Non-instantaneous polarization dynamics in dielectric media

Third-order optical nonlinearities play a vital role for generation and characterization of some of the shortest optical pulses to date, for optical switching applications, and for spectroscopy. In many cases, nonlinear optical effects are used far off resonance, and then an instantaneous temporal response is expected. Here, we show for the first time resonant frequency-resolved optical gating measurements that indicate substantial nonlinear polarization relaxation times up to 6.5\,fs in dielectric media, i.e., significantly beyond the shortest pulses directly available from commercial lasers. These effects are among the fastest effects observed in ultrafast spectroscopy. Numerical solutions of the time-dependent Schr\"odinger equation are in excellent agreement with experimental observations. The simulations indicate that pulse generation and characterization in the ultraviolet may be severely affected by this previously unreported effect. Moreover, our approach opens an avenue for application of frequency-resolved optical gating as a highly selective spectroscopic probe in high-field physics

Non-instantaneous polarization dynamics in dielectric media

Third-order optical nonlinearities play a vital role for generation1,2 and characterization 3-5 of some of the shortest optical pulses to date, for optical switching applications6,7, and for spectroscopy8,9. In many cases, nonlinear optical effects are used far off resonance, and then an instantaneous temporal response is expected. Here, we show for the first time resonant frequency-resolved optical gating measurements1012 that indicate substantial nonlinear polarization relaxation times up to 6.5 fs in dielectric media, i.e., significantly beyond the shortest pulses directly available from commercial lasers. These effects are among the fastest effects observed in ultrafast spectroscopy. Numerical solutions of the time-dependent Schrödinger equation13,14 are in excellent agreement with experimental observations. The simulations indicate that pulse generation and characterization in the ultraviolet may be severely affected by this previously unreported effect. Moreover, our approach opens an avenue for application of frequency-resolved optical gating as a highly selective spectroscopic probe in high-field physics

Ultrafast Nonlinear Nano-Optics via Collinear Characterization of Few-Cycle Pulses

Die Methode „interferometric frequency-resolved optical gating“ (iFROG) zur Charakterisierung ultrakurzer Laserimpulse wurde erweitert. Als optische Nichtlinearität werden sowohl die Erzeugung der 2. als auch der 3. Harmonischen (THG) separat verwendet. Eine iFROG-Messung stellt ein inverses Problem dar, bei dem die Amplitude und Phase des elektrischen Feldes des Laserimpulses nur durch einen iterativen Algorithmus rekonstruiert werden kann. In dieser Arbeit wird ein mathematischer Formalismus entwickelt und mit einem evolutionären Optimierungsalgorithmus kombiniert, um einen neuartigen Impuls-Rekonstruktions-Algorithmus für iFROG zu erschaffen. Während iFROG ursprünglich ausschließlich zur Charakterisierung von Laserimpulsen konzipiert wurde, kann die Technik gleichermaßen für spektroskopische Zwecke eingesetzt werden. Wird das nichtlineare Medium in iFROG durch ein Untersuchungsobjekt ersetzt und ein bekannter Laserimpuls erneut charakterisiert, so kann die Antwortfunktion des Untersuchungsobjekts mit einer sub-Femtosekunden-Auflösung entschlüsselt werden. Da für die THG-Variante bisher keine Lösung bekannt ist, ermöglicht der vorgestellte Rekonstruktion-Algorithmus die erstmalige Nutzung von iFROG zur Untersuchung ultraschneller nichtlinearer Effekte dritter Ordnung. Die spektroskopische Fähigkeit von iFROG wird durch das Studium von drei unterschiedlichen physikalischen Systemen (Nanostrukturen) geprüft. In ZnO-Nanostäben wird die Leistungsabhängigkeit der durch Multiphotonenabsorption induzierten Lumineszenz gemessen, wobei nachgewiesen werden konnte, dass diese mit einer Lokalisierung des optischen Nahfelds verknüpft ist. Eine Dreiphotonenresonanz in einem dünnen Titandioxid Film und eine Oberflächenplasmonenresonanz in Au-Nanoantennen führen beide zu einer endlichen Lebensdauer der induzierten Materialpolarisation. Die iFROG-Methode wird verwendet, um die ultraschnelle zeitliche Dynamik dieser Systeme auf der Nanometer- und wenige Femtosekunden-Skala zu messen.The ultrashort laser pulse characterization method “interferometric frequency-resolved optical gating” (iFROG) is extended. Both second- and third harmonic generation (SHG and THG) are separately employed as the optical nonlinearity. An iFROG measurement represents an inverse problem, where the electric field amplitude and phase of the underlying laser pulse can only be reconstructed by an iterative algorithm. In this work, a mathematical formalism for both the SHG and THG variants of iFROG is developed and combined with an evolutionary optimization algorithm to create a novel pulse retrieval algorithm for iFROG. While iFROG was originally conceived solely for pulse characterization, the technique can equally well be applied for spectroscopic purposes. By replacing the nonlinear medium in iFROG with an object of study, say a nanostructure, and characterizing a known pulse again such that the sample affects the harmonic generation process, the response of the object can be deciphered with sub-femtosecond precision. As no previous solution for the THG variant exists, the presented retrieval algorithm allows iFROG to be exploited in the study of ultrafast third-order nonlinear effects for the first time. The spectroscopic capability of iFROG is put to test by studying three differing physical systems, each consisting of nanostructures resting on dielectric substrates. Subjecting these specimen to few-cycle near-infrared pulses, a rich variety of nonlinear optical phenomena is observed. In ZnO nanorods, the power dependence of multiphoton-absorption induced luminescence is measured and found to be connected to a localization of the optical near-field. A three-photon resonance in a thin film of titania and a localized surface plasmon resonance in Au nanoantennas both lead to a finite lifetime of the induced material polarization. The THG-iFROG method is harnessed to measure the ultrafast temporal dynamics of these systems at the nanometer and few-femtosecond scales

Ultranopeiden optisten ilmiöiden tutkiminen taajuudenkolmentamista hyödyntävällä, interferometrisella, taajuuserotteisella optisella näytteistämismenetelmällä muutaman optisen syklin mittaisten laserpulssien luokassa

The recently introduced characterization technique of third-harmonic interferometric frequency-resolved optical gating for ultrashort laser pulses is investigated. The first pulse retrieval software for this complete characterization technique is presented and used to conduct pulse retrievals for the first time. In addition, the physics of this measurement technique is studied, and a simple equation describing the trace is derived. The subjects for the retrieval procedure are the measured traces for femtosecond laser pulses that have been guided through a thin film sample of either pure titanium dioxide or pure silicon dioxide. These experiments were conducted in the Max Born Institute of Berlin, Germany in 2012. Different combinations of modulational components of the interferometric trace are used in the simulations to produce three pulses for each of the two samples. The retrieved pulses are combined to produce two representative pulses for the thin films, completely describing the electric field envelope and the phase of the measured laser pulses. Full width at half maximum pulse widths of 10.1 fs and 15.7 fs are measured for the pulses of silicon- and titanium dioxide samples, respectively. The retrieved pulses are further examined by analyzing their spectral phases and the experienced group delay dispersion. The significant difference observed in the pulse durations for the two samples is attributed to multiphoton absorption processes in the titanium dioxide thin film, although the exact mechanism of the noninstantaneous third-order polarization remains unclear. The intensity envelopes of the reconstructed pulses are harnessed to study the lifetime of this process using a deconvolution strategy. By convolving the pulse for silicon dioxide with a one-sided exponential decay function with a time constant of 6.5 fs, a third pulse is produced, perfectly replicating the retrieved pulse shape for the titanium dioxide sample. This is one of the fastest phenomena ever measured with a Ti:sapphire laser. The measurement software presented in this work facilitates additional research on the subject, understanding of which could increase our knowledge of nonlinear optics. More light can be shed on the lifetime of the Kerr nonlinearity, a mechanism of elementary nature in the production of ultrashort pulses with the Ti:sapphire laser

Ultranopeiden optisten ilmiöiden tutkiminen taajuudenkolmentamista hyödyntävällä, interferometrisella, taajuuserotteisella optisella näytteistämismenetelmällä muutaman optisen syklin mittaisten laserpulssien luokassa

The recently introduced characterization technique of third-harmonic interferometric frequency-resolved optical gating for ultrashort laser pulses is investigated. The first pulse retrieval software for this complete characterization technique is presented and used to conduct pulse retrievals for the first time. In addition, the physics of this measurement technique is studied, and a simple equation describing the trace is derived. The subjects for the retrieval procedure are the measured traces for femtosecond laser pulses that have been guided through a thin film sample of either pure titanium dioxide or pure silicon dioxide. These experiments were conducted in the Max Born Institute of Berlin, Germany in 2012. Different combinations of modulational components of the interferometric trace are used in the simulations to produce three pulses for each of the two samples. The retrieved pulses are combined to produce two representative pulses for the thin films, completely describing the electric field envelope and the phase of the measured laser pulses. Full width at half maximum pulse widths of 10.1 fs and 15.7 fs are measured for the pulses of silicon- and titanium dioxide samples, respectively. The retrieved pulses are further examined by analyzing their spectral phases and the experienced group delay dispersion. The significant difference observed in the pulse durations for the two samples is attributed to multiphoton absorption processes in the titanium dioxide thin film, although the exact mechanism of the noninstantaneous third-order polarization remains unclear. The intensity envelopes of the reconstructed pulses are harnessed to study the lifetime of this process using a deconvolution strategy. By convolving the pulse for silicon dioxide with a one-sided exponential decay function with a time constant of 6.5 fs, a third pulse is produced, perfectly replicating the retrieved pulse shape for the titanium dioxide sample. This is one of the fastest phenomena ever measured with a Ti:sapphire laser. The measurement software presented in this work facilitates additional research on the subject, understanding of which could increase our knowledge of nonlinear optics. More light can be shed on the lifetime of the Kerr nonlinearity, a mechanism of elementary nature in the production of ultrashort pulses with the Ti:sapphire laser

Field enhancement of multiphoton induced luminescence processes in ZnO nanorods

The near-ultraviolet photoluminescence of ZnO nanorods induced by multiphoton absorption of unamplified Ti:sapphire pulses is investigated. Power dependence measurements have been conducted with an adaptation of the ultrashort pulse characterization method of interferometric frequency-resolved optical gating. These measurements enable the separation of second harmonic and photoluminescence bands due to their distinct coherence properties. A detailed analysis yields fractional power dependence exponents in the range of 3–4, indicating the presence of multiple nonlinear processes. The range in measured exponents is attributed to differences in local field enhancement, which is supported by independent photoluminescence and structural measurements. Simulations based on Keldysh theory suggest contributions by three- and four-photon absorption as well as avalanche ionization in agreement with experimental findings

Field Enhancement of Multiphoton Induced Luminescence Processes in ZnO Nanorods

The near-ultraviolet photoluminescence of ZnO nanorods induced by multiphoton absorption of unamplified Ti:sapphire pulses is investigated. Power dependence measurements have been conducted with an adaptation of the ultrashort pulse characterization method of interferometric frequency-resolved optical gating. These measurements enable the separation of second harmonic and photoluminescence bands due to their distinct coherence properties. A detailed analysis yields fractional power dependence exponents in the range of 3-4, indicating the presence of multiple nonlinear processes. The range in measured exponents is attributed to differences in local field enhancement, which is supported by independent photoluminescence and structural measurements. Simulations based on Keldysh theory suggest contributions by three- and four-photon absorption as well as avalanche ionization in agreement with experimental findings

Non-instantaneous polarization decay in dielectric media

We demonstrate experimental evidence for non-instantaneous polarization decay in dielectrics. The few-femtosecond relaxation times agree favorable with solutions of the time-dependent Schrödinger equation and relate to resonances of the quantum mechanical dipole