Statistical analysis of time-resolved emission from ensembles of semiconductor quantum dots: Interpretation of exponential decay models

We present a statistical analysis of time-resolved spontaneous emission decay curves from ensembles of emitters, such as semiconductor quantum dots, with the aim of interpreting ubiquitous non-single-exponential decay. Contrary to what is widely assumed, the density of excited emitters and the intensity in an emission decay curve are not proportional, but the density is a time integral of the intensity. The integral relation is crucial to correctly interpret non-single-exponential decay. We derive the proper normalization for both a discrete and a continuous distribution of rates, where every decay component is multiplied by its radiative decay rate. A central result of our paper is the derivation of the emission decay curve when both radiative and nonradiative decays are independently distributed. In this case, the well-known emission quantum efficiency can no longer be expressed by a single number, but is also distributed. We derive a practical description of non-single-exponential emission decay curves in terms of a single distribution of decay rates; the resulting distribution is identified as the distribution of total decay rates weighted with the radiative rates. We apply our analysis to recent examples of colloidal quantum dot emission in suspensions and in photonic crystals, and we find that this important class of emitters is well described by a log-normal distribution of decay rates with a narrow and a broad distribution, respectively. Finally, we briefly discuss the Kohlrausch stretched-exponential model, and find that its normalization is ill defined for emitters with a realistic quantum efficiency of less than 100%.\ud \u

Diagnostic performance of endoscopic tissue acquisition for pancreatic ductal adenocarcinoma in the PREOPANC and PREOPANC-2 trials

BackgroundNeoadjuvant treatment for pancreatic ductal adenocarcinoma (PDAC) has increased, necessitating histopathologic confirmation of cancer. This study evaluates the performance of endoscopic tissue acquisition (TA) procedures for borderline resectable and resectable PDAC.MethodsPathology reports of patients included in two nationwide randomized controlled trials (PREOPANC and PREOPANC-2) were reviewed. The primary outcome was sensitivity for malignancy (SFM), considering both “suspicious for” and “malignant” as positive. Secondary outcomes were rate of adequate sampling (RAS) and diagnoses other than PDAC.ResultsOverall, 892 endoscopic procedures were performed in 617 patients, including endoscopic ultrasonography (EUS)-guided TA in 550 (89.1%), endoscopic retrograde cholangiopancreatography (ERCP)-guided brush cytology in 188 (30.5%), and periampullary biopsies in 61 patients (9.9%). The SFM was 85.2% for EUS, 88.2% for repeat EUS, 52.7% for ERCP, and 37.7% for periampullary biopsies. The RAS ranged 94–100%. Diagnoses other than PDAC were other periampullary cancers in 24 (5.4%), premalignant disease in five (1.1%), and pancreatitis in three patients (0.7%).ConclusionsEUS-guided TA of patients with borderline resectable and resectable PDAC included in RCTs had an SFM above 85% for both first and repeat procedures, meeting international standards. Two percent had false positive result for malignancy and 5% had other (non-PDAC) periampullary cancers.Cellular mechanisms in basic and clinical gastroenterology and hepatolog

Light Sources in Semiconductor Photonic Materials

In photonic materials the refractive index varies on a length scale comparable to the wavelength of light; as a consequence, propagation of light is strongly modified with respect to that in homogeneous dielectric materials. In this thesis experiments are reported with light souces in crystalline and random photonic materials. It had been predicted that photonic crystals could modify the radiative decay rate of embedded light sources. However, this prediction was never verified experimentally. We have been able to prove this prediction for the first time using CdSe nanocrystals as light sources in titania inverse opal photonic crystals. Spherical CdSe nanocrystals with a diameter between 3 nm and 6 nm were made by wet-chemical synthesis. These nanocrystals show so-called 'quantum-size-effects': the colour of the emission depends on the nanocrystals' size. Nanocrystals of 3 nm size emit blue light while nanocrystals of 6 nm emit red light. We measured the rate of emission, or the radiative lifetime, of the nanocrystals as a function of emission colour and observed that blue light is emitted about three times faster than red light. The rates were found to be in good agreement with quantum-mechanical calculations on small particles. Subsequently, we embedded the nanocrystals in our photonic crystals and again measured the radiative lifetime. We observed that the lifetime of the nanocrystals varied by a factor-of-three depending on the lattice parameter of the photonic crystal. With this we have confirmed the hypothesis that photonic crystals can indeed control the radiative lifetime of embedded light sources. Porous gallium phosphide (GaP) is a random photonic material that can be made by electrochemical etching of crystalline GaP. This process has been applied for many years. We observed that during etching light is generated at the pore fronts. We were able to reveal the mechanism of light generation and attributed it to a hot-carrier process. During etching a high electric field is required. As a consequence hot carriers are generated and these hot carriers give rise to emission. Besides exploration of the emission and its mechanism we were able to use the emission as an internal light source to measure the optical properties of porous GaP during electrochemical etching. This allows one to deduce the optical characteristics of porous GaP directly and over the complete visible range in a single experiment

Hot carrier luminescence during porous etching of GaP under high electric field conditions

Electroluminescence is observed during porous etching of n-type GaP single crystals at strongly positive potential. The emission spectra, which include a supra-bandgap contribution, are markedly different from the spectra observed under optical excitation or minority carrier injection. The current density and electroluminescence intensity show a strong potential dependence and a similar hysteresis. The spectral characteristics of the luminescence suggest that both thermalised and hot charge carriers, generated by impact ionisation, are involved in light emission