We simulate multi-shot intensity-and-phase measurements of unstable ultrashort-pulse trains using frequency-resolvedoptical-gating (FROG) and spectral phase interferometry for direct electric-field reconstruction (SPIDER). Both techniques fail to reveal the pulse structure. FROG yields the average pulse duration and suggests the instability by exhibiting disagreement between measured and retrieved traces. SPIDER under-estimates the average pulse duration but retrieves the correct average pulse spectral phase. An analytical calculation confirms this behavior.When a measurement averages over many different events, it faces an impossible task: providing one result when no single result can be correct. In ultrafast optics, this issue has been particularly problematic for multi-shot intensity-autocorrelation measurements of trains of different complex pulses. The resulting measured autocorrelation trace vs. delay (see Fig. 1) consists of a narrow spike atop a broad structureless background. Given that the task is inherently impossible, it is worth asking what we should expect. The answer is that the technique should yield a pulse with some characteristics of the typical pulse in the train (e.g., its duration) and also give some indication of the stability, or randomness, of the pulses in the train. Although autocorrelation actually does yield some of this information, it yields neither the pulse intensity nor its phase for the case of a stable train of identical pulses and so is now generally considered obsolete.The next question-one whose answer is long overdue-is how more modern pulse-measurement techniques, which do yield the pulse intensity and phase for a stable train of identical pulses, react to an unstable train of random pulses. So here we consider this question for frequency-resolved optical gating (FROG)[2] and spectral-phase interferometry for direct electric-field reconstruction (SPIDER) [3], the latter of which also allows an analytical result.For the simulations, we chose a nonrandom component E() with a flat phase and Gaussian intensity of temporal FWHM 12t, where t is the temporal sampling rate. To E(), we added an equal-energy random component E rand () with the same spectrum, but with random spectral phase, which we then Fourier-filtered (made somewhat less random and hence the resulting pulse shorter) by different amounts to yield two trains of variably structured pulses with different average complexities and durations.[4] The random trains' resulting average pulse lengths (FWHM) were 26t and 54t. Figure 2 shows typical pulses in the two trains. All frequency units are in 2/(Nt), where N is the SPIDER array size (4096).We computed multi-shot traces for secondharmonic-generation (SHG) FROG and SPIDER. FROG involves measuring a self-gated spectrogram of the pulse field, while SPIDER measures a spectral interferogram of the pulse and a frequency-sheared and delayed replica of it. SPIDER requires a frequency shear, , which we chose to be 30/(Nt), and a pulse separation, T, which we chos...

We report a simple method for producing gold nanowires (AuNWs) by using silicon nanowires (SiNWs) as templates. Uniform AuNWs were formed in the core of SiNWs, when SiNWs coated with Au were furnace annealed at ∼880 °C at 10−2 Torr. Transmission electron microscopy (TEM) examination showed the AuNWs had diameters of ∼10 nm. High-resolution TEM revealed lattice fringes with an interlayer spacing of 0.235 nm, which corresponds to the spacing in Au crystal, confirming the AuNWs are crystalline.

We demonstrate a simple self-referenced single-shot method for simultaneously measuring two different arbitrary pulses, which can potentially be complex and also have very different wavelengths. The method is a variation of cross-correlation frequency-resolved optical gating (XFROG) that we call double-blind (DB) FROG. It involves measuring two spectrograms, both of which are obtained simultaneously in a single apparatus. DB FROG retrieves both pulses robustly by using the standard XFROG algorithm, implemented alternately on each of the traces, taking one pulse to be "known" and solving for the other. We show both numerically and experimentally that DB FROG using a polarization-gating beam geometry works reliably and appears to have no nontrivial ambiguities.

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