Home Professor S. T. Ho Research Publications Group member

Research project 

(1) Nanophotonics

       We have made the first realizations of various nanophotonic devices: photonic-wire lasers, various microdisk lasers, nanoscale semiconductor waveguides, thin-film nanophotonic structures, photonic bandgap along photonic wire, lasers with very high spontaneous-emission coupling factors. Current research focus is on novel nanophotonic devices with high performance capabilities, including both electro-optical and all-optical devices.

     The typical sizes of nanoscale photonic devices are tens of microns large, which are 100-1000 times smaller than that of the conventional photonic devices. With use of nanophotonic devices, an integration density of around 10,000-100,000 devices per square centimeter can be achieved, leading to very large scale integration or VLSI photonic integrated circuits. 

     Our efforts in nanophotonics are in the following areas

 1-1 Passive Nanophotonic Devices

        First Nanoscale Semiconductor Waveguide

       We realized the first semiconductor waveguide with submicron (& subwavelength) vertical and horizontal dimensions (0.2mm X 0.4 mm at l=1.55 mm) [1995; Ref]. It was used for our ¡°photonic-wire lasers¡±. Our nanoscale waveguide had refractive indices of n=1.0-1.5 for waveguide cladding and n=3.4.for waveguide core, giving an effective mode size of 0.02mm2, about 30X smaller than that of the usual semiconductor waveguides. Previously, nanoscale or strongly-guiding waveguides were thought to be too lossy to use for devices. With a combination of etching technologies (ICP, CAIBE, RIE), we had successfully demonstrated that these nano-waveguides are good enough for realizing various nanoscale photonic devices [Refs].

 

First Microresonator Mux/Demux


  
  

  We realized the first nanoscale waveguide-coupled high-Q microring and microdisk resonator [1996; Ref]. Nanofabrication techniques were developed and optimized to reduce the cavity¡¯s side-wall scattering loss, giving high cavity Q values of around 10,000 for these resonators. These high Q resonators can be used as optical switches, intensity/phase modulators, wave~~length multiplexeres, or channel adding/dropping filters. The main advantages of these devices are their small sizes, very broad (20-30nm) tuning range, narrow spectral bandwidth (0.02nm good for DWDM) and potentially high modulation speed.

Photonic Bandgap (PBG) Structures along Photonic Wire

We proposed the realization of photonic-bandgap cavity via a photonic bandgap structure along a photonic-wire waveguide [199x; Ref]. This cavity can reach a cavity volume of ~0.03 mm3.

Smallest Directional Coupler

We realized the smallest optical directional coupler with a full coupling length of 2mm [199x; Ref]. The short coupling length is because of the high refractive index contrast between the waveguide (n=3.4) and its surrounding (n=1).

 

Multimode Interference (MMI) Devices

   We realized compact MMI devices based on strongly guiding waveguides and showed that ..

1-2 Quantum Effects in Nanophotonic Devices

First Investigation of Spontaneous Emission Modification in Nanophotonic Structures and the Concept of Photonic Wires/Wells/Dots

Strongly-guiding waveguides for photons can be seen as the direct analogue of quantum wires for electrons. In the strongly-guiding nanoscale waveguide, photons are strongly confined and the properties of spontaneous emission are modified due to the modification in the photonic density of states. In a quantum wire, the electrons are strongly confined and the properties of spontaneous emission are modified due to the modification in the electronic density of states. Because of this analogy to electronic quantum wires, We introduced the concept of photonic well [III-4 to III-7, III-14], photonic wire [III-8 to III-14], and photonic dots, for which photons are strongly confined in one, two, or three of the dimensions, respectively. Among the various properties, photonic wires and photonic wells can be used to eliminate unwanted dipole emissions and increase the amount of spontaneous emission captured by the lasing mode in a microcavity lasers.

 

First Observation of Spontaneous Emission Modification in Quantum Dot Microdisk Lasers  

We showed that the quantum dots in the microdisk laser will have different spontaneous decay rate depending on whether they are on resonance or off-resonance with the high Q  microdisk cavity. A change of decay rate by ?x was observed.

1-3 Nanoscale Lasers

 

1-3.1 Photonic-Wire Lasers

First Photonic-Wire Laser

Making use of strongly-guiding photonic-wire waveguides allows us to realize very small high-Q ring microcavity lasers with ring-cavity diameters of only a few micrometers. We have realized the first microcavity laser based on photonic-wire waveguide for which about 70% (or 35% for each of the two CW & CCW ring-cavity modes) of the spontaneous emission is captured into the lasing mode. This performance is near the case of an ideal microcavity laser for which all the spontaneous emission shall be captured into a single lasing mode. We call this laser a photonic-wire laser [IV-4 to IV-6] (~1993-1996). The photonic-wire laser has a cavity volume smaller than 0.3 cubic micrometers, which is around the smallest semiconductor laser cavity ever realized [IV-5]. We realized the first photonic-wire lasers for which over 70% of the spontaneous emission is captured into the clockwise and counter-clockwise lasing mode in a micro-ring cavity formed by photonic-wire waveguide.

  First Nanolasers with Sub-Cubic Micrometer Cavity Volume

The photonic-wire laser realized in 1995 with 4mm ring diameter has a cavity volume of 0.3 mm3, making it the smallest semiconductor laser cavity then. This ring diameter can be as small as 0.8 mm without incurring excessive radiation loss, at which the cavity volume would be 0.06 mm3. Current photonic-bandgap laser has a cavity volume of about 0.02 mm3.

  First Nanolaser Coupled to Nanoscale Waveguides

We realized the first nanoscale laser (photonic-wire laser) coupled to nanoscale waveguides, opening the means to obtain useful output from annolasers.

1-3.2 Microdisk Lasers

First Vertical Coupled Structure for Microdisk Lasers

Due to the sensitivity of coupling to the gap size of a wvaeguide coupler, we devised a vertically-coupled structure using a double microdisk structure with the top disk being a passive disk and the lower disk the lasing disk.

First Dilute Nitride Based (InAsNP) Microdisk Laser Lasing at Room Temperature

Microdisk lasers, on the other hand, may be classified more as photonic-well lasers, as they have strong mode confinement primarily in only one of the three propagation directions (i.e. the top-down direction). We have realized various microdisk lasers [IV-20 to IV-36], including one based on a double-disk structure to achieve output coupling [IV-7to IV-8].

We realized the first microdisk laser using dilute nitride materials, which has a large conduction-band offset. The large conduction band offset enabled lasing at high temperature, allowing us to achieve lasing at room temperature for optically-pumped microdisk lasers.

First UV Microdisk Lasers

The modification of spontaneous emission in [III-4 to III-14](~1991-1996). This effect has been utilized in microdisk lasers and photonic-wire lasers to increase the efficiency of capturing photons into the lasing modes. For example, in microdisk lasers, up to 10-20% of the spontaneous emission can be captured into the lasing mode, as discussed in [IV-1 to IV-3] (~1993-1994).

 

First Erbium-Doped Microdisk Cavity

The modification of spontaneous emission in [III-4 to III-14](~1991-1996). This effect has been utilized in microdisk lasers and photonic-wire lasers to increase the efficiency of capturing photons into the lasing modes. For example, in microdisk lasers, up to 10-20% of the spontaneous emission can be captured into the lasing mode, as discussed in [IV-1 to IV-3] (~1993-1994).

 

We realized the first microdisk laser lasing at UV wavelength by coating ZnO on a microdisk resonator made up of silica.

 

First Quantum-Dot Microdisk Lasers

We realized the first quantum-dot microdisk laser.

 

1-3.3 Passive-Ring Resonator Lasers

 

First Passive-Ring Resonator Lasers

Also, recently (2002) we are the first to further improve the lasing characteristics of photonic-well lasers to include single frequency operation by introducing a microring filter within the laser cavity. We call this new type of laser "passive resonator-based (PMR) laser". This is represented by sample paper [E1].

   

 

NORTHWESTERN UNIVERSITY McCormick SCHOOL ELECTRICAL  & COMPUTER ENGINEERING CPCC