Characterisation of the potential of frequency modulation and optical feedback locking for cavity-enhanced absorption spectroscopy

A combination of optical feedback self-locking of a continuous-wave distributed feedback diode laser to a V-shaped high finesse cavity, laser phase modulation at a frequency equal to the free spectral range of the V-cavity and detection of the transmitted laser beam at this high modulation frequency is described for the possible application in cavity-enhanced absorption spectroscopy. In order to estimate the noise level of an absorbance baseline, the triplet of frequency modulated light, i.e. the central laser frequency and the two sidebands, were transmitted through both the V-cavity in open air and a 1.5-cm long optical cell placed behind the cavity output mirror and filled with acetylene (C₂H₂) at low pressure. The performance of the setup was evaluated from the measured relative intensity noise on the cavity output (normalised by the bandwidth) and the frequency modulation absorption signals induced by C₂H₂ absorption in the 1.5-cm cell. From these data, we estimate that the noise-equivalent absorption sensitivity of 2.1 × 10^?11 cm^?1 Hz^?1/2—by a factor of 11.7 above the shot-noise limit—can be achieved for C₂H₂ absorption spectra extracted from the heterodyne beat signals recorded at the transmission maxima intensity peaks of the successive TEM₀₀ resonances.     

Tunable Two-Frequency Lasers for Lifetime Measurements

We built a compact two-tunable-frequency longitudinally pumped solid-state laser specially designed to measure atomic lifetimes. Weak coupling between the laser eigenstates permits the obtaining of a continuously tunable frequency difference between 0 and 10 GHz. Such a source provides either a linear polarization rotating at such frequencies, or a sinusoidally modulated intensity with a 100% modulation amplitude. As an example, the fluorescence lifetime of the first excited state of Cr⁴⁺ in YAG is measured in the zero-fluence limit using a two-frequency Nd:YAG laser at 1064 nm.     

Quantum dot-based tunable inversionless laser for the far infrared and terahertz ranges

A quantum dot-based laser with a frequency continuously tunable in the far-infrared and terahertz ranges via varying the optical pumping intensity is suggested. Its operation is possible at room temperature and is based on the inversionless amplification mechanism in the proposed modified three-level ladder scheme, where the pumping frequency is equal to the frequency of a dipole-forbidden transition from the ground state to the second excited state.     

Configurational coordinate diagram of Ni2+ doped silver halide crystals,as determined by optical and luminescence measurements

Optical and luminescence measurements were carried out on Ni²⁺ doped silver halide crystals. The configurational coordinates of the ground and the first excited states made it possible to explain the important optical properties, such as the Stokes shift between the ground and excited states, the broadening of the bands with temperature and the thermal quenching. Determining the configurational coordinate diagram of the ground and first excited states is the first step for the design and fabrication of solid state and fiber lasers which are based on Ni²⁺ doped silver halides and which operate in the middle infrared.     

Modulation-free pump-probe spectroscopy of strontium atoms

We have performed polarization spectroscopy on the 5s^{2 1}S₀ →5s5p¹P₁ transition of atomic strontium. The signal is generated by saturation effects, rather than optical pumping, as the ground state is non-degenerate. This technique generated a dispersion-type lineshape suitable for laser stabilization, without the need for frequency modulation. The dependence of the amplitude and gradient on intensity and magnetic field were also investigated, and compared to a related technique based on the circular dichroism induced by a magnetic field.     

调制激光场中Rydberg原子的电磁感应透明

本文主要研究了调制探测激光场中铯Rydberg 原子阶梯型三能级系统的电磁感应透明(EIT) 效应. 铯原子基态6S_1/2, 第一激发态6P_3/2 和Rydberg 态形成阶梯型三能级系统, 探测光作用于6S_1/2 (F = 4)→6P_3/2(F' = 5) 的跃迁, 耦合光在Rydberg 跃迁线6P_3/2→49S_1/2 附近扫描, 形成Rydberg 原子EIT. 当对探测光频率施加一个几kHz 的调制时, 调制解调后的EIT 信号分裂为两个峰, 双峰间距与调制频率无关,而与调制幅度导致的失谐量大小(频率调制幅度) 成正比, 双峰间隔的一半等于探测光频率调制幅度的λ_p/λ_c = 1.67 倍. 实验结果与理论计算相一致. 本文的研究结果可应用于激光线型和频率抖动的实时监测.     

Laser excited fluorescence spectrum of nitrogen dioxide

The fluorescence spectrum of NO₂ excited by a single mode argon-ion laser was studied at a spectral resolution of 0.1 cm^?1. In particular the Stokes and the anti-Stokes ν₂ band excited by the 5145 Å line of an argon-ion laser were examined in detail. Some of the vibrational and rotational parameters of the ground electronic state and the rotational constants of the B₂ vibronic state were obtained.     

Effect of the Excitation Radiation Coherence on Oscillations of the Photon Echo Intensity

The physical reasons for observing the splitting of optical lines several orders of magnitude smaller than the spectral width of a laser pulse are investigated. A theory of coherent and incoherent photon echo (PE) in an external static magnetic field and in the presence of a pulsed magnetic field, which causes oscillations of the PE intensity, is elaborated. It is shown that the periods of oscillations in the echo intensity, the echo duration, and the dimensions of the regions in the inhomogeneous line, where the excited ions are coherent, do not depend on the degree of coherence of the laser pulse and on the external static magnetic field. As follows from the theory, in the case of the coherent excitation of the echo, the amplitude of the intensity oscillations is independent of the external static magnetic field if the inhomogeneous line is symmetric. It is shown that the amplitude of the oscillations at the incoherent excitation of the echo is equal to the autocorrelation function of the distribution function of the transition frequency along the inhomogeneous line with the argument equal to the Zeeman splitting of the optical line in the external magnetic field. In this case, the experimental values of the oscillation amplitude are in good agreement with the calculated values of the autocorrelation function for the total inhomogeneous line in LuLiF₄:Er³⁺ (⁴I_15/2?F_9/2 transition). In the same way, the autocorrelation function has been obtained for YLiF₄:Er³⁺ on the same transition.     

MOLECULAR STRUCTURE AND PHOTOPHYSICS OF N-QUATERNARY DIARYLOXAZOLIUM SALTS

ABSTRACT

The concern of this work is to study molecular structure, electronic absorption and emission spectra of several N-quaternary salts of the well-known diaryloxazole scintillating compounds: 2,5-diphenyloxazole, para-, meta- and ortho- isomers of bis-2-(5-phenyl-oxazolyl)-benzene (POPOP). All of them were obtained from the initial aryloxazoles by their methylation with dimethylsulfate.

We found, that N-methylation manifests itself in arising of sterical hindrance in the molecules of diaryloxazolium salts, which results in distortion of their planarity. On the contrary to the ground state, the investigated molecules become more planar in their lowest singlet excited state. As a result of such an excited state flattening, fluorescence Stokes shifts values of the diaryloxazolium salts exceed 9000–10,000 cm^?1. The excited state flattening rate constants, estimated for the studied oxazolium compounds, are of the 10¹⁰ s^?1 range. No considerable increase of radiationless losses, induced by the excited state structural relaxation, was found. Owing to these facts, N-quaternary diaryloxazolium salts may be considered as effective abnormally high Stokes shift organic luminophores.     

High index of refraction via quantum interference in a three-level system of Er^3＋-doped yttrium aluminium garnet crystal

A simple three-level system is proposed to produce high index of refraction with zero absorption in an Er^3＋-doped yttrium aluminium garnet （YAG） crystal, which is achieved for a probe field between the excited state 4I13/2 and ground state 4I15/2 by adjusting a strong coherent driving field between the upper excited state 4I11/2 and 4I15/2. It is found that the changes of the frequency of the coherent driving field and the concentration of Er^3＋ ions in the YAG crystal can maximize the index of refraction accompanied by vanishing absorption. This result could be useful for the dispersion compensation in fibre communication, laser particle acceleration, high precision magnetometry and so on.     

Spectral line-shapes investigation with Pound-Drever-Hall-locked frequency-stabilized cavity ring-down spectroscopy

A review of recent experiments involving a newly developed Pound-Drever-Hall-locked frequency-stabilized cavity ring-down spectroscopy (PDH-locked FS-CRDS) system is presented. By comparison to standard FS-CRDS, the PDH lock of the probe laser to the ring-down cavity optimized coupling into the cavity, thus increasing the ring-down signal acquisition rate nearly 300-fold to 14 kHz and reducing the noise-equivalent absorption coefficient by more than an order of magnitude to 7 × 10^?11 cm^?1. We discuss how averaging approximately 1000 spectra yielded a signal-to-noise ratio of 220000. We also discuss how the spectrum frequency axis was linked to an optical frequency comb, thus enabling absolute frequency measurements of molecular optical transitions at sub-MHz levels. Applications of the spectrometer to molecular line-shape studies are also presented. For these investigations, we use semi-classical line-shape models that consider the influence of Dicke narrowing as well as the speed dependence of the pressure broadening and shifting to fit spectra. We show that the improved precision and spectrum fidelity of the spectrometer enable precise determinations of line-shape parameters. We also discuss the importance of line-shape analysis with regard to the development of new spectroscopic databases as well as in the optical determination of the Boltzmann constant.     

Highly excited,normally inaccessible vibrational levels by sub-doppler modulated gain spectroscopy: The Na2 A1Σ+u state

Modulated gain spectroscopy is a sensitive, widely applicable, rovibronically state selective, sub-Doppler, triple resonance method for examining excited vibronic levels which are Franck—Condon inaccessible from thermally populated levels of the electronic ground state. A cw optically pumped molecular laser (OPL) prepares a steady-state population in a selected, vibrationally highly excited, rotation-vibration level of the electronic ground state (the lower level of the OPL transition). An intensity-modulated, single frequency dye laser excites part of this intracavity OPL-prepared population to the level of interest, thereby causing an increase in the OPL population inversion density, and, in turn, its output power. As the frequency of the dye laser is scanned, resonances are selectively detected by the appearance of modulation on the OPL output power; discrimination against dye laser excitations out of levels unconnected with the OPL is nearly perfect. Sub-Doppler (≈300 MHz FWHM) transitions are observed, thereby extending knowledge of the Na₂A¹Σ⁺_u state from v=44 to 62.     

On optical dispersion in transparent molecular systems

The inelastic optical scattering process leading to the generation of the recently reported new molecular state (being stable at 100 K) of sodium nitroprusside is investigated. The process is a strong function of the phase coherence (Glauber state) of the incident light.-With laser light of 20–200 mW/cm² the yield of new molecular states rapidly reaches saturation, about 50% of all molecules of an extended target. The generation process is a linear function of the product: light intensity × illumination time. The coherent yield function follows a very simple law which, however, is different from predictions of linear or non-linear optics. It is a strong function of the laser frequency (range at least (4900±400) Å) and of the optical polarization relative to the oriented molecules. — The yield obtained with chaotic light is one order of magnitude smaller and weaker dependent upon frequency and polarization. The chaotic yield function depends exponentially upon the light intensity and illumination time in a way as to be expected from linear optics. — The coherently generated new molecular states can be depopulated by chaotic light (or by thermal agitation at elevated temperatures) and coherently regenerated arbitrarily often. There is further evidence that the recently reported properties of the new state are predominantly determined by the internal molecular structure. The lifetimeτ?10⁷ s at 100 K.     

Neutral Dissociation of Superexcited Nitric Oxide Induced by Intense Laser Fields

利用波长800 nm、脉宽60 fs、强度0.2 PW/cm²的强激光激发NO分子,随后通过荧光光谱检测到了处于激发态的N和O的中性碎片. 说明强激光导致的多光子激发同样可以引起NO分子的超激发,并且发生中性解离. 采用离子实解离模型,建立了NO分子的超激发态势能曲线. 提出了直接解离和预解离两种解离过程,分别对应生成N^*+O或O^*+N两个通道.     

Recombination emissions and spectral blueshift of pump radiation from ultrafast laser induced plasma in a planar water microjet

We report spectroscopic investigations of an ultrafast laser induced plasma generated in a planar water microjet. Plasma recombination emissions along with the spectral blueshift and broadening of the pump laser pulse contribute to the total emission. The laser pulses are of 100 fs duration, and the incident intensity is around 10¹⁵ W/cm². The dominant mechanisms leading to plasma formation are optical tunnel ionization and collisional ionization. Spectrally resolved polarization measurements show that the high frequency region of the emission is unpolarized whereas the low frequency region is polarized. Results indicate that at lower input intensities the emission arises mainly from plasma recombinations, which is accompanied by a weak blueshift of the incident laser pulse. At higher input intensities strong recombination emissions are seen, along with a broadening and asymmetric spectral blueshift of the pump laser pulse. From the nature of the blueshifted laser pulse it is possible to deduce whether the rate of change of free electron density is a constant or variable within the pulse lifetime. Two input laser intensity regimes, in which collisional and tunnel ionizations are dominant respectively, have been thus identified.     

A New Candidate Laser Dye Based 1,4-Bis[β-(2-Naphthothiazolyle)Vinyl]Benzene. Spectroscopic Behavior,Laser Parameters and Excitation Energy Transfer

A new candidate laser dye based 1,4-bis[β-(2-naphthothisolyl) vinyl] benzene (BNTVB) were prepared, and characterized in various organic solvents. The center polarity is less sensitive than electronic absorption. A red shift was noticed in the fluorescence spectra (ca. 40 nm) with increment in the solvent’s polarity, this means that BNTVB’s polarity appreciates upon excitation. The dipole moment of ground state (μ_g) and the excited singlet state dipole moment (μ_e) are determined from Kawski – Chamma and Bakshiev–Viallet equations using the disparity of Stokes shift with solvent polarity function of ε (dielectric constant) and n (refractive index) of the solvent. The result was found to be 0.019D and 5.13D for ground and exited state, in succession. DFT/TD-DFT manners were used to understand the electronic structures and geometric of BNTVB in other solvents. The experimental and theoretical results showed a good agreement. The photochemical quantum yield (Ф_c) of BNTVB was calculated in variable organic reagents such as Dioxane, CHCl₃, EtOH and MeOH at room temperature. The values of φ_c were calculated as 2.3?×?10^?4, 3.3?×?10^?3, 9.7?×?10^?5 and 6.2?×?10^?5 in Dioxane, CHCl₃, EtOH and MeOH, respectively. The dye solutions (2?×?10^?4 M) in DMF, MeOH and EtOH give laser emission in the blue-green region. The green zone is excited by nitrogen pulse 337.1 nm. The tuning range, gain coefficient (α) and cross – section emission (σ_e) of laser were also estimated. Excitation energy transfer from BNTVB to rhodamine-6G (R6G) and N,N-bis(2,6-dimethyphenyl)-3,4:9,10-perylenebis-(dicarboximide) (BDP) was also studied in EtOH to increase the laser emission output from R6G and BDP when excited by nitrogen laser. The dye-transfer power laser system (ETDL) obeys the Foster Power Transmission (FERT) mechanism with a critical transmission distance, R_o of 40 and 32 ? and k_ET equals 2.6?×?10¹³ and 1.06?×?10¹³ M^?1 s^?1 for BNTVB / R6G and BNTVB / BDP pair, respectively.     

Absolute frequency measurement in the 28-THz spectral region with a femtosecond laser comb and a long-distance optical link to a primary standard

A new frequency chain was demonstrated to measure an optical frequency standard based on a rovibrational molecular transition in the 28-THz spectral region accessible to a CO₂ laser. It uses a femtosecond-laser frequency comb generator and two laser diodes at 852 nm and 788 nm as intermediate oscillators, with their frequency difference phase locked to the CO₂ laser. The rf repetition rate of the femtosecond laser was compared with a 100-MHz signal from a hydrogen maser, located at BNM-SYRTE. The 100-MHz signal is transmitted by amplitude modulation of a 1.55-m laser diode through a 43-km telecommunication optical fibre. As a first example, the absolute measurement of a saturation line of OsO₄ in the vicinity of the P(16) laser line of CO₂ is reported with a relative uncertainty of 10^-12, limited by the CO₂/OsO₄ frequency day-to-day reproducibility. The current limit on the stability of the frequency measurement is 4×10^-13 at 1 s. PACS 06.20.-f; 42.62.Eh; 06.30.Ft     

ʌ-type doubling and spin-rotation splitting of NO,measured in simultaneous one- and two-photon laser spectroscopy

Doppler-free two-photon excitation of NO A²Σ⁺ by visible dye laser radiation is recorded simultaneously with single-photon absorption of frequency-doubled UV laser light. The measurement of the frequency difference between corresponding lines in the one- and two-photon spectrum provides a new method for a direct optical determination of Λ-type doubling in the X²Π ground state. In addition, the spin-rotation splitting of the A²Σ⁺ excited state is resolved in the Doppler-free two-photon spectrum.     

Laser excited luminescence spectra of zirconia

Luminescence spectra of ivory zirconia (Zr0₂) excited by an argon-ion laser (19,436–21,839 cm^?1) reveal a complex pattern consisting of both sharp and diffuse peaks in the 16,000–19,000 cm^?1 region. The intensity behavior of these features depends markedly on the excitation frequency. The sharp luminescence peaks of the 18,140–18,600 cm^?1 region are attributed to phonon-mediated de-excitation of excited states of the impuriity-doped ZrO₂ lattice. The more diffuse luminescence bands of the 17,700–18,000cm^?1 region may be associated with the electron traps observed in glow experiments and/or with higher-order phonon processes.     

强激光照射下,α-LiIO3晶体中晶格振动的多模激发和模之间的相互作用

本文叙述了用调Q倍频YAG脉冲激光照射α-LiIO₃晶体,在不同的散射配置下,获得了各种散射谱图。实验表明,当用e光入射时,可以比用o光入射激发起更多不同模的晶格振动;获得了E₂模的很强的一阶、二阶斯托克斯散射线和反斯托克斯散射线,E₁模Polariton的一阶、二阶、三阶斯托克斯线和反斯托克斯线,还获得了反斯托克斯频率区一条频移为469cm^-1的新线,认为是不同模的Polariton之间的相互作用引起的。最后,提到实验中初步观察到的晶体拉曼活性的“疲劳现象”。 关键词：