Tunable Femtosecond Research Articles

High energy femtosecond mid-infrared generation pumped by a two-color Ti:sapphire multipass amplifier

Intense mid-infrared was generated by direct frequency mixing two pulses from a dual-wavelength Ti:sapphire system. From a multipass amplifier we generated two tunable wavelength femtosecond pulses with a total energy of 15 mJ. Pulse energy of 1.6 μJ and 7.4 μJ of mid-infrared light is achieved with and without its multipass amplifier at 9–11 μm, with pulse duration of 500 fs.

Highly versatile confocal microscopy system based on a tunable femtosecond Er:fiber source

The performance of a confocal microscopy setup based on a single femtosecond fiber system is explored over a broad range of pump wavelengths for both linear and nonlinear imaging techniques. First, the benefits of a laser source in linear fluorescence excitation that is continuously tunable over most of the visible spectrum are demonstrated. The influences of subpicosecond pulse durations on the bleaching behavior of typical fluorophores are discussed. We then utilize the tunable near-infrared output of the femtosecond system in connection with a specially designed prism compressor for dispersion control. Pulses as short as 33 fs are measured in the confocal region. As a consequence, 2 mW of average power are sufficient for two-photon microscopy in an organotypic sample from the mouse brain. This result shows great prospect for deep-tissue imaging in the optimum transparency window around 1100 nm. In a third experiment, we prove that our compact setup is powerful enough to exploit even higher-order nonlinearities such as three-photon absorption that we use to induce spatially localized photodamage in DNA.

The Action Spectrum for Generation of the Primary Intermediate Revealed by Ultrafast Absorption Spectroscopy Studies of Pheomelanin¶

The observation that fair-skinned individuals are more susceptible to skin cancers is commonly explained by invoking an enhanced photoreactivity of the red melanin, pheomelanin compared with the black melanin, eumelanin. For the wavelength range from 500 to 1000 nm, pump-probe spectroscopic measurements reveal the photoexcitation of pheomelanin by UVA light that generates an immediate (<100 fs) transient absorption centered at 780 nm. Using a tunable femtosecond excitation source, the action spectrum between 300 and 390 nm for generation of the primary intermediate was measured. Similar action spectra are found for the sample with molecular weight (MW) between 1000 and 10 000 and the one with MW > 10 000 fractions of pheomelanin, indicating that the reactive chromophore has a low MW but is present and its photophysics is similar in the aggregated pigment. The shape of the action spectrum differs from the absorption spectrum of bulk melanin and mass-selected fractions but resembles reported absorption spectrum of benzothiazines, oxidation products of 5-S-cysteinyl-dopa, which are formed along the biosynthetic pathway of pheomelanin.

Resonantly enhanced femtosecond second-harmonic generation and nonlinear luminescence in GaN film grown on sapphire

At room temperature, by using a tunable broadband femtosecond laser as excitation source we observed second-harmonic generation (SHG) and nonlinear photoluminescence (NPL) in GaN film grown on sapphire simultaneously or individually. In addition to the observation of the resonance effect of the nonlinear response when the SHG is tuned to coincide with the near-band-edge emission, we carefully measured dependence of the SHG and NPL signals on polarization of the excitation light. The results reveal that the reabsorption of the SHG photons with energies higher than the fundamental gap of GaN significantly contributes to generation of the efficient NPL signal.

Generation of tunable narrowband pulses initiating from a femtosecond optical parametric amplifier

We report a tunable optical pulse expander capable of producing narrowband long pulses, starting from a femtosecond optical parametric amplifier (OPA), which is based on chirp-matched frequency mixing technique. The device delivers nJ-level pulses with bandwidth of ~0.1 nm and duration of ~45 ps within its tuning range from 1000 nm to 1090 nm. The combination of a femtosecond OPA and a pulse expander may provide synchronized and both tunable femtosecond and tens of picoseconds pulses simultaneously, which should be promising in the applications where both temporal and spectral resolutions are needed.

Relaxation processes of photoexcited carriers in GaAs/AlAs multiple quantum well structures grown by molecular beam epitaxy at low temperatures

The relaxation processes of photoexcited carriers in GaAs/AlAs multiple quantum well structures grown at low temperatures by molecular beam epitaxy were studied by a tunable single-beam femtosecond pump–probe method. Concentrations of singularly ionized antisite arsenic ions, AsGa+, in the quantum wells, which were considered as traps of photoexcited carriers, were estimated from flux conditions and substrate temperatures in the growth. Transient transmittivity of the structures were measured by varying the pump–probe photon energy. The trapping rate of photoexcited carriers, which corresponded to the reciprocal of the carrier lifetime, was derived from the relaxation profile at the pump–probe photon energy close to the exciton resonant excitation energy for each structure. The trapping rate was found to increase linearly with AsGa+ in a lower concentration range and superlinearly in a higher concentration range. Photoluminescence and absorption spectra were observed at room temperature and their correlation to the carrier lifetimes were investigated.

The action spectrum for generation of the primary intermediate revealed by ultrafast absorption spectroscopy studies of pheomelanin.

The observation that fair-skinned individuals are more susceptible to skin cancers is commonly explained by invoking an enhanced photoreactivity of the red melanin, pheomelanin compared with the black melanin, eumelanin. For the wavelength range from 500 to 1000 nm, pump-probe spectroscopic measurements reveal the photoexcitation of pheomelanin by UVA light that generates an immediate (< 100 fs) transient absorption centered at 780 nm. Using a tunable femtosecond excitation source, the action spectrum between 300 and 390 nm for generation of the primary intermediate was measured. Similar action spectra are found for the sample with molecular weight (MW) between 1000 and 10 000 and the one with MW > 10 000 fractions of pheomelanin, indicating that the reactive chromophore has a low MW but is present and its photophysics is similar in the aggregated pigment. The shape of the action spectrum differs from the absorption spectrum of bulk melanin and mass-selected fractions but resembles reported absorption spectrum of benzothiazines, oxidation products of 5-S-cysteinyl-dopa, which are formed along the biosynthetic pathway of pheomelanin.

Measurement of Timing Jitter in Wavelength Tunable Femtosecond Soliton Pulses

We investigated the root mean square (RMS) timing jitter and RMS intensity noise of the wavelength tunable femtosecond soliton pulses using the technique of radio-frequency spectrum analysis. We obtained interesting results that the generated solitons have almost constant RMS timing jitter and RMS intensity noise of approximately 410 fs and 0.03% in the wavelength region of 1600-1750 nm. It is therefore believed that the magnitudes of the jitter and noise are not greatly increased in the process of wavelength tunable soliton pulse generation. The wavelength tunable soliton pulse is confirmed to be stable and useful for practical applications.

Characteristics of Wavelength Tunable Femtosecond Soliton Pulse Generation Using Femtosecond Pump Laser and Polarization Maintaining Fiber

We have investigated both experimentally and numerically the characteristics of wavelength tunable femtosecond soliton pulse generation using a pulse width variable fiber laser and two different types of polarization maintaining fibers. The generated soliton shows the feature of the pulsewidth becoming almost constant at - 250 fs under any conditions of the pump pulse for 220m fiber. High conversion efficiency from pump pulse to a generated soliton pulse accounting for as much as 73% can be obtained. This efficiency decreases with increase in the input power or wavelength shift. A superior conversion efficiency and broad wavelength shift can be obtained by using a more shortened pump pulse. In the numerical calculations, it is predicted that under a condition of constant power of pump pulse, the maximum wavelength shift is achieved when the soliton number N is -1.4. The difference of mode field diameter and the group velocity dispersion (GVD) coefficient β2 affect the wavelength shift and conversion efficiency. Using the fiber with small mode field diameter and small absolute value of GVD coefficient β2, a high conversion efficiency and large wavelength shift can be obtained.

Simultaneous generation of wavelength tunable two-colored femtosecond soliton pulses using optical fibers

Wavelength tunable two-colored femtosecond (fs) soliton pulse generation is proposed and demonstrated for the first time, using passively mode-locked fs fiber laser and polarization maintaining fibers. The wavelengths of the two soliton pulses can be changed arbitrarily by varying the power and polarization direction of the fiber-input pulse. Ideal two colored soliton pulses in which the pulsewidths are about 200 fs are generated in the wavelength region of 1.56-1.70 μm for 110-m fiber. The generated pulses are almost transform-limited ones.

Broadly Tunable Femtosecond Sources in the Mid-Infrared and Two-Wavelength Ultrafast Spectroscopy of Holes in GaAs

The recent development of periodically poled lithium niobate (PPLN) has led to a variety of highly efficient and compact all-solid-state nonlinear optical devices. Of particular interest are the optical parametric oscillators. We have most recently demonstrated for the first time a highly efficient broadly tunable femtosecond optical parametric oscillator (fs OPO) using PPLN. The threshold for oscillation of the PPLN fs OPO is remarkably low, on the order of 50 mW. As a result, it is now possible to use a diode-pumped and frequency-doubled Nd:YVO 4 laser, rather than an Ar-ion laser, as the pump source for the mode-locked Ti:sapphire laser for pumping the fs OPO. Such an OPO can operate from approximately 1 μm to 5.4 μm. Femtosecond sources have been used in a variety of experiments to study the dynamics of hot carriers in semiconductors. Because of the limited available wavelength range of earlier femtosecond sources, most such studies have been limited to electron dynamics in the conduction band of III–V compounds such as GaAs. With the extended tuning range of the recently developed fs OPO's, it is now possible to study a much wider variety of materials and transitions. We have recently obtained preliminary results on the ultrafast dynamics of holes in GaAs using two-wavelength tunable femtosecond spectroscopy, by pumping the valence-to-conduction band transition at approximately 800 nm and simultaneously probing the transitions between the split-off band and the perturbed heavy-hole band at approximately 3 μm. These recent results are summarized and discussed briefly.

Vibronic dependence of the B̃ state lifetimes of CH3I and CD3I using femtosecond photoionization spectroscopy

Tunable deep UV femtosecond photoionization spectroscopy with single photon excitation and wavelengths longer than 192 nm has been used to determine predissociation-mediated excited state lifetimes for many vibronic levels of the B̃ (6s[2] Rydberg) state of CH3I and CD3I. These include states with vibrational excitation in the ν1, ν2, ν3, and ν6 modes. We have previously reported lifetime measurements for the origin bands [Chem. Phys. Lett. 222, 335 (1994)]. The vibronic and isotopic dependences presented here qualitatively agree with various aspects of results from two indirect measurements. Our results corroborate the counterintuitive result from the resonance Raman work by Wang and Ziegler [J. Chem. Phys. 95, 288 (1991)] that the level singly excited in the C–I stretching mode (31) dissociates more slowly (we measure ∼4.0 ps for both CH3I and CD3I) than the vibrationless levels (1.38 and 1.90 ps, respectively). In contrast to the resonance Raman results and similar to those from resonance enhanced multiphoton ionization linewidth studies by Syage [Chem. Phys. Lett. 212, 124 (1993)], we find a faster predissociation rate upon excitation in the ν6 mode. The lifetimes are considerably longer than those measured for the higher lying 6p and 7s Rydberg states by femtosecond mass-resolved photoionization studies by Janssen et al. [Chem. Phys. Lett. 214, 281 (1994)]. In that case, a faster dissociation rate was measured for the 31 compared to the vibrationless level. This work provides further evidence of the multidimensional nature of the B̃ state predissociation mechanism and an opportunity to critically test high level calculations of the Rydberg state photodissociation dynamics.

Ultrabroadband chirped mirrors for femtosecond lasers

We report on the performance of widely tunable femtosecond and continuous-wave Ti:sapphire lasers that use a newly developed ultrabroadband mirror set. The mirrors exhibit high reflectivity (R > 99%) and smooth variation of group delay versus frequency over a wavelength range from 660 to 1060 nm. Mode-locked operation with pulse durations of 85 fs was achieved from 693 to 978 nm with only one set of ultrabroadband mirrors.

Narrow-band seeding of an optical parametric amplifier in the femtosecond regime

Nearly transform limited femtosecond pulses tunable between 2.56 and 3.16 μm have been generated by optical parametric amplification. The single stage parametric amplifier is pumped by a tunable high power femtosecond Ti:sapphire laser system at 1 kHz repetition rate and seeded by quasi-continuous wave (cw) radiation from the Q-switched Nd:YLF laser used to pump the regenerative amplifier. The ≈100 fs idler pulses are shorter than the pump pulses. The mechanism of the achieved pulse compression is discussed and experimental results are compared with numerical simulations.

Electron dynamics at semiconductor surfaces and interfaces

The application of femtosecond laser photoemission to the study of electron dynamics at semiconductor surfaces and interfaces is described. Fundamental processes such as electron-electron and electron-phonon scattering, diffusion and recombination are discussed. The dynamical evolution of photoexcited electrons on the As/Ge(111) surface is studied with broadly tunable harmonics. Extension of high harmonic generation to produce photon energies approaching 80 eV have resulted in the first atomic core level photoemission spectra from a tunable femtosecond source. Possible applications of harmonics to studies of surface photochemistry are discussed.

Development of Tunable Femtosecond Solid-State Lasers.

Since a new mode-locking technique based on the Kerr nonlinearity in a laser rod wasdemonstrated for generation of highly stable and broadly tunable femtosecond pulses by asolid-state Ti: Al203 laser, experimental and theoretical studies on this technique have beenextensively developed. As a result, extremely short pulses, as short as 8 fs, were directly producedfrom the higher-order dispersion-compensated, self-mode-locked Ti: Al203 laser cavity. The tunable solid-state lasers have opened the door to the study of ultrafast quantum electronicsand related new fields.This paper describes recent advances in the tunable femtosecond solid-state lasers including Kerr-lens mode-locking, solitary laser, effect of higher-order dispersion and compact LDpumpedlasers.

High power tunable femtosecond visible and infrared light from a synchronized Ti:sapphire/Nd:YAG laser system by difference frequency mixing

High power, 20 Hz repetition rate difference frequency mixing of externally synchronized femtosecond Ti:sapphire laser radiation with harmonics of a phase locked picosecond Nd:YAG laser, in a BBO crystal, is shown to produce femtosecond pulses in the visible (666 nm) and infrared (1.77 μm) regions. Using 50 μJ of 760 nm amplified Ti:sapphire light, outputs of tens of microjoules at 666 nm, or several microjoules at 1.77 μm, were achieved. This technique can be used to extend the tuning range (720–1000 nm) of the Ti:sapphire laser into the visible (550–700 nm) and the infrared (1.1–2.0 μm) regions of the spectrum.

Characteristics of a noncritically phasematched Ti: sapphire pumped femtosecond optical parametric oscillator

The operating characteristics of a singly-resonant famtosecond optical parametric oscillator (OPO) based on a noncritically phasematched geometry in KTP are described. The parametric oscillator is synchronously pumped by a self-modelocked Ti: sapphire laser and is operated in a standing-wave cavity configuration. With extracavity dispersion compensation, chirped pulses of 90 fs are generated, whilst shorter transform-limited pulses of 40 fs are generated with intracavity dispersion compensation. A numerical model of the effects of self-phase-modulation in the OPO has been developed and gives results which are in good qualitative agreement with experiment. Experimental results are presented relating to non-phasematched processes in the OPO, and the implications for the development of a tunable femtosecond source in the blue spectral region are discussed. A characterisation of the amplitude and phase noise of the OPO has also been performed, and provides insight into the important physical processes governing the stability of OPO operation.

Femtosecond wave packet and chemical reaction dynamics of iodine in solution: Tunable probe study of motion along the reaction coordinate

One- and two-color time-domain probing of the resonant dichroic response of iodine in n-hexane following femtosecond B-X excitation at 580 nm is described. The detected signals contain both ground and excited state vibrational coherence contributions to the third-order polarization. The dichroic response can be separated into positive and negative amplitude contributions: B-X absorption and stimulated emission are positive but absorption from the B-state can yield either positive or negative signals depending on the direction of the transition moment. Wave packet motion on both the ground and excited states of iodine is studied with a frequency tunable femtosecond probe. It is shown that the positive signals can be interpreted as B-X dichroic response using the classical Franck principle. The classical Franck principle also provides information about the potential probed in absorption from the B state. From the probe wavelength dependent delay in the signal appearance, it is concluded that the absorptive signal for blue probe wavelengths arises from a repulsive state reached by solvent-induced predissociation of the B state. Dephasing of B state vibrational coherence results from this solvent-induced predissociation of iodine. We discuss the evolving reaction in terms of possible dissociative potential energy curves a1g(3Π) and a′0g+(3Σ−). The time evolution of the bluest probe dichroism signals is representative of continuing atom separation; the experiments have not yet probed large enough internuclear separations to evidence a buildup of dissociated product or momentum reversal, i.e., caging.

Tunable sub-100 femtosecond dye-laser pulses generated with a nanosecond pulsed pumping

Subpicosecond pulses at a fixed wavelength produced with a low-Q cavity dye laser pumped by a single, nanosecond laser (Q-switched Nd:YAG) are converted into tunable high-power sub-100 femtosecond pulses by generation, spectral selection, amplification and compression of a supercontinuum. The tunable, chirped, high-energy pulses obtained are compressed with a prism pair. Energies up to 50 μJ in sub-100 fs pulses were obtained in the 540 to 650 nm range using 40 mJ of the Nd: YAG-laser pumping pulses at 532 nm. The whole sub-100 fs system including the low-Q dye laser uses only one Nd:YAG laser as a pump source.