Srinivasan Anand

Temperature tuning of the optical properties of planar photonic crystal microcavities

We report on the temperature tuning of the optical properties of planar Photonic Crystal (PhC) microcavities. Studies were made on one and two dimensional PhCs that were etched in InP and GaAs vertical waveguides. Two dimensional (hexagonal) and one-dimensional (Fabry-Perot) cavities were optically investigated by an internal light source technique. The samples were mounted on a Peltier-stage which allowed temperature variation from T = 20 °C up to T = 76 °C. A linear dependence of the resonance wavelengths with respect to temperature is observed. A gradient of dλ/dT = 0.09 nm/°C and 0.1 nm/°C for the GaAs and InP based cavities was observed, respectively. These results are in agreement with the theoretical calculations based on the thermal dependence of the refractive index of the PhC semiconductor component.

Optical study of two-dimensional InP-based photonic crystals by internal light source technique

We present the first optical study of 2-D photonic crystals (PCs) deeply etched in an InP/GaInAsP step-index waveguide. Following the same internal light source approach proposed by Labilloy et al. (1997,1999) for the investigation of GaAs-based 2-D PCs, transmission measurements through simple PC slabs and 1-D Fabry-Perot (FP) cavities between PC mirrors were performed. Details are given on the experimental setup which has been implemented with respect to the original scheme and adapted to InP-based systems working at 1.5-/spl mu/m. 2-D plane-wave expansion and finite difference time-domain (FDTD) methods are used to fit the experimental data. Out-of-plane losses were evaluated according to a recently introduced phenomenological model. In spite of the complex hole morphology in the measured samples, preliminary results are presented which indicate the possibility of separating different loss contributions from finite etch depth and hole shape. As for 1-D cavities, both FDTD and classical theory for planar resonators are applied in order to deduce the optical properties of the PC mirrors. The origin of an anomalously high transmission observed inside the stopgap is discussed and arguments are given to demonstrate the need for further modeling efforts when working in the bandgap regime.

Deep level transient spectroscopy of InP quantum dots

We report on the application of deep level transient spectroscopy to the study of electron emission from quantum dots. The results are presented for coherently grown InP dots embedded in Ga0.5In0.5P. We determine an emission activation energy of 220 meV for the one electron ground state of the dots. With increased average electron occupation in the dots we observe a systematic shift of the DLTS peak towards lower temperatures. This we interpret as being due to Coulomb charging of the dots. We extract an average Coulomb charging energy of 8–12 meV per added electron in the dot in agreement with our estimated value of 9 meV.

Photonic crystal optical filter based on contra-directional waveguide coupling

Wavelength-selective operation of an optical filter (add/drop) based on a contra-directional photonic crystal waveguide coupler is demonstrated. The waveguides are defined as line defects in a two-dimensional triangular photonic crystal fabricated in an InP/GaInAsP heterostructure. The device is characterized using the end-fire method for the drop functionality. The experimental data are in good agreement with the theoretical results predicted by finite-difference time-domain simulations.

ELECTRICAL CHARACTERIZATION OF INP/GAINP QUANTUM DOTS BY SPACE CHARGE SPECTROSCOPY

An investigation of coherently grown InP quantum dots embedded in Ga0.5In0.5P by conventional space charge spectroscopy methods is reported. Deep level transient spectroscopy (DLTS) is used to obtain quantitative information on the electron emission from the dots. The applied field is found to significantly enhance the electron emission rates as seen by shifts in the peaks towards lower temperatures with increased field. Taking the field induced barrier lowering into account, the emission energy for the one electron ground state of the dot is determined as 240±10 meV. The correlation between the measured signal and the observed electron accumulation in capacitance–voltage measurements is clearly demonstrated. Further, studies of the electron emission when the average electron population in the dots was varied show that the emission energies are modified by the coulomb charging energy. Admittance measurements as a function of temperature, bias and frequency were also performed, and the results are qualitat...

Surface Second-Harmonic Generation from Vertical GaP Nanopillars

We report on the experimental observation and analysis of second-harmonic generation (SHG) from vertical GaP nanopillars. Periodic arrays of GaP nanopillars with varying diameters ranging from 100 to 250 nm were fabricated on (100) undoped GaP substrate by nanosphere lithography and dry etching. We observed a strong dependence of the SHG intensity on pillar diameter. Analysis of surface and bulk contributions to SHG from the pillars including the calculations of the electric field profiles and coupling efficiencies is in very good agreement with the experimental data. Complementary measurements of surface optical phonons by Raman spectroscopy are also in agreement with the calculated field intensities at the surface. Finally, polarization of the measured light is used to distinguish between the bulk and surface SHG from GaP nanopillars.

Nanofabrication of two-dimensional photonic crystal mirrors for 1.5 μm short cavity lasers

We have developed a fabrication scheme for two-dimensional (2D) triangular photonic crystals (PCs) on InP-based material systems involving high resolution electron beam lithography, pattern transfer to a SiO2 etch mask, and a Cl2/Ar electron cyclotron resonance reactive ion etch step yielding PCs with periods of a=300–450 nm and air fill factors of f=18%–63%. These PCs are employed as high reflectivity mirrors for 1.5 μm short cavity lasers, which are key components in future highly integrated PC based photonic circuits. We have fabricated lasers with 2 PC mirrors and cavity lengths down to 100 μm. Threshold currents as low as 7.6 mA were achieved for the shortest lasers with 2 PC mirrors. The short laser cavity results in a large Fabry–Perot mode spacing and mode competition leads to single mode lasing over a reasonably large current range up to 4.5× threshold. Lasers with one PC back mirror and a cleaved output facet show a higher threshold current of 13 mA and a maximum output power of more than 4 mW. ...

Compound cavity measurement of transmission and reflection of a tapered single-line photonic-crystal waveguide

Measurements on a single-line defect photonic crystal waveguide demonstrate propagation losses as low as 140 cm−1 and coupling efficiency to a ridge access guide increased from 20% to 50% thanks to a tapered access. These results are obtained by analyzing the internal cavities created by residual reflections, hence, requiring a single sample. They are nevertheless crosschecked by measurements on distinct samples by the Fabry–Perot resonance method.

Liquid crystal infiltration of InP-based planar photonic crystals

A procedure for the infiltration of planar photonic crystals (PhCs) with liquid crystals (LCs) is presented. InP-based PhCs are infiltrated with the nematic LC-K15 in a specially designed high-vacuum chamber. The infiltration technique is validated and systematically characterized by measuring the transmission through the infiltrated PhCs at different temperatures and for different polarizations. The reproducibility and reliability of our procedure are demonstrated and a high filling efficiency is obtained.

Low-loss InP-based photonic-crystal waveguides etched with Ar/Cl2 chemically assisted ion beam etching

We demonstrate low-loss photonic-crystal (PC) waveguides realized in InP by Ar/Cl2 based chemically assisted ion beam etching. The waveguides are obtained as line defects in a triangular lattice of holes etched through a three-layer InP/GaInAsP/InP heterostructure. By optimizing the etching parameters so that the physical and the chemical components are balanced we succeed in obtaining holes deeper than 2 μm even for a hole diameter as small as 220 nm. The quality of the PCs etched by two different process conditions is compared by using the shape and the position of one of the mode gaps as an assessment tool. The measured transmissions spectra indicate that the PC waveguides etched with an optimized process exhibit losses smaller than 1 dB/100 μm. This is to date the lowest loss value reported for PC waveguides in semiconductor heterostructures at optical communication wavelengths.