Spatial homogeneity of optically switched semiconductor photonic crystals and of bulk semiconductors

This paper discusses free carrier generation by pulsed laser fields as a mechanism to switch the optical properties of semiconductor photonic crystals and bulk semiconductors on an ultrafast time scale. Requirements are set for the switching magnitude, the time-scale, the induced absorption as well as the spatial homogeneity, in particular for silicon at lambda= 1550 nm. Using a nonlinear absorption model, we calculate carrier depth profiles and define a homogeneity length l_hom. Homogeneity length contours are visualized in a plane spanned by the linear and two-photon absorption coefficients. Such a generalized homogeneity plot allows us to find optimum switching conditions at pump frequencies near v/c= 5000 cm^{-1} (lambda= 2000 nm). We discuss the effect of scattering in photonic crystals on the homogeneity. We experimentally demonstrate a 10% refractive index switch in bulk silicon within 230 fs with a lateral homogeneity of more than 30 micrometers. Our results are relevant for switching of modulators in absence of photonic crystals

Ultrafast Nonlinearities In Semiconductor-Laser Amplifiers

The bound-electronic optical nonlinearities in highly excited semiconductors (i.e., semiconductor lasers) have been calculated using a two-parabolic-band model. The nonlinear absorption spectrum is first obtained using a dressed-state formalism taking into account the contributions from two-photon absorption, electronic Raman, and optical Stark effects. The nonlinear refractive index ( n 2 ) is then found by performing a Kramers-Kronig transformation on the nonlinear absorption spectrum. It is also shown that the quadratic Stark splitting of the bands leads to a shift in the quasi-Fermi levels, which introduces additional absorptive and refractive nonlinearities. The sign, magnitude, and the current-density dependence of the calculated n 2 agree well with some recently published experimental results for Al-Ga-As and In-Ga-As-P diode lasers

Enhancement of optical switching parameter and third-order optical nonlinearities in embedded Si nanocrystals: a theoretical assessment

Third-order bound-charge electronic nonlinearities of Si nanocrystals (NCs) embedded in a wide band-gap matrix representing silica are theoretically studied using an atomistic pseudopotential approach. Nonlinear refractive index, two-photon absorption and optical switching parameter are examined from small clusters to NCs up to a size of 3 nm. Compared to bulk values, Si NCs show higher third-order optical nonlinearities and much wider two-photon absorption threshold which gives rise to enhancement in the optical switching parameter.Comment: 8 pages, 3 figures, to be published in Optics Communication

All-Optical Switching Devices Based On Large Nonlinear Phase-Shifts From 2Nd Harmonic-Generation

We show that the large nonlinear phase shifts obtained from phase-mismatched second harmonic generation can be used to implement all-optical switching devices such as a nonlinear Mach-Zehnder interferometer and a nonlinear directional coupler

Saturation of the all-optical Kerr effect in solids

We discuss the influence of the higher-order Kerr effect (HOKE) in wide band gap solids at extreme intensities below the onset of optically induced damage. Using different theo- retical models, we employ multiphoton absorption rates to compute the nonlinear refractive index by a Kramers-Kronig transform. Within this theoretical framework we provide an esti- mate for the appearance of significant deviations from the standard optical Kerr effect pre- dicting a linear index change with intensity. We discuss the role of the observed saturation behavior in practically relevant situations, including Kerr lens mode-locking and supercon- tinuum generation in photonic crystal fibers. Furthermore we present experimental data from a multi-wave mixing experiment in BaF2 which can be explained by the appearance of the HOKE

Modified spectrum autointerferometric correlation (MOSAIC) for single-shot pulse characterization

A method for generation of the modified spectrum autointerferometric correlation that allows single-shot pulse characterization is demonstrated. A sensitive graphical representation of the ultrashort pulse phase quality is introduced that delineates the difference between the presence of temporal and spectral phase distortions. Using these schemes, full-field reconstruction of ultrashort laser pulses is obtained in real time using an efficient iterative technique. (a) Single-shot characterization using a combination of fringe-free (noninterferometric) autocorrelation and second-harmonic spectrum (b) A hybrid graphical representation that distinguishes between spectral and temporal phase distortions (c) Real-time full-field reconstruction using the above schemes with an efficient sequential search algorithm Naganuma et al. showed that the pulse spectrum and IAC provide a sufficient dataset to uniquely reconstruct the complex electric field, with only a timedirection ambiguity The increased SNR found on averaged MOSAIC traces extends the utility of all retrieval techniques using the dataset outlined by Naganuma et al. The principle of computing a MOSAIC can be described in the frequency domain as follows: a secondorder IAC waveform with a fringe frequency ⍀ is Fourier transformed to generate a spectrum. Spectral filtering is then performed to remove the ⍀ component and amplify the 2⍀ component by a factor of 2. An inverse Fourier transform generates a new time-domain signal known as a fringe-resolved MOSAIC In the (delay) time-domain analysis, the maximum and minimum envelopes of MOSAIC are given by the intensity autocorrelation, g͑͒ = ͐f͑t͒f͑t + ͒dt, and the difference computation, S min = g͑͒ − ͉g p ͉͑͒, respectivel