Pice Chen

Photoluminescence fatigue and inhomogeneous line broadening in semi-insulating Tl6SeI4 single crystals

Photoluminescence (PL) properties of semi-insulating Tl6SeI4 have been investigated. A broad emission band centered at 1.63 ± 0.02 eV was observed in all samples. The PL emission band is excitonic in nature and is tentatively attributed to a bound exciton emission. PL fatigue (a reduction in PL intensity under prolonged laser excitation) was always observed. The amount of PL fatigue depended on excitation power and temperature. PL fatigue kinetics are described by a stretched exponential with nominal lifetimes in the 10–265 s range. The recovery of the PL occurred within a few seconds of light cessation. The magnitude of PL fatigue in different samples correlated with inhomogeneous line broadening of the 1.63 eV emission band, such that broader bands exhibited more fatigue. An additional luminescence band centered at 1.78 eV was observed which increased in intensity under prolonged laser irradiation. The fatigue phenomenon is tentatively attributed to two mechanisms—the formation of photo-induced defects and the formation of quasi-stable particles. Both of these mechanisms introduce additional radiative and non-radiative recombination channels that lead to a decrease in the PL intensity under prolonged laser irradiation. Since inhomogeneous line broadening and PL fatigue are related to the concentration of defects or impurities, the measurement of these two parameters is an effective method to screen sample quality.

Charge Transport and Observation of Persistent Photoconductivity in Tl6SeI4 Single Crystals

The chalcohalide compound Tl6SeI4 is a promising wide-bandgap semiconductor for efficient hard radiation detection at room temperature due to its high density, average atomic number and mobility-lifetime product. However, the nature of its charge transport kinetics, especially the role of defects in recombination, has not been examined in detail. To determine the charge transport kinetics in Tl6SeI4 single crystals, electrical conductivity and photoinduced current transient spectroscopy were measured over the temperature range 105-330 K. These measurements reveal the existence of multiple defect states with energy levels in the range 0.10-0.90 eV, within the bandgap of Tl6SeI4. Large persistent photoconductivity (PPC) is observed at low temperature that shows strong thermal quenching at 160 K. The quenching of PPC is described using a configuration coordinate model involving a deep level donor state, which is tentatively attributed to the presence of iodine vacancies or Si interstitial impurities.

Integrated BaTiO3 modulator with 8 dB extinction at 50 GHz and 25 km reach

We demonstrate the high frequency and long-reach performance of a compact modulator using thin film BaTiO₃. An extinction of 12 dB is measured at 25 GHz for 2.2 V_pp operation through a 25 km fiber with 8 dB extinction at 50 GHz for 9.1 V_pp.

Small footprint barium titanate photonic crystal modulators for photonic integrated circuits

We report on the design, fabrication, and characterization of photonic crystal phase modulators on epitaxial barium titanate thin films. Modeling indicates that >50 GHz bandwidth and 0.25 V·cm voltage-length product are achievable in sub-millimeter long devices.

Ultrahigh Bandwidth, Low V π Photonic Crystal BaTiO 3 Modulators

BaTiO3 modulators with as low as 3.9 V Vπ and as high as 28 GHz bandwidth are reported. We demonstrate photonic crystal waveguides with a group index of 4 and predict > 50 GHz bandwidth and < 5 V operation in optimized photonic crystal modulators.

BaTiO3 Modulator Devices for Silicon Photonics

The microwave characteristics of thin-film BaTiO3 traveling-wave electro-optic modulators were measured between 10 MHz and 50 GHz. S-parameter measurements show a smooth response to 50 GHz and up to 50 GHz electrical bandwidth.

50 GHz electro-optic modulators with BaTiO3 epitaxial thin film platform for short distance optical communications

Future datacom and telecom applications as well as optical interconnects require high bandwidth, low-power, small area optical modulator devices [1], [2], [3], [4], [5]. No device technology to date has been shown with the concomitant properties of high electro-optic (EO) bandwidth, wide optical range, low power consumption, and compact size. We have taken an alternative approach of developing thin film BaTiO3 as an optoelectronic material platform for high frequency, low operation voltage, and compact modulators. The BaTiO3 thin film platform has competitive advantages over other legacy platforms for optical modulator applications. In brief, ferroelectric oxide thin film BaTiO3 modulators have advantages of: (1) large EO coefficient, more than ten times higher compared to those of LiNbO3 devices currently dominating optical modulator markets; (2) low driving voltage of <1.5 V, thus low power consumption; (3) bandwidth >50 GHz demonstrated with potential reaching 100 + GHz regime; (4) potential for medium scale integration; and (5) integration with Si electronics leading to ultrahigh compact electrooptical components at low cost [6]. Using BaTiO3 thin films with 1 mm long interaction length, we have demonstrated optical modulators with voltage-length products nearly an order of magnitude smaller than that of silicon [7]. We have also shown the potential for high frequency operation by demonstrating modulation out to 50 GHz [7], [8], [9].

Enhancement of the pockels effect in photonic crystal modulators through slow light

We report a photonic crystal (PhC) optical modulator operating in the optical C band (1530-1565 nm) using barium titanate epitaxial thin films as the electro-optic (EO) medium. The PhC has hexagonal lattice symmetry and an extinction ratio of 9 dB. Due to the slow light enhancement of the EO coefficient near the PhC band edge, the driving electrode can be as short as a one millimeter. We report for the first time, to the best of our knowledge, at microwave frequencies from 10 to 45 GHz the effective EO coefficient and its enhancement through slow light effects. A monotonic increase of the effective EO coefficient from 60 to 110 pm/V across the stopband edge is obtained, resulting in an enhancement as high as 1.8.