Ethan Schonbrun

3D interferometric optical tweezers using a single spatial light modulator

Hexagonal arrays of micron sized silica beads have been trapped in three-dimensions within an optical lattice formed by the interference of multiple plane-waves. The optical lattice design with sharply peaked intensity gradients produces a stronger trapping force than the traditionally sinusoidal intensity distributions of other interferometric systems. The plane waves were generated using a single, phase-only, spatial light modulator (SLM), sited near a Talbot image plane of the traps. Compared to conventional optical tweezers, where the traps are formed in the Fourier-plane of the SLM, this approach may offer an advantage in the creation of large periodic array structures. This method of pattern formation may also be applicable to trapping arrays of atoms.

Polarization beam splitter based on a photonic crystal heterostructure.

The design and characterization of a photonic crystal (PC) polarization beam splitter (PBS) that operates with an extinction ratio of greater than 15 dB for both polarizations are presented. The PBS is fabricated on a silicon-on-insulator (SOI) wafer where the input and output ports consist of 5 mum wide ridge waveguides. A large spectral shift is observed in the dispersion plots of the lowest-order even (TE-like) and odd (TM-like) modes due to the SOI confinement. Because of this shift, the TE-like mode is close to a directional gap at the top of the band, and the TM-like mode is in a low-frequency regime where the dispersion surface is almost isotropic. We show that the TE-like mode has very high reflection at the interface between the two PCs, whereas the TM-like mode exhibits a very high transmission.

Negative refraction in a Si-polymer photonic Crystal membrane

We have observed negative refraction in a photonic crystal (PC) membrane structure at near-infrared wavelengths. The device is fabricated on a silicon-on-insulator substrate and consists of a silicon rod matrix suspended in a polymer where optical energy is delivered by ridge waveguides with various incident angles. The propagation direction inside the PC shows extraordinary refraction with an angle consistent with our two-dimensional numerical simulations. To our knowledge, this is the first experimental observation of isotropic negative refraction in a Si-based planar PC structure at optical frequencies. The realization of negative refraction in a planar Si-based system enables applications in negative index imaging to be integrated into compact optical circuits.

Silicon-Based 2-D Slab Photonic Crystal TM Polarizer at Telecommunication Wavelength

We report an extremely compact (15.4 mum x 8 mum) silicon-based 2D slab nano photonic crystal (PC) transverse-magnetic (TM) polarizer which blocks propagation of the transverse-electric (TE) polarized light but passes TM polarized light around telecommunication wavelength (1550 nm). The TE polarized light totally vanishes but the TM polarized light propagates with some attenuation in a length of mere 4.9 mum and it has a great potential to be integrated in a complex photonic integrated circuits. To our knowledge, this is the first experimental demonstration of a silicon-based PC TM polarizer at 1.55-mum wavelength. The plane wave expansion method (PWEM) and 2-D and 3-D finite-difference time-domain (FDTD) simulation were utilized to design a periodic triangular array of air holes in 340-nm-thick silicon with a diameter of 170 nm and pitch distance of 347 nm for the TM polarizer and 371 nm for the input and output waveguide. Such a PC TM polarizer was fabricated in silicon-on-insulator wafer using focused ion beam and reactive ion etching. The device was characterized using tunable lasers in the wavelength range of 1528 nm~1604 nm. Transmitted light intensities of the TE and TM polarized lights were measured which clearly showed the TE polarized light is filtered out around 1.55-mum wavelength.

Wave front evolution of negatively refracted waves in a photonic crystal

Using a heterodyne near-field scanning microscope, the authors analyze the phase and amplitude of the electric field of an optical wave across the boundary of positive to negative refraction media. The photonic crystal acts as an extremely anisotropic material with a negative curvature of its dispersion surface whose shape is resolved experimentally. This extreme anisotropy results in the beam having a peculiar phase evolution through propagation that does not occur in isotropic media. A focusing wave is produced by negative refraction, which has diverging wave fronts before the internal focus and converging wave fronts after the focus.

Tunable superlensing by a mechanically controlled photonic crystal

We investigated negative refraction and subwavelength imaging by a mechanically tunable photonic crystal (PC) slab. A honeycomb-structured PC composed of a silicon-polyimide membrane was used because it exhibits isotropic negative refraction within the second photonic band. Using the finite-difference time-domain (FDTD) method, we demonstrated focusing properties of the PC lenses at various frequencies and mechanical stresses. Analyses based on a ray optics model and equifrequency surface also confirmed the behavior observed by the FDTD simulations. These results suggested a mechanically tunable superlens, whose achievable frequency bandwidth was 12.9% of the center frequency for a mechanical stress of ±10%.

Optical vortices for localized optical lattice site manipulation

We present a method for manipulating individual sites in a 2-D optical lattice that enables the creation of defects within otherwise periodic light patterns. The modified optical lattice is created by interference of plane waves and spiral phase waves. Spiral phase modulation creates an optical vortex that is well suited for addressing single rows of the optical lattice owing to the localized wrap of the phase front. Local fringes can be split, annihilated, or have their size tuned by controlling the charge and location of the vortex cores. These properties provide a connection between dislocations in crystals, waves, and optical lattices.

Image inversion and magnification by negative index prisms

A Dove prism and an anamorphic prism pair are investigated in negative index imaging systems. An equilateral triangular prism with refractive index of −1 operates as a negative index Dove prism that inverts as well as images the incident field. A negative index anamorphic prism pair acts as a negative index imaging system with magnification. The relationship between achievable magnification and aberrations is discussed. Both prism systems can be implemented by using negative index photonic crystals, and their performance is demonstrated numerically by the finite-difference time-domain method. These negative index prism structures enhance the functionalities of negative index flat lenses and broaden the applications of negative index materials.

Total internal reflection photonic crystal prism.

An integrated total internal reflection prism is demonstrated that generates a transversely localized evanescent wave along the boundary between a photonic crystal and an etched out trench. The reflection can be described by either the odd symmetry of the Bloch wave or a tangential momentum matching condition. In addition, the Bloch wave propagates through the photonic crystal in a negative refraction regime, which manages diffraction within the prism. A device with three input channels has been fabricated and tested that illuminates different regions of the reflection interface. The reflected wave is then sampled by a photonic wire array, where the individual channels are resolved. Heterodyne near field scanning optical microscopy is used to characterize the spatial phase variation of the evanescent wave and its decay constant.

Silicon-based 2D slab nano photonic crystal TM polarizer in telecommunication wavelength

We report an extremely compact (15.4 mum x 8 mum) silicon-based 2D slab nano photonic crystal transverse magnetic (TM) polarizer which blocks propagation of the transverse electric (TE) polarized light around telecommunication wavelength (1,550 nm). TM polarization occurs in a length of mere 4.9 mum and it has a great potential to be integrated in a complex photonic integrated circuits. To our knowledge, this is the first ever demonstration of silicon-based TM polarizer in telecommunication wavelength. 2D and 3D finite difference time domain (FDTD) simulation was utilized to design a triangular array of air holes in silicon. Such photonic crystal TM polarizer was fabricated in silicon-on-insulator wafer using focused ion beam and reactive ion etch with air hole diameter of 170 nm and pitch distance of 347 nm for the TM polarizer and 371 nm for the input and output waveguide. The device was fully characterized using tunable lasers in the wavelength range of 1,528 nm ~ 1,604 nm. Transmitted light intensities of the TE and TM polarized lights were measured which clearly showed the TE polarized light is filtered out around 1.55 mum wavelength.