Gombojav O. Ariunbold

Single-shot detection of bacterial endospores via coherent Raman spectroscopy

Recent advances in coherent Raman spectroscopy hold exciting promise for many potential applications. For example, a technique, mitigating the nonresonant four-wave-mixing noise while maximizing the Raman-resonant signal, has been developed and applied to the problem of real-time detection of bacterial endospores. After a brief review of the technique essentials, we show how extensions of our earlier experimental work [Pestov D, et al. (2007) Science 316:265–268] yield single-shot identification of a small sample of Bacillus subtilis endospores (≈104 spores). The results convey the utility of the technique and its potential for “on-the-fly” detection of biohazards, such as Bacillus anthracis. The application of optimized coherent anti-Stokes Raman scattering scheme to problems requiring chemical specificity and short signal acquisition times is demonstrated.

Coherent versus incoherent Raman scattering : molecular coherence excitation and measurement

We present a comparative analysis of spontaneous and coherent Raman scattering on pyridine. The instantaneous excitation of the molecular coherence is done by a pair of ultrashort preparation pulses. Then, a long narrowband probe pulse is scattered off the molecular vibrations. The described hybrid technique allows for the single-shot acquisition of a background-free coherent Raman spectrum within the excitation band and its straightforward comparison with the spontaneous Raman measurements, performed in the same setup. We report a 10(5)-fold increase in the efficiency of the Raman scattering process due to the broadband pump-Stokes preparation. The coherence magnitude (approximately 0.5x10(-3)) is inferred experimentally, without a priori knowledge about the molecular structure.

Third and fifth harmonic generation by tightly focused femtosecond pulses at 2.2 μm wavelength in air

We report experiments on the generation of third and fifth harmonics of millijoule-level, tightly focused, femtosecond laser pulses at 2.2 μm wavelength in air. The measured ratio of yields of the third and fifth harmonics in our setup is found equal to 2 · 10(-4). This result contradicts the recent suggestion that the Kerr effect in air saturates and changes sign in ultra-intense optical fields.

Pulse shaping for mode-selective ultrafast coherent Raman spectroscopy of highly scattering solids

We report on mode-selective ultrafast coherent anti-Stokes Raman spectroscopy (CARS) of a powder of sodium dipicolinate. We produce a pair of stretched laser pulses with precisely adjusted matching chirp rates. We use the pulses for frequency-resolved excitation of coherent molecular vibrations in this highly scattering medium. The induced oscillations are probed with the third delayed ultrashort laser pulse. Since the attained spectral width of the pump-Stokes excitation band is on the order of the spacing between the Raman lines, time-resolved CARS measurements reveal single-mode as well as double-mode coherence decay dynamics, depending on the timing between the pump and Stokes pulses. For a fixed probe pulse delay, the sweeping of the arrival time for one of the preparation pulses maps out the CARS spectrum of the analyte.

Complex line shapes in surface-enhanced coherent Raman spectroscopy

Peaks, dips, and intermediate line shapes have been observed in surface-enhanced coherent Raman spectroscopy. Here, we report an experimental observation of a peculiar line shape revealing both a peak and a dip as two vibrational transitions of pyridazine in the presence of aggregated gold nanoparticles. We propose a simple model based on plasmonic phase effects and quantum chemistry calculations, and compare the simulated coherent (SECARS) and incoherent (SERS) Raman signals from several complexes. Complex SECARS line shapes provide additional information compared to SERS and can be used as a tool in nanoscale sensing and spectroscopy.

Temporal coherent control of superfluorescent pulses.

We demonstrate that collective atomic interferences can be investigated by measuring the superfluorescence (SF) time delay. A pair of broadband (≈20  nm), ultrashort (≈80  fs), collinear pulses with a variable delay coherently excites rubidium (Rb) atoms. The generated superfluorescent pulses at 420 nm on the cascade transition are recorded by a picosecond streak camera. Both intensity and SF time delay of the 420 nm pulse are altered as the delay between input pulses varies. In particular, the SF time delay of the normalized 420 nm pulse exhibits oscillations with different periods. This can be understood in terms of atomic and quantum interferences due to two possible two-photon excitation pathways through the intermediate levels (Rb D-lines).

Observation of picosecond UV pulses produced by coherent scattering of IR femtosecond pulses in atomic rubidium vapor

We report the observation of coherent UV light pulses by the coherent scattering of IR pulses from atomic rubidium vapor. Rubidium atoms were first excited by a 100 fs pulse from the 5S ground state to the 5D state via a two-photon transition. The atoms were then pumped by an IR pulse resonant to the 5D–12P transition. The presence of the IR pulse triggered the instantaneous emission of a UV light pulse on the 12P–5S transition. The pulse had a time duration of tens of picoseconds, which was measured by a picosecond-resolution streak camera. The temporal shape of the generated light is explained by a simplified atom–field interaction theory.

In vivo diagnostics of early abiotic plant stress response via Raman spectroscopy

Significance Feeding a population of 9 billion in 2050 coupled with the changing climate and environmental stresses motivate us to develop advances in plant science and technology. We present a high-throughput plant phenotyping platform for detection of abiotic stress. The proposed Raman spectroscopic technique for high-throughput stress phenotyping and early stress detection in vivo improves sensitivity with the ability to interrogate individual molecules simultaneously in plants. This technology holds promise for mobile automated systems and precision agriculture. Development of a phenotyping platform capable of noninvasive biochemical sensing could offer researchers, breeders, and producers a tool for precise response detection. In particular, the ability to measure plant stress in vivo responses is becoming increasingly important. In this work, a Raman spectroscopic technique is developed for high-throughput stress phenotyping of plants. We show the early (within 48 h) in vivo detection of plant stress responses. Coleus (Plectranthus scutellarioides) plants were subjected to four common abiotic stress conditions individually: high soil salinity, drought, chilling exposure, and light saturation. Plants were examined poststress induction in vivo, and changes in the concentration levels of the reactive oxygen-scavenging pigments were observed by Raman microscopic and remote spectroscopic systems. The molecular concentration changes were further validated by commonly accepted chemical extraction (destructive) methods. Raman spectroscopy also allows simultaneous interrogation of various pigments in plants. For example, we found a unique negative correlation in concentration levels of anthocyanins and carotenoids, which clearly indicates that plant stress response is fine-tuned to protect against stress-induced damages. This precision spectroscopic technique holds promise for the future development of high-throughput screening for plant phenotyping and the quantification of biologically or commercially relevant molecules, such as antioxidants and pigments.

Ultrafast laser control of backward superfluorescence towards standoff sensing

We study infrared backward cooperative emission in a rubidium vapor induced by ultrafast two-photon optical excitations. The laser coherent control of the backward emission is demonstrated by using a pair of 100 fs pulses with a variable time delay. The temporal variation (quantum beat) of the backward beam intensity due to interference of atomic transitions in the rubidium atomic level system 5S-5P-5D is produced and controlled. Based on the obtained experimental results, we discuss possible applications of the developed approach for creation of an effective “guide star” in the sodium atomic layer in the upper atmosphere (mesosphere).

Intensity correlation and anti-correlations in coherently driven atomic vapor

Motivated by the recent experiment [Sautenkov, V.A.; Rostovtsev, Yu.V.; Scully, M.O. Phys. Rev. A 2005, 72, 065801], we study the field intensity fluctuations due to interaction between a laser with a finite bandwidth and a dense atomic medium. The intensity–intensity cross-correlation of two orthogonal, circular polarized beams can be controlled by the applied external magnetic field. A smooth transition from perfect correlations to anti-correlations (at zero delay time) of the outgoing beams is observed.