Ultrafast Nonlinear Spectrometer for Material Characterization

This work describes the use of a broadband spectral source for nonlinear spectroscopy to characterize various materials with potential applications in confocal microscopy, biological sample markers, optical limiting devices and optical switches. The goal is to study the spectrum of nonlinear absorption and the dispersion of nonlinear refraction as well as the dynamics of the nonlinearities by means of femtosecond excite-­probe experiments. The principle is quite simple: if a sample is under the influence of a strong fs excitation pulse and a pro be pulse beam is incident at the same time, or shortly after (within the decay time of the nonlinearity), then the probe pulse will sense the nonlinearity induced by the excitation. If the probe pulse is broadband, a femtosecond white-light continuum (WLC) in our case, we can monitor the nonlinearity induced over the entire continuum spectrum in one laser shot . The use of femtosecond laser pulses to generate WLC will provide femtosecond time resolution for time-resolved spectroscopy. We built the nonlinear spectrometer and allowed for many degrees of flexibility in terms of choice of wavelengths for pump and probe beams and a dual detection system to cover both visible and infrared spectral ranges. We have the possibility of performing broad band spectral measurements using a spectrometer or selected narrow bandwidth probes incident on Si or Ge photodiodes for improved S/N ratios. The intrinsic properties of the continuum probe demand a careful characterization of its spatial and temporal profile. Know ledge of the dispersion of the index of refraction in various optical elements, including the sample itself, is also required for a correct analysis of the transient absorption raw data, especially for short time-scale dynamics of nonlinear processes. We tested the system using well-characterized semiconductor samples, and the results came out in excellent agreement with those from previous picosecond Z-scan measurements and theoretical modeling. With confidence, we can now measure various organic dyes with enhanced two-photon and excited-state absorption. Our setup is used to conduct a systematic study on similar compounds with modified molecular structures in order to learn about structure-property relations and draw guidelines for future design work

Alignment procedure for a dual grating pulse compressor

Grating pulse compressors are an integral part of chirped pulse amplification (CPA) lasers.1 Accurate alignment of the compressor is required to obtain minimum pulse-width at the output of the system. Dual grating compressors are difficult to align because they don\u27t function unless they are close to optimum alignment. The procedure outlined here provides a simple step-wise method of aligning a dual grating pulse compressor so that the gratings will be parallel with one another. Once this condition has been established, only the distance between the gratings needs to be adjusted to start the system operating. At this point, the compressor can be critically aligned

Alignment procedure for a dual grating pulse compressor

Grating pulse compressors are an integral part of chirped pulse amplification (CPA) lasers.1 Accurate alignment of the compressor is required to obtain minimum pulsewidth at the output of the system. Dual grating compressors are difficult to align because they don’t function unless they are close to optimum alignment. The procedure outlined here provides a simple step-wise method of aligning a dual grating pulse compressor so that the gratings will be parallel with one another. Once this condition has been established, only the distance between the gratings needs to be adjusted to start the system operating. At this point, the compressor can be critically aligned. © 1998 Optical Society of America

White-Light-Continuum Spectroscopy To Determine Third-Order Nonlinear Optical Properties

The realization of all-optical switching schemes is mostly hindered by the lack of suitable materials with a refractive index change that is large and fast enough. The characterization of the linear and nonlinear optical properties of potential materials is therefore of prime importance. Various characterization methods have been proposed and are employed, yielding different parameters of the nonlinear optical response at the involved laser frequencies. However, in most techniques the resulting nonlinearities are measured only at one point in the spectral dispersion. To generate the whole nonlinear spectra, the laser source has to be tuned over the desired wavelength range and consecutive measurements have to be taken. We propose and demonstrate here a novel technique to measure the nonlinear optical response for a broad wavelength region in a pump-probe scheme that requires no laser tuning. In order to detect the two-photon absorption at several wavelengths simultaneously, we use a white-light-continuum as the probe beam. As the pump beam is held constant, the Kramers-Krönig transformation can be used to calculate the dispersion of the nonlinear refractive index from the two-photon absorption spectrum. By delaying the probe beam relative to the pump beam, the temporal behavior of the nonlinearity can be obtained

Experiment And Analysis Of Two-Photon Absorption Spectroscopy Using A White-Light Continuum Probe

We present an experimental technique along with the method of data analysis to give nondegenerate two-photon absorption (2PA) spectra. We use a femtosecond pump pulse and a white-light continuum (WLC) probe to rapidly generate the 2PA spectra of a variety of materials. In order to analyze data taken with this method, the spectral and temporal characteristics of the WLC must be known, along with the linear dispersion of the sample. This allows determination of the temporal walk-off of the pump and probe pulses as a function of frequency caused by group-velocity mismatch. Data correction can then be performed to obtain the nonlinear losses. We derive an analytical formula for the normalized nonlinear transmittance that is valid under quite general experimental parameters. We verify this on ZnS and use it for the determination of 2PA spectra of some organic compounds in solution. We also compare some of the data on organics with two-photon fluorescence data and find good agreement

Two-Photon Photochromism Of An Organic Material For Holographic Recording

We report the two-photon-induced photoisomerization of 3-[1-(1,2-dimethyl-1H-indol-3-yl)-ethylidene]-4-isopropylidene- dihydrofuran-2,5-dione (1), a photochromic compound with λmax = 385 nm, using 775-nm femtosecond pulsed laser irradiation. The resulting photoisomer had λmax = 582 nm. The kinetic rate constant for the two-photon-induced electrocyclic isomerization reaction was measured at two different intensities (two different powers), showing a quadratic dependence with respect to the pump intensity. Results of pump-probe solution phase experiments and guest/host polymer thin film interferometric imaging studies are reported. A two-photon absorption molecular cross section σ2 = 10.3 x 10-48 cm4·s/photon was measured using Z-scan, further supporting a two-photon-induced isomerization process. Two-photon-induced interferometric recording in a fulgide-containing polymer film was demonstrated

Chi-3\u27s: Their characterization and understanding

The third-order nonlinear optical susceptibility χ(3), where the values for this quantity are quoted that vary in magnitude depending on the material and source used, in particular the source pulsewidth, is discussed. The causal relations between the refractive and absorptive components are also discussed and show experiments where the dispersion of the nonlinear refraction can be directly calculated from the nonlinear absorption spectrum. A method for directly obtaining nonlinear spectra as well as determining the response time of the nonlinearity uses a fixed frequency pump and a white-light-continuum probe

Two-Photon Spectroscopy And Analysis With A White-Light Continuum Probe

We present a powerful experimental tool and analysis for characterization of two-photon absorption (2PA) spectra. We demonstrate this method with ZnS and then apply it to organic dyes in solution. We also compare the results with those from other methods such as two-photon fluorescence spectroscopy. This femtosecond pump-probe method uses a white-light continuum (WLC) as the probe to produce a nondegenerate 2PA spectrum. The extreme chirp of the WLC requires that transmittance data be collected over a range of temporal delays between pump and probe pulses. These data then need to be corrected for the effects of this chirp as well as for the temporal walk-off of the pulses in the sample that result from the frequency nondegenerate nature of the experiment. We present a simple analytic solution for the transmitted fluence through the sample, which is applicable for most practical cases. © 2002 Optical Society of America

Femtosecond continuum spectroscopy and Kramers-Kronig relations

A nonlinear spectrophotometer that can measure nondegenerate spectra from ≅1.7 μm in the infrared (IR) to ≅200 nm in the UV is developed. Dual diode arrays for the IR and visible are used to measure the spectral changes in the transmittance against a reference beam. The measured nonlinear spectra are subsequently merged and the Kramers-Kronig integral is performed. The Kramers-Kronig method gives a reliable nonlinear refraction dispersion from the nonlinear absorption spectrum. This separates ultrafast nonlinear absorption from cumulative effects such as excited state absorption. The resulting excited state refractive spectra agree with independent measurements performed using Z-scans with a picosecond optical parametric oscillator