Boshen Gao

On-chip spectroscopy with thermally tuned high-Q photonic crystal cavities

Spectroscopic methods are a sensitive way to determine the chemical composition of potentially hazardous materials. Here, we demonstrate that thermally tuned high-Q photonic crystal cavities can be used as a compact high-resolution on-chip spectrometer. We have used such a chip-scale spectrometer to measure the absorption spectra of both acetylene and hydrogen cyanide in the 1550 nm spectral band and show that we can discriminate between the two chemical species even though the two materials have spectral features in the same spectral region. Our results pave the way for the development of chip-size chemical sensors that can detect toxic substances.

Design of flat-band superprism structures for on-chip spectroscopy.

We present a systematic design procedure of photonic crystal (PhC) superprism structures for on-chip spectroscopic applications. In specific, we propose a new figure of merit, namely the angular-group-dispersion-bandwidth-product (AGDBP) to quantitatively describe the spectroscopic performance of PhC superprism structures, and an optimum PhC structure for spectroscopic applications should have large angular group dispersion over a large bandwidth, i.e., a flat-top dispersion profile. We demonstrate the advantage of such a new design consideration by optimizing the geometry of a two-dimensional parallelogram-lattice PhC superprism structure. The performance of such a superprism spectrometer is further analyzed numerically using finite-difference time-domain simulations, which out-performs current implementations in terms of the number of achievable output spectral channels.

Enhanced spectral sensitivity of a chip-scale photonic-crystal slow-light interferometer

We experimentally demonstrate that the spectral sensitivity of a Mach-Zehnder (MZ) interferometer can be enhanced through structural slow light. We observe a 20-fold resolution enhancement by placing a dispersion-engineered, slow-light, photonic-crystal waveguide in one arm of a fiber-based MZ interferometer. The spectral sensitivity of the interferometer increases roughly linearly with the group index, and we have quantified the resolution in terms of the spectral density of interference fringes. These results show promise for the use of slow-light methods for developing novel tools for optical metrology and, specifically, for compact high-resolution spectrometers.

Distributed angular double-slit interference with pseudo-thermal light

We propose and perform an interference experiment involving a distributed angular double-slit and the orbital angular momentum (OAM) correlations of thermal light. In the experiment, two spatially separated angular apertures are placed in two correlated light beams generated by splitting the thermal light beam via a beam splitter. The superposition of the two spatially separated slits constitutes an angular double-slit in two-photon measurements. The angular interference pattern of the distributed double-slit is measured even though each beam interacts with a different part of the object. This scheme allows us to discriminate among different angular amplitude objects using a classical incoherent light source. This procedure has potential applications in remote sensing or optical metrology in the OAM domain.

Development of a slow-light spectrometer on a chip

We discuss the design and development of a slow-light spectrometer on a chip with the particular example of an arrayed waveguide grating based spectrometer. We investigate designs for slow-light elements based on photonic crystal waveguides and grating structures. The designs will be fabricated using electron-beam lithography and UV photolithography on a silicon-on-insulator platform. We optimize the geometry of these structures by numerical simulations to achieve a uniform and large group index over the largest possible wavelength range.

Ultra-wide-band structural slow light

The ability of using integrated photonics to scale multiple optical components on a single monolithic chip offers key advantages to create miniature light-controlling chips. Numerous scaled optical components have been already demonstrated. However, present integrated photonic circuits are still rudimentary compared to the complexity of today’s electronic circuits. Slow light propagation in nanostructured materials is a key component for realizing chip-integrated photonic devices controlling the relative phase of light and enhancing optical nonlinearities. We present an experimental record high group-index-bandwidth product (GBP) of 0.47 over a 17.7 nm bandwidth in genetically optimized coupled-cavity-waveguides (CCWs) formed by L3 photonic crystal cavities. Our structures were realized in silicon-on-insulator slabs integrating up to 800 coupled cavities, and characterized by transmission, Fourier-space imaging of mode dispersion, and Mach-Zehnder interferometry.

Broadband slow light in genetically optimized coupled-cavity waveguides with GBP exceeding 0.45

Slow light propagation through engineered band dispersion in photonic structures is a highly promising tool for realizing integrated optical delay lines and efficient photonic devices through enhanced optical nonlinearities [1, 2]. A primary goal is to achieve devices with large, approximately constant group index over the largest possible bandwidth, thus enabling multimode and pulsed operation [2]. We present an experimental proof of record high group-index bandwidth product (GBP = ng Αω/ω) [2] in genetically optimized coupled-cavity waveguides (CCWs) made of staggered L3 photonic crystal cavities (Fig.1(a) and (b)). The optimization procedure [3] was applied to the unit cell (Fig. 1(a)) to achieve maximal GBP combined with low losses. The resulting designs [4] were realized in Si slabs (Fig. 1(b)), where CCWs of length ranging between 50 and 800 cavities were fabricated. The samples were characterized by measuring the CCW transmission (Fig. 1(c) and (d)), the mode dispersion through Fourier-space imaging, and the group index ng with Mach-Zehnder interferometry (Fig. 1(e)). Various cavity designs were investigated, with theoretical group index ranging from ng=37 to ng>100. Record-high GBP=0.45 was demonstrated over a bandwidth approaching 20nm (Fig. 1(e)), with ng=37, a very homogeneous flat-top transmission profile (Fig. 1(c) and (d), variations lower than 10 dB) and losses below 67 dB/ns. On a different design [3], an average ng=107 with 15% variation over 7.4nm was measured. These values range among the best ever demonstrated for a silicon device.

Chemical Species Discrimination by a Photonic Crystal Cavity Array

We demonstrate that a properly-tuned array of photonic crystal cavities can be used to discriminate between the absorption spectra of acetylene and hydrogen cyanide.

Slow-Light Enhanced Integrated Spectrometers on Chip

We propose using slow light to enhance the spectral performance of on-chip spectrometers. We use an optimized calzone photonic crystal waveguide to analyze the pontential of integrated slow-light spectrometers under practical considerations.