Eric C. Fox

Femtosecond time‐resolved refractive index changes in CdS0.75Se0.25 and CdS

A pump‐probe, light by light deflection technique employing 120 fs pulses at λ=620 nm has been used to measure the temporal and irradiance (I) dependence of the refractive index (n) in 100‐μm‐thick single crystals of CdS0.75Se0.25 and CdS at 295 K. For I<3 GW/cm2 only instantaneous negative changes in n are observed and are attributed to Stark and two‐photon resonance effects while for higher I negative changes with onset times of approximately 2 ps are attributed to the cooling of two‐photon generated carriers. For I≳30 GW/cm2 a partial recovery of n occurs within 10 ps and is attributed to possible threshold dependent recombination processes. The variation of probe‐beam deflection with pump irradiance has been determined for different probe delays and related to two photon absorption of the pump beam.

Third harmonic generation as a structural probe of ion-implanted silicon

Third harmonic generation in samples of crystalline silicon implanted at room temperature with 80 and 100 keV As/sup +/ ions at fluxes up to 3*10/sup 16/ cm/sup -2/ is discussed. 1.06 mu m, 35 ps pulses of a Nd:YAG laser have been used in a reflection geometry to determine the magnitude, phase, and anisotropy of chi /sup (3)/. Loss of anisotropy for ion fluxes above approximately=10/sup 14/ cm/sup -2/ has been identified with amorphization of the near surface region, but little variation in the phase of chi /sup (3)/, which was earlier suggested by others as a possible origin of the minimum in the third harmonic intensity at the amorphization threshold, is observed. The minimum is interpreted as one in the magnitude of chi /sup (3)/ and it is shown that the linear optical properties, through the linear reflectivity, also display an extremum at the amorphization threshold. It is suggested that these variations are probably due to variations in the short range electronic order. Third harmonic generation is therefore seen to be a unique diagnostic of ion-implanted materials in that it is capable of showing variation in electronic response both below and above the amorphization threshold. >

Femtosecond Probing of Photoinduced Refractive Index Changes in Semiconductors

For decades, Information technology has been dominated by electronics. Increasingly, however, the physical limitations of electronics are being or have been reached and scientists are exploring new technologies for transmitting, storing and processing information. Many believe that light or photons will form the new “current” for information in the next century and that photonics could possibly supplant electronics in several devices. Certainly photonics is now making significant inroads in areas such as transmission and storage. However, the same can’t be said of routing and switching, since such functions are still carried out using all electronic or hybrid, opto-electronic technologies. Increasing demands for integration call for all-optical switching devices and it has become the “holy grail” of the emerging optical communication technologies to find suitable materials which display a large enough and fast enough optical response to be considered for such devices. The underlying physical mechanism which is being researched in many of these quests is photo-induced refractive index changes [Shen, 1984; Gibbs,1985]. It is envisioned that a gate optical pulse can be used to alter the local refractive index in a device and thus modify the direction of propagation, phase, or transmission of an optical pulse passing through the device in what is commonly referred to as light-by-light switching. Many different types of materials have been and continue to be investigated for these applications including semiconductors, glasses, semiconductor-doped glasses, and polymers [Miller, 1981; Stegeman, 1985; Haug, 1988; Gibbs, 1990]. Also, several different geometries have been researched for switching applications based on Fabry-Perot interferometers, etalons, waveguides, diffraction, and scattering [Stegeman, 1985].

Femtosecond time-resolved refractive index changes in CdSSe

Light deflection and diffraction scattering techniques have been used in pump-probe geometries to measure the temporal and irradiance dependences of refractive index changes induced by intense 120 fsec (lambda) equals 620 nm pulses in 100 micrometers thick crystals of bulk CdS and CdS0.75Se0.25 at 295 K. Instantaneous and long lived negative refractive index changes are observed. The instantaneous changes are attributed to optical Stark and two- photon resonance effects while the long lived changes are attributed to free carrier bandfilling effects. The irradiance dependences of the different contributions are discussed in terms of a model in which attenuation is dominated by two-photon absorption. The rise and decay times associated with the bandfilling nonlinearity have allowed us to identify a carrier cooling time of 2 psec, and a carrier lifetime which can be as short as 6 psec for pump irradiances in excess of 300 GW/cm2. Possible mechanisms for the carrier recombination are discussed.

Separation of bound and free carrier contributions to the refractive index change induced in II-VI semiconductors by femtosecond pulses

Time-resolved probe beam deflection and degenerate four-wave mixing (DFWM) have both been used to determine the temporal evolution of the refractive index changes induced in ZnSe and CdS0.75Se0.25 single crystals at T=300 K by lambda =620 nm, 120 fs pulses with irradiances (I) up to 500 GW cm-2. Negative refractive index changes which follow the pump pulse profile are attributed to virtual electronic transitions and two-photon resonances, while two-photon-generated hot carriers and their cooling rate determine the magnitude and temporal evolution of the refractive index change for probe delays up to 4 ps. No further evolution is observed up to 100 ps unless I>30 GW cm-2, in which case the refractive index appears to recover on a 10 ps timescale following carrier relaxation: the possible role of a threshold dependent recombination process in this recovery is discussed.

High-speed VGA CMOS image sensor

We present some design details and characterization results for a VGA CMOS image sensor designed for high sped inspection applications. The sensor has 16 analog outputs, which can each operate at 50 MHz data rate, and can capture images at 1600 frames per second. The image sensor has exposure control functionality, antiblooming capability and a on-rolling shutter architecture to implement snap-shot image capture mode. The pixel architecture incorporates 5 transistors on a 15.3 micron pitch with 50 percent fill factor.

Nondegenerate two-photon absorption in CdS

A femtosecond continuum, with a bandwidth of 90 nm, has been used in a pump-probe geometry to investigate nondegenerate two-photon absorption (2 PA) from 660 nm to 570 nm, with pump wavelength (lambda) equals 620 nm, in hexagonal CdS at 300 K. A dependence upon probe wavelength and relative beam polarization is observed. The polarization anisotropy has allowed us to measure the relative magnitudes of particular (chi) (3) tensor elements. The nondegenerate 2 PA coefficient increases by a factor of about two from 660 nm to 570 nm. The anisotropy and dispersion are discussed in terms of a two-parabolic-band model.