Claudiu M. Cirloganu

Extremely nondegenerate two-photon absorption in direct-gap semiconductors [Invited]

Two-photon absorption (2PA) spectra with pairs of extremely nondegenerate photons are measured in several direct-gap semiconductors (GaAs, CdTe, ZnO, ZnS and ZnSe) using picosecond or femtosecond pulses. In ZnSe, using photons with a ratio of energies of ~12, we obtain a 270-fold enhancement of 2PA when comparing to the corresponding degenerate 2PA coefficient at the average photon energy (ηω1 + ηω2)/2. This corresponds to a pump photon energy of 8% of the bandgap. 2PA coefficients as large as 1 cm/MW are measured. Thus, by using two widely different wavelengths we are able to access the large 2PA observed previously only in narrow gap semiconductors. We also calculate the corresponding enhancement of nonlinear refraction, consisting of two-photon, AC-Stark and Raman contributions. The net effect is a smaller enhancement, but exhibits very large dispersion within the 2PA regime.

Femtosecond-to-nanosecond nonlinear spectroscopy of polymethine molecules

The linear and nonlinear optical properties of a series of polymethine molecules are investigated to study the effects of molecular structure and the host environment on overall nonlinear absorption performance. The linear characterization includes measuring the solvatochromic shifts between absorption and fluorescence peaks and studying the excited-state orientational diffusion kinetics. The nonlinear characterization involves measuring the excited-state absorption spectra with a femtosecond white-light-continuum pump-probe technique and performing Z scans and nonlinear transmission measurements from the picosecond to the nanosecond time regimes. The results of these experiments allow us to develop an energy-level structure for the polymethines, which accurately predicts nonlinear absorption properties from the picosecond to the nanosecond time regimes. From this model we are able to identify the key molecular parameters for improved nonlinear absorption.

Optimization of the Double Pump–Probe Technique: Decoupling the Triplet Yield and Cross Section

The double pump-probe technique (DPP), first introduced by Swatton et al. [Appl. Phys. Lett. 1997, 71, 10], is a variant of the standard pump-probe method but uses two pumps instead of one to create two sets of initial conditions for solving the rate equations, allowing a unique determination of singlet- and triplet-state absorption parameters and transition rates. We investigate the advantages and limitations of the DPP theoretically and experimentally and determine the influence of several experimental parameters on its accuracy. The accuracy with which the DPP determines the triplet-state parameters improves when the fraction of the population in the triplet state relative to the ground state is increased. To simplify the analysis of the DPP, an analytical model is presented, which is applicable to both the reverse saturable and the saturable absorption regimes. We show that the DPP is optimized by working in the saturable absorption regime. Although increased accuracy is in principle achievable by increasing the pump fluence in the reverse saturable absorption range, this can cause photoinduced decomposition in photochemically unstable molecules. Alternatively, we can tune the excitation wavelength to the spectral region of larger ground-state absorption, to achieve similar accuracy. This results in an accurate separation of triplet yield and excited-state absorption cross section. If the cross section at another wavelength is then desired, a second pump-probe experiment at that wavelength can be utilized given the previously measured triplet yield under the usually valid assumption that the triplet yield is independent of excitation wavelength.

Femtosecond to nanosecond characterization of the excited-state properties of polymethine molecules

Saturation limits excited-state absorption (ESA) in many organic molecules. We studied polymethines using continuum spectroscopy to determine ESA spectra, two-color pump-probe anisotropy for transition dipole moment orientations, picosecond and nanosecond z-scans and nonlinear transmission measurements.

Sub-bandgap detection using extremely nondegenerate two-photon absorption

Femtosecond gated detection of low power mid-IR radiation using ultraviolet gating pulses in a GaN detector at room temperature is demonstrated by utilizing two-to-three orders of magnitude enhancement from nondegenerate two-photon absorption.

Extremely nondegenerate two-photon detection of sub-bandgap pulses

Our observation of two-to-three orders of magnitude enhancement of nondegenerate two-photon absorption in direct-gap semiconductors has led to gated detection of subgap femtosecond pulses, e.g. measurement of 200fs 5.6μm pulses in a GaN photodiode.

Extremely Nondegenerate Two-Photon Absorption and Detection in Direct Gap Semiconductors

Two-to-Three orders of magnitude enhancement of nondegenerate two-photon absorption (2PA) compared to degenerate 2PA are observed. Femtosecond gated detection of low power mid-IR radiation using ultraviolet gating pulses using a GaN detector at room temperature is demonstrated.

Large enhancement of two-photon absorption in semiconductors using highly non-degenerate photons

We performed frequency non-degenerate pump-probe experiments in several direct-gap semiconductors using femtosecond and picosecond pulses. Tuning the long wavelength photons in the IR region, we observed a 125-fold enhancement of the two-photon absorption coefficient.

Three-photon Absorption In Semiconductors

The bandgap and wavelength scaling of three-photon absorption is studied in several semiconductors by the Z-scan technique. The 3PA coefficient is found to vary as Eg-7as predicted by theory.

Spectral and temperature dependence of nonlinear absorption in InSb

Temperature dependent two-photon and free-carrier absorption spectra of InSb are measured using a tunable picosecond source via temperature controlled Z-scan experiments and show good agreement with theoretical predictions. Three-photon absorption is also observed.