Note: An analyzer for field detection of H2S by using cavity ring-down at 1.57 µm.

A prototype analyzer for the detection of hydrogen sulfide (H2S), based on cavity ring-down spectroscopy, is described. The device exploits, whenever possible, optical fibers, in order to simplify the alignment and to improve the stability. A trade-off between low detection level and simplicity has been pursued. The experimental results obtained during tests on different kinds of H2S samples are shown.

High-Q resonant cavities for terahertz quantum cascade lasers.

We report on the realization and characterization of two different designs for resonant THz cavities, based on wire-grid polarizers as input/output couplers, and injected by a continuous-wave quantum cascade laser (QCL) emitting at 2.55 THz. A comparison between the measured resonators parameters and the expected theoretical values is reported. With achieved quality factor Q ≈ 2.5 × 10(5), these cavities show resonant peaks as narrow as few MHz, comparable with the typical Doppler linewidth of THz molecular transitions and slightly broader than the free-running QCL emission spectrum. The effects of the optical feedback from one cavity to the QCL are examined by using the other cavity as a frequency reference.

Mid-infrared frequency comb for broadband high precision and sensitivity molecular spectroscopy.

We report on the experimental demonstration of the metrological and spectroscopic performances of a mid-infrared comb generated by a nonlinear downconversion process from a Ti:sapphire-based near-infrared comb. A quantum cascade laser at 4330 nm was phase-locked to a single tooth of this mid-infrared comb and its frequency-noise power spectral density was measured. The mid-infrared comb itself was also used as a multifrequency highly coherent source to perform ambient air direct comb spectroscopy with the Vernier technique, by demultiplexing it with a high-finesse Fabry-Perot cavity.

Association of APOA1 and APOA5 polymorphisms and haplotypes with lipid parameters in a Brazilian elderly cohort.

Apolipoproteins have an important role in lipid metabolism and transport. Polymorphisms in the APOA1/C3/A4/A5 gene cluster have been associated with lipid alterations and cardiovascular diseases. We investigated APOA1 XmnI, APOA5 S19W, and APOA5 -1131T>C polymorphisms in 377 individuals from a cohort of a longitudinal Brazilian elderly study. Allele frequencies, genotype distribution, and association with major morbidities as well as with lipids, creatinine, albumin, urea, glycated hemoglobin, and fasting glucose serum levels were investigated. Linkage disequilibrium and haplotype associations were also analyzed. This is the first time that haplotypes involving these polymorphisms were evaluated. Genotyping was performed by PCR-RFLP. Minor allele frequencies were 0.119, 0.071, and 0.158 for XmnI, S19W, and -1131T>C polymorphisms, respectively. We found a significant association of the -1131C allele with low LDL-C levels. We also observed that XmnI and S19W polymorphisms were in linkage disequilibrium. The C/G haplotype, which is composed of the wild-type allele of XmnI and the minor allele of S19W, was associated with high total cholesterol serum levels in this elderly population. We conclude that the -1131T>C polymorphism and the C/G haplotype, including XmnI and S19W polymorphisms, are associated with alterations in lipid levels and may be risk factors for cardiovascular disease in the Brazilian elderly.

High-coherence mid-infrared frequency comb.

We report on the generation of a frequency comb around 4330 nm with an unprecedented coherence of the single teeth. Generating the comb within a Ti:sapphire laser cavity by a difference-frequency process and using a phase-lock scheme based on direct digital synthesis, we achieve a tooth linewidth of 2.0 kHz in a 1-s timescale (750 Hz in 20 ms). The generated per-tooth power of 1 µW ranks this comb among the best ever realized in the mid-infrared in terms of power spectral density.

Subkilohertz linewidth room-temperature mid-infrared quantum cascade laser using a molecular sub-Doppler reference.

We report on the narrowing of a room-temperature mid-IR quantum cascade laser by frequency locking it to a CO2 sub-Doppler transition obtained by polarization spectroscopy. A locking bandwidth of 250 kHz has been achieved. The laser linewidth is narrowed by more than two orders of magnitude below 1 kHz, and its absolute frequency is stabilized at the same level.

Whole blood genome-wide gene expression profile in males after prolonged wakefulness and sleep recovery.

Although the specific functions of sleep have not been completely elucidated, the literature has suggested that sleep is essential for proper homeostasis. Sleep loss is associated with changes in behavioral, neurochemical, cellular, and metabolic function as well as impaired immune response. Using high-resolution microarrays we evaluated the gene expression profiles of healthy male volunteers who underwent 60 h of prolonged wakefulness (PW) followed by 12 h of sleep recovery (SR). Peripheral whole blood was collected at 8 am in the morning before the initiation of PW (Baseline), after the second night of PW, and one night after SR. We identified over 500 genes that were differentially expressed. Notably, these genes were related to DNA damage and repair and stress response, as well as diverse immune system responses, such as natural killer pathways including killer cell lectin-like receptors family, as well as granzymes and T-cell receptors, which play important roles in host defense. These results support the idea that sleep loss can lead to alterations in molecular processes that result in perturbation of cellular immunity, induction of inflammatory responses, and homeostatic imbalance. Moreover, expression of multiple genes was downregulated following PW and upregulated after SR compared with PW, suggesting an attempt of the body to re-establish internal homeostasis. In silico validation of alterations in the expression of CETN3, DNAJC, and CEACAM genes confirmed previous findings related to the molecular effects of sleep deprivation. Thus, the present findings confirm that the effects of sleep loss are not restricted to the brain and can occur intensely in peripheral tissues.

Direct link of a mid-infrared QCL to a frequency comb by optical injection.

A narrow-linewidth comb-linked nonlinear source is used as master radiation to injection lock a room-temperature mid-infrared quantum cascade laser (QCL). This process leads to a direct lock of the QCL to the optical frequency comb, providing the unique features of narrow linewidth, absolute frequency, higher output power, and wide mode-hop-free tunability. The QCL reproduces the injected radiation within more than 94%, with a reduction of the frequency-noise spectral density by 3 to 4 orders of magnitude up to about 100 kHz, and a linewidth narrowing from a few MHz to 20 kHz.

Measuring frequency noise and intrinsic linewidth of a room-temperature DFB quantum cascade laser.

The frequency-noise power spectral density of a room-temperature distributed-feedback quantum cascade laser emitting at λ = 4.36 µm has been measured. An intrinsic linewidth value of 260 Hz is retrieved, in reasonable agreement with theoretical calculations. A noise reduction of about a factor 200 in most of the frequency interval is also found, with respect to a cryogenic laser at the same wavelength. A quantitative treatment shows that it can be explained by a temperature-dependent mechanism governing the transport processes in resonant tunnelling devices. This confirms the predominant effect of the heterostructure in determining shape and magnitude of the frequency noise spectrum in QCLs.

Apolipoprotein E polymorphisms and sleep quality in obstructive sleep apnea syndrome.

BACKGROUND: The purpose of this study was to evaluate the influence of polymorphism on sleep parameters of Obstructive Sleep Apnea Syndrome (OSAS) patients. METHODS: Patients were genotyped after a full-night polysomnography using the large Epidemiologic Sleep Study of São Paulo population-based sample. RESULTS: Individuals who carry the APOE ε2 allele showed longer sleep latency, lower sleep efficiency and higher numbers of arousals/hour, when compared to ε3 allele homozygous and carriers of ε4 allele (p<0.05). These findings remained significant even after correction for potential confounders, such as sex, age and African genetic ancestry. CONCLUSION: The APOE polymorphisms may modulate the effects of intermittent hypoxia and sleep fragmentation in the sleep architecture of OSAS patients, and that the presence of the ε2 allele may serve as a biological marker for the identification of a subgroup of patients who are more likely to suffer with OSAS detrimental effects on sleep, impacting not only the daily functioning, but also their quality of life.

Molecular gas sensing below parts per trillion: radiocarbon-dioxide optical detection.

Radiocarbon ((14)C) concentrations at a 43 parts-per-quadrillion level are measured by using saturated-absorption cavity ringdown spectroscopy by exciting radiocarbon-dioxide ((14)C(16)O(2)) molecules at the 4.5 µm wavelength. The ultimate sensitivity limits of molecular trace gas sensing are pushed down to attobar pressures using a comb-assisted absorption spectroscopy setup. Such a result represents the lowest pressure ever detected for a gas of simple molecules. The unique sensitivity, the wide dynamic range, the compactness, and the relatively low cost of this table-top setup open new perspectives for ^{14}C-tracing applications, such as radiocarbon dating, biomedicine, or environmental and earth sciences. The detection of other very rare molecules can be pursued as well thanks to the wide and continuous mid-IR spectral coverage of the described setup.

Ti:sapphire laser intracavity difference-frequency generation of 30 mW cw radiation around 4.5 µm.

A cw mid-IR coherent source based on difference-frequency generation is designed and characterized. For mid-IR generation, a periodically poled MgO:LiNbO(3) crystal is placed inside a compact Ti:sapphire laser cavity. This provides high-power pump radiation for the nonlinear process. Optical injection by an external-cavity diode laser ensures single-frequency operation of the Ti:sapphire laser, while signal radiation is provided by a fiber-amplified Nd:YAG laser. Mid-IR radiation can be generated with 3850-4540 nm tuning range, narrow linewidth, Cs-standard traceability, and TEM(00) spatial mode. 30 mW power is obtained at 4510 nm.

Saturated-absorption cavity ring-down spectroscopy.

We report on a novel approach to cavity ring-down spectroscopy with the sample gas in saturated-absorption regime. This technique allows us to decouple and simultaneously retrieve the empty-cavity background and absorption signal, by means of a theoretical model that we developed and tested. The high sensitivity and frequency precision for spectroscopic applications are exploited to measure, for the first time, the hyperfine structure of an excited vibrational state of 17O12C16O in natural abundance with an accuracy of a few parts in 10{-11}.

Observing the intrinsic linewidth of a quantum-cascade laser: beyond the Schawlow-Townes limit.

A comprehensive investigation of the frequency-noise spectral density of a free-running midinfrared quantum-cascade laser is presented for the first time. It provides direct evidence of the leveling of this noise down to a white-noise plateau, corresponding to an intrinsic linewidth of a few hundred hertz. The experiment is in agreement with the most recent theory on the fundamental mechanism of line broadening in quantum-cascade lasers, which provides a new insight into the Schawlow-Townes formula and predicts a narrowing beyond the limit set by the radiative lifetime of the upper level.

Ultra-stable, widely tunable and absolutely linked mid-IR coherent source.

We report on a new coherent source that, using a phase-lock scheme to an optical frequency-comb synthesizer, achieves a 10-Hz intrinsic linewidth, is tunable from 4 to 4.5 microm with a presettable absolute frequency and, when coupled to a high-finesse cavity, can provide a short-term absorption sensitivity of 1.3 x 10(-11) cm(-1)Hz,(-1/2). These unique spectral features make this source a precise tool for molecular physics.

Lamb-dip-locked quantum cascade laser for comb-referenced IR absolute frequency measurements.

The frequency of a DFB quantum cascade laser (QCL) emitting at 4.3 microm has been long-term stabilized to the Lamb-dip center of a CO2 ro-vibrational transition by means of first-derivative locking to the saturated absorption signal. Thanks to the non-linear sum-frequency generation (SFG) process with a fiber-amplified Nd:YAG laser, the QCL mid-infrared (IR) radiation has been linked to an optical frequency-comb synthesizer (OFCS) and its absolute frequency counted with a kHz-level precision and an overall uncertainty of 75 kHz.

Frequency-comb-referenced quantum-cascade laser at 4.4 microm.

We report what we believe to be the first absolute frequency measurement performed using a quantum-cascade laser (QCL) referenced to an optical frequency comb synthesizer (OFCS). A QCL at 4.43 microm has been used for producing near-infrared radiation at 858 nm by means of sum-frequency generation with a Nd:YAG source in a periodically poled lithium niobate nonlinear crystal. The absolute frequency of the QCL source has been measured by detecting the beat note between the sum frequency and a diode laser at the same wavelength, while both the Nd:YAG and the diode laser were referenced to the OFCS. Doppler-broadened line profiles of (13)CO(2) molecular transitions have been recorded with such an absolute frequency reference.

Frequency-comb-based absolute frequency measurements in the mid-infrared with a difference-frequency spectrometer.

We demonstrate the possibility of extending the well-established metrological performance of optical frequency-comb synthesizers to the mid-IR region by phase locking the pump and signal lasers of a difference-frequency source to two near-IR teeth of an optical comb. An uncertainty of 800 Hz (1.1 x 10(-11)) in the absolute frequencies of CO2 transitions near 4.2 microm has been measured by cavity-enhanced saturated-absorption spectroscopy. Prospects for the creation of a new dense set of high-quality molecular frequency standards in the IR are discussed.

Spatial mode control of radiation generated by frequency difference in periodically poled crystals.

We demonstrate the possibility to control the spatial mode of a narrow-linewidth, continuous-wave, infrared radiation beam generated by difference frequency in a periodically poled crystal. This can be achieved by acting directly on a few experimental parameters. We show that hollow beams can be generated. A numerical routine has been developed and results agree with experimental observations, without requiring any free parameters. The relevance of these results for high-resolution spectroscopy and atom manipulation is discussed.

Search for exchange-antisymmetric states for spin-0 particles at the 10(-11) level.

We set a new upper limit of 1.7x10(-11) to the probability that two spin-0 16O nuclei are in forbidden exchange-antisymmetric states, testing the validity of fundamental principles of quantum mechanics. A newly designed difference-frequency spectrometer is used to look for the existence of these states. The experimental test was performed by tuning the spectrometer across a molecular transition of 12C 16O (2) belonging to the rovibrational band at 4.25 microm, connecting the forbidden states (00(0)0,J = 25) and (00(0)1,J = 26).