Intense Femtosecond Optical Pulses Research Articles

Generation of intense femtosecond optical vortex pulses with blazed-phase grating in chirped-pulse amplification system of Ti:sapphire laser

We demonstrate the generation of an intense femtosecond optical vortex (OV) pulse by employing an OV converter set between two laser amplifiers in a chirped-pulse amplification (CPA) system of a Ti:sapphire laser. The OV converter is composed of a liquid-crystal spatial light modulator (LC-SLM) exhibiting a blazed-phase computer-generated hologram, a concave mirror, and a flat mirror in the 4f setup. Owing to the intrinsic nature of the 4f setup, the OV converter is free from chromatic and topological-charge dispersions, which are always induced in a spiral phase plate conventionally used to convert an intense Gaussian laser pulse to an OV pulse, while we can avoid damage to the LC-SLM by the irradiation of a low-energy pulse before the second amplifier. We have increased the throughput of the OV converter to 42% by systematically investigating the diffraction efficiency of the blazed-phase hologram on the LC-SLM, which relaxes the gain condition required for the second amplifier. The combination of the high-throughput OV converter and the two-stage amplification enables us to generate OV pulses with an energy of 1.63 mJ and a pulse duration of 60 fs at a wavelength of ~720 nm, at which the gain of the Ti:sapphire laser is only 60% of the peak gain around 800 nm.

Nonlinear absorbance in dielectric multilayers

Within the last two decades dispersive dielectric multilayer mirrors (DMs), also known as chirped mirrors (CMs), have played a significant role in the progress of ultrafast science. Their ability to manipulate the phase of a light pulse has advanced the synthesis of intense femtosecond optical pulses followed by remarkable progress in the disciplines of nonlinear optics. Meanwhile, the performance of the mirrors themselves has been strictly limited to the linear regime, as essential mirror characteristics such as reflectance, transmittance, and dispersion are evaluated with only intensity-independent values of refractive indices and extinction coefficients taken into the design formalism. Here, we report, to the best of our knowledge, the first observation of a strong nonlinear response of the DMs. We have found that the DM’s multilayer stack causes very significant enhancement of the internal electric field that becomes sufficient to enable third-order nonlinearity. Remarkably, in our particular case, the response is solely emerging in the form of nonlinear absorbance. By modifying the multilayer structure of the mirror, we gained control over observed nonlinearity and were able to predict and to some extent to tune the magnitude of the response, without perturbing the dispersive properties of the DMs. This demonstration not only expands the functionality of DMs into the nonlinear domain, but also marks a new approach to the development of multilayer coatings for applications in ultrafast science.

Spatially-uniform temporal recompression of intense femtosecond optical pulses

A specially designed telescope with defocusing lens and off-axis parabolic mirror, which is working as a nonlinear element and producing self-phase modulation, was implemented for intense (3.1 TW/cm2) Fourier Transform Limit femtosecond laser pulses with Gaussian beam profiles. The pulse spectrum was broadened quasi- homogeneously over the beam cross-section due to the change in the lens thickness compensating for the reduction of the beam intensity from its center to periphery. In experimental demonstrations a set of chirped mirrors allowed for the spectral phase correction to a final pulse compression of 20 fs from 40 fs.

Arrays of Ag split-ring resonators coupled to InGaAs single-quantum-well gain

We study arrays of silver split-ring resonators operating at around 1.5-µm wavelength coupled to an MBE-grown single 12.7-nm thin InGaAs quantum well separated only 4.8 nm from the wafer surface. The samples are held at liquid-helium temperature and are pumped by intense femtosecond optical pulses at 0.81-µm center wavelength in a pump-probe geometry. We observe much larger relative transmittance changes (up to about 8%) on the split-ring-resonator arrays as compared to the bare quantum well (not more than 1-2%). We also observe a much more rapid temporal decay component of the differential transmittance signal of 15 ps for the case of split-ring resonators coupled to the quantum well compared to the case of the bare quantum well, where we find about 0.7 ns. These observations are ascribed to the evanescent coupling of the split-ring resonators to the quantum-well gain. All experimental results are compared with a recently introduced analytical toy model that accounts for this evanescent coupling, leading to excellent overall qualitative agreement.

Terahertz Radiation from Atomic Clusters Illuminated by Intense Femtosecond Optical Pulses

Radiation of the electromagnetic waves (THz waves) in the terahertz frequency region from atomic clusters illuminated by intense femtosecond laser pulses is studied. The power of the THz wave from argon clusters is ∼40 times larger than that from argon molecular gas. The enhancement of the THz radiation originates from strong laser energy absorption by the clusters. The angular distribution of the power and the polarization state of the radiated THz wave are compared to the predictions of the several models of the THz radiation from the laser produced plasma.

Nanoacoustic waves in nanomaterials

We discuss the formation and detection of acoustic soliton trains in crystal slabs. High-amplitude amplitude strain pulses are generated by impact of intense femtosecond optical pulses on a metallic film and injected into the crystal slab. Nonlinear acoustic propagation leads to the formation of shock fronts (N-waves), that, in absence of viscous damping and by virtue of dispersion, may develop into soliton trains at the leading edge and high frequency tails at the trailing edge. Interferometric pump-probe optical experiments are discussed to directly detect the ultrafast surface displacements when a soliton is reflected at the surface of the crystal slab. Finally, we experimentally prove that these acoustic solitons are capable of impulsively exciting THz transitions in electronic centers in solids. The coherent terahertz acoustic pulse trains are applied to manipulate the optical response of two-dimensional excitons in a III-V quantum well on the ultrafast timescale. By virtue of the deformation potential, the coherent exciton emission becomes strongly "chirped" when the acoustic pulse train passes the quantum well. This yields the prospect to perform pump-probe terahertz acoustic experiments with nanoacoustic waves in semiconductor nanomaterials.

Analysis of instantaneous profiles of intense femtosecond optical pulses propagating in helium gas measured by using femtosecond time-resolved optical polarigraphy

We analyze instantaneous profiles of intense femtosecond optical pulses propagating in helium gas measured using femtosecond time-resolved optical polarigraphy (FTOP). Considering the characteristics of the optical Kerr effect, relations linking images obtained by FTOP with intensity distributions of optical pulses are derived under approximations applicable to a relatively wide range of experimental conditions. By inversely applying the derived equations, images proportional to instantaneous intensity distributions of propagating light pulses are constructed from snapshot images taken in the experiment. From an analysis of the images obtained, we discuss the temporal changes in the optical-pulse profiles. We can directly observe the energy reduction of the optical pulses after they pass through the simultaneously generated laser plasma. It is confirmed that each filament can be separately investigated by this method even under multifilament conditions. Furthermore, the images clearly reveal intensity distributions along the propagation axis as well as cross-section distributions of femtosecond-pulse filaments, and show temporal shape modulations shorter than the incident pulse width in the middle of the propagation. We specify the volume where the light energy concentrates at a particular instant of time, and directly observe the cross-section expansion of the volume at the focal point brought about due to nonlinear effects.

Determination of the nonlinear refractive indices of Ca_2Ga_2SiO_7 and SrLaGa_3O_7 with intense femtosecond optical pulses

The ordinary and extraordinary nonlinear refractive indices of SrLaGa3O7 and Ca2Ga2SiO7 were determined from the measured spectral changes in intense 800-nm, 150-fs pulses, propagated through these uniaxial crystals. A numerical analysis involving both group-velocity dispersion and self-phase modulation gave n2=(11±1)×10-20 m2/W in SrLaGa3O7 and n2=(6.5±1)×10-20 m2/W in Ca2Ga2SiO7 for ordinary polarization; results for extraordinary beams agreed with these values to within experimental error. These high values suggest that these crystals are viable host matrices for pulsed solid-state lasers, capable of producing subpicosecond pulses through Kerr-lens mode locking.

Generation of intense femtosecond optical pulses near 3 μm with a kilohertz repetition rate

A simple extension of a commercial laser system permits the generation of intense femtosecond infrared pulses near 3 µm by optical parametric generation and amplification at kilohertz repetition rates. Pulse durations of a few hundred femtoseconds and pulse energies of tens of microjoules can be routinely produced, thus permitting nonlinear optical studies of molecular vibrations.

Third-order electronic susceptibilities of liquids measured by femtosecond kinetics of optical Kerr effect

Intense femtosecond optical pulses are used to discriminate temporally among the different processes involved in the third-order-induced polarization. The tensor elements deduced from the instantaneous measured components are compared with previous frequency-domain experimental results.

NONLINEAR INTERACTIONS WITH INTENSE FEMTOSECOND OPTICAL PULSES

Recent advances in optical pulse generation techniques have led to the production of optical short pulses in the femtosecond time domain. This talk will discuss the interaction of intense femtosecond optical pulses in solids and liquids. The use of nonlinear interactions to generate a compressed optical pulse as short as 30 femtoseconds will be described. In addition, the generation of white light continuum in the femtosecond domain will be discussed.