James H. Glownia

Terahertz emission from ultrafast ionizing air in symmetry-broken laser fields

A transient photocurrent model is developed to explain coherent terahertz emission from air irradiated by a symmetry-broken laser field composed of the fundamental and its second harmonic laser pulses. When the total laser field is asymmetric across individual optical cycles, a nonvanishing electron current surge can arise during optical field ionization of air, emitting a terahertz electromagnetic pulse. Terahertz power scalability is also investigated, and with optical pump energy of tens of millijoules per pulse, peak terahertz field strengths in excess of 150 kV/cm are routinely produced.

Multiphoton ionization of ammonia: Mass analysis and photoelectron spectra

Mass and photoelectron spectra following the resonantly enhanced four‐photon ionization of ammonia have been recorded. The mass spectral data show that extensive fragmentation occurs at a distinct wavelength threshold. This threshold energetically corresponds to the minimum number of photons required to reach the first excited state of the ion. The branching ratio for the various fragments (H+, H+2, N+, NH+, NH+2, NH+3) is independent of the neutral resonantly excited vibronic state, and the ratios differ from those recorded by conventional ionization techniques. Photoelectron energy measurements show that low (∼0 eV) kinetic energy electrons are produced following ionization of all but one of the resonantly excited Rydberg states investigated. The origin of these zero energy electrons is thought to be due to vibrational autoionization. Internal conversion may give rise to the neutral vibronic levels responsible for the autoionization. For one excited state, higher electron energies are recorded correspon...

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.

High-Power Broadband Terahertz Generation via Two-Color Photoionization in Gases

We review high-energy, broadband terahertz (THz) generation in two-color laser-produced gaseous plasma. We first describe our microscopic plasma current model for directional plasma current and far-field THz radiation generation. Experimental results for THz yield dependence on laser energy, optical phase difference, gas species, and gas pressure are presented. We also describe ultrabroadband THz generation and detection in our experiments and numerical simulations. Finally, we discuss 2-D plasma currents for THz polarization control and macroscopic phase-matched THz generation.

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.

Resonantly enhanced vacuum ultraviolet generation and multiphoton ionization in carbon monoxide gas

Competition between three‐photon resonantly enhanced vacuum ultraviolet third‐harmonic generation and six‐photon multiphoton ionization using the A state in gaseous carbon monoxide is observed. Excitation spectra of the third‐harmonic emission exhibit increasing blue shifts and broadening with increasing pressure due to the phase matching requirements. Estimates for the efficiency and tunability show that third‐harmonic generation in carbon monoxide molecules is a promising source for coherent vacuum ultraviolet light.

Terahertz radiation from shocked materials

Distinct physical mechanisms for the generation of temporally coherent, narrow bandwidth optical radiation are few and rare in nature. Such sources, including lasers, have widespread applications ranging from spectroscopy to interferometry. We review the recent theoretical prediction of a new type of temporally coherent optical radiation source in the 1–100 THz frequency range that can be realized when crystalline polarizable materials like NaCl are subject to a compressive shock wave.

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.

Terahertz-frequency electrical conductivity measurements of ultrashort laser-ablated plasmas

We investigate the phase transitions of intense ultrashort laser-heated solids, from the cold solid to the hot dense plasma state, by measuring the complex electrical conductivity (or refractive index) transients at terahertz (1 THz = 1012 Hz) frequencies. Using optical-pump, terahertz-probe spectroscopy, we measured the phase shifts and absorption of terahertz probe pulses that were reflected from the warm dense plasma. To characterize the THz field, we developed and used a single-shot, high-temporal-resolution THz diagnostic capable of measuring free-space electromagnetic pulse fields in time and space. Due to relatively large focal spot sizes of the THz probe (~mm), mainly limited by the diffraction properties of THz radiation, the optical pump pulse was weakly focused onto the target in order to overfill the THz probe spot size with a peak intensity of ~1013 W/cm2. In contrast to the previous measurements of conductivities at optical frequencies, our THz non-contact probe method can directly measure quasi-DC electrical conductivities, providing insight into the transport nature of warm dense matter and any present discrepancies with the Drude model. In case of warm dense aluminum, we observe a noticeable deviation from the Drude model even in the ~1013 W/cm2 laser intensity regime. In addition, we observe strong coherent THz emission produced by a current surge in the laser-produced plasma.