Stimulated emission dynamics in InGaN-based multiple quantum wells (MQWs) is analyzed. The lasing threshold
measurements of the In<sub>0.09</sub>Ga<sub>0.91</sub>N/In<sub>0.02</sub>Ga<sub>0.98</sub>N MQWs revealed non-monotonous threshold dependence on the growth
temperature of the active MQW region. The optimal growth temperature range with the lowest stimulated emission
threshold (100 kW/cm<sup>2</sup>) in the active region was found to be 780 - 800°C. The influence of indium nano-clusters on
stimulated emission threshold is discussed. Optical gain in InGaN MQWs was measured using variable excitation stripe
length technique. The optical gain dependence on excitation stripe length and excitation power density was studied. The
onset of the gain saturation was observed on the high energy side of the stimulated emission peak. The onset exhibited
red-shift with increasing stripe length due to reduced electron-hole density caused by high optical transition rate.
Increase of excitation power density resulted in the strong blue-shift of the optical gain spectra. The maximal optical gain
coefficient values of 200 cm<sup>-1</sup> and 300 cm<sup>-1</sup> were obtained for the samples with the lowest and the highest stimulated
emission thresholds, respectively. The calculated optical confinement factor (3.4 %) for the samples yielded the net gain
coefficient of about 5900 cm<sup>-1</sup> and 8800 cm<sup>-1</sup>, respectively
On the basis of investigation of photoluminescence (PL) dynamics of highly excited structures with different In content
under strong excitation by short laser pulses and comparison with model calculations we analyze emission mechanisms
in these In-containing systems. The obtained PL spectra in various In-containing samples exhibit stimulated emission
line on the short-wavelength side of the spontaneous PL band. We suggest theoretical model for optical transitions in
InGaN compounds with strong compositional fluctuations leading to two distinct types of the regions, one of which is
In-rich islands. We assume that optical band-gap within those two regions is randomly fluctuating with Gaussian
distribution. Calculated PL spectra as a function of excitation and time are in fair agreement with experimental results
demonstrating all the observed peculiarities of luminescence dynamics obtained experimentally.
A set of UV light-emitting diodes (LEDs) with the peak wavelengths ranging from 255 nm to 375 nm was applied for
the investigation of spectral and decay-time fluorescence signatures in dry <i>B. globigii </i>spores and common airborne
interferants (albuminous, epithelium, and cellulosous materials as well as aromatic hydrocarbons). The fluorescence
decay signature was represented by a phase shift of the sinusoidal fluorescence waveform in respect of excitation
provided by high-frequency modulated LEDs. The obtained data matrix was used for the optimization a bioparticle
fluorescence sensor with a minimized number of excitation sources and detection channels and maximized
discrimination ability of bioparticles against common interferants. Based on the optimization, a new concept for a UV
LED based "detect-to-warn" bioparticle fluorescence sensor is proposed. The sensor contains a single deep-UV LED
emitting at 280 nm that is harmonically modulated at a high frequency (of about 70 MHz) and a dual-channel
fluorescence detector with the spectral windows peaked at 320 nm and 450 nm. The output parameters of the sensor are
the ratio of the fluorescence intensity in the two windows and the phase shift of the fluorescence waveform in the
320-nm detection channel in respect of the excitation one. Such a sensing scheme has a smaller number of optical
components and a potentially higher discrimination ability of bioparticles against common interferants in comparison
with the conventional approach based on just fluorescence intensity measurement under dual-wavelength excitation
(280 nm and 340 nm).
Recently developed deep-UV light-emitting diodes (LEDs) are already used in prototype fluorescence sensors for detection of hazardous biological agents. However, increasing of the sensor ability of discrimination against common interferents requires further development of measurement technique. In particular, LED-based fluorescence lifetime measurements are to be considered as a technique supplementary to fluorescence spectral and excitation measurements. Here we report on application of UVTOP<sup>®</sup> series deep-UV LEDs developed by Sensor Electronic Technology, Inc. for real-time measurements of fluorescence lifetime in the frequency domain. LEDs with the wavelengths of 280 nm (targeted to protein excitation) and 340 nm (for excitation of coenzymes NADH and flavins) were used. The output of the LEDs was harmonically modulated at frequencies up to 100 MHz and fluorescence lifetime on the nanosecond and subnanosecond scale was estimated by measuring the phase angle of the fluorescence signal in respect of the LED output. Dual-wavelength LED-based phase-resolved measurement technique was tested for discrimination of <i>B. globigii</i> against a variety of interferents such as diesel fuel, paper, cotton, dust, etc. We conclude that fluorescence phase measurements have potential to improve the discrimination ability of the "detect-to-warn" optical bioparticle sensors.
Recent progress in wide-bandgap semiconductor optoelectronics resulted in an appearance of deep-UV light-emitting diodes (LEDs), which can be used for fluorescence excitation in a variety of chemical and biological compounds. We used two generations of AlGaN-based UVTOP series deep ultraviolet LEDs developed by Sensor Electronic Technology, Inc. The peak wavelength of these fully packaged devices is 340 nm and 280 nm, line width at half maximum approximately 10 nm, wall-plug efficiency up to 0.9% and output power in the milliwatt range. The second-generation emitters are shown to have an extremely low level of unwanted long-wavelength emission what is important for fluorescence measurements. The UV LEDs were tested for fluorescence excitation in standard fluorophores (organic dyes), autofluorescent biological compounds (riboflavin, NADH, tryptophan, and tyrosine) and medical specimens (fluid secreted by prostate gland). Fluorescence lifetime measurements in the frequency domain were demonstrated using UVTOP-340 and -280 devices. The output of the LEDs was modulated at frequencies up to 200 MHz by high-frequency current drivers and the phase angle of the fluorescence signal was resolved using a radio-frequency lock-in amplifier. Nanosecond-scaled measurements of fluorescence lifetimes, which are the “fingerprints” of chemical and biological compounds, were demonstrated.
Fluorescence and reflectance spectra of dipolar N,N-dimethylaminobenzylidene 1,3-indandione (DMABI) molecular crystals of α and β crystallographic modifications have been studied over a wide temperature range. The luminescence spectral properties have been discussed by means of the self-trapped exciton model. The crystal phase transition in both α and β modifications resulting in the deeply-trapped excitonic state formation has been observed at low temperatures, below 60 K.
The excitation dynamics in indandione-1,3 Pyridinium Betaine (IPB) intramolecular charge transfer molecules in various environments was studied. By comparing the excitation properties of the IPB molecules in solution and those in the crystal form, the influence of the intermolecular interactions on the excited state dynamics is considered. Two types of excited states are revealed in the IPB crystal: the Frenkel exciton states, which cause ultrafast nonradiative excitation decay, and the intermolecular charge transfer exciton states positioned below the Frenkel states, that have a longer lifetime and are responsible for pronounced photocurrent efficiency of IPB solids.
Transient and quasi-steady-state photoluminescence of a dense electron-hole plasma was studied in GaN epilayers under high photoexcitation at room-temperature. High initial carrier heating up to 1100 K was observed. Decay of nonthermalized electron-hole plasma was analyzed both in homo- and heteroepitaxial GaN layers. The heating is shown to significantly influence the luminescence peak position and the rate of spontaneous and stimulated recombination. After the thermalization process is completed, the luminescence decay is exponential and the room-temperature carrier lifetime can be extracted. The lifetime in the heteroepitaxial layer grown on sapphire was found to be 190 ps, while the homoepitaxial layer exhibited an essentially higher value of 890 ps, which is one of the highest reported for free-carrier recombination in GaN. Additionally, optical gain spectra were studied using variable-stripe method. The threshold for stimulated emission was found to be considerably lower and the gain at a certain pump intensity was shown to be much higher in the homoepitaxial layer than in the heteroepitaxial one. Maximum net gain value of 300 cm<SUP>-1</SUP> was observed.
Strain Energy Band Engineering of Group III-N heterostructures should allow us to prevent defect formation at the heterointerfaces ad to reduce the built-in electric field in the quantum wells. The strain, caused by lattice mismatch, may be decreased by incorporation of In into AlGaN. To monitor structural perfection of the quaternary compound AlInGaN and to evaluate electronic potential profile, we employed optical methods: reflectivity, site- selectively excited photoluminescence, photoluminescence excitation and time-resolved luminescence. AlGaN with the molar fraction of Al of 9% and two samples with the lattice mismatch reduced by partial substitution of Al by 1% and 2% of In were investigated. In AlGaN, the luminescence excited resonantly with the exciton position is red shifted. The photoluminescence excitation spectra indicate that the mobility edge is above the optical band gap, and the localization vanishes. These results show that the incorporation of approximately equals 2% indium into AlGaN leads to the disappearance of the band tail states and smoothing of the potential profile.
The origin of the excited states and the spectroscopic features of polar molecular films and crystals of N,N- dimethylaminobenzylidene 1,3-indandione (DMABI) was considered. The formation of charge transfer exciton states in the absorption and luminescence spectra by increase of the thicknesses of DMABI films was observed. The evidence of a strong exciton-phonon interaction and its effects on the exciton absorption and luminescence spectra was discovered. The coexistence of free and self-trapped excitons at elevated temperatures was found in DMABI films. The dynamics of both shallow short-lived and deeply trapped long-lived self-trapped exciton states in various films and crystal of DMABI was discussed.
Near-to-bandgap luminescence has been studied in bulk-like CdS nanocrystals as a function of the average radii ranging above the exciton Bohr radius (from 3.3 nm to 100 nm). The surface recombination was shown to be the main recombination route under experimental conditions. Comparing the size dependences of the luminescence under high and low excitation, a striking increase in the luminescence intensity of nanocrystals with one excited electron-hole pair was discovered for average radii below 10 nm. The small-volume enhanced bimolecular recombination was shown to account for the observed data.