to random fluctuations of the electric field with respect to the envelope, washing out the timing information for the sub-cycle phenomenon. CEP controlled laser pulses open up new avenues in control and observation of atomic and molecular dynamics on extremely short time scales.
High energy density plasma physics
In even higher intensity regime, we have undertaken femtosecond pump-probe studies pertaining to the hot-electron generation and the control of harmonic and X-ray generation. This work done at TIFR has led to understanding of plasma wave excitation and breaking in ultrafast solid density plasmas. The plasma wave-breaking is an extremely non-linear phenomenon in which plasma waves driven above certain amplitude limit catastrophically break and transfer the energy to fast electrons. The wave breaking, thus, leads to ultrashort bursts of very fast electrons in the range of keV to few MeV. However, the harmonic emission which occurs due to coupling between plasma and EM waves undergoes the opposite behavior. As plasma wave amplitude undergoes decay upon wave breaking, the harmonic efficiency drops rapidly. We used this as signature, for the first time, to follow the time evolution of wave breaking. We also developed a theoretical model for understanding the criteria for the plasma wave breaking process in solid density plasma. Plasma length scale and polarization optimizations lead to X-ray enhancements by more than factor of 100 and second harmonic by factor of 10. This work has important applications in terms of developing plasma sources of X-rays and fast electrons. The fast electrons generated in ultrashort laser plasma interaction are associated with high magnetic field pulses. These pulsed magnetic fields are largest available terrestrially. We have for the first time studied the temporal behavior of these Megagauss magnetic fields near high-density critical layer.
For complete list of publications see -
Arvinder Sandhu CV
The research results highlighed above are from the projects that I initiated at JILA (KM group) and TIFR (Atmol group) labs.
To visit these labs please click the links below -