Differential coded aperture single-snapshot spectral imaging.

We propose a novel, to the best of our knowledge, concept of the differential coded aperture snapshot spectral imaging (D-CASSI) technique exploiting the benefits of using a {-1,+1} random mask, which is demonstrated by a broadband single-snapshot hyperspectral camera using compressed sensing. To double the information, we encode the image with two complementary random masks, which proved to be superior to two independent patterns. We utilize dispersed and non-dispersed encoded images captured in parallel on a single detector. We explored several different approaches to processing the measured data, which demonstrates significant improvement in retrieving complex hyperspectral scenes. The experiments were completed by simulations in order to quantify the reconstruction fidelity. The concept of differential CASSI could be easily implemented also by multi-snapshot CASSI without any need for optical system modification.

Enhancement of CASSI by a zero-order image employing a single detector.

Coded aperture snapshot spectral imaging (CASSI) makes it possible to recover 3D hyperspectral data from a single 2D image. However, the reconstruction problem is severely underdetermined, and efforts to improve the compression ratio typically make the imaging system more complex and cause a significant loss of incoming light intensity. In this paper, we propose a novel approach to CASSI that enables capturing both a spectrally sheared and an integrated image of a scene with a single camera. We performed hyperspectral imaging of three different testing scenes in the spectral range of 500-900 nm. We demonstrate the prominent effect of using the nondiffracted image on the reconstruction of data from our camera. The use of the spectrally integrated image improves the reconstruction quality, and we observed an approximately fivefold reduction in reconstruction time.

Fluorescence lifetime imaging via spatio-temporal speckle patterns in a single-pixel camera configuration.

Photoluminescence (PL) spectroscopy offers excellent methods for mapping the PL decay on the nanosecond time scale. However, capturing maps of emission dynamics on the microsecond timescale can be highly time-consuming. We present a new approach to fluorescence lifetime imaging (FLIM), which combines the concept of random temporal speckles excitation (RATS) with the concept of a single-pixel camera based on spatial speckles. The spatio-temporal speckle pattern makes it possible to map PL dynamics with unmatched simplicity. Moreover, the method can acquire all the data necessary to map PL decay on the microsecond timescale within minutes. We present proof-of-principle measurements for two samples and compare the reconstructed decays to the non-imaging measurements. Finally, we discuss the effect of the preprocessing routine and other factors on the reconstruction noise level. The presented method is suitable for lifetime imaging processes in several samples, including monitoring charge carrier dynamics in perovskites or monitoring solid-state luminophores with a long lifetime of PL.

Random temporal laser speckles for the robust measurement of sub-microsecond photoluminescence decay.

Time-resolved photoluminescence (PL) is commonly used to track dynamics in a broad range of materials. Thus, the search for simplification of the acquisition of PL kinetics attracts continuous attention. This paper presents a new robust and straightforward approach to the measurement of PL decay, which is based on randomly fluctuating excitation intensity. The random excitation waveform is attained by using laser speckles generated on a rotating diffuser. Owing to this, the presented technique is able to utilize any coherent excitation source without the necessity to generate short pulses or to controllably modulate the light. PL decay can be computationally reconstructed from the Fourier image of the PL trace. The paper demonstrates the performance of the method, which is able to acquire sub-microsecond dynamics as the impulse response function reaches 300 ns. The reconstructed PL decays were compared to streak camera measurements to verify the method. Finally, potential limitations and applications of the technique are discussed.

Femtosecond luminescence spectroscopy of core states in silicon nanocrystals.

We present a study of ultrafast carrier transfer from highly luminescent states inside the core of silicon nanocrystal (due to quasidirect transitions) to states on the nanocrystal-matrix interface. This transfer leads to a sub-picosecond luminescence decay, which is followed by a slower decay component induced by carrier relaxation to lower interface states. We investigate the luminescence dynamics for two different surface passivation types and we propose a general model describing spectral dependence of ultrafast carrier dynamics. Our results stress the crucial role of the energy distribution of the interface states on surface-related quenching of quasidirect luminescence in silicon nanocrystals. We discuss how to avoid this quenching in order to bring the attractive properties of the quasidirect recombination closer to exploitation.

Data processing correction of the irising effect of a fast-gating intensified charge-coupled device on laser-pulse-excited luminescence spectra.

Intensified charge-coupled devices (ICCDs) comprise the advantages of both fast gating detectors and spectrally broad CCDs into one device that enables temporally and spectrally resolved measurements with a few nanosecond resolution. Gating of the measured signal occurs in the image intensifier tube, where a high voltage is applied between the detector photocathode and a microchannel plate electron multiplier. An issue arises in time-resolved luminescence spectroscopy when signal onset characterization is required. In this case, the transient gate closing process that causes the detected signal always arises in the middle of the ICCD chip regardless of the spectral detection window--the so-called irising effect. We demonstrate that in case when the detection gate width is comparable to the opening/closing time and the gate is pretriggered with respect to the signal onset, the irising effect causes the obtained data to be strongly distorted. At the same time, we propose a software procedure that leads to the spectral correction of the irising effect and demonstrate its validity on the distorted data.

Nonlinear optical properties of nanocrystalline diamond.

We report on investigation of nonlinear optical phenomena in nanocrystalline diamond prepared by microwave plasma enhanced chemical vapour deposition. We observed the upconverted photoluminescence, the second and the third harmonic generation and Z-scan signal. The value of the third order nonlinear susceptibility was estimated. Our results show that nonlinear optical properties of nanocrystalline diamond have many features of the bulk diamond affected to some extent by the presence of grain boundaries.