Intense self-compressed, self-phase-stabilized few-cycle pulses at 2 microm from an optical filament

We report the compression of intense, carrier-envelope phase stable mid-IR pulses down to few-cycle duration using an optical filament. A filament in xenon gas is formed by using self-phase stabilized 330 microJ 55 fs pulses at 2 microm produced via difference-frequency generation in a Ti:sapphire-pumped optical parametric amplifier. The ultrabroadband 2 microm carrier-wavelength output is self-compressed below 3 optical cycles and has a 270 microJ pulse energy. The self-locked phase offset of the 2 microm difference-frequency field is preserved after filamentation. This is to our knowledge the first experimental realization of pulse compression in optical filaments at mid-IR wavelengths (lambda>0.8 microm).     

Generation of mid-infrared pulses by X(3) difference frequency generation in CaF(2) and BaF(2)

Tunable mid-IR pulses in the range 1300-4200 cm(-1) (7.7-2.4 microm) are generated through a phase-matched four-wave mixing process in ordinary mid-IR window materials such as CaF(2) and BaF(2) . In this process the difference frequency v(3)=2v(2)-v(1) is generated from pump fields v(1) and v(2) . The process can be phase matched to different frequencies by adjustment of the angle between the pump fields.     

Generation of narrowband subpicosecond mid-infrared pulses via difference frequency mixing of chirped near-infrared pulses

The widely used setup for the generation of femtosecond infrared (IR) pulses based on parametric amplifiers (OPAs) and difference frequency mixing (DFM) is extended to produce tunable narrowband mid-IR pulses. The insertion of pairs of silicon prisms after the OPA induces adjustable chirp, which leads to the generation of narrowband pulses in the DFM stage. Rapid tunability of the mid-IR wavelength via a computer-controllable actuator can be achieved in a range of approximately 200 cm(-1) at a bandwidth of the IR-pulses between approximately 15 and approximately 50 cm(-1) and pulse energies up to 0.4 microJ. The narrowband mid-IR pulses are well suited for 2D IR spectroscopy.     

Mid-infrared ZnGeP2 parametric oscillator directly pumped by a pulsed 2 microm Tm-doped fiber laser

We have demonstrated what we believe to be the first mid-infrared optical parametric oscillator (OPO) pumped directly by a pulsed Tm-doped fiber laser. The Tm-fiber pump laser produces 30 ns pulses with a repetition rate of 30 kHz at a wavelength of 2 microm. The ZnGeP2 (ZGP) OPO produces 20 ns mid-IR pulses in the 3.4-3.9 microm and 4.1-4.7 microm spectral regions simultaneously. More than 658 mW of mid-IR output power has been generated with a total OPO slope efficiency greater than 35%.     

Wavelength-agile mid-infrared (5-10 microm) generation using a galvano-controlled KTiOPO4 optical parametric oscillator

A wavelength-agile mid-infrared (IR) ZnGeP2 (ZGP) optical parametric oscillator (OPO) using a galvano-controlled double-crystal KTiOPO4 (KTP) OPO was demonstrated. The mid-IR wavelength was tuned by varying the KTP OPO pump wavelength while the ZGP crystal angle remained fixed. Rapid tuning of the KTP OPO was achieved by changing the crystal angle by using the galvano scanner. Our mid-IR source can jump to a different wavelength without scanning through the intermediate wavelengths while also permitting continuous-wavelength scanning. The mid-IR source can be tuned from approximately 5to10 microm at a phase-matching angle of 51 degrees , while the pump wavelength is controlled in the 1.95-2.2 microm range.     

Efficient mid-IR spectral generation via spontaneous fifth-order cascaded-Raman amplification in silica fibers

Spontaneous cascaded Raman amplification is demonstrated as a practical and efficient means of power transfer from telecommunications wavelengths to mid-IR wavelength bands through use of conventional silica fibers and amplifiers. We show that silica fibers possessing normal dispersion over all near-IR and mid-IR wavelengths can facilitate 37% and 16% efficient Raman power conversion from 1.53 microm to 2.15 and 2.41 microm wavelength bands, respectively, using nanosecond pulses from an all-fiber laser source. In contrast to supercontinuum-based techniques for long-wavelength generation, the high levels of Raman gain generated at these wavelength bands could produce useful optical amplification necessary for the development of numerous mid-IR laser sources.     

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.     

Generation of tunable octave-spanning mid-infrared pulses by filamentation in gas media

The continued development of femtosecond mid-infrared (IR) sources with ultrabroad spectral width is critical for probing and controlling complex molecular structural dynamics on an ultrafast timescale. We report on a sub-20 fs, coherent mid-IR source with an octave-spanning spectral bandwidth (>2000 cm(-1)) tunable from 2-8 micrometers (37.5-150 THz), with energy >0.4 μJ/pulse at 1 kHz. The mid-IR pulses are generated by four-wave mixing during the filamentation of intense 800 nm and 400 nm pulses in various gas media. Spectral tunability is achieved by the choice of gas, pressure and input 800 nm pulse energy.     

Ultrafast continuum mid-infrared spectroscopy: probing the entire vibrational spectrum in a single laser shot with femtosecond time resolution

Until now, ultrafast IR spectroscopy has been limited by the bandwidth of optical parametric amplifiers, typically 100-400 cm(-1). Here we present the first example of transient IR spectroscopy using a continuum laser source to probe the entire mid-IR region with ultrafast time resolution. The continuum source is based on focusing the fundamental, second harmonic, and third harmonic of 1 mJ, 25 fs, 800 nm pulses in air, generating ～150 fs continuum mid-IR pulses that span the frequency range of <400 to >5000 cm(-1) or, conversely, <2 to >25 μm. We characterize the spectral and temporal properties of dicarbonylacetonato rhodium(I) in hexane. We further demonstrate the versatility of the method by measuring the very fast and broad (>1500 cm(-1)) spectral changes following IR excitation associated with the 7-azaindole-acetic acid heterodimer in carbon tetrachloride.     

Mercury thiogallate mid-infrared femtosecond optical parametric generator pumped at 1.25 microm by a Cr:forsterite regenerative amplifier

We demonstrate a novel traveling-wave-type optical parametric generator based on 1.25-microm pumping of HgGa(2)S(4) that produces tunable, high-power, transform-limited infinity 200-fs pulses in the mid-IR from 5 to 9 microm. Output idler energies on the microjoule level are obtained with maximum conversion efficiency of 11% for the amplifier stage, which is more than two times better than the results obtained with an analogous sample of the widely spread material AgGaS(2).     

Midinfrared optical breakdown in transparent dielectrics

Optical breakdown measurements for transparent dielectrics are reported for 1 ps laser pulses as a function of mid-IR wavelength from 4.7 to 7.8 microm. For wide-gap dielectrics seed electrons are generated by tunnel ionization with subsequent avalanche ionization and laser absorption by dense plasma. For narrow-gap dielectrics tunnel ionization alone leads to dense plasma formation.     

Optical parametric oscillation in quasi-phase-matched GaAs

We demonstrate an optical parametric oscillator (OPO) based on GaAs. The OPO utilizes an all-epitaxially-grown orientation-patterned GaAs crystal that is 0.5 mm thick, 5 mm wide, and 11 mm long, with a domain reversal period of 61.2 microm. Tuning either the near-IR pump wavelength between 1.8 and 2 microm or the temperature of the GaAs crystal allows the mid-IR output to be tuned between 2.28 and 9.14 microm, which is limited only by the spectral range of the OPO mirrors. The pump threshold of the singly resonant OPO is 16 microJ for the 6-ns pump pulses, and the photon conversion slope efficiency reaches 54%. We also show experimentally the possibility of pump-polarization-independent frequency conversion in GaAs.     

5.2-5.6-microm source tunable by frequency conversion in a GaAs-based waveguide

A tunable mid-IR source obtained by difference-frequency generation is demonstrated in a selectively oxidized GaAsAlAs multilayer waveguide. We designed the waveguide to present the required form birefringence for phase matching of the nonlinear interaction. We took special care to lower losses for the mid-IR radiation. IR tunability from 5.2 to 5.6 mum was achieved by variation of the waveguide temperature and one pump wavelength. IR output power as great as 0.12 muW was obtained with the product of two pump powers of 7 mW(2). Losses of ~50 cm(-1) were measured for the mid-IR radiation. These losses are attributed to surface scattering.     

Controlling vibrational excitation with shaped mid-IR pulses

We report selective population of the excited vibrational levels of the T(1u) CO-stretching mode in W(CO)(6) using phase-tailored, femtosecond mid-IR (5.2 microm, 1923 cm(-1)) pulses. An evolutionary algorithm was used to optimize specific vibrational populations. Stimulated emission peaks, indicative of population inversion, could be induced. Systematic truncation of each optimized pulse allowed for increased understanding of the excitation mechanism. The pulses and techniques developed herein will have broad applications in controlling ground state chemistry and enhancing vibrational spectroscopies.     

Phase-coherent light pulses for atom optics and interferometry

We have developed a novel source for the generation of powerful phase-coherent light pulses. Our setup uses an acousto-optic modulator (AOM) inside an external high-finesse resonator. By applying a short rf pulse to the AOM, we dump the cavity and extract a large part of the stored and enhanced power within a short optical pulse and with a controllable optical phase. In preliminary experiments we reached 100 W of peak power in a 15-ns optical pulse. The mutual phase coherence of successive light pulses is demonstrated with a molecular iodine interferometer experiment in a cell.     

Third- and fifth-harmonic generation by mid-infrared ultrashort pulses: beyond the fifth-order nonlinearity

Third- and fifth-harmonic generation by ultrashort laser pulses in the mid-infrared (mid-IR) reveals nonlinear-optical effects beyond the fifth-order nonlinearity and enables, because of an extraordinarily long coherence length, efficient multiplex frequency upconversion of ultrashort mid-IR pulses. We show that harmonic generation by mid-IR pulses provides an access to the key optical constants of gas media, allowing metrology of linear and nonlinear-optical susceptibilities in the mid-IR and offering a tool for the remote sensing of the atmosphere.     

Midinfrared resonant magnetic nanostructures exhibiting a negative permeability

We experimentally demonstrate the first midinfrared (mid-IR) resonant magnetic nanostructures exhibiting a strong magnetic response corresponding to a negative permeability. This result is an important step toward the achievement of a negative refractive index in the IR. The possibility of extending negative permeability to higher frequencies is discussed; a structure with a negative effective permeability at a near-IR resonance frequency of 230 THz (1.3 microm) is proposed.     

Characteristic of pulsed fiber laser induced by switching time

Laser-diode pumped Q-switched ytterbium-doped switching time of acousto-optic modulator （AOM） fiber laser is studied experimentally by controlling the The characteristics of Q-switched pulses with different rise time of AOM regulated by the laser beam size along the window of AOM are presented. Meanwhile, the behaviors of Q-switched pulses are achieved by regulating the switching time of AOM. The singlerepetition-rate and half-repetition-rate phenomena are described and discussed. The experimental results confirm that the fiber laser with lower level inversion population can be more easily operated for half- repetition-rate generation.     

Amplitude and phase measurement of mid-infrared femtosecond pulses by using cross-correlation frequency-resolved optical gating

We describe a cross-correlation-based frequency-resolved optical gating (XFROG) technique for simultaneously measuring the amplitude and phase of two ultrashort pulses that have different wavelengths but are derived from a common mode-locked oscillator. A measurement is presented in which 4.0-mum mid-IR pulses from a synchronously pumped femtosecond optical parametric oscillator (OPO) are characterized by mixing with the 770-nm OPO pump pulses. Details of the pulse-retrieval algorithm are included, together with examples of pulse data retrieved from the experimentally measured XFROG trace.     

Application of the nitroanisole as an infrared detector used in middle infrared interferometer

We propose the application of nitroanisole as a detector for middle infrared (mid-IR) interferometry or holography. The present experiment utilizes the liquid form of nitroanisole, which has a thermal lens effect, i.e. a temperature dependent refractive index. Since the nitroanisole absorbs IR radiation as heat, it is possible to estimate the IR intensity distribution on the nitroanisole from the diffraction pattern made by visible laser light that is transmitted through the nitroanisole. In this study, the time resolution and the diffraction efficiency of the nitroanisole was measured under various conditions. The experimental results show that the nitroanisole has a time resolution as high as that of a standard video camera, as well as a high diffraction efficiency and the spatial resolution equivalent to that of a conventional IR camera. Furthermore, we confirmed that the phase shift in mid-IR region can be estimated by analyzing the change in the visible diffraction pattern.