J.-M. Lourtioz

Band‐edge and deep level photoluminescence of pseudomorphic Si1−x−yGexCy alloys

Photoluminescence of strained Si1−x−yGexCy alloys grown by rapid thermal chemical vapor deposition on Si(100) is investigated. Two dominant features are reported: At low pump intensities, the photoluminescence is dominated by a deep level broad luminescence peak around 800 meV whereas at high pump intensities, a well‐resolved band‐edge luminescence (no phonon and transverse optic replica) is observed. At 77 K, we attribute this band‐edge feature to an electron‐hole plasma luminescence of the ternary alloy. The dependences of the deep level and band‐edge peaks versus the excitation power density are, respectively square‐root‐like or superlinear. A blue shift of the energy gap of Si1−x−yGexCy alloys with respect to Si1−xGex alloy is observed. The blue shift increase with carbon content corresponds to what is expected for the bulk alloy. An eventual influence of the strain relaxation cannot be excluded.

Optical saturation of intersubband absorption in GaAs‐AlxGa1−xAs quantum wells

We have investigated intersubband absorptions between the conduction ground state and the first excited state of two AlxGa1−xAs/GaAs/AlxGa1−xAs multiple quantum well structures with x=0.3 and 85 A well width, and with x=0.57 and 96 A well width. Small‐signal measurements show absorption peaks at 10.45 and 10.15 μm, respectively. Under an intense resonant excitation from a pulsed CO2 laser, saturation of the intersubband absorption occurred. The saturation intensity is estimated to be 340±120 kW/cm2 for the first sample and 375±120 kW/cm2 for the second. From these values, we have deduced subband decay times of the order of 10.6±3.5 ps for the first sample and 15.5±5 ps for the second.

Experimental demonstration of electrically controllable photonic crystals at centimeter wavelengths

Electrically controllable photonic crystals have been fabricated by inserting p-i-n diodes in two-dimensional metallic lattices. A first structure uses a square lattice of thin and discontinuous metallic wires. A second structure is fabricated using stacks of printed circuits with metallic strips. The p-i-n diodes are soldered along the different metallic wires or strips. The crystals have been characterized between 1 and 20 GHz. We show that they can be operated as wideband switchable electromagnetic windows with high transmission or reflection contrast between on and off states. A ∼25 dB transmission modulation is reported within the first transmission band of a two-period crystal. We also show that the switching domain and modulation rate can be varied with a separate bias control for each crystal plane. Finally, the distance between crystal planes is used to tune the operating frequency range.

Photoluminescence of strained Si1−yCy alloys grown at low temperature

The photoluminescence of strained Si1−xCx alloys grown at low temperature by rapid thermal chemical vapor deposition is investigated. The photoluminescence spectra are mainly characterized by a deep level broadband at low energy which subsists up to room temperature. This low energy emission is associated with the low‐temperature growth process required for the incorporation of carbon into substitutional sites. The stability of the layers after thermal annealing is monitored using this low energy radiative recombination. A blue shift of the photoluminescence energy peak is observed and the peak intensity presents a maximum versus annealing time. The blue shift and the associated increased linewidth is explained in terms of the local strain induced band‐gap fluctuations. Infrared transmission spectra of the annealed samples suggest that silicon carbide precipitates appear during the anneal and that oxygen and carbon complexes contribute to the formation of the deep level band.

Experimental demonstration of complete photonic band gap in graphite structure

We experimentally demonstrate the existence of complete photonic band gap in graphite-type photonic crystals, thereby confirming theoretical predictions reported in previous studies. Experiments are performed at microwave frequencies from 27 to 75 GHz using hexagonal lattices of alumina rods. Transmission spectra measured for E (TM) and H (TE) polarizations and for different orientations of the two-dimensional lattice are found to be in excellent agreement with numerical calculations. The complete photonic band gap results from the overlap of E7 and H5 forbidden bands. Attenuations larger than 30 dB are measured for structures comprised of only four rows of alumina rods.

Intersubband absorption of GaAs/AlGaAs quantum wells in MBE grown mid-infrared slab waveguides

A structure to enhance the absorbance due to intersubband transitions in GaAs/AlGaAs quantum wells is discussed. Mid-infrared slab waveguides including 30 quantum wells were grown using molecular-beam epitaxy (MBE). Photoluminescence experiments revealed an excellent uniformity of the samples. Absorption measurements over the whole 9-13.4- mu m spectral range were performed for the first time using the combination of CO/sub 2/ and NH/sub 3/ lasers. Effective absorbance due to intersubband transitions as high as 14 dB were measured for 3-mm-long waveguides. The waveguide structure is expected to be a good candidate for optoelectronic devices in the 10- mu m region.<<ETX>>

Ultrafast response (∼2.2 ps) of ion-irradiated InGaAs photoconductive switch at 1.55 μm

A 2.2 ps full-width-at-half-maximum impulse response is measured for ion-irradiated InGaAs photoconductive switches triggered by ultrashort 1.55 μm laser pulses. Correspondingly, the −3 dB bandwidth is estimated to be ∼120 GHz. Measurements of the electrical signals delivered by photoconductive switches are performed using an electro-optic sampling technique. As is shown, the ion irradiation reduces the carrier lifetime to less than 1 ps. The sheet resistance is 0.6×105 Ω/square. The photoconductive switch responsivity is found to exhibit a nonlinear dependence with optical power. The results are qualitatively interpreted.

Novel all‐optical 10 μm waveguide modulator based on intersubband absorption in GaAs/AlGaAs quantum wells

Efficient all‐optical modulation of mid‐infrared radiations based on photoinduced intersubband absorption of a GaAs/AlGaAs multiquantum well structure is demonstrated at room temperature. The sample designed for mid‐infrared waveguiding was grown by molecular beam epitaxy. A double‐resonance spectroscopy of both interband and intersubband transitions of the quantum wells is presented. The intersubband resonance is found at 9.56 μm. An AlGaAs laser diode is used to optically pump the quantum wells. On/off modulation ratios as high as 150:1 are obtained with only 45 mW pump power and for mid‐infrared input powers up to 0.5 W. The frequency bandwidth of the device is presently comparable to that of acousto‐optic modulators available at mid‐infrared wavelengths.

Evidence of four-wave parametric interaction in optically pumped NH3

Abstract This paper reports the first evidence of a near-resonant four-wave parametric interaction in gaseous ammonia using a single-pass NH 3 cell pumped by a 9R16 CO 2 TEA laser. Experimental signatures of these processes are clearly revealed by an accurate time-history of the FIR/MIR emission and by detuning measurements. A theoretical model is used to identify the relevant mechanisms responsible for each emission and confirms that both the 12.2 μm and 12.08 μm lines are produced by a four-wave parametric interaction involving the pump and two FIR lines.

First demonstration of room temperature intersubband-interband double-resonance spectroscopy of GaAs/AlGaAs quantum wells

Intersubband-interband double-resonance experiments in undoped GaAs/Al/sub 0.33/Ga/sub 0.67/As multiple quantum well (MQW) structures at room temperature are discussed. The well width is 78 AA. A Ti:sapphire laser is used to pump the interband transitions, while the first intersubband transition is probed with a CO/sub 2/ laser. The intersubband absorption is found to peak at 10.6 mu m, and a 10-meV linewidth is measured. The absorption signal is also recorded at a fixed CO/sub 2/ tuning while varying the pump laser wavelength from 700 to 850 nm. A high-resolution spectrum is obtained, reflecting the steplike density of states with sharp peaks at the exciton resonances.<<ETX>>