Balakishore Yellampalle

Single-shot terahertz pulse characterization via two-dimensional electro-optic imaging with dual echelons

A single-shot measurement of terahertz electromagnetic pulses is implemented using two-dimensional electro-optic imaging with dual echelon optics. The reported embodiment produces sequentially delayed multiprobe beamlets, routinely providing a time window of >10 ps with ~25 fs temporal step sizes. Because of its simplicity and robustness, the technique is ideally suited for real-time ultrashort relativistic electron bunch characterization.

Algorithm for high-resolution single-shot THz measurement using in-line spectral interferometry with chirped pulses

Z. P. Jiang and X. C. Zhang demonstrated a single-shot THz diagnostic based on spectral encoding of a chirped optical probe pulse [Appl. Phys. Lett. 72, 1945 (1998)]. This technique is thought to have an inherent uncertainty principle-imposed temporal resolution limitations. In this letter, we describe a method to recover the THz field without distortions, surpassing previous resolution limitations. Our approach is based on interpreting the spectral encoding experiment as in-line spectral interferometry, analogous to Gabor’s in-line spatial holography [D. Gabor, Nature (London) 161, 777 (1948)]. We recover the THz field from the interferogram and the characterized probe by using Tikhonov regularization combined with lower and upper triangular decomposition.

Single-shot, interferometric, high-resolution, terahertz field diagnostic

We present a single-shot, high-temporal-resolution terahertz diagnostic capable of measuring free-space far-infrared electromagnetic fields in time and space. We show that by using a chirped probe electro-optic sampling technique, in combination with a recently described interferometric retrieval algorithm [Appl. Phys. Lett. 87, 211109 (2005)], the diagnostic can provide transform-limited temporal resolution, mainly limited by the spectral bandwidth of the optical probe pulse, regardless of its chirp.

Unambiguous chirp characterization using modified-spectrum auto-interferometric correlation and pulse spectrum

Modified-spectrum auto-interferometric correlation (MOSAIC), derived from a conventional second order interferometric autocorrelation trace, is a sensitive and visual chirp diagnostic method for ultrashort laser pulses. We construct several pairs of example pulse shapes that have nearly identical MOSAIC traces and demonstrate that chirp ambiguity can result when the field amplitude or spectrum are not known, thus making MOSAIC a qualitative tool for chirped pulses. However, when the pulse spectrum is known, a unique chirp reconstruction is possible. With the help of a new reconstruction technique, we experimentally demonstrate complete pulse characterization using MOSAIC envelopes and the pulse spectrum.

Spectral interferometric coherent Raman imaging

We propose a low-background and sensitive coherent Raman imaging technique that uses selective excitation of Raman levels with chirped pulses from a single femtosecond laser. The proposed scheme uses spectral interferometric detection to obtain the real and imaginary parts of chi(3) simultaneously. We combine three advantages desirable for microscopy namely, use of single femtosecond laser, suppression of non-resonant background, and measurement of complex resonant Raman chi(3).

Comment on "Temporally resolved electro-optic effect".

We comment on the recent Letter by S. P. Jamison et al. [Opt. Lett.31, 1753 (2006)] where the analysis of a chirped probe pulse that is electro-optically modulated by a terahertz pulse reportedly results in a new expression for the electric field. While in principle the derived expressions for the total field after the crystal are correct, in their treatment the authors implicitly assumed that the derived total field is identical to the measured field, without regard to the residual birefringence of a typical electro-optic crystal or a crossed analyzer. Based on this analysis neglecting birefringence, they report that earlier expressions of the temporal field are incorrect. Here we show, on the contrary, that for chirped single-shot terahertz measurement schemes that include residual birefringence, the temporally resolved electro-optic effect is described correctly by the commonly used expression in the literature. We verify this result with our experimental data.

Three envelope approach for ultrafast pulse characterization in a pump-probe experiment

We demonstrate an approach for the complete characterization of temporally identical ultrashort pulses at the focal point in a pump-probe experiment for potential use of pump-probe data deconvolution. Our approach uses three envelope measurements, autocorrelation, fundamental, and second harmonic spectra, combined with an error minimization pulse retrieval scheme. The three envelope approach is suitable when the measured envelopes have low noise and minimal systematic errors.

Amplitude ambiguities in second-harmonic-generation frequency-resolved optical gating: reply to comment

We construct field shapes with distinct amplitude profiles that have nearly identical second-harmonic generation frequency-resolved optical gating (SHG FROG) traces. Although such fields are not true mathematical ambiguities, they result in experimentally indistinguishable FROG traces. These fields are neither time-reversed copies nor pulselets with a mere relative phase difference, which are well known nontrivial ambiguities for SHG FROG. We also show that for certain example fields, second-order interferometric autocorrelation is more sensitive to the pulse shape than is SHG FROG.

Electrical conductivity measurements of warm dense matter with time-resolved terahertz spectroscopy

The quasi-DC electrical conductivity of warm dense matter is directly measured with terahertz-probe reflection spectroscopy. The measurements show a noticeable deviation from the Drude model in warm dense aluminum.

Intense Coherent THz Pulse Generation by Two-Color Photoionization in Air

Intense coherent terahertz radiation via two-color photoionization in air is examined experimentally and interpreted as a photocurrent effect by the symmetry-broken laser field. THz power scalability is also tested experimentally.