Excitation

Excitation types

Temporal properties

The temporal properties of the sources are stored in the FDTD object.

Sinusoidal excitation

For simulations where a continuous wave is required, a sinusoidal excitation is the method of choice which is simply defined by its frequency.

A sinusoidal excitation is added by

FDTD = SetSinusExcite(FDTD,f0)

with the parameters

• FDTD: the original FDTD object of the simulation
• f0: the frequency of the excitation

Gaussian excitation

A Gaussian type excitation is needed when simulating the response to a light pulse or the steady state response to a range of frequencies. A Gaussian excitation is a sinusoidal wave with a Gaussian envelope function. It is defined by its its center frequency and the cutoff frequency.

A Gaussian excitation is added by

FDTD = SetGaussExcite(FDTD,f0,fc);

with the parameters

• FDTD: the original FDTD object of the simulation
• f0: the center frequency of the pulse
• fc : 20dB cutoff frequency --> bandwidth is 2*fc

Custom excitation

If a source cannot be described by a continuous wave or a Gaussian pulse one can define a custom excitation. The source can be defined by any function of time.

A custom source is added by

FDTD = SetCustomExcite(FDTD,f0,funcStr)

with the parameters

• FDTD: the original FDTD object of the simulation
• f0: nyquist rate
• funcStr : string desribing the excitation function e(t)

Spatial properties

Standard excitation

A standard E-field or H-field excitation is added with the AddExcitation() function. The source has to be assigned to a box primitive which defines its position and size. The standard excitation is added by

CSX = AddExcitation(CSX, name, type, excite, varargin)

with the parameters

• CSX: CSX-struct created by InitCSX
• name: property name for the excitation
• type:
• 0=E-field soft excitation
• 1=E-field hard excitation
• 2=H-field soft excitation
• 3=H-field hard excitation
• 10=plane wave excitation
• excite: e.g. [2 0 0] for excitation of 2 V/m in x-direction

• Delay  : setup an excitation time delay in seconds
• PropDir: direction of plane wave propagation (plane wave excite only)

A hard excitation forces the field at the source points to exactly take the field values of the source, ignoring any other incoming wave e.g. from reflections. For soft sources the field values at source points are the superposition of the defined source and other travelling waves

Total-field/scattered-field excitation

To create a plane wave excitation in the sense of a total-field/scattered field approach, the AddPlaneWaveExcite() function is used. This type of source is very useful if only the scattered field of an object is of interest. The field from the excitation is confined to the box defined for the source, the scattered field will propagate beyond the box. A plane wave excitation must not intersect with any kind of material. This excitation type can only be applies in air/vacuum and completely surrounding a structure. The plane wave source has to be assigned to a box primitive which defines the position and extend of the field. The excitation is added by

function CSX = AddPlaneWaveExcite(CSX, name, k_dir, E_dir, <f0, varargin>)

With the parameters

• CSX: CSX-struct created by InitCSX
• name: property name for the excitation
• k_dir: unit vector of wave progation direction
• E_dir: electric field polarisation vector (must be orthogonal to k_dir)
• f0: frequency for numerical phase velocity compensation (optional)

Examples

Add a gaussian pulse as a total-field/scattered-field source:

FDTD = SetGaussExcite( obj.FDTD, 0.5*(f_start+f_stop), 0.5*(f_stop-f_start) );
inc_angle = 0 /180*pi; %incident angle on the x-axis
k_dir = [cos(inc_angle) sin(inc_angle) 0]; % plane wave direction
E_dir = [0 0 1]; % plane wave polarization --> E_z
f0 = 500e6;      % frequency for numerical phase velocity compensation

CSX = AddPlaneWaveExcite(CSX, 'plane_wave', k_dir, E_dir, f0);
start = [-100 -100 -100];
stop = [100 100 100];
CSX = AddBox(obj.CSX, 'plane_wave', 0, start, stop); % source is in the box defined by start and stop

Add a sinusoidal line excitation (short dipole):

FDTD = SetSinusExcite(FDTD,f0)
CSX = AddExcitation( CSX, 'infDipole', 1, [1 0 0] );
start = [-dipole_length/2, 0, 0];
stop  = [+dipole_length/2, 0, 0];
CSX = AddBox( CSX, 'infDipole', 1, start, stop );