Apratim Majumder

Barrier-free absorbance modulation for super-resolution optical lithography

Absorbance-Modulation-Optical Lithography (AMOL) enables super-resolution optical lithography by simultaneous illumination of a photochromic film by a bright spot at one wavelength, λ1 and a node at another wavelength, λ2. A deep subwavelength region of the transparent photochromic isomer is created in the vicinity of the node. Light at λ1 penetrates this region and exposes an underlying photoresist layer. In conventional AMOL, a barrier layer is required to protect the photoresist from the photochromic layer. Here, we demonstrate barrier-free AMOL, which considerably simplifies the process. Specifically, we pattern lines as small as 70nm using λ1 = 325nm and λ2 = 647nm. We further elucidate the minimum requirements for AMOL to enable multiple exposures so as to break the diffraction limit.

Ultra-compact polarization rotation in integrated silicon photonics using digital metamaterials

Polarization controlling devices such as polarization splitters and rotators are critical elements in integrated-photonic circuits that function via polarization-diversity schemes. Here, we present the design of an ultra-compact nanophotonic-polarization rotator (NPR) that rotates the polarization state from TE to TM with a simulated extinction ratio of 23dB over a coupling length of 5µm and an operating bandwidth of 40nm. This all-silicon device can be fabricated in a single lithography step and we have fabricated and characterized a preliminary device exhibiting 9dB extinction ratio. To emphasize the generality of our methodology, we also designed a NPR that can rotate the polarization state from TM to TE as well. A small device footprint is enabled by the evanescent coupling of guided modes enabled by computationally designed digital metamaterials.

A comprehensive simulation model of the performance of photochromic films in absorbance-modulation-optical-lithography

Optical lithography is the most prevalent method of fabricating micro-and nano-scale structures in the semiconductor industry due to the fact that patterning using photons is fast, accurate and provides high throughput. However, the resolution of this technique is inherently limited by the physical phenomenon of diffraction. Absorbance-Modulation-Optical Lithography (AMOL), a recently developed technique has been successfully demonstrated to be able to circumvent this diffraction limit. AMOL employs a dual-wavelength exposure system in conjunction with spectrally selective reversible photo-transitions in thin films of photochromic molecules to achieve patterning of features with sizes beyond the far-field diffraction limit. We have developed a finite-element-method based full-electromagnetic-wave solution model that simulates the photo-chemical processes that occur within the thin film of the photochromic molecules under illumination by the exposure and confining wavelengths in AMOL. This model allows us ...

Reverse-absorbance-modulation-optical lithography for optical nanopatterning at low light levels

Absorbance-Modulation-Optical Lithography (AMOL) has been previously demonstrated to be able to confine light to deep sub-wavelength dimensions and thereby, enable patterning of features beyond the diffraction limit. In AMOL, a thin photochromic layer that converts between two states via light exposure is placed on top of the photoresist layer. The long wavelength photons render the photochromic layer opaque, while the short-wavelength photons render it transparent. By simultaneously illuminating a ring-shaped spot at the long wavelength and a round spot at the short wavelength, the photochromic layer transmits only a highly confined beam at the short wavelength, which then exposes the underlying photoresist. Many photochromic molecules suffer from a giant mismatch in quantum yields for the opposing reactions such that the reaction initiated by the absorption of the short-wavelength photon is orders of magnitude more efficient than that initiated by the absorption of the long-wavelength photon. As a resul...

Passive and active light control using computational metamaterials

We report on our latest developments in computational metamaterials based nanophotonics devices. Non-linear optimization is used to design passive metamaterials for ultracompact on-chip polarization rotator and waveguide cloaks. We also report a novel method of implementing active control to such devices and present the design of an all-optical modulator. Our devices exhibit efficient performance at a much smaller footprint compared to similar conventional devices.

Ultra-compact nanophotonic devices designed by computational metamaterials

Computationally designed metamaterials are used to realize a number of novel nanophotonic devices. We report on the development of ultra-compact on-chip polarization rotators, waveguide cloaks and all optical modulators made possible by the use of a non-linear optimization algorithm.

Modelling the performance of photochromic thin films to achieve super-resolution nanopatterning by absorbance modulation at low light intensity

This paper presents a comprehensive model and simulation results of the performance and governing factors of the photochromic layer in Absorbance-Modulation-Optical-Lithography (AMOL) and examines the possibility of performing AMOL at low light intensities.

Super-Resolution Optical Lithography via Barrier-Free Absorbance Modulation and Separable Substrate Technique

This paper reports two new techniques of performing the sub-wavelength lithographic process - Absorbance Modulation Optical Lithography (AMOL) without barrier layers and using separable substrates and its most recent developments and characterization.

Exploiting photochromism for optical nanopatterning

This paper reports the application of photochromic molecules to optical subwavelength nanopatterning. Two distinct approaches will be described.