Molecular excitation by the simultaneous absorption of two photons provides intrinsic three-dimensional resolution in laser scanning fluorescence microscopy. The excitation of fluorophores having single-photon absorption in the ultraviolet with a stream of strongly focused subpicosecond pulses of red laser light has made possible fluorescence images of living cells and other microscopic objects. The fluorescence emission increased quadratically with the excitation intensity so that fluorescence and photo-bleaching were confined to the vicinity of the focal plane as expected for cooperative two-photon excitation. This technique also provides unprecedented capabilities for three-dimensional, spatially resolved photochemistry, particularly photolytic release of caged effector molecules.

https://materias.df.uba.ar/TemasNano2013/files/2013/07/1990_Denk_Science_Two-photon-fluo-microscopy.pdf

Two-Photon Laser Scanning Fluorescence Microscopy

Over the last decade there have been spectacular developments in ultrafast laser technology, due to the introduction of solid state active materials and of new mode-locking and amplification techniques. These advances, together with the discovery of new nonlinear optical crystals, have fostered the introduction of ultrafast optical parametric amplifiers as a practical source of femtosecond pulses tunable across the visible and infrared spectral ranges. This article summarizes the recent progress in the development of ultrafast optical parametric amplifiers, giving the basic design principles for different frequency ranges and in addition presenting some advanced designs for the generation of ultrabroadband, few-optical-cycle pulses. Finally, we also briefly discuss the possibility of applying parametric amplification schemes to large-scale, petawatt-level systems.

Ultrafast optical parametric amplifiers

A variety of frequency-modulation methods for high-sensitivity absorption detection of gas-phase species has evolved in recent years. The distinctions among these methods are mostly semantic. The mathematical derivations for wavelength-modulation spectroscopy and one- and two-tone frequency-modulation spectroscopies are presented; a common terminology is used to permit a comprehensive comparison of predicted detection sensitivities. Applying this formalism, I compare the optimum detection sensitivities of these different methods for a typical laser system, using the same parameters. As long as residual amplitude modulation is minimized by proper adjustment of the detection phase angle, high-frequency wavelength modulation and one- and two-tone frequency-modulation methods all achieve approximately the same sensitivities. The choice among techniques is most strongly driven by the individual laser tuning characteristics, the absorption linewidth, and the detection bandwidth. It is shown that excess laser noise cannot always be excluded from consideration, even at megahertz detection frequencies. Also, detection at harmonics of the modulation or beat frequency may present certain advantages in minimizing residual amplitude-modulation noise.

Frequency-modulation spectroscopy for trace species detection: theory and comparison among experimental methods

Ultrafast vibrational dynamics of hydrogen bonds in the condensed phase.

Wavelength modulation spectroscopy (WMS) and one-tone and two-tone frequency modulation spectroscopy (FMS) are compared by measuring the minimum detectable absorbances achieved using a mid-IR lead-salt diode laser. The range of modulation and detection frequencies spans over 5 orders of magnitude. The best results, absorbances in the low-to-mid 10−7 range in a 1-Hz bandwidth, are obtained by using high-frequency WMS (10-MHz detection frequency) and are limited by detector thermal noise. This sensitivity can provide species detection limits well below 1 part per billion for molecules with moderate line strengths if multiple-pass cells are used. High-frequency WMS is also tested by measuring the absorbance due to tropospheric N2O at 1243.795 cm−1. WMS at frequencies <100 kHz is limited by laser excess (1/f) noise. Both of the FMS methods, which require modulating the laser at frequencies ≥150 MHz, give relatively poor results due to inefficient coupling of the modulation waveform to the laser current. The results obtained agree well with theory. We also discuss the sensitivity limitations due to interference fringes from unintentional etalons and the effectiveness of etalon reduction schemes.

Frequency modulation and wavelength modulation spectroscopies:comparison of experimental methods using a lead-salt diode laser

A broadly tunable femtosecond optical parametric oscillator (OPO) based on KTiOPO4 is externally pumped by a self-mode-locked Ti:sapphire laser. The laser is capable of continuous tuning from 1.2 micrometers to 1.37 micrometers in the signal branch and 1.8 to 2.15 micrometers in the idler branch, when using one set of OPO optics. Other optics expand the tuning range of the OPO from 1.0 micrometers to 2.75 micrometers, for example, by using three sets of mirrors and two different crystals. Without prisms in the OPO cavity, 215 mW of chirped pulses is generated in the signal branch, while 235 mW is generated in the idler branch. The total conversion efficiency, as measured by pump depletion, is 50%. With prisms in the cavity, nearly transform-limited pulses of 135 femtoseconds are generated, which can be shortened to 75 fs by increasing the output coupling.

Ti:sapphire-pumped high repetition rate femtosecond optical parametric oscillator

The method of doing dual-beam single-detector wavelength-modulation spectroscopy originally proposed by Bonfiglioli and Trench is demonstrated experimentally, using a tunable laser. This system monitors both the reference and the sample beams continuously in phase quadrature and measures the derivative and the direct spectra simultaneously. When this system was used in conjunction with an electro-optically tuned cw dye laser, a sensitivity of (1/T) (dT/dlambda) = 3 x 10(-6) A(-1) was experimentally achieved for modulation depths from 1 to 50 A and can be extended down to 0.1 A. The derivative spectrum corresponding to the extremely weak fifth overtone of the C-H stretching vibration in the electronic ground state of benzene has been observed in a path length as short as 1 cm.

High-sensitivity laser wavelength-modulation spectroscopy.

We describe dispersion compensation of a cw mode-locked optical parametric oscillator based on a thin crystal of KTiOPO4 pumped intracavity by a standard colliding-pulse mode-locked dye laser, The device provides potential cw tunability from 0.72 to 4.5 μm with milliwatt outputs. Transform-limited pulses should be possible throughout the tuning range;here we report pulse widths of 105 fsec at 840 nm and 108 Hz. Active stabilization of the oscillator cavity is also discussed.

Continuous-wave mode-locked and dispersion-compensated femtosecond optical parametric oscillator

We describe a high-repetition-rate femtosecond optical parametric oscillator (OPO) that was broadly tunable in the mid infrared. The all-solid-state-pumped OPO was based on quasi-phase matching in periodically poled lithium niobate. The idler was tunable from approximately 1.7 µm to beyond 5.4 µm, with maximum average power levels greater than 200  mW and more than 20  mW of average power at 5.4 µm. We used interferometric autocorrelation to characterize the mid-infrared idler pulses, which typically had durations of 125  fs. This OPO had a pumping threshold as low as 65  mW of average pump power, a maximum conversion efficiency of >35% into the near-infrared signal, a slope efficiency for the signal of approximately 60%, and a maximum pump depletion of more than 85%.

/pdf/broadly-tunable-mid-infrared-femtosecond-optical-parametric-156fvfnuof.pdf

Broadly tunable mid-infrared femtosecond optical parametric oscillator using all-solid-state-pumped periodically poled lithium niobate

We demonstrate a Ti:sapphire-pumped intracavity-doubled optical parametric oscillator (OPO) that generates a total of up to 240 mW of sub-100-fs pulses tunable in the visible. The OPO consists of a 1.5-mm-thick KTiPO4 (KTP) crystal configured in a ring cavity that is synchronously pumped by a self-mode-locked Ti:sapphire laser operating at an 81-MHz repetition rate and 2.1-W average power, producing 115-fs pulses at λ = 790 nm. Intracavity doubling of the OPO is accomplished by inserting a 47-μm-thick β-BaB2O4 crystal into an additional focus in the OPO cavity. We demonstrate continuous tuning of the second-harmonic output from 580 to 657 nm. The potential tuning range of this intracavity-doubled KTP OPO is approximately 500 to 800 nm.

/pdf/high-power-high-repetition-rate-femtosecond-pulses-tunable-3lsqc6w0cm.pdf

C. L. Tang

Papers

Ti:sapphire-pumped high repetition rate femtosecond optical parametric oscillator

High-sensitivity laser wavelength-modulation spectroscopy.

Continuous-wave mode-locked and dispersion-compensated femtosecond optical parametric oscillator

Broadly tunable mid-infrared femtosecond optical parametric oscillator using all-solid-state-pumped periodically poled lithium niobate

High-power, high-repetition-rate femtosecond pulses tunable in the visible.