GUIDED BY SUBMITTED BY

UDAY VEER SIR VISHAL SINGH 00320903613 MAE-2SEMESTER

INTRODUCTION & APPLICATION

NANOPHOTONICS APPLIED PHYSICS

INTRODUCTION

Nanophotonics or Nano-optics is the study of the behavior of light on

the nanometer scale. It is considered as a branch of optical engineering which

deals with optics, or the interaction of light with particles or substances, at

deeply sub wavelength length scales. Technologies in the realm of Nano-optics

include near-field scanning optical microscopy (NSOM), photo assisted

scanning tunneling microscopy, and surface Plasmon optics. Traditional

microscopy makes use of diffractive elements to focus light tightly in order to

increase resolution. But because of the diffraction limit (also known as the

Rayleigh Criterion), propagating light may be focused to a spot with a

minimum diameter of roughly half the wavelength of the light. Thus, even with

diffraction-limited confocal microscopy, the maximum resolution obtainable is

on the order of a couple of hundred nanometers.

Electrons play crucial role of information carriers between light and

matter in various photonic devices. Electrons have the split personality of

wave properties in terms of wavelength and corpuscular properties in terms of

mass and charge. Thus by proper manipulation of the interaction of light with

matter, information processing speed can be increased substantially. The

interaction of light with matter can be effectively modified when the spatial

inhomogeneities that are present in the medium of various photonic devices

are not negligible when compared to the electron wavelength. Variations in

the electric and magnetic fields or inhomogeneity in electronic charge or

electronic mass displacement can contribute to the spatial inhomogeneities

for electrons.

When the spatial inhomogeneities can be extended to the atomic or sub

atomic level, the interaction of light with matter becomes the prerogative of

various processes that are involved in the electron subsystem of atoms. The

atoms in turn may form molecules and solids. Hence Nanophotonics can be

characterized as the science and technology of confined light waves in

complex media and confined electron waves in various nanostructured solids

that in turn determine a plethora of versatile physical phenomena.

ADVANTAGES/USES

Enormous data transmission rate

Trans-oceanic fibre optic caller have been deployed in the Pacific and

Atlantic oceans for long distance communication technology. They are also

being used as local area networks. Thus owing to the speed by which various

data types can be sent from one place to another with minimal loss, electronic

technology has been replaced by photonics technology. Photonic integrated

circuits are devices that are similar to electronic integrated circuits which

integrate multiple photonic functions. Photonic integrated circuits are

replacing electronic integrated circuits owing to their higher capability and

performance in signal processing. This can be attributed mainly due to high

switching speeds offered by photonic integrated circuits. Hence with photonic

integrated circuit technology, one can achieve high-speed data processing

with an average processing speed of the order of tera bits per second.

High optical memory storage density

An optical disk memory has the capacity for storing enormous amount

of digital signals in numerous small pits present on its surface, where each pit

stores one bit of data. The stored digital signals are read by illuminating the

disk surface by a focused laser beam and by detecting the laser light reflected

from the disk surface Nanophotonics devices, which are capable of large-scale

data storage and processing and thereby lay the foundation for the

fabrication, measurement, control and functional requirements of novel

optical science and technology.

Diffraction limited Nanophotonics

Diffraction limited nanophotonics, as a broader perspective, encompasses

photonic crystals, plasmonics, silicon photonics and quantum dot lasers, that

employ conventional propagating light.

Photonic crystals Photonic crystals are mainly used for controlling optical interference and light scattering by devising a sub wavelength-sized periodic structure in the photonic device material. Hence they are mainly used as filter device. The principal laying behind the working of a photonic crystal is that at the centre of the device material, constructive interference occurs between scattered light. Thus optical energy is concentrated. In order to filter out the scattered light, it is made to interference destructively at the edge of the device material. Constructive Interference is maintained at the centre of the device material only when the rim of the material is made sufficiently larger than the wave length of the conventional propagating light. Otherwise, light that is concentrated at the centre leaks to the rim, thereby playing a spoil sport for constructive interference to occur. As photonic crystals employs conventional propagating light and as its size cannot be reduced beyond a certain values, the spatial dimensions of the photonic crystal is limited by diffraction.

Plasmonics In plasmonics technology, by exciting free electrons, resonant enhancement of light takes place in a metal. As a result of strong interaction with the free electrons, optical energy gets concentrated on the metal surface in the form of a surface plasmon, which represents in general, the quantum mechanical picture of plasmon oscillation of free electrons on the metal surface. But this quantum mechanical picture is lost as the plasma oscillation of electrons has a short phase relaxation time. Hence plasmonics is essentially governed by wave optics in the metal and hence is limited by diffraction.

Silicon Photonics In silicon photonics, narrow-striped optical wave guides that use high-refractive index silicon crystals are employed to confine light effectively. As wave optics is solely responsible for this light confinement, this is essentially an application of wave optics in silicon and hence is limited by diffraction.

Quantum dot lasers Nanometre-sized semiconductor quantum dots are used as the gain media in a quantum dot laser. Large number of quantum dot lasers are required in order to confine light effectively because semiconductor quantum dots are much smaller than the wavelength of light and hence an individual quantum dot cannot be employed for effective light confinement due to scattering and diffraction. This again leads to the scenario that the device size becomes limited by diffraction. Thus all the above mentioned cases are based on diffraction limited wave optics. Also, even if nanometre- sized materials are used for the above mentioned cases in the future, as long as conventional propagating light is used, the size of these photonic devices cannot be reduced beyond the diffraction limit. Hence to go beyond the diffraction limit, the only other option left is to use non-propagating or stationary nanometre-sized light to induce primary excitations in a nanometre-sized material such that the spatial phase of the excitation is independent of that of the incident light.

Electromagnetic radiation in vacuum

E denote the macroscopic electric field strength, D-the electric displacement, H-the magnetic field strength, P-the electric polarization of the macroscopic medium, M-the magnetization of the macroscopic medium, B-the magnetic induction, r-the charge density and J-the electric current density. For electric displacement are free charges

E and H are responsible for the generation of each other and that the macroscopic current density J is responsible for the generation of the magnetic field strength H can be represented by the following respective relations

Vacuum is represented by the following conditions

Solving,

Hence, the equation for plane wave is given by

Diffraction of light Diffraction of light is generally formed as the encouragement of light on any obstacle and is attributed to the wavelength of light. Huygens’ theory of light states that as a point source is responsible for a spherical wavefront, any point propagated by a light wave is solely responsible for the origin of secondary spherical waves that spread out in all directions. By adding the concept of interference to Huygens’ theory, Fresnel stated that the complex amplitude of a light wave beyond the wavefront can be superimposed to that of all elementary waves which propagate from each point of the wavefront to the observed point

where A is the amplitude, r0 is the radius and k is the propagation constant of the plane wave.

where K (q ) is termed as the obliquity factor such that 0 ≤ K(q ) ≤ M, where K(q )→0 as q →1.57 and tends to a maximum value M, when q →0. q is the angle subtended by a line between a points of the wavefront to the observed point to a line normal to the wavefront in the amplitude of the elementary waves in the plane wave. When there is no obstacle, the process of light propagation is given by the equation

where ds is the infinitesimal area.

APPLICATIONS

NEMS

Nanoelectromechanical systems (NEMS) are a class of devices integrating electrical and mechanical functionality on the nanoscale. NEMS form the logical next miniaturization step from so-called microelectromechanical systems, or MEMS devices. NEMS typically integrate transistor-like nanoelectronics with mechanical actuators, pumps, or motors, and may thereby form physical, biological, and chemical sensors. The name derives from typical device dimensions in the nanometre range, leading to low mass, high mechanical resonance frequencies, potentially large quantum mechanical effects such as zero point motion, and a high surface-to-volume ratio useful for surface-based sensing mechanisms. Uses include accelerometers, or detectors of chemical substances in the air.

A key application of NEMS is atomic force microscope tips. The increased sensitivity achieved by NEMS leads to smaller and more efficient sensors to detect stresses, vibrations, forces at the atomic level, and chemical signals. AFM tips and other detection at the nanoscale rely heavily on NEMS. If implementation of better scanning devices becomes available, all of nanoscience could benefit from AFM tips. FABRICATION Two complementary approaches to fabrication of NEMS can be found.

The top-down approach uses the traditional micro fabrication methods, i.e. optical and electron beam lithography, to manufacture devices. While being limited by the resolution of these methods, it allows a large degree of control over the resulting structures. Typically, devices are fabricated from metallic thin films or etched semiconductor layers.

Bottom-up approaches, in contrast, use the chemical properties of single molecules to cause single-molecule components to

(a) Self-organize or self-assemble into some useful conformation (b) Rely on positional assembly. These approaches utilize the concepts of

molecular self-assembly and/or molecular recognition. This allows fabrication of much smaller structures, albeit often at the cost of limited control of the fabrication process.

Nanotube nanomotor

A device generating linear or rotational motion using carbon nanotube(s) as the primary component, is termed a nanotube nanomotor. Nature already has some of the most efficient and powerful kinds of nanomotors. Some of these natural biological nanomotors have been re-engineered to serve desired purposes. However, such biological nanomotors are designed to work in specific environmental conditions (pH, liquid medium, sources of energy, etc.). Laboratory-made nanotube nanomotors on the other hand are significantly more robust and can operate in diverse environments including varied frequency, temperature, mediums and chemical environments. The vast differences in the dominant forces and criteria between macro scale and micro/nanoscale offer new avenues to construct tailor-made nanomotors. The various beneficial properties of carbon nanotubes makes them the most attractive material to base such nanomotors on.

NEMS nanomotor The nanoactuator consists of a gold plate rotor, rotating about the axis of a multi-walled nanotube (MWNT). The ends of the MWNT rest on a SiO2 layer which form the two electrodes at the contact points. Three fixed stator electrodes (two visible 'in-plane' stators and one 'gate' stator buried beneath the surface) surround the rotor assembly. Four independent voltage signals (one to the rotor and one to each stators) are applied to control the position, velocity and direction of rotation. Empirical angular velocities recorded provide a lower bound of 17 Hz (although capable of operating at much higher frequencies) during complete rotations. Arrays of nanoactuators

Due to the minuscular magnitude of output generated by a single nanoactuator the necessity to use arrays of such actuators to accomplish a higher task comes into picture. Conventional methods like chemical vapor deposition (CVD) allow the exact placement of nanotubes by growing them directly on the substrate. However, such methods are unable to produce very high qualities of MWNT. Moreover, CVD is a high temperature process that would severely limit the compatibility with other materials in the system. A Si substrate is coated with electron beam resist and soaked in acetone to leave only a thin polymer layer. The substrate is selectively exposed to an low energy electron beam of an SEM that activates the adhesive properties of the polymer later. This forms the basis for the targeting method. The alignment method

exploits the surface velocity obtained by a fluid as it flows off a spinning substrate. MWNTs are suspended in orthodicholrobenzene (ODCB) by ultrasonication in an aquasonic bath that separates most MWNT bundles into individual MWNTs. Drops of this suspension are then pipetted one by one onto the center of a silicon substrate mounted on a spin coater rotating at 3000 rpm.

Arc-discharge evaporation technique

This technique is a variant of the standard arc-discharge technique used for the synthesis of fullerenes in an inert gas atmosphere. The experiment is carried out in a reaction vessel containing an inert gas such as helium, argon, etc. flowing at a constant pressure. A potential of around 18 V is applied across two graphite electrodes (diameters of the anode and cathode are 6 mm and 9 mm) separated by a short distance of usually 1–4 mm within this chamber. The amount of current (usually 50–100 A) passed through the electrodes to ensure nanotube formation depends on the dimensions of the electrodes, separation distance and the inert gas used. As a result, carbon atoms are ejected from the anode and are deposited onto the cathode hence shrinking the mass of the anode and increasing the mass of the cathode. The black carbonaceous deposit (a mixture of nanoparticles and nanotubes in a ratio of 1:2) is seen growing on the inside of the cathode while a hard grey metallic shell forms on the outside. The total yield of nanotubes as a proportion of starting graphitic material peaks at a pressure of 500 torr at which point 75% of graphite rod consumed is converted to nanotubes. The nanotubes formed range from 2 to 20 nm in diameter and few to several micrometres in length. There are several advantages of choosing this method over the other techniques such as laser ablation and chemical vapor deposition such as fewer structural defects (due to high growth temperature), better electrical, mechanical and thermal properties, high production rates (several hundred mg in ten minutes), etc. Applications

1. The rotating metal plate could serve as a mirror for ultra-high-density optical sweeping and switching devices as the plate is at the limit of

visible light focusing. An array of such actuators, each serving as a high frequency mechanical filter, could be used for parallel signal processing in telecommunications.

2. The plate could serve as a paddle for inducing or detecting fluid motion in microfluidic applications. It could serve as a bio-mechanical element in biological systems, a gated catalyst in wet chemistry reactions or as a general sensor element.

3. A charged oscillating metal plate could be used as a transmitter of electromagnetic radiation.

Thermal gradient driven nanotube actuators Fabrication

The MWNT are fabricated using the standard arc-discharge evaporation process and deposited on an oxidized silicon substrate. The gold plate in the centre of the MWNT is patterned using electron-beam lithography and Cr/Au evaporation. During the same process, the electrodes are attached to the nanotube. Finally, electrical-breakdown technique is used to selectively remove a few outer walls of the MWNT. This enables low friction rotation and translation of the shorter nanotube along the axis of the longer tube. The application of the electrical-breakdown technique does not result in the removal of the tube(s) below the cargo. This might be because the metal cargo absorbs the heat generated in the portion of the tube in its immediate vicinity hence delaying or possibly even preventing tube oxidation in this part. Principle

The interaction between the longer and shorter tubes generates an energy surface that confines the motion to specific tracks – translation and rotation. The degree of translational and rotational motion of the shorter tube are highly dependent on the chirality’s of the two tubes as shown in Figure 2.3. Motion in the nanoactuator displayed a proclivity of the shorter tube to follow a path of minimum energy. This path could either have a roughly constant energy or have a series of barriers. In the former case, friction and vibrational motion of atoms can be neglected whereas a stepwise motion is expected in the latter scenario. Mechanism for actuation

Many proposals were made to explain the driving mechanism behind the nanoactuator. The high current (0.1 mA) required to drive the actuator is likely

to cause sufficient dissipation to clean the surface of contaminants; hence, ruling out the possibility of contaminants playing a major role. The possibility of electro migration, where the electrons move atomic impurities via momentum transfer due to collisions, was also ruled out because the reversal of the current direction did not affect the direction of displacement. Similarly, rotational motion could not have been caused by an induced magnetic field due to the current passing through the nanotube because the rotation could either be left or right-handed depending on the device. Stray electric field effect could not be the driving factor because the metal plate staid immobile for high resistive devices even under a large applied potential. The thermal gradient in the nanotube provides the best explanation for the driving mechanism. Thermal gradient induced motion

The induced motion of the shorter nanotube is explained as the reverse of the heat dissipation that occurs in friction wherein the sliding of two objects in contact results in the dissipation of some of the kinetic energy as phononic excitations caused by the interface corrugation. The presence of a thermal gradient in a nanotube causes a net current of phononic excitations traveling from the hotter region to the cooler region. The interaction of these phononic excitations with mobile elements (the carbon atoms in the shorter nanotube) causes the motion of the shorter nanotube. This explains why the shorter nanotube moves towards the cooler electrode. Changing the direction of the current has no effect on the shape of thermal gradient in the longer nanotube. Hence, direction of the movement of the cargo is independent of the direction of the bias applied. The direct dependence of the velocity of the cargo to the temperature of the nanotube is inferred from the fact that the velocity of the cargo decreases exponentially as the distance from the midpoint of the long nanotube increases. Applications

Some of the main applications of the electron windmill include: 1. A voltage pulse could cause the inner element to rotate at a calculated

angle hence making the device behave as a switch or a nanoscale memory element.

2. Modification of the electron windmill to construct a nanofluidic pump by replacing the electrical contacts with reservoirs of atoms or molecules under the influence of an applied pressure difference.

Electron windmill Structure

The nanomotor consists of a double-walled CNT (DWNT) formed from an achiral outer tube clamped to external gold electrodes and a narrower chiral inner tube. The central portion of the outer tube is removed using the electrical-breakdown technique to expose the free-to-rotate, inner tube. The nanodrill also comprises an achiral outer nanotube attached to a gold electrode but the inner tube is connected to a mercury bath. Principle

Conventional nanotube nanomotors make use of static forces that include elastic, electrostatic, friction and van der Waals forces. The electron windmill model makes use of a new "electron-turbine" drive mechanism that obviates that need for metallic plates and gates that the above nanoactuators require. When a DC voltage is applied between the electrodes, a "wind" of electrons is produced from left to right. The incident electron flux in the outer achiral tube initially possesses zero angular momentum, but acquires a finite angular momentum after interacting with the inner chiral tube. By Newton's third law, this flux produces a tangential force (hence a torque) on the inner nanotube causing it to rotate hence giving this model the name – "electron windmill". For moderate voltages, the tangential force produced by the electron wind is much greatly exceed the associated frictional forces. Applications

Some of the main applications of the electron windmill include: 1. A voltage pulse could cause the inner element to rotate at a calculated

angle hence making the device behave as a switch or a nanoscale memory element.

2. Modification of the electron windmill to construct a nanofluidic pump by replacing the electrical contacts with reservoirs of atoms or molecules under the influence of an applied pressure difference.

Nanolithography

Nanolithography is the branch of nanotechnology concerned with the study and application of fabricating nanometre-scale structures, meaning patterns with at least one lateral dimension between the size of an individual atom and approximately 100 nm. Nanolithography is used during the fabrication of leading-edge semiconductor integrated circuits or Nanoelectromechanical systems (NEMS). Assisted photolithography On employing a visible light source, the dressed photon-coherent phonon assisted photolithography enhances the patterning of commercial photoresists. The main highlight is that the propagating light does not pattern the photoresist as the photoresist is sensitive only to UV propagating light. However a dressed-photon coherent phonon gets generated at the photoresist edge. The photoresist gets activated by the transfer of energy from the dressed photon coherent phonon to the photoresist and thereby gets patterned due to the phonon assisted process. Moreover, as the energy gets transferred not only to the surface of the photoresist but also to its interior, the photoresist gets effectively patterned within a short exposure time. Thus by properly manipulating the exposure time, the photoresist can be patterned to have a stable spatial profile. By employing a light source of appropriate laser frequency, high resolution can be achieved when the wavelength of the light source is greater than the wavelength of absorption band edge of the photoresist. Hence phonon assisted photolithography is not expensive as it does not require either short wavelength X-ray or UV light source for patterning.

Advantages

1. Complicated patterns can be obtained with high resolution when subject to multiple exposures as the photoresist is insensitive to incident visible light. Phonon assisted photolithography has the ability to pattern even an optically inactive film.

2. Photon-coherent phonons can be generated and applied to a nanometre rough surface material, when illuminated with a light source. On employing dressed photon technology, for repairing surface roughness, namely, etching and desorption.

Near-field scanning optical microscope

Near-field scanning optical microscopy (NSOM/SNOM) is a microscopy technique for nanostructure investigation that breaks the far field resolution limit by exploiting the properties of evanescent waves. This is done by placing the detector very close (distance much smaller than wavelength λ) to the specimen surface. This allows for the surface inspection with high spatial, spectral and temporal resolving

power. With this technique, the resolution of the image is limited by the size of the detector aperture and not by the wavelength of the illuminating light. In particular, lateral resolution of 20 nm and vertical resolution of 2–5 nm have been demonstrated. As in optical microscopy, the contrast mechanism can be easily adapted to study different properties, such as refractive index, chemical structure and local stress. Dynamic properties can also be studied at a sub-wavelength scale using this technique. Near-field spectroscopy

As the name implies, information is collected by spectroscopic means instead of imaging in the near field regime. Through Near Field Spectroscopy (NFS), one can probe spectroscopically with subwavelength resolution. Raman SNOM and fluorescence SNOM are two of the most popular NFS techniques as they allow for the identification of nanosized features with chemical contrast. Some of the common near field spectroscopic techniques are:

Direct local Raman NSOM: Aperture Raman NSOM is limited by very hot and blunt tips, and by long collection times. However, apertureless NSOM can be used to achieve high Raman scattering efficiency factors (around 40). Topological artefact’s make it hard to implement this technique for rough surfaces.

Surface enhanced Raman spectroscopy (SERS) NSOM: This technique can be used in an apertureless shear-force NSOM setup, or by using an AFM tip coated with gold. The Raman signal is found to be significantly enhanced under the AFM tip. This technique has been used to give local variations in the Raman spectra under a single-walled nanotube. A highly

sensitive optoacoustic spectrometer must be used for the detection of the Raman signal.

Fluorescence NSOM: This highly popular and sensitive technique makes use of the fluorescence for near field imaging, and is especially suited for biological applications. The technique of choice here is the apertureless back to the fibre emission in constant shear force mode. This technique uses merocyanine based dyes embedded in an appropriate resin. Edge filters are used for removal of all primary laser light. Resolution as low as 10 nm can be achieved using this technique. Near field infrared spectrometry and near field dielectric microscopy.

References

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o www.nanophotonics.de/Â

o www.ece.rice.edu/~halas/

o nanohub.org/courses/nanophotonics

o Hewakuruppu, Y., et al., Plasmonic “ pump – probe ” method to study

semi-transparent nanofluids

o A. Harootunian, E. Betzig, M. Isaacson, and A. Lewis (1986). "Super-

resolution fluorescence near-field scanning optical microscopy". Appl.

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