Background-free Method Research Articles

Broadband background-free vibrational spectroscopy using a mode-locked Cr:ZnS laser.

We demonstrate high-sensitivity vibrational absorption spectroscopy in the 2-micron wavelength range by using a mode-locked Cr:ZnS laser. Interferometric subtraction and multichannel detection across the broad laser spectrum realize simultaneous background-free detection of multiple vibrational modes over a spectral span of >380 cm-1. Importantly, we achieve detection of small absorbance on the order of 10-4, which is well below the detection limit of conventional absorption spectroscopy set by the detector dynamic range. The results indicate the promising potential of the background-free method for ultrasensitive and rapid detection of trace gases and chemicals.

Terbium Functionalized Schizochytrium-Derived Carbon Dots for Ratiometric Fluorescence Determination of the Anthrax Biomarker.

Efficient and instant detection of biological threat-agent anthrax is highly desired in the fields of medical care and anti-terrorism. Herein, a new ratiometric fluorescence (FL) nanoprobe was elaborately tailored for the determination of 2,6-dipicolinic acid (DPA), a biomarker of anthrax spores, by grafting terbium ions (Tb3+) to the surface of carbon dots (CDs). CDs with blue FL were fabricated by a simple and green method using schizochytrium as precursor and served as an FL reference and a supporting substrate for coordination with Tb3+. On account of the absorbance energy transfer emission effect (AETE), green emission peaks of Tb3+ in CDs-Tb nanoprobe appeared at 545 nm upon the addition of DPA. Under optimal conditions, good linearity between the ratio FL intensity of F545/F445 and the concentrations of DPA was observed within the experimental concentration range of 0.5–6 μM with the detection limit of 35.9 nM, which is superior to several literature studies and significantly lower than the infectious dosage of the Bacillus anthracis spores. Moreover, the CDs-Tb nanoprobe could sensitively detect DPA in the lake water sample. This work offers an efficient self-calibrating and background-free method for the determination of DPA.

Coherently driving a single quantum two-level system with dichromatic laser pulses

The excitation of individual two-level quantum systems using an electromagnetic field is an elementary tool of quantum optics, with widespread applications across quantum technologies. The efficient excitation of a single two-level system usually requires the driving field to be at the same frequency as the transition between the two quantum levels. However, in solid-state implementations, the scattered laser light can dominate over the single photons emitted by the two-level system, imposing a challenge for single-photon sources. Here, we propose a background-free method for the coherent excitation and control of a two-level quantum system using a phase-locked dichromatic electromagnetic field with no spectral overlap with the optical transition. We demonstrate this method experimentally by stimulating single-photon emission from a single quantum dot embedded in a micropillar, reaching single-photon purity of 0.988(1) and indistinguishability of 0.962(6). The phase-coherent nature of our two-colour excitation scheme is demonstrated by the dependence of the resonance fluorescence intensity on the relative phase between the two pulses. Our two-colour excitation method represents an additional and useful tool for the study of atom–photon interaction, and the generation of spectrally isolated indistinguishable single photons. A quantum two-level system can be coherently excited by a phase-locked dichromatic electromagnetic field. This technique can make single-photon generation more efficient as the pump light does not overlap in frequency with the emitted single photons.

Rotational constants and structure of para-difluorobenzene determined by femtosecond Raman coherence spectroscopy: A new transient type.

Femtosecond Raman rotational coherence spectroscopy (RCS) detected by degenerate four-wave mixing is a background-free method that allows to determine accurate gas-phase rotational constants of non-polar molecules. Raman RCS has so far mostly been applied to the regular coherence patterns of symmetric-top molecules, while its application to nonpolar asymmetric tops has been hampered by the large number of RCS transient types, the resulting variability of the RCS patterns, and the 10(3)-10(4) times larger computational effort to simulate and fit rotational Raman RCS transients. We present the rotational Raman RCS spectra of the nonpolar asymmetric top 1,4-difluorobenzene (para-difluorobenzene, p-DFB) measured in a pulsed Ar supersonic jet and in a gas cell over delay times up to ∼2.5 ns. p-DFB exhibits rotational Raman transitions with ΔJ = 0, 1, 2 and ΔK = 0, 2, leading to the observation of J -, K -, A -, and C-type transients, as well as a novel transient (S-type) that has not been characterized so far. The jet and gas cell RCS measurements were fully analyzed and yield the ground-state (v = 0) rotational constants A0 = 5637.68(20) MHz, B0 = 1428.23(37) MHz, and C0 = 1138.90(48) MHz (1σ uncertainties). Combining the A0, B0, and C0 constants with coupled-cluster with single-, double- and perturbatively corrected triple-excitation calculations using large basis sets allows to determine the semi-experimental equilibrium bond lengths re(C1-C2) = 1.3849(4) Å, re(C2-C3) = 1.3917(4) Å, re(C-F) = 1.3422(3) Å, and re(C2-H2) = 1.0791(5) Å.

Single-cell screening and quantification of transcripts in cancer tissues by second-harmonic generation microscopy.

Fluorescence-based single molecule techniques to interrogate gene expression in tissues present a very low signal-to-noise ratio due to the strong autofluorescence and other background signals from tissue sections. This report presents a background-free method using second-harmonic generation (SHG) nanocrystals as probes to quantify the messenger RNA (mRNA) of human epidermal growth receptor 2 (Her2) at single molecule resolution in specific phenotypes at single-cell resolution directly in tissues. Coherent SHG emission from individual barium titanium oxide (BTO) nanoprobes was demonstrated, allowing for a stable signal beyond the autofluorescence window. Her2 surface marker and Her2 mRNA were specifically labeled with BTO probes, and Her2 mRNA was quantified at single copy sensitivity in Her2 expressing phenotypes directly in cancer tissues. Our approach provides the first proof of concept of a cross-platform strategy to probe tissues at single-cell resolution in situ.

Photomodulated Rayleigh Scattering of Single Semiconductor Nanowires: Probing Electronic Band Structure

The internal electronic structures of single semiconductor nanowires can be resolved using photomodulated Rayleigh scattering spectroscopy. The Rayleigh scattering from semiconductor nanowires is strongly polarization sensitive which allows a nearly background-free method for detecting only the light that is scattered from a single nanowire. While the Rayleigh scattering efficiency from a semiconductor nanowire depends on the dielectric contrast, it is relatively featureless as a function of energy. However, if the nanowire is photomodulated using a second pump laser beam, the internal electronic structure can be resolved with extremely high signal-to-noise and spectral resolution. The photomodulated Rayleigh scattering spectra can be understood theoretically as a first derivative of the scattering efficiency that results from a modulation of the band gap and depends sensitively on the nanowire diameter. Fits to spectral lineshapes provide both the band structure and the diameter of individual GaAs and InP nanowires under investigation.

Electron capture branching ratio measurements in an ion trap for double beta decay experiments at TITAN

Double beta decay (ββ) is a nuclear decay mode expected to appear in at least two varieties, the double-neutrino (2ν) and the zero-neutrino (0ν) mode. The 0νββ-decay is of particular interest as it requires the neutrino to be a Majorana particle. The search for such a decay is presently being carried out or planned in a number of experiments, such as EXO, MAJORANA, GERDA, CUORE, COBRA, NEMO-III and SNO+. The 0ν-decay rate depends on the neutrino mass but, unfortunately, also on a rather complex nuclear matrix element, making the extraction of the mass heavily dependent on the underlying theoretical nuclear model. However, all theoretical models can readily be tested against the 2ν mode, which, unlike its 0ν counterpart, only involves simple Gamow–Teller nuclear matrix elements. These elements can be determined experimentally either through charge-exchange reactions or, for the ground-state transition, through the electron capture (EC) or single β-decay of the intermediate odd–odd nucleus. The present program is geared towards the measurement of the EC branching ratios (BR). In most cases, these ratios are poorly known or not known at all, because EC is usually suppressed by several orders of magnitude compared to the β-decay counterpart due to energy considerations. Traditional methods for measuring these ratios have so far suffered from overwhelming background generated by these high-energy electrons. Recently, a unique background-free method for measuring EC branching ratios was proposed using the TITAN ion trap at the TRIUMF ISAC (Isotope Separator and ACcelerator) radioactive beam facility. The measurements will make use of the EBIT (Electron Beam Ion Trap) operating in Penning mode where electrons from the β−-decay will be confined by the magnetic field. K-shell X-rays from EC will be detected by seven X-ray detectors located around the trap, thus providing orders of magnitude background suppression and thus ideal low-BR measurement environment.

Two-beam femtosecond high-field interactions with a solid target. Surface modification and second harmonic generation under the conditions of surface electromagnetic wave excitation

Very intense interfering beams generated a high-temperature near-surface femtosecond plasma with periodically modulated parameters. The dynamics of the efficiency of diffraction by such an induced 'grating' was studied. A background-free method for measuring the duration of high-intensity laser pulses on the basis of noncollinear second-harmonic generation was demonstrated. An increase in the second-harmonic signal was observed for the first time under the conditions of resonant excitation of a surface electromagnetic wave by high-intensity radiation of femtosecond duration.

Determination of picosecond relaxation times of bleachable dyes by the method of cross polarizers

A background-free method is proposed for the determination of the time constants of fast relaxation of bleachable dyes. The method utilizes exciting and probe ultrashort light pulses. The absence of background is ensured by the use of crossed polarizers. The results are given of a determination (by the new method) of picosecond relaxation times of bleachable dyes of the kind used for mode locking of neodymium laser radiation. These results are in good agreement with the data obtained by the familiar absorption method.