Raluca A. Negres

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 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.

The nature of excited-state absorption in polymethine and squarylium molecules

Subpicosecond transient absorption measurements were performed for several polymethine and squarylium dyes in ethanol solution and a polymeric host over the spectral range 400-1500 nm. A variety of nonlinear effects including saturable absorption, reverse saturable absorption, and gain were observed and analyzed. We observe strong excited-state absorption (ESA) in all dyes in the range 450-600 nm. We also report the first prediction and observation of additional ESA bands in the near-infrared range. The predictions were based on quantum chemical calculations and the ESA experiments were performed with femtosecond pump-continuum probe techniques. For polymethine dye 2-[2-[3-[(1,3-dihydro-3,3-dimethyl-1-phenyl-2H-indol-2-ylidene) ethylidene]-2-phenyl-1-cyclohexen-1-yl]ethenyl]-3,3-dimethyl-1-phenylindolium perchorate, an additional ESA band was detected near 1250 nm, and for squarylium dye 1,3-Bis-[(1,3-dihydro-1-butyl-3,3-dimetyl-2H-benzo[e]indol-2-ylidene)methyl]squ araine, two additional ESA bands were found around 870- and 1380-nm, respectively. To further study the nature of these transitions, the steady-state excitation anisotropy was also studied and compared with predictions. The relationship between ESA spectra of organic dyes and their molecular structure is discussed.

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’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.

Nonlinear spectrometer for characterization of organic and polymeric molecules

We have developed a femtosecond continuum spectrometer to measure nonlinear absorption spectra from 300 nm in the UV to 1.7 micrometers in the IR. This method is applied for measuring NLA spectra of semiconductor, organic and polymeric materials. The pump-probe nature of the experiment also allows the temporal response to be determined, thus helping in the determining of the underlying physical mechanisms for the nonlinearity. We describe studies of two-photon absorption in a series of alkyl fluorenes and excited state absorption dynamics in a series of polymethines using this spectrometer.

Nonlinear spectrometry of chromophores for optical limiting

We describe two methods for the spectral measurement of nonlinear absorption and refraction in reverse-saturable absorber materials. In the first, we use a picosecond optical parametric oscillator to perform Z-scan at many different wavelengths to measure excited state refraction and absorption cross sections throughout the visible. The second methods uses a chirped-pulse amplification scheme to produce 100 fs pulses at 840 nm. Focusing these into sapphire generates a white light continuum that is used as a probe in an excite-probe experiment. The excitation beam is derived from the second harmonic of the remaining 840 nm light. By measurement of the transmission spectrum of the probe as a function of excite- probe delay time, we can determine the spectral dependence of the excited-state absorption cross section. Moreover, by use of Kramers-Kronig relations, the excited state refraction can also be extracted from this data. We describe our measurements using both methods in a Zn:tetrabenzporphyrine derivative (TBP). The fact that both methods give excellent agreement not only verifies the utility of continuum measurements, but also reveals some interesting properties of the excited states of TBP.

Two-photon photochromism of a photorefractive 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-dihydro-furan- 2,5-dione (1), a photochromic compound with (lambda) maxequals385nm, using 775 fs pulses. The resulting photoisomer has a (lambda) maxequals582 nm. The kinectic rate contant for the isomerization reaction w as 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 shown. A two-photon absorption molecular cross-section (sigma) 2equals10.3 x 10- 45 cm4 s/photon was measured using Z-scan, supporting a two photon induced isomerization process.

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-Kroenig transformation can be used to calculate the dispersion of the nonlinear refractive index from the two-photon spectra. By delaying the probe beam relative to the pump beam, the temporal behavior of the nonlinearity can be obtained.

Optical Limiting: Characterization & Numerical Modeling

Progress in making effective optical limiting devices requires careful characterization of the material nonlinearitics as well as modeling of the propagation of optical beams through the material. We present a method to study the spectral properties of the nonlinear response as well as the results of modeling nanosecond pulse propagation in optically absorbing media. We specifically look at two-photon absorbing and reverse saturable absorbing materials in liquid hosts. The characterization technique is an excitation-femtosecond continuum probe technique. The modeling includes beam propagation through thick media (i.e. thickness much greater than the diffraction length or depth of focus) and includes the effects of index changes associated with acoustic waves generated by any absorption process. This requires a simultaneous solution to the acoustic and electromagnetic wave equations. A graphical user interface to a C++ numerical code has been developed for modeling such devices including the possibility of multiple nonlinear elements. We have extended this code for a tight focusing geometry beyond the paraxial ray approximation, but assuming cylindrical symmetry.

Femtosecond white-light continuum for characterization of organic molecules

Summary form only given. Measurements of the nonlinear absorption spectra and the dispersion of the nonlinear refraction are of interest in modeling nonlinear optical materials and determination of structure/properties relations. In the past, most of these experiments were performed at a single wavelength. Tuning over a broad spectral region can provide with a comprehensive information but, often times, one has to perform calibration of the experiment at each wavelength. An alternative is to use a white-light continuum produced by focusing short pulsed lasers into suitable media (e.g. sapphire) and using a pump-probe geometry where the probe is the continuum and the pump is an intense pulse that induces the nonlinearity. We have developed a femtosecond continuum spectrometer to measure nondegenerate spectra from 300 nm in the UV to 1.7 /spl mu/m in the IR. The pump beam is the output of a regeneratively amplified fiber ring oscillator at 775 nm or the output of a subsequent optical parametric generator/amplifier producing wavelengths from 500 nm to 1650 nm. The continuum is produced by focusing /spl sim/2 /spl mu/J of different frequencies from these two sources. Dual diode arrays for the IR and visible are used to measure the spectral changes in the transmittance against a reference beam. The spectra can be obtained as a function of time delay between pump and probe pulses. This method is applied for measuring nonlinear absorption spectra of organic materials.