Jean-François Roux

Grating-assisted coupling of terahertz waves into a dielectric waveguide studied by terahertz time-domain spectroscopy

We report on the efficient coupling of terahertz (THz) waves into a dielectric waveguide by means of a diffraction grating engraved at the top of the waveguide. The waveguide is made of a 201-microm-thick high-resistivity silicon wafer. The transmission of the device, measured versus frequency by terahertz time-domain spectroscopy, shows usual m lines when a frequency component of the THz pulse spectrum satisfies the phase-matching condition and is coupled into the waveguide. The experimental data are well modeled with the differential electromagnetic method to compute the diffraction pattern of the grating device. The dispersion curve of the first four modes of propagation is determined from the frequency position of the m lines recorded for different angles of incidence of the THz beam. The waveguide exhibits a weak group velocity dispersion at high frequencies.

Photovariation of grating-assisted coupling of terahertz waves into a silicon waveguide

Using terahertz (THz) time-domain spectroscopy, we study the coupling of THz waves into a silicon waveguide by means of a grating coupler. We vary the efficiency of the coupling coefficient for selected frequencies up to 20% in amplitude (40% in intensity) by generating photocarriers in the silicon material through illumination of the waveguide with white light. The experimental data are well fitted using the differential method to model the grating diffraction.

Time-resolved reflectivity characterization of polycrystalline low-temperature-grown GaAs

Using femtosecond time resolved reflectivity, we have characterized the dynamics of photoinduced generated carriers in a polycrystalline low-temperature-grown GaAs sample. Our measurements are fitted with an analytical expression reliable for low pump power experiments. The sample, which presents no As precipitates, shows an ultrafast subpicosecond response together with a longer picosecond tail that we attribute to the midgap defect states. Moreover, we have observed the influence of surface roughness on the differential reflected signal.

Stable dual-wavelength microlaser controlled by the output mirror tilt angle

A continuous-wave dual-wavelength solid-state microlaser is presented and a technique for regulating the gain competition between the two wavelengths is proposed, based on the angular tilt of the laser cavity output mirror. Laser behavior is studied and balanced dual-wavelength emission is obtained with output power levels as high as 200 mW for 2 W pump power. Sum frequency mixing is demonstrated making the source promising for Terahertz generation in the 0.5-0.7 THz range through difference frequency generation.

Grating-assisted phase-matched second-harmonic generation from a polymer waveguide

Diffraction gratings can be used to achieve phase matching between the fundamental modes of a slab or confined waveguide. Compared with the usual techniques such as quasi-, Cerenkov, and intermodal phase matching, the matching method used here involves spatial harmonics of the guided electromagnetic field that are generated by the corrugated grating. This grating acts simultaneously as a linear waveguide coupler at both the pump and harmonic frequencies. Using the third spatial harmonic, we report what we believe to be the first observation of grating-assisted phase-matched second-harmonic generation between counterpropagating TM(0) modes of an organic waveguide.

Simultaneous passively Q-switched dual-wavelength solid-state laser working at 1065 and 1066 nm

A passively Q-switched dual-wavelength solid-state laser is presented. The two wavelengths are emitted by two different crystals in order to avoid gain competition, and the synchronization between the pulses is obtained by external triggering of the saturable absorber. Sum frequency mixing is demonstrated, proving the interest of this source for terahertz generation in the 0.3-0.4 THz range through difference frequency generation.

Principles and Applications of THz Time Domain Spectroscopy

Terahertz Time-Domain Spectroscopy (THz-TDS) is one of the most powerful techniques to characterise the far infrared response of materials and devices. Based on the use of short electromagnetic waveforms generated by rectifying femtosecond optical pulses delivered by a mode-locked laser, it allows, within one single experiment, the determination of the optical constants of materials over a wide frequency spectrum ranging from 100 GHz up to several THz. Thanks to its large dynamics and its ability to give both amplitude and phase of the transmitted, reflected or scattered THz fields, this technique addresses a large panel of scientific studies. In this chapter, one introduces the principles of THz-TDS. First, the characteristics and performances of a classical set-up are described, then the extraction of the refractive index and absorption coefficient of the studied device from its complex experimental transmission or reflection coefficients are explained. As a demonstration of the power of this technique, one presents some results of thin film spectroscopy and recent studies of left-handed metamaterials. Finally, a detailed analysis of the precision of determined optical constants is given. One will treat not only the classical transmission scheme, but also the reflection regime that is mandatory in the study of strongly absorbing media.

All-optical high-frequency characterization of optical devices for optomicrowave applications

Presents an original method to investigate the frequency response of optomicrowave devices in the GHz-THz range. In order to reach such a high bandwidth, we take advantage of the wide spectrum associated with femtosecond laser pulses. Using nonlinear crosscorrelation optical technique under appropriate conditions, the transfer function of a device is derived from its measured impulse response. We experimentally demonstrate our method by testing an integrated Mach-Zehnder interferometer, which acts as a notch filter for modulation frequencies that are odd multiples of 15 GHz.

Ultralarge-band optical characterization of optomicrowave devices

We present an original method for characterizing opto- microwave devices up to THz modulation frequencies. This method is based on the measurement of the temporal response of devices excited by a laser pulse. The transfer function of the device is obtained by a numerical FFT of the experimental temporal response. Using ultrashort laser pulses, we are able to obtain this response from DC up to a few THz. In order to demonstrate the feasibility of the method, we have characterized an integrated optical Mach- Zehnder interferometer. The optical path difference between the two arms of the interferometer is about 6.6 mm leading to interferences in the tens of GHz frequency domain. This opto-microwave device has been characterized from 10 GHz to 2.0 THz. The first rejected frequency is around 15 GHz and the free spectral range is 30 GHz. This device may be used as a sub-carrier frequency filter for all-optical signal processing in high-rate optical communication systems.