Photodiffraction in InGaAs/InGaAsP multiple quantum wells enclosed in a microcavity

We report new results on the diffraction properties of photoinduced gratings in InGaAs/InGaAsP MQW structures. The original feature of this device is that the QWs are enclosed in an asymmetric Fabry–Perot microcavity in order to increase the diffraction efficiency. We observe oscillations in the diffraction efficiency due to resonant effects in the microcavity. The experimental spectra are compared with theory. Diffraction efficiency at 1.55 μm attains a maximum value of 2.7% at a write beam fluence of 260 μ J cm⁻², and then decreases at higher fluences. We explain this phenomenon by an absorption saturation at high excitation.     

Elastic scattering of light from quantum-well exciton-polarization fluctuations in a microcavity

A model of exciton polarization fluctuations in a quantum well of a randomly variable lateral width is proposed. The stochastic part of the nonlocal susceptibility of quasi-two-dimensional excitons is expressed through random functions of the shape of quantum well boundaries. A theory of elastic light scattering from a quantum well placed in a Fabry-Perot cavity or a semiconductor microcavity is constructed in the lowest (Born) approximation in interface roughness height. The scattering cross section is calculated for an arbitrary statistics of interface roughness. The spectral and angular dependences of the intensity of light scattered by a quantum well have been studied using Gaussian correlation functions of the interface shape. It follows from numerical estimates that elastic resonant scattering in quantum wells should be observed at an rms roughness height of the order of the atomic monolayer thickness.     

Fabry–Perot interferometer using curved plates

Fabry–Perot interferometer with curved plates is described. It is found that the partially reflecting surfaces of the Fabry–Perot interferometer need not be necessarily flat. The pair of Fabry–Perot plates may be curved but the surfaces must be well matched with an accuracy better than λ/100.     

Intersubband absorption in highly photoexcited semiconductor quantum wells

Transient mid infrared (MIR) absorption spectroscopy is used to investigate transitions between higher electronic subbands in semiconductor quantum well (QW) structures after interband photoexcitation with intense picosecond pulses in the visible spectral range. Our investigation focuses on the e₂–e₃ intersubband transition in an asymmetric undoped GaAs/AlGaAs QW structure. At an injected nonequilibrium carrier density of 1×10¹³ cm⁻²/QW, an e₂–e₃ absorption band at 99 meV with a spectral width of 5 meV is found. For a higher density studied, 3×10¹³ cm⁻²/QW, the band is broadened and blueshifted by 30 meV. Intersubband absorption signals are distinguished from free-carrier absorption signals in the MIR by their characteristic time behavior.     

Photon recycling in Fabry–Perot micro-cavities based on Si3N4 waveguides

We present a numerical analysis and preliminary experimental results on one-dimensional Fabry–Perot micro-cavities in Si₃N₄waveguides. The Fabry–Perot micro-cavities are formed by two distributed Bragg reflectors separated by a straight portion of a waveguide. The Bragg reflectors are composed of a few air slits produced within the Si₃N₄ waveguides. In order to increase the quality factor of the micro-cavities, we have minimized, with a multiparametric optimization tool, the insertion loss of the reflectors by varying the length of their first pairs (those facing the cavity). To explain the simulation results, the coupling of the fundamental waveguide mode with radiative modes in the Fabry–Perot micro-cavities is needed. This effect is described as a recycling of radiative modes in the waveguide. To support the modelling, preliminary experimental results of micro-cavities in Si₃N₄ waveguides realized with the focused ion beam technique are reported.     

The spectral measurement of a high repetition rate tunable dye laser output using Fabry–Perot fringe

We present the measurement of spectral distribution of a high repetition rate dye laser by deriving explicit relationship of the parameters with ring diameter of the Fabry–Perot fringe pattern. The output characteristics and its variation has been represented by generating a composite picture of a single line scan across the ring diameter of the Fabry–Perot fringe, captured through CCD camera and frame grabber card.     

New insights on amorphous silicon-nitride microcavities

All amorphous silicon-nitride planar optical microcavities operating in the visible range have been grown by plasma enhanced chemical vapor deposition. The luminescence intensity of the N-rich silicon-nitride layer from a microcavity with 6 period distributed Bragg reflectors (DBRs) is two order of magnitude higher than that of the luminescent layer without the cavity. Moreover, a strong directionality of the microcavities emission can be observed. Such results can be ascribed to the anisotropic optical density of states induced in the Fabry–Perot structure. The quality factors of the resonators are strictly correlated to the number of periods of the DBRs.     

Self-mixing interference effect of DFB semiconductor lasers

This paper analyzes the self-mixing interference in DFB (distributed feedback) semiconductor lasers. The general expression of the threshold gain and frequency in the DFB lasers was deduced. Numerical simulations indicate that, for specific coupling coefficient and cavity length value, the DFB lasers are more sensitive to optical feedback than the Fabry–Perot semiconductor laser. These results show that high-accuracy self-mixing sensors can be obtained by using the DFB lasers. PACS 42.55.Px; 42.60.Lh; 42.87.Bg     

Surface states and accumulation nanolayer induced by Ba and Cs adsorption on the n-GaN(0 0 0 1) surface

The Ba and Cs adsorption on the n-GaN(0 0 0 1) surface has been studied in situ by the threshold photoemission spectroscopy using s- and p-polarized light excitation. Two surface bands induced by Ba (Cs) adsorption are revealed in surface photoemission spectra below the Fermi level. The surface-Fermi level position is found to be changed from significantly below the conduction band minimum (CBM) at clean n-GaN surface to high above the CBM at Ba, Cs/n-GaN interfaces, with the transition from depletion to electron accumulation occurring at low coverages. Photoemission from the accumulation nanolayer is found to excite by visible light in the transparency region of GaN. Appearance of an oscillation structure in threshold photoemission spectra of the Ba, Cs/n-GaN interfaces with existing the accumulation layer is found to originate from Fabry–Perot interference in the transparency region of GaN.     

Light Squeezing in a Fabry–Perot Cavity

A quantum mechanical model for the study of quadrature squeezing in radiation coming out of Fabry–Perot cavity containing nonlinear Kerr medium has been proposed. We have incorporated the vacuum fluctuations entering in the cavity through unused ports. The analysis has been applied to a sample of GaAs filled in the Fabry–Perot cavity and irradiated by an off-resonant Co:MgF₂ laser. Limitations on achievable squeezing due to incident pump power, interaction time, nonlinear coupling parameter and facet reflectivities have been discussed and it is seen that low reflectivity of front facet and high reflectivity of rear facet of the cavity produces substantial squeezing.     

Estimation of the Dispersion of Absorptance in the Region of Resonant Transitions Using the Reflection Spectra in a Fabry–Perot Interferometer

It is shown that in the region of resonant transitions the dependence of the area under the contour of the Fabry–Perot interference bands on the frequency minus the area under the envelope of the minima reflects the character of the dispersion of light absorptance.     

Transmission of a Gaussian beam after incidenting nonnormally on a Fabry–Perot etalon: a nonresonant case

Under the nonresonant case where the carrier frequency of a Gaussian beam deviates from the resonant frequency of a Fabry–Perot etalon, the transmission of a Gaussian beam after incidenting nonnormally on a Fabry–Perot etalon has been investigated. The results show that under the nonresonant case, variations of the peak intensity, the position of the peak intensity and the spot size of the transmitted beam with the input angle behave differently and even with a reversed tendency compared with those obtained under the resonant case.     

Photoluminescence of “dark” excitons in CdMnTe quantum well, embedded in a microcavity

A diluted magnetic semiconductor (DMS) quantum well (QW) microcavity operating in the limit of the strong coupling regime is studied by magnetoptical experiments. The interest of DMS QW relies on the possibility to vary the excitonic resonance over a wide range of energies by applying an external magnetic field, typically about 30 meV for 5 T in our sample. In particular, the anticrossing between the QW exciton and the cavity mode can be tuned by the external field. We observe the anticrossing and formation of exciton polaritons in magneto-reflectivity experiments. In contrast, magneto-luminescence exhibits purely excitonic character. Under resonant excitation conditions an additional emission line is observed at the energy of the dark exciton. The creation of dark excitons is made possible due to heavy hole–light hole mixing in the QW. The emission at this energy could be due to a combined spin flip of an electron and a bright exciton recombination.     

Measurement of the electro-optic coefficient of polymer thin films with better spatial resolution

A method for measuring the electro-optic coefficient of polymer films on the basis of an asymmetry Fabry–Perot cavity is introduced. The sample layer is located between two aluminium layers, which are deposited on glass substrates by thermal evaporation. This layer structure is objected to a laser beam, and a variable voltage is applied to the aluminium films resulting in a modulation of the transmitted laser power. The electro-optic coefficient γ₁₃ of the poled polymer film can be calculated by evaluating the Fabry–Perot equation. The spatial resolution is tested with a polymer film that was poled by a needle corona discharge in air through a metal grating with a period of 120 μm. By scanning the sample plate in the direction perpendicular to the grating lines, the spatial resolution is also demonstrated according to the spacing of the poled structure.     

Influence of cavity de-tuning in semiconductor microcavity LEDs

Data are presented showing the precise role of the cavity de-tuning on the emission properties of semiconductor microcavity light emitting diodes (MC-LEDs). Enhanced output power and narrow line width emission have been observed over a wide range of cavity de-tunings, with a power reduction of less than −1 dB observed for ±12 nm de-tuned devices. For a resonantly de-tuned MC-LED, the slope efficiency extracted from the power output versus current characteristics decreases monotonically with increasing temperature. However, for a MC-LED de-tuned +5.6 nm to long wavelength with respect to the room temperature quantum well (QW) peak, the extracted slope efficiencies vary by less than ±0.5 dB over a temperature range of greater than 65°C. Compared to a conventional LED, narrower beam divergence is observed for MC-LEDs de-tuned to short wavelength with respect to the bare QW peak. For positively de-tuned MC-LEDs the beam divergence broadens, and the far-field emission profiles exhibit a double-lobed pattern that is sensitive to pump level.     

Dissipative optical bistability in a Fabry–Perot cavity

From an interferometric point of view the bistable behavior due to a nonlinear absorptive material inside a Fabry–Perot cavity is studied. The effect of building standing wave and nonstanding wave on the bistable phenomena is discussed.     

Resonant Raman scattering by N-related local modes in AlGaAs/InGaAsN multiquantum wells

We report resonant Raman scattering and secondary ion mass spectrometry measurements on InGaAsN/AlGaAs multiquantum wells grown by plasma-assisted molecular beam epitaxy. The appearance of a strong TO band at resonance with nitrogen (N)-related electronic levels has been observed. The N-induced vibration mode at 470 cm⁻¹ changes in intensity and shape with increasing N and In content. A new vibration mode has been observed at 320 cm⁻¹, whose intensity scales with the N concentration. This mode is not present in InGaAsN films, so it is linked to the presence of Al. Its frequency is close to the B₁ silent mode of wurtzite GaN. It is attributed to the formation of GaN pairs, near the MQW interfaces as a consequence of the preferential Al–N bonding.     

Experimental investigation of the line center of Hg intercombination spectral line 253.7 nm perturbed by Kr

The collision broadening and shift of the Hg intercombination spectral line 253.7 nm (6¹S₀–6³P₁) perturbed by Kr has been investigated using a high-resolution scanning Fabry–Perot interferometer. The values of the pressure broadening and shift coefficients β and δ, respectively, for the studied line have been obtained. The obtained coefficients β and δ are compared with their corresponding published experimental values and also those calculated using Lindholm–Foley impact theory.     

Generation of optimized wavelength-tunable optical pulse from an uncooled gain-switched Fabry–Perot semiconductor laser using optical amplifier as external light injection

We present a novel system design that can generate the optimized wavelength-tunable optical pulse streams from an uncooled gain-switched Fabry–Perot semiconductor laser using an optical amplifier as external light source. The timing jitter of gain-switched laser has been reduced from about 3 ps to 600 fs and the pulse width has been optimized by using our system. The stability of the system was also experimentally investigated. Our results show that an uncooled gain-switched FP laser system can feasibly produce the stable optical pulse trains with pulse width of 18 ps at the repetition frequency of 5 GHz during 7 h continuous working. We respectively proved the system feasibility under 1 GHz, 2.5 GHz and 5 GHz operation.     

Spontaneous emission and elastic scattering of light from quantum-well excitons in a Fabry-Perot microcavity

A theory of spontaneous emission and elastic light scattering by quasi-two-dimensional excitons in a quantum well placed in a Fabry-Perot microcavity is developed. The problem is solved by means of electrodynamic Green’s functions with inclusion of fluctuations of the quantum-well width and cavity wall shape treated as a perturbation. General expressions are found in a zero approximation of perturbation theory (plane interfaces) for the radiative decay rates of quasi-two-dimensional excitons and for their energy shifts in the cavity. The boundary conditions for the electromagnetic field are taken into account through the coefficients of inward light reflection from the cavity walls. Resonance contributions to the scattering cross sections, which differ in the polarizations (p or s) of the incident and scattered waves, are derived in the lowest (Born) approximation in quantum-well width fluctuations. The spectral and angular dependences of elastic light scattering are studied numerically for Gaussian and exponential correlation functions. It is shown that the contribution from quantum-well width fluctuations to light scattering exceeds that due to single interfaces (surfaces) of a heterostructure by two orders of magnitude.